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Clinical Microbiology Reviews, April 2004, p. 390-412, Vol. 17, No. 2
0893-8512/04/$08.00+0 DOI: 10.1128/CMR.17.2.390-412.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Animal Pneumoviruses: Molecular Genetics and Pathogenesis
Andrew J. Easton,1 Joseph B. Domachowske,2 and Helene F. Rosenberg3*
University
of Warwick, Coventry, United Kingdom,1
State University of New York
Upstate Medical University, Syracuse, New
York,2
Laboratory of Allergic
Diseases, National Institute of Allergy and Infectious
Diseases, National Institutes of Health, Bethesda,
Maryland3

SUMMARY
Pneumoviruses are single-stranded, negative-sense, nonsegmented
RNA
viruses of the family
Paramyxoviridae, subfamily
Pneumovirinae,
and include pathogens that infect humans
(respiratory syncytial
virus and human metapneumovirus), domestic
mammals (bovine,
ovine, and caprine respiratory syncytial viruses),
rodents (pneumonia
virus of mice), and birds (avian metapneumovirus).
Among the
topics considered in this review are recent studies focused
on
the roles of the individual virus-encoded components in promoting
virus
replication as well as in altering and evading innate antiviral
host
defenses. Advances in the molecular technology of pneumoviruses
and
the emergence of recombinant pneumoviruses that are leading
to
improved virus-based vaccine formulations are also discussed.
Since
pneumovirus infection in natural hosts is associated with
a profound
inflammatory response that persists despite adequate
antiviral therapy,
we also review the recent experimental treatment
strategies that have
focused on combined antiviral, anti-inflammatory,
and immunomodulatory
approaches.

INTRODUCTION
Pneumoviruses (family
Paramyxovirida, subfamily
Pneumovirinae)
are enveloped viruses with
negative-sense, nonsegmented RNA
genomes. The human pathogen,
respiratory syncytial virus (hRSV)
remains the best
characterized of this group. Despite considerable
efforts toward the
development of effective treatments for hRSV
disease, acute
infection remains associated with significant
morbidity and mortality,
particularly among immunocompromised
patients and infants born
prematurely. Supportive therapy alone
remains the standard of care for
the treatment of severe cases
of hRSV infection, although
prophylactic treatments are now
available and encouraging progress has
been made toward formulating
a vaccine. The recently discovered human
pathogen human metapneumovirus
(hMPV) also belongs to this virus
subfamily, as do the veterinary
pathogens avian metapneumovirus (APV),
bovine respiratory syncytial
virus (bRSV), ovine and caprine RSVs, and
pneumonia virus of
mice (PVM). The text begins with a review of
pneumovirus biology
and molecular genetics, continues with a discussion
of current
concepts related to the pathogenesis of pneumovirus disease
as
it occurs in vivo and as it is modeled in vitro, and concludes
with
a discussion of recent therapeutic options for the treatment
of
hRSV in human subjects.

MOLECULAR GENETICS
Virion Structure
Electron
microscopic analysis of the virions of all four members
of the
subfamily
Pneumovirinae demonstrates that they are
structurally
similar to one another and similar to other members of the
family
Paramyxoviridae. Virions, while pleomorphic, are
typically spherical
(diameters of 150 to 200 nm), although filamentous
particles
of up to 400 nm in length have been described
(
9,
22,
192).
PVM virions are
predominantly filamentous, 100 to 120 nm in
diameter, and up to 3
µm in length
(
71). The functional
differences
(if any) based on virion structure are not known, and the
differences
observed may be a consequence of the cell type used for
infection.
A diagrammatic representation of a typical pneumovirus is
shown
in Fig.
1. The pneumovirus virion is surrounded by a lipid envelope
derived from
the plasma membrane of the host cell into which
the three virus
glycoproteins, the attachment (G), fusion
(F),
and small hydrophobic (SH) proteins, are inserted
(
59,
176).
The F protein
spans the lipid membrane, with the amino terminus
located outside the
virion and a short cytoplasmic carboxy-terminal
"tail"
located inside. In contrast, the G protein is a type
2 membrane
protein, with internal amino and external carboxy-terminal
components.
The F and G proteins comprise the 10- to 14-nm spikes
on the virion
surface (
71). The precise
localization of the
SH glycoprotein has not
yet been established.
The F and G
glycoproteins interact with the virus matrix
(M)
protein layer on the internal surface of the membrane. The details
of
the molecular interaction(s) between the M protein and the surface
F
and G glycoprotein(s) are not
clear.
The helical nucleocapsid is located within the M-protein
layer, and includes the 13- to 15-kb single-stranded, non-segmented RNA
genome in association with the virus nucleoprotein (N) and large (L)
proteins and the phosphoprotein (P). The pneumovirus nucleocapsids
(diameter, 13 to 14 nm) are significantly smaller than those that have
been described for other paramyxoviruses (18 nm)
(22). Electron
microscopic studies indicate that hRSV nucleocapsids are less
rigid and narrower than those of measles virus and Simian virus 5
(SV5), further confirming differences in nucleocapsid morphology
between the Pneumovirinae and the Paramyxovirinae
(23). The L protein is
often referred to as the RNA polymerase since it is thought to contain
all of the catalytic activities necessary for virus RNA synthesis,
although it does not function as such without the N and P proteins. The
M2-1 transcriptional enhancer protein is also thought to be associated
with the nucleocapsid, although this has not yet been determined
directly.
Virus Attachment and Entry
The conventional model for pneumovirus attachment involves the
virus
G protein interacting with a molecule or molecules on
the target cell
surface (
232). While the
precise nature of the
cellular receptor has not been identified,
current evidence
suggests that one or more cellular glycosaminoglycans
or heparin-like
molecules are involved
(
116,
163,
164,
218,
342). Following
G-protein-mediated
attachment, the model proposes that the F
glycoprotein then
promotes pH-independent
fusion between the cell membrane and
the virus envelope via a mechanism
involving the hydrophobic
amino-terminal region of the F1 component.
The fusion process
introduces the internal components of the virion
into the cytoplasm
of the host cell, where the remainder of the
infectious cycle
takes place. While pneumovirus infection results in
alteration
of host cell gene expression, there is no absolute
requirement
for a functional nucleus, since virus development can
proceed
in enucleated cells
(
122).
Recent
analyses of deletion mutants of RSV have raised questions about the
conventional model of attachment. Karron et al.
(205) have characterized
a mutant hRSV strain, cp-52, that grows in tissue culture but
is attenuated in animals and humans. Sequencing of the genome of this
mutant showed that it was lacking both the G and SH coding regions but
retained the F-protein gene. Using a recombinant vaccinia virus system.
Heminway et al. (172)
suggested that the hRSV fusion event required the presence of
the F, G, and SH proteins acting in concert. However, Kahn et al.
(199) demonstrated that
heterologous expression of the hRSV F protein alone promoted
infection and cell fusion. Several mutants which lack either the G gene
or the SH gene or both have been generated using a reverse genetics
approach (341,
342; see below). All of
these mutants are viable in tissue culture; their growth rates vary
depending on the target cell and are significantly attenuated in vivo
compared to that of the wild-type parental strain. There are also data
indicating a binding interaction between the RSV G protein and the
fractalkine receptor, CX3CR1
(353), although the
latter is not expressed on human epithelial cells. These data suggest
that the attachment and fusion steps may be more complex than the
conventional model proposes.
Genome Organization of the Pneumovirinae
The genomes of the
pneumoviruses are

15,000 nucleotides in length
(
262,
332;
L. Thorpe and A.
Easton, unpublished data). Metapneumovirus
genomes are smaller
(

13,000 kb) than those of the pneumoviruses.
While the precise
sizes of the genomes are strain dependent,
the number of nucleotides
does not follow the "rule of six"
(
42,
43,
312),
a feature that
differentiates the
Pneumovirinae from the
Paramyxovirinae.
The reasons underlying the discrepancy have
not been determined,
but it has been proposed that length restriction
among the
Paramyxovirinae may be due to specific N-protein to
RNA interactions (e.g.,
binding units of six nucleotides) and/or the
need to ensure
the integrity of the genome in viruses susceptible to
non-template-dependent
nucleotide insertion.
Figure
2 summarizes the genome organization of the pneumoviruses and
metapneumoviruses together with representatives of the
Paramyxovirinae. While the general strategy for genome
organization is similar for all of the viruses, there are differences
in specific detail. The genomes of the pneumoviruses and
metapneumoviruses encode a greater number of mRNAs (10 and 8,
respectively) than the 6 or 7 mRNAs encoded by genomes of the
Paramyxovirinae. While all members of the family
Paramyxoviridae encode a standard set of structural proteins
(293), one feature which
differentiates the Paramyxovirinae from the
Pneumovirinae are different coding potentials of P genes and
the use of an RNA editing mechanism to express multiple proteins from
this gene (82).
Furthermore, the Pneumovirinae lack counterparts of the C and
V proteins found in Paramyxovirinae, although the PVM P gene
does direct the synthesis of a polypeptide by using a second open
reading frame in a manner analogous to the expression of the
paramyxovirus C proteins
(14). Another unique
feature of the Pneumovirinae is the gene encoding the M2
proteins which have not been identified in any other virus
genome.
