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Spectrum of Kaposi's Sarcoma-Associated Herpesvirus, or Human Herpesvirus 8, Diseases

Advanced Biotechnologies Inc., Columbia, Maryland,¹ Department of Microbiology and Immunology, Georgetown University School of Medicine, Washington, D.C.,² Department of Pathology, Weill Medical College of Cornell University, New York, New York³

SUMMARY

INTRODUCTION

STRUCTURE AND MORPHOLOGY

BIOLOGY, INFECTIVITY, TRANSFORMATION, AND TUMORIGENESIS

Biology

Infectivity of HHV-8

Transformation

Viral Genes and Transformation

Receptor for HHV-8

METHODS OF DETECTING VIRAL INFECTION

Nucleic Acids

In Situ Hybridization

Immunohistochemistry

Serology

TRANSMISSION

PBMCs

Saliva and Mucosal Shedding

Mucosal Shedding

Semen and Prostate Glands

Sexual Transmission

Transmission during Childhood and Adolescence

Experimental Transmission

SEROEPIDEMIOLOGY

KAPOSI'S SARCOMA

PRIMARY EFFUSION LYMPHOMAS

MULTICENTRIC CASTLEMAN’S DISEASE

EVIDENCE OF HHV-8 IN OTHER DISEASES

HHV-8 in Patients with Pemphigus

KS in Association with Bullous Phemigoid

Presence of HHV-8 in Other Skin Diseases

HHV-8 in Salivary Gland Tumors

HHV-8 in Nonneoplastic Lymphadenopathies and Chronic Interstitial Pneumonitis

KS and Sarcoidosis Coexistence in Lesions of an HIV-Seronegative Patient

HHV-8 in Kikuchi's Disease

Multiple Myeloma and HHV-8 Infection

HHV-8 in Hemophagocytic Syndrome with KSHV-Related Effusion Lymphomanone

TRANSPLANTATION RISKS

ANTIVIRAL THERAPY

CONCLUSIONS AND POINTS FOR FUTURE RESEARCH

ACKNOWLEDGMENTS

REFERENCES

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INTRODUCTION

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Most human herpesviruses are ubiquitous in most populations. They usually persist as long-term latent infections, and asymptomatic shedding of infectious virus is common. This shedding is responsible for horizontal primary transmission, usually from mother to child, so that initial infection occurs very early in life. Because they are so common, it has been very difficult to prove their role in the pathogenesis of malignant or nonmalignant diseases.

An important exception to this rule, because of its limited and uneven distribution, is human herpesvirus 8 (HHV-8), also called Kaposi's sarcoma-associated herpesvirus (KSHV). In sub-Saharan Africa, antibodies to HHV-8 can be found in upwards of 30% of the general population (55, 134, 258, 263). From 10 to 25% of people from the Mediterranean area are seropositive for the virus. Geographic pockets in this area with higher or lower prevalences can be found. In the rest of the world, the seroprevalency is low, 2 to 5% (58).

HHV-8 was first detected by Chang et al. (56) in Kaposi's sarcoma (KS) tissues from a patient with AIDS by representational difference analysis. Since its initial discovery, HHV-8 has been found in all forms of KS: classical, endemic, and AIDS-associated iatrogenically acquired KS (265). In situ hybridization techniques have pinpointed the location of HHV-8 in the vascular endothelial cells and perivascular spindle-shaped cells in KS lesions (31,172). This association has been supported both by molecular analysis (33, 50, 212, 262) and by seroepidemiological studies (11, 51, 134, 258, 263). The pathogenic role of HHV-8 in other malignancies, such as multicentric Castleman's disease and primary effusion lymphoma, was again based on molecular, seroepidemiological, and cell biology studies (264).

On the basis of phylogenic analysis (205, 248), HHV-8 is the first human rhadinovirus (gamma-2 herpesvirus) identified. HHV-8 is related to the rhadinoviruses herpesvirus saimiri, found in squirrel monkeys, and herpesvirus ateles, found in spider monkeys. Both primates are native to South America. HHV-8 is also in the lineage of rhadinoviruses that infect macaques and African green monkeys (30, 70). More recent studies (5, 70, 121, 122, 261, 267) have found additional rhadinoviruses that are closely related to HHV-8 infecting monkeys and chimpanzees. PCR has detected the DNA polymerase from rhadinoviruses in rhesus monkeys and pigtail macaques suffering from retroperitoneal fibromatosis (virus strains FHVMm and RFHVMn) and also in asymptomatic African green monkeys (virus strain ChRV-1). Retroperitoneal fibromatosis is characterized by a proliferation of spindle cells that is somewhat similar to KS. HHV-8 homologues were also detected in drill, mandrill, and a hybrid of Mandrillus leucophaeus-Mandrill sphinx, nonhuman primates living in Cameroon and Gabon, central Africa (155). Gamma-2 herpesviruses of higher primates closely related to HHV-8 were isolated from chimpanzees and gorillas after finding that they expressed KHSV antigens (154).