The genomes of the
Pneumovirinae contain
regulatory sequences
at each terminus. The 3' end of the genome
sense RNA directs
both replication and transcription, while the
5' end of the
genome RNA (i.e., the 3' end of the
antigenome) contains signals
that direct replication of the antigenome
for synthesis of progeny
virions. The metapneumovirus genome encodes
eight distinct transcription
units in a linear array, with each unit
separated by a short
segment of untranscribed sequence. In contrast,
the pneumovirus
genome encodes 10 transcription units,
including nonstructural
proteins NS1 and NS2
(
106). There is one
notable difference
in gene order: the F-protein gene follows the
G-protein gene
in the pneumovirus genomes, the reverse order from that
observed
among the metapneumoviruses
(
293). In evolutionary
terms, this
difference is very striking since recombination among these
virus
genomes has been observed only in the laboratory setting
(
329).
While the
PVM genome is organized with a tandem array of distinct
genes similar to that of the metapneumoviruses, the hRSV
genome includes a 68-nucleotide overlap of the gene encoding the M2
proteins with the start of the L gene, discussed further in
"Current model for genome transcription"
(below).
Pneumovirus Gene Products
As indicated in Fig.
2,
the nucleocapsid, phosphoprotein, matrix,
fusion, and polymerase
proteins are common to all members of
the family
Paramyxoviridae. Much of our understanding of the
functions of
these proteins has been derived by extrapolation
from experiments
performed with one or more specific viruses,
particularly Sendai virus.
More recently, it has been possible
to confirm these functions directly
by targeted mutagenesis
and reverse
genetics.
Nonstructural (NS1 and NS2) proteins.
The NS1 and NS2 proteins are
unique to the pneumoviruses. The genes are located at the
3'-most positions of the genome and, as a consequence, are the
most abundantly transcribed genes. The NS1 and NS2 proteins are
relatively small, with the NS1 proteins of hRSV and PVM being
139 and 113 amino acids in length, respectively, and the NS2 proteins
being 124 and 156 amino acids, respectively
(50,
57,
58).
The
hRSV NS1 protein interacts with the M protein in infected
cells, and two-hybrid analysis identified an additional interaction
between NS1 and the P protein
(106,
173). In reverse
genetics studies, expression of the NS proteins inhibited RNA
synthesis, suggesting a regulatory role for the NS proteins in vivo
(8,
343). While the finding
of hRSV mutants lacking the NS1 or NS2 genes confirmed that
the NS proteins are dispensable for virus replication in vitro,
recombinant viruses grew more poorly than the parental virus and the
mutants were attenuated in chimpanzees
(188,
344,
347,
378).
Other
studies suggest that the pneumovirus NS proteins play a role in
inhibiting responses to interferon
(30,
31,
315). Recombinant rabies
viruses each expressing one of the bovine RSV NS genes were resistant
to interferon treatment if cells were dually infected with the virus
but not if they were infected alone
(315), suggesting that
the NS1 and NS2 proteins function together to inhibit
responses to interferon in this setting. Most recently,
Bossert et al. (31) have
demonstrated that NS1 and NS2 proteins inhibit the
activation of interferon regulatory factor 3 and thus the production of
beta interferon in bRSV- infected MDBK cells. Valarcher et al.
(358) likewise
demonstrated that recombinant, NS-2 lacking, bRSV are potent inducers
of interferon in bovine nasal fibroblasts and bronchoalveolar
macrophages. Studies using recombinant rabies viruses that express the
PVM or hRSV NS1 and NS2 genes have yielded similar results
(30, B. Bossert,
A. J. Easton, and K. K. Conzelmann,
unpublished data). The mechanism by which the interferon
inhibition is achieved is not yet
understood.
Nucleocapsid (N) protein.
The N protein forms an integral
part of the nucleocapsid complex of the virion and is an essential
component of the polymerase complex. The N protein is thought to be
responsible for giving the RNA genome its helical structure through a
tight association with the virus genome RNA; interestingly,
hRSV N protein expressed in insect cells spontaneously formed
nucleocapsid structures containing RNA
(24). While the RSV N
protein (391 amino acids) is smaller than its counterparts among the
Paramyxovirinae (489 to 553 amino acids) and shows little
sequence similarity, alignments with the amino-terminal 400 amino acids
of the N proteins of various nonsegmented negative-sense RNA viruses
suggested similarities among predicted secondary structures
(13). The hRSV N
protein has a relatively high level of amino acid identity (60%)
to the N protein of PVM
(13) and has
regions of specific sequence homology to the N proteins of APV and hMPV
(234,
361).
Deletion
mutant analysis of N protein expressed in cells suggests that a large
segment of this protein is required for interacting with the
P protein (128). Barr
and Easton (12)
demonstrated that the amino-terminal half of the PVM N protein was
unable to bind to immobilized P protein while the carboxy-terminal half
of the N protein bound with only 17.6% of the
affinity seen with the full-length protein, results suggesting that the
complete N protein is important for thisinteraction.
Phosphoprotein (P).
The P protein is an essential component of the
replication and transcription complexes of the pneumoviruses
(14,
225,
234,
314,
361). Analysis of the
interaction between the P and N proteins has shown that the carboxy
terminus of the P protein contains most of the elements necessary to
bind to the N protein
(128,
173,
207,
242,
247,
326). The P-protein
genes of the Pneumovirinae encode several different proteins
using alternative start codons. In addition to the full-length
sequence, the P gene of hRSV encodes an mRNA for an
additional, short polypeptide of unknown function
(45). While the PVM and
hRSV P proteins have significant amino acid identity, the two
proteins align well only at the extreme amino terminus and within the
carboxy-terminal half of the protein, and there are large regions near
the amino terminus with limited to nonexistent sequence identity
(14), including the
second open reading frame of the PVM P gene. The protein
product of this second open reading frame has been detected in infected
cells, as have four additional proteins generated by initiation at
internal AUG translation initiation codons within the P-protein gene
(14). No additional
proteins have been identified from the P genes of the
meta-pneumoviruses.
The P protein is phosphorylated at
specific serine residues
(98). Villanueva et al.
(366) demonstrated that
most of the phosphorylation, occurring at residues 116, 117, 119, and
232, is not essential for transcription or
replication.
Matrix (M) protein.
The M protein is associated with
the inner face of the lipid membrane of infected target cells
(284,
382). All M proteins
include a carboxy-terminal hydrophobic domain. The M proteins of
nonsegmented minus-strand viruses are thought to function by rendering
the nucleocapsid transcriptionally inactive before packaging and also
by promoting the association of the nucleocapsid complex with the
nascent envelope (140,
343).
Small hydrophobic (SH) protein.
The SH proteins
of pneumoviruses are small integral membrane proteins. The sizes of the
proteins vary considerably, with hRSV having the smallest SH
protein (64 amino acids) and hMPV having the largest (183 amino acids)
(58,
100,
361). The RSV SH protein
is anchored in the membrane by a hydrophobic signal-anchor sequence
(275) and is a type 2
membrane protein (59,
117). The precise
function of the SH protein is unknown, and analysis of hRSV
mutants lacking the SH and/or G genes suggests that the SH protein is
not absolutely necessary for attachment, infectivity, or virion
assembly. However, the hRSV G and SH proteins have also been
implicated in impairing the Th1-mediated host antiviral responses
(352).
Attachment (G) protein.
The hRSV G protein
was identified as the viral attachment protein based on the observation
that G-specific polyclonal antibody blocked absorption of the virus to
the surface of target cells
(232). Similarly, a
monoclonal antibody that inhibited the hemagglutinating activity of PVM
likewise bound to the G protein
(235). The pneumovirus G
proteins do not show any sequence or structural homology to the HN or H
attachment proteins of other viruses of the family
Paramyxoviridae, although all are type 2
glycoproteins
(373). The G proteins of
pneumoviruses are heavily glycosylated with both N- and O-glycosylated
sites, adding approximately 55 kDa to the mass of the
hRSV polypeptide. While the roles of these glycosyl groups
have not been completely clarified, extensive glycosylation of the
extracellular domain of the G protein could reduce its antigenicity by
shielding the virus protein with host-specified sugars. Alternatively,
partial utilization of glycosylation sites can result in antigenic
heterogeneity, representing a source of diversity within a genetically
homogenous virus population. The immune response to hRSV
correlates with and provides selection pressure for G-protein variation
(127,
261).
The
hRSV G protein in infected cells is present in two forms. The
first is translated from the entire open reading frame and contains an
amino-terminal sequence prior to the putative signal/membrane anchor
region. A second, secreted form of the protein has been observed
(304) which is generated
by the initiation of translation at an internal AUG initiation codon
that is in the same reading frame as that used for the full- length G
protein. The function of the secreted protein is not known, but it may
alter the immune response of the host, possibly by acting as a
"decoy" or by altering specific aspects of the host
inflammatory response
(190,
191).
While the G
protein of PVM shares structural features with those of hRSV
and the metapneumoviruses
(295), the G-protein
genes of two strains of PVM have been sequenced and show intriguing
differences. The G protein of the nonpathogenic PVM strain 15 does not
have an amino-terminal sequence prior to the putative membrane anchor,
while the G gene of a pathogenic PVM strain contains a single base
deletion and several mutations
(295).
A final
interesting feature of the G genes of hRSV and PVM is the
presence of a small open reading frame just 5' to the main
coding region, of function unknown
(295,
373).
Fusion (F) protein.
The F protein of
hRSV was first was identified by immunoprecipitation using
monoclonal antibodies which inhibit the formation of multicell syncytia
in cell culture. Although the pneumovirus F proteins show only limited
amino acid sequence identity to the F proteins of other viruses of the
family Paramyxoviridae, they show significant structural
similarities to members of this group
(15,
18,
51,
64,
361,
386). The F protein of
hRSV is synthesized as an inactive precursor (F0) that
assembles into a homo-oligomer in the rough endoplasmic reticulum (ER),
forming a structure that is presumed to represent a single virion spike
(62). When the F0 protein
reaches the trans Golgi network, it is activated by cleavage
into two disulfide-linked subunits, F1 and F2, at two furin consensus
sites (142,
392,
393). The short
(27-amino-acid) peptide between the two furin cleavage sites does not
remain associated with the F-protein subunits
(19) and a novel
biological activity for this protein has recently been described
(314; see "Virus
evasion of innate immunity"
below).
M2 proteins.
The genes encoding the M2 proteins of pneumoviruses
and metapneumoviruses contain two overlapping open reading frames (Fig.
3) (4,
58,
165,
237,
361,
387). The first open
reading frame, initiated by the 5'-proximal AUG initiation
codon, encodes the M2-1 protein, which is involved in virus RNA
synthesis as described below. Immunocytochemical analysis of
cytoplasmic inclusions showed that genome RNA, N, P, L, and M2-1
proteins were colocalized in hRSV-infected cells
(126). Reverse genetics
studies have shown that while the minimal trans-acting
requirements for hRSV (i.e., proteins that must be added for
efficient first-round virus replication) are the N, P, and L proteins,
the presence of the M2 gene significantly enhanced the production of
full length virus mRNA
(62,
63,
112,
152,
388). In cells
expressing the N, P, and L proteins, virus-specific transcription
resulted in the production of prematurely terminated mRNA
(62) and expression of
the M2-1 protein served to promote the production of full- length
mRNA without altering genome RNA replication
(63). These data suggest
that the M2-1 protein functions as a transcription elongation
factor.
The product of the second open reading frame, the M2-2
protein,
has been detected in cells infected with hRSV and PVM
(
4). No
M2-2 protein was
detected in APV-infected cells, and to date
there is no information
about the putative M2-2 protein of hMPV.
Expression of the M2-2 open
reading frame resulted in the inhibition
of virus gene expression from
synthetic genomes. The RSV M2-2
protein accumulates during infection,
and it has been proposed
that the role of this protein is to switch the
virus from replicative
to assembly mode prior to virion release
(
4,
22).
Large (L) polymerase protein.
By analogy to
other viruses, the pneumovirus L protein is thought to be the major
component of the viral RNA-dependent RNA polymerase complex, which is
responsible for the synthesis of all viral RNA, including
mRNA, replicative intermediates, and the progeny RNA genomes.
The L protein is also thought to be responsible for mRNA
methylation and capping, although this has not been shown directly. As
the name indicates, the L protein is very largethe
hRSV L protein contains 2,165 amino acids, and the hMPV L
protein contains 2,000 amino acids
(297,
361). Poch et al.
(289,
290) described six
conserved domains for polymerase proteins of minus-strand RNA viruses,
including three functional domains and distinct interdomain regions
acting as hinges to assemble the functional domains into an appropriate
conformation. Domain III contains the polymerase GDNQ motif, and domain
IV contains a putative ATP binding site
(200). There have been
few direct studies of the specifics of the L proteins of the
Pneumovirinae. Most recently, Cartee et al.
(47) demonstrated that a
single amino acid change (N1049D) resulted in aberrant transcriptional
termination without altering the rate of virus
replication.
Current Model for Transcription
The details of the current model of transcription
and replication
of the pneumovirus genomes are consistent with those
proposed
for all nonsegmented negative-sense RNA viruses. The virus
genome
contains a single polymerase entry site, termed the leader or
promoter
sequence, at the 3' end of the RNA. The polymerase
complex initiates
transcription at the conserved transcriptional start
sequence
that is present at the beginning of all transcribed genes. The
polymerase
then travels along the genome until it reaches a conserved
signal
that marks the point at which transcription terminates. At some
point
during the transcription process, the nascent mRNA is
methylated
to produce a cap structure that ensures translation on host
cell
ribosomes; there are no reports directly addressing this process.
The
signal marking the end of transcription among the pneumoviruses
contains
a short run of U residues, and this marks the position at
which
the polyadenylated tail is added to the mRNA. When the
polyadenylation
process is complete, the mRNA is released for
translation on
either free or membrane-bound ribosomes, depending on
the nature
of the protein product. At this point, the polymerase does
not
automatically progress along the genome. Rather, only a proportion
of
the active polymerase molecules do so, with estimates suggesting
that
approximately 50% of the polymerase molecules are able to
continue.
The dissociated polymerases can reinitiate transcription only
by
translocation to the 3'-terminal leader region to begin
transcription
at the first gene. The remaining polymerases traverse the
genome
without transcribing until they encounter the conserved
transcriptional
start sequence of the second gene. At the end of this
gene,
the remaining polymerases repeat the
dissociation-versus-continuation
process. The result is that genes
proximal to the 3' leader
region are represented in
mRNA more abundantly than are those
closer to the 5'
end in a gradient of transcription
(
215). The
intergenic
regions of the pneumoviruses are variable in length
and tolerate
mutation and insertion of additional nucleotides
(
18,
42,
49,
361).
The process
of termination and reinitiation is poorly understood, but the M2-1
protein of RSV has been shown to be essential for production of
full-length virus mRNA
(62,
63,
112,
166). The
antitermination activity during transcription permits the viral
polymerase to remain associated with the template, thereby increasing
the production of full-length mRNA and enhancing the
production of products of genes that are distal to the 3'
leader region. The eight RSV gene junctions, which vary in their
ability to terminate transcription, all direct the production of more
readthrough mRNA in the presence of M2-1 protein, although
they vary in their sensitivity to the presence of the M2-1 protein
(167).
The genome
structure of hRSV contains an exception to the process
described above. The transcriptional stop signal for the M2 gene is
located 68 nucleotides downstream of the transcriptional start sequence
for the L gene (65).
Thus, a large proportion of the mRNA initiated at the L-gene
start sequence terminates at the M2 transcriptional stop sequence
(65,
111).
Elements
within intergenic regions play a vital role in the
regulation of transcription
(114). The intergenic
regions of the Pneumovirinae show considerable sequence
diversity and vary from 2 to 56 nucleotides in length. The
transcriptional start sequences are thought to be 10 nucleotides in
length, and the consensus sequences, while conserved, are
slightly different for each pneumovirus
(61,
221). The PVM
transcriptional start signals are more variable than those of RSV or
APV, which are absolutely conserved with the exception of the L gene
(49,
296,
332).
The
transcriptional stop signals of hRSV consist of a conserved
pentanucleotide sequence together with a 1- to 4-nucleotide AU-rich
region and a 4- to 7-nucleotide poly(U) tract. Saturation
mutagenesis of the hRSV gene transcriptional start sequences
showed that residues 1, 3, 6, 7, and 9 were critical in directing
transcription initiation, although there was some variability in
efficiency, particularly with the NS1 and NS2 gene start sequences,
which were approximately 40% less efficient than the others
(219,
220,
221).
Deletion of
a transcriptional stop signal from an upstream gene resulted in
transcriptional readthrough to the next gene without termination.
Harmon et al. (168)
analyzed the hRSV M-gene transcriptional stop sequence and
determined that the integrity of the pentanucleotide and AU-rich
regions was essential for efficient termination. The residue following
the poly(U) tract might also be important for efficient transcription
termination
(335).
No
significant difference in transcriptional activity or efficiency was
seen with different sized RSV intergenic sequences
(167,
219). Bukreyev et al.
(42) described the
recovery of a recombinant hRSV containing an intergenic region
of 160 nucleotides. This very long intergenic region had little effect
on sequential transcription.
The genome structure of
hRSV contains an exception to the process described above. The
gene end signal for the M2 gene is located 68 nucleotides downstream of
the gene start sequence for the L gene
(65), and much of the
mRNA that initiates at the L transcriptional start site
terminates at the M2 gene end sequence
(65,
111). The role (if any)
of this truncated L transcript remains to be
elucidated.
Current Model for Genome Replication
The current model, based on that determined for
other nonsegmented
negative-sense RNA viruses, proposes that the
nucleocapsid complex
is the functional unit for both replication and
transcription
and that the L, N, and P proteins are involved in both
processes.
Transcription and replication promoters of hRSV
overlap but
are not identical
(
114). In the
replicative mode, the polymerase
binds to the 3' end of the
genome RNA and initiates RNA synthesis
de novo. Once
initiated, the polymerase is committed to continue
to the
end of the template to produce an antigenome. The polymerase
complex
then uses the antigenome as a template, binding to and
initiating
RNA synthesis at the 3' end of the
antigenome RNA. In the genome
sense RNA, the trailer region
contains the antisense copy of
the sequence necessary for initiating
replication to produce
more genome RNA from the
antigenome.
It is not yet clear precisely how the
polymerase complex alters its activity and switches from the
transcription to the replication mode. Fearns et al.
(113) showed that
increased expression of the N protein stimulated hRSV
replication. This is consistent with the proposal made for other
negative-sense RNA viruses, suggesting that replication is dependent on
RNA encapsidation (224).
However, there are some conflicting data suggesting that high levels of
N protein may also stimulate transcription, and, more recently, a role
for M2-2 protein has been proposed
(21).
The
3' and 5' termini of the pneumovirus genome contain the
sequences that direct replication. Reverse genetics studies showed that
the 3'-terminal 44-nucleotide leader region and the
5'-terminal 40 nucleotides of the trailer region of
hRSV were necessary for replication, encapsidation, and
assembly (66). The
immediate termini of the pneumovirus genomes have complementary
sequences, as has been described for other negative-sense RNA viruses
(262,
296,
361). The trailer
sequences are more efficient than the leader sequences at directing
replication, as would be expected from their role in replication and
the requirement for the leader region to direct both replication and
transcription
(114).