STRUCTURE AND MORPHOLOGY

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Before the discovery of HHV-8 (56), herpesvirus particles were described in a short-term tissue culture of a KS lesion (92, 115), and they were subsequently identified as cytomegalovirus (116). HHV-8 exhibits typical herpesvirus morphology: 100- to 150-nm particles with a lipid envelope and an electron-dense central core (242). Figure 1 is an electron micrograph showing the morphology of HHV-8 particles in KS-1 cells, a primary effusion body cavity B-cell lymphoma cell line which is persistently infected with HHV-8 (250). Its capsid is icosahedral, with a 110-nm diameter, and consists of 162 hexagonal capsomeres (295). Mature virions with a glycoprotein coat are 140 nm in diameter. The tegument is a protein-filled region between the capsid and the envelope. The 75-nm torus-shaped core is a complex of DNA and protein.

View larger version (102K): [in this window] [in a new window]   FIG. 1. Morphology of KSHV (HHV-8) virions produced in PEL cell line KS-1. Arrows indicate fully mature virions.

When the envelope glycoproteins of HHV-8 bind the proteoglycans on the surface of the host cell, penetration can occur by fusion of the viral envelope with the plasma membrane of the cell (134). HHV-8 infects dividing B cells (CD45⁺) during mitosis, much like EBV, the human gammaherpesvirus. Following circularization of the viral genome DNA replication and capsid assembly occur in the nucleus of the host cell.

Pulsed-field gel electrophoresis of DNA extracted from purified HHV-8 virions shows that the full-length genome is 165 to 170 kb (Fig. 2). Primary characterization of the genome was done by Moore et al. (205), and others (213, 248) have done additional studies. The genome of HHV-8 is similar to that of herpesvirus saimiri in that it has a single contiguous region, 140 to 145 kb, containing all the coding regions. Some permissive and nonpermissive tumor cell lines harbor forms of HHV-8 viral DNA up to 270 kb in size (14). The genome has repeats of 803 bp in length that are 85% guanidine and cytosine. Each molecule harbors 35 to 45 such repeats, but they are not arrayed uniformly or symmetrically at each end (158). Like EBV, the latent HHV-8 genome appears to have a circular conformation, but the active DNA found during lytic replication is linear (242).

View larger version (32K): [in this window] [in a new window]   FIG. 2. Structure of the KSHV (HHV-8) genome. The central portion of the genome is flanked by the terminal repeats, labeled TR. The KSHV genome contains close to 100 open reading frames. Many of these are conserved in most herpesviruses; these are present in the conserved blocks (white boxes) and are not indicated. Other open reading frames are unique to rhadinoviruses, gammaherpesviruses, or KSHV and are present in more divergent areas of the genome (indicated by gray boxes). ORFs that have homology with herpesvirus saimiri are assigned the corresponding numbers, and ORFs without recognizable homologues were numbered separately and given the prefix K (K1 to K15) (248). Adapted from references 212 and 248 with permission of the publishers.

HHV-8 has many homologies with closely related viruses (134, 267) but also has many unique sequences (30). ORF-26 of HHV-8, which encodes the minor capsid protein, has 51% homology to VP23 of herpesvirus saimiri and is 39% homologous to the EBV open reading frame (ORF) for tegument, BD LF1 (56). Of the approximately 95 genes in the HHV-8 genome, nearly 25 encode novel proteins not found in other human herpesviruses. Many of these represent captured and diverged homologues of cellular genes that are referred to by ORF-K numbers if they do not have homologues in the herpesvirus saimiri genome. A number of genes seem to be responsible for KS pathogenesis: K1, K2, vMIPS, K4, K4.1, K5, K9, K12, ORF-6, ORF-71, ORF-73, ORF-74, and K15 (212). The products and functions of these genes in pathogenesis have been reviewed by Neipel and Fleckenstein (212).