Analysis of
the leader and trailer sequences suggested that nucleotide positions 1,
2, 3, 5, 6, and 7 were particularly important for replication, while
position 4 was found to be tolerant of alteration
(285). Using synthetic
chimeric minigenomes with terminal sequences from both APV
and RSV in virus infected cells, Marriott et al.
(250) showed that paired
termini from the same virus were required for replication to occur. It
is not clear whether the critical step is replication or encapsidation,
but the data suggest that an association between the leader and trailer
is necessary for productive infection
(250).
Packaging and Assembly
Reverse genetics experiments suggest that the
accumulation of
the M protein may be a component of the trigger to
initiate
the assembly of virions
(
343). It is anticipated
that the nucleocapsid
complex/M-protein structure associates with the
internal tails
of the major glycoproteins
that are inserted into the membrane
of the infected cell after
processing through the Golgi complex.
In parallel with studies
performed with other viruses, a progressive
series of interactions
between the glycoproteins and the M protein,
which
acts as a bridge with the nucleocapsids, results in the progeny
virions
budding from the surface of the cell in the form of filament
structures
(
280). In
polarized cells, such as those of the respiratory
tract, hRSV
buds from the apical surface and is shed into the
airways, from which
it can be transmitted to new hosts
(
303).

PATHOGENESIS OF DISEASE
This section is divided into four parts. APV, hMPV,
and bRSV
infections are discussed individually. In the fourth part, we
consider
the human pneumovirus pathogen hRSV. The current
state of understanding
of the human disease as assessed by direct
studies of patients
and patient-derived materials is reviewed, followed
by the results
obtained from cell culture and animal models of
hRSV. Consideration
of the mouse pneumovirus pathogen mPVM has
been interwoven with
the information about hRSV, since it has
been developed specifically
as a surrogate for severe hRSV
infection in rodent models.
Avian Metapneumovirus Infection
APV, a metapneumovirus previously
known as turkey rhinotracheitis
virus, is the cause of severe
respiratory infection in turkeys
(reviewed in references
73,
244 and
274) and may also be the
etiologic
agent of swollen head syndrome in chickens
(
73). Originally
identified
in South Africa in the late 1970s, APV was later found in
Europe,
Asia, and South America and has only recently emerged among
flocks
of domestic birds in the United States. Sequence divergence
among
the genes encoding the APV viral G (attachment) proteins
suggested
division of the European isolates into two subtypes, APV/A
(in
the United Kingdom) and APV/B (in continental Europe). A new
subtype,
APV/C, first emerged in the United States in 1996 in Colorado
and
was followed by a 1997 outbreak in Minnesota. Sequence analysis
of
APV/C isolates indicates that it is the most closely related
to the
newly discovered human metapneumovirus pathogen, hMPV
(
5,
274,
360).
The U.S. outbreaks
of APV infection have occurred in a seasonal
pattern (spring and
autumn), and several studies have implicated
wild migratory birds in
the spread of the disease
(
321). This
disease has
had devastating consequences for the turkey industry
in Minnesota,
resulting in much recent attention to disease
detection, pathogenesis,
and prevention (
74,
322).
In the wild,
APV leads to severe upper respiratory infection, often resulting in
death of the infected bird. The clinical syndrome and the pathology
observed in response to experimental infection with APV/A, APV/B, and
APV/C (the last of these from the Colorado outbreak) have been
described in several reports
(75,
76,
193,
194,
277), in addition to a
recent experimental study describing the natural history of APV/C from
Minnesota (189). In the
natural history study, respiratory symptoms were detected as early as 2
days postinoculation and included open mouth breathing, sneezing,
swollen sinuses, and nasal discharge. Microscopically, inflammatory
infiltrates (lymphocytes, macrophages, plasma cells, and heterophils)
were detected in nasal turbinate and sinus tissues, although,
interestingly, the ciliated epithelium, which was subjected to
significant disruption in the earlier studies (including that
describing APV/C from Colorado), remained intact. Anti-APV antibody
responses emerged at 7 days postinoculation, although it is unclear
whether this antibody response is protective
(271). While the events
promoting disease and tissue destruction are not yet understood at the
molecular level, Munir and Kapur
(266) have recently
presented the results of a subtractive hybridization and gene
microarray approach and have reported increased expression of
transcripts encoding interferon response proteins and other elements of
the inflammatory response and ubiquitination pathways in cultured
chicken embryo cells infected with APV/C.
As might be
anticipated, significant research has gone into vaccine studies. In
various trials, several groups have demonstrated protection in response
to immunization with high-passage virus
(154,
283,
363,
381), with one report
using high-passage virus from the Minnesota outbreak
(APV/MN/turkey/1-a/97) together with the immunomodulatory agent S-28828
(299), an
imidazoquinolinamine agent that induces interferon production and
macrophage activation in avian species
(203).
Human Metapneumovirus Infection
The very recently discovered hMPV is
the first human pathogen
of this family to be identified and is most
closely related
to the APVs described above; evolutionary analysis
suggests
that hMPV has existed in the human population for at least 50
years.
This virus was first identified in the Netherlands by van den
Hoogen
et al. (
360), and
evidence of infection has since been collected
worldwide
(
26,
105,
110,
273,
286), with one reported
fatality
in a stem cell transplant recipient
(
44). The clinical
syndrome
is currently indistinguishable from that resulting from
hRSV
infection, with some cases characterized by upper
respiratory
tract infection and others characterized by severe
bronchiolitis
and pneumonia
(
123). While several
reports have associated hMPV
infection with asthma exacerbation
(
123,
187,
247,
337), one
report
suggests that the association is less strong than that
of asthma and
rhinovirus infection
(
300). The current
diagnostic
approach involves reverse transcriptase PCR (RT-PCR)
(
245),
facilitated by
the substantial amount of virus gene sequence
that is available
(
18,
245,
360,
361). Similar to what
has been
reported for hRSV infection, one group has reported
acute wheezing
and elevated concentrations of interleukin-8 (IL-8) in
nasal
secretions from patients infected with hMPV
(
187). For more
detail,
the reader is referred to two excellent recent reviews
on this subject
(
197,
198).
Why was this
virus invisible prior to 2001? The authors of the original study
(360) discuss some
possibilities, including very slow replication of hMPV in the standard
cell lines used to isolate respiratory virus pathogens. Among the other
possibilities, most clinical cases of hMPV disease are not
life-threatening, and, as such, the impetus to identify novel pathogens
among the pool of hRSV-negative respiratory infections has not
been overwhelming. One can only wonder what other pathogens have been
overlooked.
Bovine Pneumovirus Infection
Research on bRSV has focused primarily on issues
related to
its prevalence as a respiratory virus pathogen in dairy and
beef
cattle worldwide (reviewed in references
10,
103,
227, and
355).
There are also
distinct ovine and caprine RSV pathogens, infecting
sheep and goats,
respectively (reviewed in references
34 and
97).
bRSV was first
isolated and identified as a unique pathogen
in the 1970s, and it is
currently the closest known phylogenetic
relative of hRSV
(
360), although there is
no evidence for antigenic
grouping into A/B strains
(
104). Similar to
hRSV, bRSV results
in seasonal infections among domestic
cattle herds, and transmission
is thought to occur via contact with
respiratory secretions,
with a short incubation period followed by
signs and symptoms
of upper and lower respiratory tract infection.
Interestingly,
and unlike hRSV infection, natural bRSV
infection is often accompanied
by concomitant bacterial infection
(
Mannheimia haemolytica,
Pasteurella multocida, and
Haemophilus somnus), resulting in
what has been defined as the
bovine respiratory disease complex.
One study has documented sequence
clustering based on geography
(
357),
while yet another
reports extensive sequence divergence in outbreaks
distributed over
time within a single herd
(
228).
There are
several vaccine formulations in existencemodified live and
inactivated, in both adjuvanted and nonadjuvanted forms
(102,
374,
375)and there
is significant debate about the relative efficacy and modes of
appropriate response. Modified live vaccines preferentially stimulate
the production of neutralizing antibodies that detect the virus F
(fusion) protein and stimulate lymphocyte proliferation and cytokine
responses. Inactivated vaccines also stimulate lymphocyte proliferation
and production of virus neutralizing antibodies, but the latter are
produced together with nonneutralizing antibodies. There is some
information relating to a potential immunopotentiation in response to
challenge after formalin-inactivated vaccine administration
(138,
318), analogous to what
was observed in the human trials. While much of the focus has been on
the development of humoral immunity to bRSV, recent results suggest
that vaccine efficacy may relate to the development of cellular
immunity, specifically Th1 (gamma interferon) responses
(102,
121,
125,
374).
Experimental
models of bRSV infection in calves involve aerosol administration of
virions, resulting in early-onset cough and nasal discharge and
progressing to signs and symptoms of lower respiratory tract infection
(287,
350,
367,
380). In the study
performed by Philippou et al.
(287), lungs from
animals sacrificed at 12 weeks postinoculation were atelectatic with
evidence of bronchiolar obstruction and profound inflammatory
infiltration. Disease progression has been associated with increased
expression of IL-2, IL-4, and/or gamma interferon in cells drained from
pulmonary lymph (139,
214) and in peripheral
blood mononuclear cells
(257), detection of
tumor necrosis factor alpha and leukotriene B4 in
respiratory secretions
(302,
308), and elevations in
levels of serum acute-phase response proteins
(171). Aside from the
challenging aspects of working with large experimental animals,
research in this field is hampered by the fact that there are
comparatively few bovine cytokine gene sequences available in the
public domain and even fewer commercially available
reagents.
Buchholz et al.
(35) described the
creation of recombinant bRSV virions in vitro utilizing the complete
sequence genome of bRSV
(389) by methods
analogous to those developed for the study of hRSV. Among the
results as they relate to disease pathogenesis are the finding that the
bRSV NS1 and NS2 proteins function in synchrony to antagonize
alpha/beta interferon-mediated antiviral activities in a
species-specific fashion
(30,
315) and that the SH and
G genes are dispensable for bRSV replication in vitro
(204). This technology
has already been applied to the development of the next generation of
bRSV and hRSV vaccines
(36,
317).
Human Respiratory Syncytial Virus Infection
Human RSV is a seasonal
infection, which in North America is
observed most frequently in winter
and spring. Transmission
between individuals occurs via contact with
respiratory secretions,
either directly or via handled surfaces and
objects or via large
aerosolized droplets. The incubation period is
typically 2 to
8 days, with virus replication spreading from the upper
respiratory
tract epithelial cells to the lower respiratory tract. The
clinical
signs and symptoms of acute hRSV infection have been
described
in significant detail in several recent reviews
(
86,
141,
155,
161,
162,
216,
372).
The complex
interplay of factors involved in acquired immunity
to hRSV is
discussed in the section on vaccines. The focus here
is on several
specific topics of recent interest.
Genetic susceptibility to severe infection.
While
hRSV infection appears to be universal, there is great
interest in understanding why and how some children proceed from
initial infection to severe, life-threatening forms of lower
respiratory tract disease. There are several straightforward and
clearly understood conditions that lead to increased susceptibility to
severe infection, including premature birth, congenital dysplasias, and
immunodeficiency disorders (reviewed in references
52,
96,
260,
340, and
371). Less well
understood are the conditions under which otherwise
"normal" children succumb to severe hRSV
disease. Several groups have approached this question via genotyping
and have focused on questions relating to normal variation within the
human population. Among the results, Hallman and colleagues
(223,
239) have documented
correlations between specific alleles of the genes encoding surfactant
A and D with increased disease severity and Kwiatkowski and colleagues
(177,
178) have provided
evidence supporting a relationship between disease and a susceptibility
locus related to the gene encoding IL-8. Two groups, Choi et al.
(56) and Hoebee et al.
(175), have identified
alleles of IL-4, and IL-4 and its receptor, respectively, that are also
associated with more severe sequelae of RSV infection in vivo. At
present, it is not clear how these genetic factors influence responses
to hRSV infection at the molecular
level.
Inflammatory responses to infection.
To understand the concepts
underlying inflammatory responses to hRSV, it is important to
recognize the there are two different forms of hRSV-related
disease discussed in the literature. Primary hRSV infection,
or natural hRSV infection, occurs in a naive (i.e.,
unvaccinated) host, and the inflammatory responses are those of the
innate immune system responding to virus replication in situ. This is
to be contrasted with what has been termed "enhanced"
or "augmented" disease, which describes the aberrant
inflammatory responses observed in children who had been vaccinated
with formalin-fixed virus preparations and later challenged "in
the wild" with natural virus infection. The postvaccination
response has been characterized extensively, and the profound
lymphocytic and eosinophilic inflammation observed has been attributed
to an induced Th1-Th2 imbalance, with dominance of Th2 lymphocyte
responses in this setting
(32,
148,
180).
With respect
to primary hRSV infection, the earliest histologic studies
were, of necessity, examinations of postmortem specimens. Severe
infection progressed to include necrosis of airway epithelium,
interstitial inflammation, and mucus secretion leading to airway
obstruction and respiratory compromise. While lymphocytes were
predominant in postmortem specimens, more recent studies have pointed
to neutrophil influx as an earlier acute cellular response and to
neutrophil-mediated activities as a major cause of respiratory symptoms
(107,
209,
328,
370). Neutrophils are
proinflammatory leukocytes of the granulocyte series that develop from
pluripotent stem cells in the bone marrow, released into the
circulation, and recruited into tissues via the coordinated actions of
various cytokines and chemokines. Activated neutrophils participate in
phagocytosis and degranulation and release reactive oxygen metabolites
into the surrounding tissue. While the role of the neutrophil is to
promote host defensevery clearly established for bacteria and
fungi, less clearly for virusesneutrophils at the same time
can promote tissue damage and destruction, representing the two sides
of their classically defined "double-edged sword" (see
reference 309 for a
general review of the role of neutrophils in innate immunity).
Eosinophils are also recruited to the lungs in response to primary
hRSV bronchiolitis
(133,
169), and both
neutrophils and eosinophils are recruited to lung tissue in the PVM
model of acute pneumovirus infection in vivo
(88). Virus-specific
cytotoxic T lymphocytes have been detected within 10 days of infection
in human subjects (54,
181), and roles for
CD4+ and CD8+ T lymphocytes in
virus clearance have been inferred from rodent studies
(81,
146,
147).
Proinflammatory
cytokines have been detected in respiratory secretions and in
bronchoalveolar lavage samples from individuals infected with
hRSV (1,
169,
182,
263). Three of the major
chemokines detectedIL-8, RANTES, and macrophage inflammatory
protein 1
(MIP-1
)have each been implicated
in neutrophil and eosinophil chemotaxis and recruitment. MIP-1
has been of particular interest, since its levels have been shown to
correlate with levels of eosinophil degranulation products
(169) and with severity
of disease
(136).
Several
recent studies have described a role for the pattern recognition
receptors CD14 and/or toll-like receptor 4 (TL4) in innate host defense
against hRSV in vivo
(158,
170,
222). Among the
findings, these studies documented persistence of hRSV virions
in TLR-4-deficient mouse lungs, impaired NK cell trafficking and
function, and reduced early NF-
B-dependent inflammatory
responses. However, analogous experiments performed with TLR-4
deficient mice and the natural rodent paramyxovirus pathogen Sendai
virus (mouse parainfluenza 1) yielded completely contradictory results
(362). The use of
natural host-pathogen pairs for the study of acute respiratory virus
infection is discussed further in "Biochemical responses in
mouse models"
(below).
Modeling hRSV Infection in Cell Lines and in Experimental Animals
PVM.
The human pathogen,
hRSV, and the mouse pathogen, PVM, can induce
similar
disorders in their natural hosts. As such, the mouse
data that are
available are compared and contrasted to what
is currently understood
about pneumovirus infection in a human
host.
Target cells.
The primary targets of hRSV in vivo are
respiratory epithelial cells, although infection of alveolar
macrophages has been reported
(278). This has been
modeled in tissue culture, and the epithelial cell lines HEp-2 and A549
and primary human epithelial cultures have been used to study the
cellular and molecular responses to hRSV infection. Other
cells are susceptible to hRSV infection in culture, including
the fibroblast cell line MRC-5, macrophage and macrophage-like cell
lines (301,
313), and, for uptake if
not outright infection, eosinophils
(211). Mouse PVM has
similar target affinity and infects respiratory epithelial cells in
vivo and the mouse epithelial cell line LA4
(265).
Cellular receptors.
The receptor for PVM has not
been characterized. While no one has yet identified a single specific
protein, lipid, or carbohydrate entity that serves as a unique,
virus-specific receptor for the hRSV pathogen, the ongoing
story is an interesting one, with multiple virus and cell determinants
coming into play. Among the earliest findings, Krusat and Streckert
(218) demonstrated that
the glycosaminoglycan (GAG) heparin, when included in an in vitro
culture system, could inhibit the infection of target cells with
hRSV. This finding has withstood the test of time, and several
groups have followed through with significant elucidations and
refinements. Taking the whole-virus approach, Hallak et al.
(163,
164) reported that only
N-sulfated moieties of GAGs (and not C6-O or C2-O sulfation) are
important mediators of RSV infection and later documented the specific
role of iduronic acid-substituted forms. This is consistent with the
current overall thinking on GAGs as heterogeneous structures that
promote specificity to a greater degree than initially appreciated. The
reader is referred to several excellent reviews of GAG biology for a
larger sense of the role of these carbohydrates in pathogen invasion
(174,
238,
270,
330).
From the
perspective of the virus, initial studies focused on the RSV G protein.
Feldman et al. (115,
116) confirmed the
interaction of the RSV G protein with heparin, identified a linear
heparin binding domain within the G protein, and documented an
interaction of heparin with the RSV F protein as well. However, Teng et
al. (347) later
demonstrated that deletion of the G-protein binding domain did not
alter sensitivity to heparin, and this was followed by a study by
Techaarpornkul et al.
(341) demonstrating that
recombinant RSV with deleted G and SH proteins not only was able to
infect epithelial cells in culture but also was able to do so in a
partially GAG-independent fashion. Teng et al.
(347) also demonstrated
that the membrane-bound form of the G protein was not absolutely
required for replication in vitro and that the secreted form, without
cytoplasmic and transmembrane domains, served adequately in its stead;
a subsequent study (345)
demonstrated that the conserved "cystine noose" region
separating the two extracellular glycosylated domains of the
hRSV G protein were likewise dispensible.
In summary,
the issue of cellular receptors for hRSV remains one with many
unanswered questions. GAGs clearly play a significant role in the
infectivity of wild-type virus, and the recent identification of
protein binding domains with high affinity for specific GAGs may be of
some help in this endeavor
(174). One of the most
important questions that remains unanswered is that of species
specificity; on that topic, a very recent and intriguing observation by
Schlender et al. (316)
documents the role of the F2 subunit domain in modulating the species
specificity observed for human versus bovine RSV infection. Also of
note is the report by Tripp et al.