BIOLOGY, INFECTIVITY, TRANSFORMATION, AND TUMORIGENESIS

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View larger version (107K): [in this window] [in a new window]   FIG. 3. Purified HHV-8 virions. Supernatant from PMA-butyrate-induced KS-1 cells was harvested and fractionated by sucrose gradient centrifugation. The micrograph shows a cluster of negatively stained, mature HHV-8 virions (arrows), having a nucleocapsid core, defined tegument region, and outer lipid envelope. Surface spikes are evident on some particles.

View larger version (53K): [in this window] [in a new window]   FIG. 4. Detection of KSHV (HHV-8) DNA in various clinical samples from KS patients and infected cells. Genomic DNA was extracted with DNAzol reagent (Gibco-BRL, Gaithersburg, Md.), and 1 µg of total DNA was used in the PCR. For the detections of KSHV/HHV-8 expression, two primers were synthesized from the minor capsid region of the HHV-8 sequence. The sense primer was 5'-TCG AGC AGC TGT TGG TGT ACC ACA T, and the antisense primer was 5'-TCC GTG TTG TCT ACG TCC AG. These primers, used in the PCR, were designed to amplify 142 bp of HHV-8. Each PCR cycle consisted of denaturation at 94°C for 1 min, primer annealing at 60°C for 45 s, and extension at 72°C for 2 min. The samples were amplified for 30 cycles. A positive reaction for PCR of HHV-8 showed amplified product of 142 bp. Molecular weight markers (marker VI) were from Boehringer Mannheim, Indianapolis, Ind. (A) PBMCs from classic KS patients and HIV-infected KS patients, KS lesions, and skin from HIV-infected KS patients. (B) HHV-8-infected cells: PBMCs, monocytes, primary monkey peripheral blood cells, PBMCs from KS patients, the KS-1 cell line, which is persistently infected with HHV-8, and KS lesions from two patients. (C) HHV-8 DNA detected in plasma of two Israeli patients from Ethiopia.

Cultures of primary human monocytes-macrophages from patients with AIDS-associated KS can be infected with HHV-8, and treatment with cytokines seems to enhance viral production and allow the cells to maintain the virus for a longer period (21). An infection of primary cells with HHV-8 obtained from induced KS-1 cells is shown in Fig. 4.

Microvascular endothelial cells transformed with the papillomavirus type 16 E6 and E7 genes were also permissive for HHV-8 derived from the BCB-1 cell line (206). Successful transmission was demonstrated by DNA PCR, RT-PCR, immunofluorescent assay for cellular proteins, electron microscopy for morphological changes, and morphological changes in soft-agar colonies of the infected cells. The viral genome was maintained indefinitely and remained in latent form, as indicated by reaction with monoclonal antibody for ORF-73, an indicator gene of HHV-8 latency. The virus in these infected endothelial cells could be induced to go into the lytic cycle. This transition caused the cells to go from a cobblestone growth pattern to a spindle-shaped morphology, a growth pattern typical of KS lesions.

Untransformed primary fetal dermal microvascular endothelial cells, derived from large blood vessels or capillaries, have also been successfully infected with HHV-8 from PEL cell lines (JSC-1, BC-3, and BCP-1) (39, 294). In this system, the best results were obtained with virus from the JSC-1 line. Infection caused the dermal microvascular endothelial cells to change morphology from a cobblestone pattern to spindle-shaped cells, similar to the change seen in the transformed endothelial cells described above. This change is accompanied by loss of contact inhibition. Infected cells expressed latent nuclear antigen (LNA or LANA) and showed increased mitosis. Ten percent of the spindle-shaped cells spontaneously go into the lytic cycle and express K8 and other lytic cycle proteins, as demonstrated by immunofluorescence assay. The JSC-1 line is coinfected with HHV-8 and EBV, but EBV expression in dermal microvascular endothelial cells is extremely low. It is not yet clear what, if any, role EBV may play in HHV-8 infectivity in dermal microvascular endothelial cells.