(353) on
chemokine mimicry by the hRSV G protein, leading
some to conclude that the chemokine receptor for
fractalkine, CX3CR1, might serve as a coreceptor for
hRSV infection. However, while epithelial cells synthesize and
secrete fractalkine, there is no evidence for CX3CR1 expression in
these major target cells for hRSV infection. Finally, the most
recent report by Malhotra et al.
(246) suggests a role
for annexin II, which itself is a heparin binding protein and as such
may interact with the glycosylated virus proteins and glycosylated
target cell proteins in unexplored and unexpected
ways.
Cellular and biochemical responses. (i) Syncytium formation.
The
classic multicellular cytopathic syncytia formed in epithelial cell
culture in response to infection with hRSV have been described
in significant detail
(195). While several
lines of evidence suggest a role for the hRSV F protein in
inducing syncytium formation, the molecular details of this morphologic
response remain largely unknown. Recent evidence has demonstrated
increased expression of the cytoskeleton filament protein
cytokeratin-17 and its localization at sites of syncytium formation
(91) as well as
involvement of the small GTPase RhoA
(281,
282).
(ii) Biochemical responses in cell culture.
The
biochemical responses of epithelial cell cultures infected with
hRSV have been reviewed recently
(78,
92,
131); this will permit
us to cover the highlights and focus here on the most recent studies.
Among the most prominent biochemical responses of epithelial cells is
the synthesis and secretion of the potent neutrophil chemoattractant
IL-8. The transcriptional control of IL-8 relies on the activation of
the master transcriptional activator, NF-
B
(119,
135,
253,
255), with the
involvement of NF-IL-6
(185,
254), ERK-2
(53), and unique
hRSV-responding enhancer elements in the IL-8 gene promoter
(48). hRSV
infection of epithelial cells in vitro induces the production of other
chemokines, including RANTES, monocyte chemotactic protein 1 (MCP-1),
and MIP-1
, the last chemokine being of particular note because
it is directly related to the inflammatory responses to both
hRSV and PVM observed in vivo
(90,
136,
169). Other prominent
biochemical responses to hRSV infection in vitro include the
production of inducible nitric oxide synthase and nitric oxide
(155,
201,
356) beta interferon,
IL-1
, IL-6, and IL-11 and increased expression of cell surface
markers intercellular cell adhesion molecule 1
(ICAM-1; CD54), CD18, vascular cell adhesion molecule
(VCAM), major histocompatibility complex classes I and II, CD14, and
CD15 (reviewed in reference
92). See also the section
on gene microarray approaches in "Gene microarray expression
studies" below.
(iii) Biochemical responses in mouse models.
A number of
groups have studied the pathogenesis of hRSV inoculation in
various strains of inbred mice (primarily BALB/c) and cotton rats. All
describe the moderate influx of inflammatory cells, with reports
including lymphocytes and eosinophils as components of the respiratory
infiltrates (120,
145,
147,
319,
320). Blanco et al.
(25) documented the
increased expression of transcripts encoding several cytokines,
chemokines, alpha and gamma interferons, and the interferon response
gene, IP-10, in response to inoculation of cotton rats with
105 PFU of hRSV-A Long strain. These
results were similar to those reported by Haeberle et al.
(157), who evaluated the
responses of inoculated BALB/c mice, although the latter group also
reported the inducible expression of MIP-1
. However, others
have shown that while MIP-1
is produced in this model, the
levels detected do not correlate with the extent of the inflammatory
response (89), as has
been observed in both human disease
(136,
169) and the PVM mouse
model (90).
This
latter point leads us to an important caveat. It is important to
understand that, just as there are two distinct disease entities
studied in humans (discussed in "Inflammatory responses to
infection" above), there are two distinct rodent models
involving hRSV inoculation. The immunopotentiation model, in
which mice are inoculated with live, attenuated, or fixed hRSV
or hRSV components and then challenged with hRSV, is
a model of acquired immunity and has been developed in conjunction with
studies related to vaccine development. This application of
hRSV needs to be distinguished from primary infection models,
in which one is attempting to evaluate innate responses and innate
immunity to infection. In many studies of the pathogenesis of primary
infection in these nonnatural rat and mouse hosts, it is difficult to
determine whether the responses observed are to the very limited virus
replication observed or simply to large quantities of inoculated
foreign antigens that happen to be viral in origin. The responses to
infection and the responses to antigen bolus are not interchangeable,
since they reflect different capacities and different reactivities of
the innate inflammatory response. The remarkable evolutionary
divergence of proteins involved in innate host defense
(269) suggests that
experiments relating to disease pathogenesis and innate host response
need to be performed using appropriate species-pathogen pairs. Studies
using natural host-pathogen pairs may be limited and limiting in other
ways, but they ultimately provide better representations of the
pathogenesis of pneumovirus infection in vivo.
PVM has recently
been developed as an alternative model for the study of the
pathogenesis of severe pneumovirus disease
(27,
77,
88,
90,
93). Intransal
inoculation of fewer than 30 PFU of virus results in clear-cut
replication in mouse lung tissue, yielding titers as high as
108 PFU/g. Virus replication is accompanied by a profound
inflammatory response, mucus production, and airway obstruction,
leading to significant morbidity and mortality. As noted above, the
inflammatory response is promoted by local production of the CC
chemokine MIP-1
and its signaling through the receptor, CCR1,
a pathway under exploration for combined antiviral-immunomodulatory
therapy, as discussed in "Combination therapy"
(below).
(iv) Gene microarray expression studies.
The gene microarray is a more
global approach to the study of transcriptional responses to defined
physiologic or pathophysiologic stimuli. As of this writing, three
published studies have used this approach to evaluate the
transcriptional responses of epithelial cells to hRSV in vitro
and a fourth study has focused on the responses of whole
mouse lung to infection with PVM in vivo. The first of these systematic
studies, reported by Zhang et al.
(390), presents an
evaluation of the responses of the human epithelial cell line A549 and
primary human SAE cells to infection with the A2 strain of RSV. The
A549 responses included the anticipated increases in the levels of
transcripts encoding the chemokines IL-8, RANTES, MCP-1, and
MIP-1
, and have extended the list to include, among others,
I-309 and fractalkine. Most interesting was the fact that the pattern
observed in the primary SAE cells was not precisely identical,
demonstrating the existence of distinct responses in what are often
thought to be completely interchangeable model systems. The gene array
study of the transcriptional responses in whole-mouse lung
mRNA to infection with the pathogenic PVM strain J3666
likewise demonstrates increased expression of a specific array of
proinflammatory mediators, including transcripts encoding murine MCP-1,
RANTES, MCP-3, beta interferon, and 10 characterized interferon
response and interferon-related genes
(93). The second
publication focused on the in vitro response
(349) has provided us
with a more general look at the responses directly related to the
activation of the proinflammatory transcriptional master switch,
NF-
B. Using a clever, dominant-negative mutant approach, the
authors compared the responses of hRSV-A2-infected HeLa cells
designed to express the I
B
-dominant negative mutant
under doxycycline control and established that in addition to what we
already know are NF-
B regulated transcripts (the I
B
inhibitors and IL-8), several interferon response factors (IRF-1 and
IRF-7B) and the transcription factor STAT-1 are also under
NF-
B control. The third publication from this group
(391) focuses on the
antiviral agent ribavirin and its interference with hRSV-
A2-mediated transcriptional activation of the A549 cell line. Among the
conclusions, the authors determined that ribavirin, while an effective
antiviral agent, actually does not interfere in a very substantial way
with the inflammatory cascade initiated by hRSV replication, a
result that has been echoed by the findings of Bonville et al.
(27) in their study of
the cellular inflammatory responses to PVM in vivo. Another important
conclusion from the gene array study was that ribavirin administration
resulted in enhanced expression of several components of the
interferon-signaling pathway via as yet undefined interactions with the
interferon-stimulated response enhancer elements common among these
up-regulated transcripts
(391), adding to our
understanding of the more subtle immunomodulatory properties of
ribavirin.
Variants and Mutants with Altered Pathogenicity
Natural variation.
Naturally occurring variants of
hRSV fall into two diagnostic
subgroups, hRSV-A and
hRSV-B, initially identified on the basis
of differential
reactivity with monoclonal antibodies and later
confirmed on the basis
of gene sequence, with particular divergence
noted within the sequence
of the virus G gene
(
129). While the
evolutionary
significance of this divergence remains largely unknown,
Woelk
and Holmes (
379)
have identified six sites within the sequence
of the hRSV G
protein that have been subjected to positive selection,
all within the
extracellular domain and all associated to some
degree with specific
anti-hRSV immunoglobulin epitopes and/or
glycosylation,
suggesting immune-driven natural selection. The
question of the
relative pathogenicity of hRSV-A and hRSV-B
variants
has been addressed in a recent study by Martinello
et al.
(
251) in which they
found no association of disease severity
with either the
hRSV-A or hRSV-B subgroup (137 and 84 isolates,
respectively),
but went on to identify a specific A subgroup clade
(GA3) with
this characteristic. In contrast, a study of 28 isolates by
Brandenburg
et al. (
33)
found no relationship between hRSV-A lineages and
disease
severity. Further study is necessary to distinguish
between these sets
of results.
Tissue culture attenuation and random mutagenesis.
Prior to the
development of reverse genetics, studies of the contributions of virus
sequence to disease severity were dependent on the identification,
isolation, and sequence analysis of naturally occurring virus variants,
typically the result of the relaxation of functional constraints
provided by replication in tissue culture. A good example of this comes
from the PVM literature, specifically the study by Randhawa et al.
(295) in which G-protein
sequences of the mouse-passaged PVM strain J3666 and the tissue
culture-passaged, attenuated PVM strain 15 were compared; the authors
demonstrated different translational start sites leading to the absence
of a putative amino-terminal cytoplasmic domain in the latter. The pace
of discovery was enhanced by the addition of random mutagenesis
methods, which led to the identification of important mutations in the
P (phosphoprotein) (242)
and L (polymerase) (351)
proteins of hRSV.
Antibody escape mutants.
Escape mutants are
essentially a technological form of immune-drive natural selection (see
reference 261 for an
excellent review and details of earlier studies). Escape mutants have
been created from hRSV grown in tissue culture in the presence
of neutralizing monoclonal antibodies directed against the virus G or F
proteins (241,
252,
253,
368). This work has
generated detailed antigenic maps of these proteins, has provided a
virtual "snapshot" of the way in which hRSV
virions respond to immunoglobulins in vivo, and has provided
significant guidance on the development of passive immunoprophylaxis
for hRSV
(80).
Recombinant viruses.
Collins et al.
(67) were the first to
report the successful production of infectious hRSV from
cloned cDNA in tissue culture. Cotransfection of HEp-2 cells with a
plasmid containing the 15,223-nucleotide antigenome together
with expression plasmids encoding the N, P, L, and M2 proteins
permitted the recovery of infectious virions and led the way to the
important next step, the creation of point, replacement, addition, and
deletion mutations in the virus backbone. The use of this technology,
known as reverse genetics, in studies of pneumovirus biology in general
(68,
249) and its application
to the vaccine effort
(60,
69,
268) have recently been
reviewed. With respect to the vaccine studies, among the more
intriguing applications of this technology has been the inclusion of
immunomodulatory cytokines IL-2
(40) and
granulocyte-macrophage colony-stimulating
factor (41) within the
virus backbone in an attempt to alter the host response as part of the
overall vaccination strategy. Deletion and substitution mutational
analyses using this or alternative genome-based methods have led to a
greater understanding of the role of various virus-encoded proteins
(46,
188,
242,
339,
341), particularly that
of F and G proteins in hRSV infectivity and species
specificity (130,
316,
347,
392), described in
detail in "Cellular receptors"
(above).
Virus Evasion of Innate Immunity
RNA viruses use a large number and variety of
mechanisms to
subvert innate antiviral host defenses; many of those
mechanisms
involve evasion of interferon and interferon-regulated
responses.
Comparatively little is known about such strategies among
the
pneumoviruses (see references
143 and
385 for reviews
discussing
strategies used by other viruses of the family
Paramyxoviridae).
However, Atreya and Kulkarni
(
7) demonstrated that
hRSV replication
was not diminished in response to treatment
with either alpha
or beta interferon in several standard tissue culture
model
systems. Given that there is ample evidence for induced
production
of beta interferon by hRSV-infected epithelial
cells (
7,
134,
186,
356),
the question of
resistance remains acute. The most clear-cut
evidence for resistance
strategies to date comes from Conzelmann
and colleagues
(
315), who demonstrated
that the NS1 and NS2
proteins of bRSV and hRSV function
coordinately and in a species-restricted
fashion to antagonize the
antiviral activities of alpha and
beta interferons. Also, as noted
above, bRSV NS-1 and NS-2 proteins
inhibit the production
of beta interferon by reducing the activation
of interferon-regulatory
factor 3 (
28) and bRSV
NS-2 specifically
is involved in interferon production in bovine nasal
fibroblasts
and bronchoalveolar macrophages
(
358). Teng, Whitehead,
and
colleagues (
346,
378) have demonstrated
the attenuation of hRSV
NS-1 and M2-2 or NS-1 and SII deletion
mutants in the chimpanzee
model of infection, although without specific
reference to interferon-related
phenomena. Interestingly,
hRSV is sensitive to inhibition by
gamma interferon
(
11,
20,
39,
99,
276,
365), and potential
hRSV-mediated
modulation pathways have beenidentified (
6,
17).
Chemokine
mimicry is another means by which viruses are known to subvert innate
antiviral host defense. The potential role of the hRSV G
protein in chemokine mimicry is an intriguing lead to be explored
further (353). In
another recent development, Zimmer et al.
(394) describe
virokinin, a post-translationally modified cleavage product of bRSV F
protein that contains a classical motif identifying it as a member of
the conserved tachykinin family of peptide hormones. The role of this
virus-encoded protein in binding to tachykinin receptors in the
respiratory tract and potentially altering the course of bRSV infection
remains to be clarified.
Regarding virus evasion in vivo, the
responses are complex and often difficult to evaluate or pinpoint with
any assurance at the molecular level. One interesting interpretation of
viral evasion in vivo comes from the work of Sakurai et al.,
(311), who isolated and
evaluated naturally occurring antibodies reactive with the
immunodominant F glycoprotein and found a
subset of nonneutralizing antibodies with a high affinity for the
immature, viral lysate forms of F, together with neutralizing subset
with high affinity for mature F on infected cells and on virions. The
authors consider the intriguing possibility that by evolving and
maintaining two developmental forms of the F protein with distinct
antigenicity, the virus can evade and subvert humoral immunity more
effectively.

CLINICAL STRATEGIES
Passive Immunoprophylaxis for hRSV Disease
There are two formulations of anti-RSV
immunoglobulins in use
for prophylaxis against hRSV infection
in high-risk patient
groups. The earliest studies with experimental
animals suggested
that immunoglobulin preparations containing
high-titer antibodies
to hRSV (hRSV-Ig) protected
against development of severe disease
(
150,
291,
323).
While passive
prophylaxis could attenuate the severity of hRSV
disease, it
did not prevent infection, and an increased number
of unexplained
deaths occurred in hRSV-Ig recipients with congenital
heart
disease, leading to the recommendation that RSV-Ig not
be used in
patients with this underlying condition
(
324); considerable
controversy
ensued regarding specific indications due primarily to the
high
cost of administering this agent
(
294). Other problems
associated
with the use of hRSV-Ig included the potential for
transmission
of blood-borne pathogens, the need to administer a large
volume
of product (15 ml/kg of body weight), and the associated fever
and
desaturation that can accompany the administration of any blood
product.
In response to the need for a better prophylactic agent,
a humanized monoclonal antibody against the hRSV F protein was
developed for clinical use (reviewed in references 151 and 325). A
randomized trial of humanized anti-F monoclonal antibody, palivizumab,
was performed at 139 centers in North America and the United Kingdom,
with intramuscular administration of this agent to high-risk infants
once per month during hRSV season. This approach was shown to
be highly effective in reducing hRSV-related hospitalizations
(180a). Palivizumab was
licensed by the Food and Drug Administration in June 1998. Emerging
data demonstrating the safety of this monoclonal antibody product in
patients with congenital heart disease will very probably lead to a
Food and Drug Administration-approved indication for use in this group
of high-risk individuals.
While hRSV-Ig and monoclonal
anti-F antibody already play an important role in the prevention of
serious hRSV infection, neither appears to be effective in the
treatment of this disease. In a study of previously healthy infants
randomized to receive RSV-Ig or placebo, no significant differences
were observed in the duration of hospitalization, intensive care unit
stay, mechanical ventilation, supplemental oxygen use, or adverse
events between the two groups
(306). One explanation
for the lack of clinical efficacy during acute hRSV infection
is that by the time respiratory symptoms are present, the virus is
sequestered from host humoral immunity
(294). Furthermore, gene
microarray (391) and
mouse model (27,
29) studies suggest that
the inflammatory responses to pneumovirus infection are initiated very
early on and that antiviral therapy alone is not effective in
controlling the negative sequelae. Despite these observations, there is
growing evidence that using palivizumab in transplant patients with
documented hRSV upper respiratory tract infection may prevent
the spread of infection to the lower lung when used in combination with
ribavirin (85). This
strategy is currently under investigation by the Collaborative
Antiviral Study Group and may be of particular importance in this very
high-risk patient population.
Active Immunoprophylaxis for hRSV Disease
The morbidity and
mortality associated with hRSV bronchiolitis,
coupled with its
frequency and worldwide distribution, make
it a prime target for the
development of a vaccine that can
be used early in life. There have
been several forces at work
that have hampered progress on a vaccine
against hRSV. Most
important is the possibility that
vaccination will result in
adverse reactions to naturally occurring
hRSV disease, as observed
with the formalin-inactivated
vaccine used in the 1960s
(
55,
124,
202,
210).
Furthermore,
newborns and very young infants may not mount a
protective immune
response because of relative immunologic immaturity
or suppression of
their immune response due to circulating maternally
derived
anti-hRSV antibodies
(
68,
79,
108,
267). Natural
hRSV
infection itself elicits only partial immunity, and
reinfection
in later life is commonplace. Several strategies have been
implemented
recently in an attempt to generate safe and effective
subunit,
inactivated, and live-attenuated hRSV vaccines
(
60,
109). Currently,
two
promising candidate vaccines under study in clinical trials
are an
hRSV F subunit vaccine for immunization of patients who
have
already experienced primary RSV infection, such as the
elderly and
older hRSV-seropositive children with conditions
predisposing
them to severe hRSV disease
(
108,
279,
288,
354),
and cold-passaged,
temperature sensitive (cpts) attenuated hRSV
strains
(
196,
206). cpts mutant virus
strains are difficult,
if not impossible, to "fine
tune" by serial passage in tissue
culture, emphasizing the
importance of using newer recombinant
(reverse genetics) technologies
to advance the field. Recombinant
hRSV vaccines under
evaluation in clinical trials are largely
based on the cpts 248/404
"backbone" virus, with molecular deletions
and/or point
mutations designed to enhance several desirable
features. Another
promising area involves hRSV vaccine strains
that lack the NS1
and/or NS2 genes since deletion of these sequences
augments the host
cell interferon response pathways
(
31,
315).