Virus from PEL cell lines and virus from KS lesions passed to 293 cells were used to infect the EBV-negative Loukes B-cell line (99). After infection, viruses from both sources induced apoptotic cell death. Transient expression of the HHV-8 vBCL-2 homologue delayed apoptosis and prolonged the survival of the infected 293 cells. From this, it was concluded that HHV-8 induces apoptosis through a BCL-2-dependent pathway. The virus derived from KS lesions and grown in 293 cells has distinct characteristics compared to the virus from PEL cell lines and therefore may play different roles in the pathophysiology of KS.

Six days postinfection, cultures of primary human keratinocytes infected with HHV-8 have demonstrated transcription of viral genes by RT-PCR and protein expression by immunofluorescence assay for ORF-73 (49). This infection was tried because HHV-8 DNA can be found in endothelial keratinocytes in the basal layer of the epidermis in nodular lesions of the plaque stage of KS. HHV-8 DNA can also be found in the epithelial cells of the eccrine glands within KS lesions, in salivary glands, and rarely in prostate tissues (75, 227). Infected cultures of keratinocytes could be induced to go into the lytic cycle and transcribe lytic genes such as ORF-26. The cells continued to proliferate, and the growth pattern of the culture was changed from the pattern of uninfected cultures. They lost contact inhibition and demonstrated telomerase activity, anchorage-independent growth, and changes in cytokine production. In the study by Flore et al. (92) on primary bone marrow endothelial cells, HHV-8 could always be detected in a subset of transformed cells after culture for more than 1 year, but in the keratinocyte study, HHV-8 was undetectable after 8 weeks by nested PCR.

Most KSHV genes with oncogenic potential (Fig. 2) appear to be transcribed to some extent in PELs. Still, patterns of KSHV gene expression in de novo infection and during initial steps of lymphomagenesis are not known. KSHV carries 11 open reading frames that encode homologues to cellular proteins involved in signal transduction, cell cycle regulation, and/or inhibition of apoptosis (248) (Fig. 2). Four of these genes, K9, the viral interferon regulatory factor (vIRF) (107, 170, 299), ORF-74, the KSHV viral G protein-coupled receptor (GPCR) (15, 17), ORF-K1 (165, 166), and ORF-K12 (kaposin) (208, 209), can transform rodent cells and/or cause tumors in animal models.

The KSHV GPCR and K1 oncogenes are potentially important in PEL lymphomagenesis because they can trigger signaling cascades relevant for B- and T-cell growth. KSHV GPCR is a constitutively active G protein-coupled receptor able to trigger the mitogen-activated protein kinase signaling cascades and induce secretion of vascular endothelial growth factor (17). Mitogen-activated protein kinase cascades such as those triggered by KSHV GPCR are activated by inflammatory cytokines and mitogens. Vascular endothelial growth factor is an angiogenic and vascular permeability factor that could contribute to the effusion phenotype. K1 has an ITAM motif that can activate cytoplasmic tyrosine kinases and mimic signaling by the B-cell antigen receptor (165, 166).

KSHV also encodes homologues of cytokine and cytokine response genes: a viral interleukin 6 (vIL-6), K2, and the vIRFs (Fig. 2). vIL-6 can bind the gp130 receptor to activate IL-6-responsive genes and promote B-cell survival (201, 204, 215). vIRF1 can inhibit interferon-induced transcriptional activation (107, 170, 299) (Fig. 5). KSHV also contains ORFs homologous to cellular oncogenes involved in lymphomagenesis. These are the viral cyclin D (ORF-72), which is homologous to the BCL-1 gene, and the viral BCL-2 (ORF-16). The KSHV cyclin homologue is a functional cyclin that can associate with CDK6, induce phosphorylation of retinoblastoma (Rb) protein, and overcome Rb-mediated cell cycle arrest (169; Y. Chang, P. S. Moore, S. J. Talbot, C. Boshoff, T. Zarkowska, D. Godden-Kent, R. Weiss, and S. Mittnacht, letter to the editor, Nature 382:410, 1996). The viral cyclin D differs from the cellular cyclin D in its ability to induce degradation of the CDK inhibitor p27^Kip when complexed with CDK6 (85, 182). The viral BCL-2 can block apoptosis as efficiently as cellular BCL-2, BCL-xL, or the EBV BCL-2 homologue BHRF1 (61). Interestingly, the KSHV BCL-2 cannot homodimerize or heterodimerize with other BCL-2 family members, suggesting that it may have evolved to escape any negative regulatory effects of the cellular Bax and Bak proteins. In addition to BCL-2, KSHV carries a viral FLIP (vFLIP, K13), which is an inhibitor of the proapoptotic molecule FLICE/caspase-8 (281).

View larger version (102K): [in this window] [in a new window]   FIG. 5. KS, PEL, and MCD: morphology and immunohistochemistry for viral antigens and detection of latent and lytic antigens to KSHV by IFA, with human sera and monoclonal antibody. (A) KS lesions in the lower extremities typical of a sporadic case. (B) Hyperpigmented KS lesions in the upper arms. (C) Histological section stained with hematoxylin and eosin of a nodular tumor stage lesion of KS. Note the spindle cell proliferation and abundant vasculature. (D) KSHV LANA (ORF-73) expression in KS. Staining with a rat monoclonal antibody revealed LANA positivity (diaminobenzidine, brown) in the nuclei of many spindle cells in a KS lesion. Positivity was also identified in endothelial cells lining the larger vascular spaces that may represent lymphatic vessels. (E) Histology of multicentric Castleman's disease. Hematoxylin- and eosin-stained section of a lymph node with HIV-associated Castleman's disease showing a single follicle with a large, concentrically arranged mantle zone surrounding a germinal center. The interfollicular area contains a network of small vessels. (F) KSHV vIL-6 expression in MCD. Immunohistochemical staining with polyclonal antiserum to vIL-6, showing cells with cytoplasmic positivity (diaminobenzidine, brown) in the mantle zone surrounding an atrophic germinal center. (G) Wright-Giemsa stain air-dried cytocentrifuge preparation of a KSHV-positive primary effusion lymphoma. The two tumor cells in this image are considerably larger than normal benign lymphocytes and neutrophils. The cells display significant polymorphism and possess moderately abundant basophilic cytoplasm. A prominent, clear perinuclear Golgi zone can be appreciated in the largest cell. The nuclei vary from large and round to highly irregular, multilobated, and pleomorphic and often contain one or more prominent nucleoli. (H) KSHV LANA (ORF-73) expression in KSHV-positive lymphomas. Staining with a rat monoclonal antibody revealed LANA positivity (alkaline phosphatase, red) in the nuclei of large, atypical lymphoma cells seen infiltrating reactive lymphoid tissue. This section was double stained with a polyclonal antiserum to kappa light chains (diaminobenzidine, brown), showing cytoplasmic positivity in a few of the surrounding cells but not the tumor cells. (I) KSHV vIL-6 expression in PELs. Immunohistochemical staining of cell block containing the BC-3 cell line was performed with a polyclonal rabbit antiserum to a vIL-6-specific peptide. Abundant expression is seen (diaminobenzidine, brown) in numerous lymphoma cells. (J) Detection by IFA of KSHV latent IgG antibody in serum from a classic KS patient. Typical nuclear speckles in the PEL cell line KS-1 are evident at a 1:50 dilution of the serum. (K) Presence of LANA-2 protein in KSHV-infected BCBL-1 cells. Diffuse finely speckled nuclear pattern of LANA-2 (green) is observed by IFA with LANA-2 monoclonal antibody. (L) Detection by IFA of KSHV lytic antibody in serum from a classic KS patient, using induced KS-1 cells. Apple green diffusely stained cells carry lytic antigen. Original magnifications: x200 (D, E, F, and I), x600 (C and H), and x1,000 (G). Panels C to I copyright Amy Chadburn (Weill Medical College of Cornell University, New York, N.Y.). Panel K reproduced from reference 247 with permission.

LNA-1 (ORF-73), the protein most frequently identified in KS tumor cells (Fig. 5), has also been found to be tumorigenic. It transforms primary rat embryo cells (236), seems to act as a transcription cofactor, and contributes to HHV-8-induced oncogenesis by targeting the retinoblastoma protein E2F transcriptional regulatory pathway.

METHODS OF DETECTING VIRAL INFECTION

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Serological and molecular methods have been employed independently and in concert to detect the presence of HHV-8 KSHV in cells, tissues, and body fluids.