Therapy for hRSV Disease
Antiviral agents.
While significant advances in our
knowledge of hRSV biology,
immunology, pathophysiology, and
epidemiology have been made
in the past 50 years, there continues to be
a fair amount of
controversy regarding the optimum management of
infants and
children with hRSV infection
(
369). Supportive
therapy, including
hospitalization, supplemental oxygen delivery, nasal
and pulmonary
toilet, and, in the most severe cases, intubation and
mechanical
ventilation, is the mainstay of clinical
management.
(i) Ribavirin.
Lack of consensus regarding the use of ribavirin for
moderate to severe forms of hRSV-induced bronchiolitis cannot
be attributed to a paucity of clinical trials. Results from at least 14
trials of bronchodilators, 3 trials of adjunctive systemic
glucocorticoid treatment, and 11 randomized trials of the antiviral
synthetic nucleoside ribavirin have been published
(16,
149,
159,
160,
229,
233,
259,
264,
294,
305,
327,
336,
369,
376). In six controlled,
randomized studies, ribavirin treatment was found to reduce the
severity of hRSV illness by decreasing viral shedding,
improving oxygen saturation, and improving clinical scores
(16,
159,
160,
207,
327,
336), but in 1996, a
systematic overview of these studies highlighted potential flaws in
study design and interpretation
(296). The results of
this meta-analysis were discouraging but were consistent with the
clinical experience of pediatricians, specifically that ribavirin
treatment alone did not lead to significant improvement in clinically
important outcomes. The growing concern regarding the efficacy of
ribavirin and the high cost of its use
(118) led to a wording
change in the American Academy of Pediatrics recommendation for its
usage from "should be used" to "may be
considered" for selected infants and young children at high
risk for serious hRSV disease
(70).
Severe
hRSV infection during infancy is a known risk factor for the
development of subsequent wheezing illnesses during the first decade of
life. Studies of the outcome of bronchiolitis during infancy have
included studies that evaluated the effects of ribavirin therapy during
acute illness on the long-term sequelae of recurrent wheezing during
childhood (101,
132,
208,
240,
256,
310), with promising
results. In contrast, Krilov et al.
(217) found no
differences between ribavirin-treated and untreated infants in wheezing
or other pulmonary function measures 6 to 8 years beyond their
infection. Interestingly, new attention has also been raised regarding
potential long-term benefits of using the leukotriene receptor
antagonist montelukast during acute infection for the prevention of
post-hRSV infection wheezing
(24). A large multicenter
clinical trial to test this hypothesis is under
way.
(ii) New antiviral strategies: fusion inhibitors, RNases, and others.
New
strategies for therapeutic intervention during moderate to severe
hRSV disease are clearly needed. A variety of antiviral
compounds have been tested for their ability to impair hRSV
replication in vitro. Speculation that inhibiting cell-cell fusion
(i.e., syncytium formation) might lessen the severity of clinical
disease has led to attempts to develop hRSV-specific fusion
inhibitors. Peptides derived from epitopes of the
hRSV fusion or F protein blocked
hRSV-induced syncytium formation at concentrations as
low as 0.015 µM
(226). Small-molecule
hRSV-specific fusion inhibitors have neutralizing activity
against A and B group hRSV isolates in vitro and in vivo via
blockade of cell-cell fusion
(179,
258,
272), while a
benzodithiin compound has demonstrated ability to interfere with
intracellular processing of the hRSV fusion protein, thereby
resulting in decreased infectivity
(333). Other agents
reported to have anti-hRSV activity have included crude plant
protein extracts (3), some
forms of superoxide dismutase
(384), an
L-aspartate transcarbamoylase inhibitor
(383),
eosinophil-derived neurotoxin, a potent RNase
(87), and lovastatin
(144).
While
nucleoside derivatives other than ribavirin have generated some
interest by investigators in this field
(359), enthusiasm for
the development of antisense technology has been more robust. Specific
cleavage of hRSV genome RNA has been detected at antisense
oligonucleotide binding sites
(184). Because of the
pattern of RNA cleavage observed, it was suggested that endogenous
RNase H participated in hRSV genomic degradation. Other RNases
have been studied for their antiviral properties. For example,
recruitment and activation of RNase L to an hRSV mRNA
using 2-5A antisense oligonucleotide technology showed potent
inhibition of hRSV replication in vitro
(2) and, more recently, in
vivo using an African green monkey model of hRSV infection
(230). As of this
writing, there are no publications describing antisense therapy in
human clinical trials.
Combination therapy: antivirals with antibody, anti-inflammatory, and immunomodulatory strategies.
Clinically,
the negative sequelae of pneumovirus infection involve active viral
replication in conjunction with the host cell inflammatory response.
Combination therapy with the antiviral ribavirin and systemically
administered immunoglobulin has been tried with some success in the
cotton rat model of hRSV infection
(153), and one report of
combined aerosolized ribavirin and systemic immunoglobulin treatment in
bone marrow transplant recipients who developed hRSV disease
showed a 22% mortality among patients treated before the onset
of respiratory failure and 100% mortality in patients who went
untreated or in whom treatment was initiated within 24 h of
respiratory failure
(377). Parenteral
therapy with a monoclonal anti-F antibody in the cotton rat model of
human hRSV infection resulted in clearance of hRSV
within 24 h but had no effect on early pulmonary pathology,
while glucocorticoid treatment reduced the inflammatory component of
the pneumonitis but delayed virus clearance; when the two interventions
were combined, an antiviral and anti-inflammatory effect was
appreciated
(292).
Increasing
attention is being paid to the need for concurrent anti-inflammatory
therapy for moderate to severe hRSV disease. To this end,
glucocorticoids are generally considered to have broad-reaching
anti-inflammatory properties and in fact have been the focus of several
clinical hRSV trials. Unfortunately, these trials suggest that
when used alone in the absence of antivirals, glucocorticoid treatment
has only marginal, if any, benefit in the treatment of
hRSV-infection and its associated inflammatory response
(72,
83,
84,
212,
213,
231,
307,
331,
334,
338,
365). A meta-analysis of
several these studies demonstrated that systemic glucocorticoids
reduced the total length of hospital stay by only 0.43 day per patient,
an effect reduced to only 0.29 day per patient when patients with
previous episodes of wheezing were excluded from the analysis
(137), a statistically
significant difference with overall minor clinical impact.
Interestingly, several recent studies showed that glucocorticoid
treatment during hRSV bronchiolitis in vivo had no effect on
hRSV titers measured in upper or lower respiratory secretions
or on the release of the proinflammatory chemokine IL-8
(37; H. S.
Jafri, C. M. Carubella, S. C. Buckingham, P.
Sheeran, J. Hatfield, R. D. Hardy, J. P. DeVincenzo
and O. Ramilo, Pediatr, Acad. Soc. Joint Meet, abstr. 1564, 2000) or on
the release of IL-8 from hRSV-infected epithelial cells
(28). The observation
that glucocorticoids failed to blunt the virus-directed IL-8 response
suggests the possibility that hRSV-infected epithelial cells
may be resistant to the anti-inflammatory effects usually seen with
steroid administration, an observation that was reproduced in mice
infected with the natural pneumovirus pathogen PVM
(94).
The approach
involving a combined strategy that includes antiviral and agents
directed at specific virus-induced proinflammatory events has been best
characterized using the PVM mouse model
(92,
94), described in detail
in "Pathogenesis of disease" (above). The
proinflammatory chemokine MIP-1
has been identified as an
essential link between virus infection and granulocyte recruitment to
infected lung tissue
(90). However, once the
virus infection is established, the continued production of
MIP-1
and the cellular inflammatory response that ensues are
not tightly linked to virus replication, and, while ribavirin has
significant efficacy in reducing virus replication, it does nothing to
ameliorate the virus-induced proinflammatory cascade
(27). Dramatic clinical
improvement was observed in response to systemic administration of
ribavirin in combination with either neutralizing anti-MIP-1
antibody (27) or by
pharmacologic blockade of the primary MIP-1
receptor CCR1
(29,
95). Translation of these
laboratory findings may result in a clinically achievable therapeutic
strategy for the management of severe hRSV infection in human
infants.

CONCLUSIONS
Pneumovirus infection remains a significant problem for
practitioners
of both human and veterinary medicine. Advances in our
understanding
of pneumovirus biology have provided pathways toward new
vaccine
formulations. At the same time, advances in our understanding
of
the pathogenesis of pneumovirus disease in specific natural
hosts
have provided strategies for combined antiviral and
anti-inflammatory/immunomodulatory
therapies.

ACKNOWLEDGMENTS
We are grateful
to John Ellis, Department of Veterinary Microbiology,
Western College
of Veterinary Medicine at the University of
Saskatchewan, Saskatoon,
Canada, for his guidance in locating
and interpreting
relevant information from the veterinary
literature.

FOOTNOTES
* Corresponding
author. Mailing address: Building 10, Room 11N104,
Laboratory of Allergic Diseases, NIAID, National Institutes of Health,
9000 Rockville Pike, Bethesda, MD 20892. Phone: (301) 402-1545. Fax:
(301) 402-4369. E-mail:
hrosenberg{at}niaid.nih.gov.


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Clinical Microbiology Reviews, April 2004, p. 390-412, Vol. 17, No. 2
0893-8512/04/$08.00+0 DOI: 10.1128/CMR.17.2.390-412.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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