The PEL cell lines BCP-1, KS-1, BC-3, and BCBL-1 are commonly used as substrate cells for immunofluorescence assay (IFA) for the detection of antibodies to both latent and lytic proteins (Fig. 5J and L). In order to detect latent antibody, a harsher fixation method has been used to permeate the cells so that the test sera can react with LNA (108, 268). This latent nuclear protein is 226 to 236 kDa and is localized in the cell nucleus of infected cells. Typical IFA of LNA shows dotted fluorescence in the nucleus of infected cells, found in more than 95% of PEL cells (Fig. 5J). LNA IFA detection was the primary method first used in HHV-8 serology to detect antibody prevalence in patients and healthy donors. Recently, an ELISA with recombinant LNA (ORF-73) baculovirus protein has been developed and is available commercially; this assay can detect latent antibody in more specimens from patient groups and healthy blood donors than the traditional IFA-LNA. The recombinant ORF-73 ELISA and IFA-LNA gave similar results in over 90% of testing trials (C. R. Lee, M. Roman, D. Thomas, D. Bourboulia, C. Boshoff, O. Flore, A. Friedman-Kien, P. S. Gill, R. Masood, T. Schulz, J. E. Whitman, B. Chandran, and D. V. Ablashi, HHV-8 ORF-73 latent antigen [LNA-1] ELISA: development and performance comparison with LANA IFA, Fifth International AIDS Malignancy Conference, 23-25 April 2001, National Cancer Institute, abstract 31). This ELISA to detect latent antibody is highly specific. The assay has >10% more sensitivity than IFA-LANA.

For the detection of lytic antibody to HHV-8, both IFA (Fig. 5L) and ELISA have been used. In order to test for lytic antibodies by IFA, the PEL cell lines must be chemically induced to express the lytic antigens. Positive cells range from 20 to 70% depending on the cell line used, the time course of induction, and other biological factors. Chatlynne et al. (59) described an IFA with the KS-1 cell line (253) and a lytic antibody ELISA that uses sucrose-purified whole virus from the same cell line. Both of these assays have proven to be sensitive and specific, correlate very well with other assays and disease incidence, and are commercially available (Advanced Biotechnologies Inc.). Homemade ELISAs for antibodies to small viral capsid antigen (175) and to the major capsid protein with recombinant ORF-65 have also demonstrated good results (38, 234, 268).

KS patients are 80 to 90% positive and demonstrate elevated lytic antibody titers in the whole-virus ELISA. By the same test, normal healthy donors show a prevalence of 2 to 5%, varying slightly with the population except for areas of central Africa, where the virus is endemic (1). Gao et al. (108) used ORF-73 isolated from the nuclei of BC-1 PEL cell line for immunoblots, with which 80% of AIDS-related KS sera tested positive. With a combination of Western blots and an ORF-65 ELISA to test serological samples, the following percentages were positive: 75 to 85% of AIDS-KS, 31% of homosexual men, 2% of hemophiliacs in the United Kingdom, 1.7 to 5% of healthy blood donors from the United Kingdom and United States, and 35% of control groups from Uganda (134, 175, 268). An ELISA using targeted peptides from ORF-65 and ORF-K8.1 to detect lytic IgG HHV-8 antibodies found the following were positive: 92% of KS patients, 3% of United States blood donors, and 55% of HIV-positive men (40).

Rabkin et al. (234) and later Enbom et al. (86) did comparative studies of most of the serological methods available at that time and found considerable variations in the specificity and sensitivity of the assays. They found no reliable consistency between all the assays. In Enbom's study, the samples that were positive by the K8.1 ELISA were positive in all the other assays used, but very few samples were positive by this assay. More recently, Schatz et al. (260) compared many second-generation assays plus some novel ones and found much better concordance than had been seen previously. Five panels of serum were sent to seven European laboratories and tested with 18 different assays, 9 of which were used to screen all five panels. The panels contained samples from Uganda, Germany, Switzerland, and Italy, from people both with and without KS, people both positive and negative for HIV-1, and various combinations of these states. Since no "gold standard" for determining positivity for HHV-8 antibodies is yet available, they assumed that all KS patients were infected with HHV-8 because all KS lesions are HHV-8 positive by PCR.

Using this assumption as a basis, they compared the nine assays for specificity and sensitivity by statistical methods. All nine showed excellent concordance with AIDS-KS sera, HIV-1-positive/KS-negative sera, and Italian blood donors. The combination of the lytic IFA from the Laboratory of Virology, Istituto Superiore di Sanità, in Rome and the latent nuclear antigen IFA from the Department of Medical Microbiology of the University of Liverpool was found to be the most specific, but only marginally better than the ELISA ORF-K8.1 and ELISA ORF-K12 from the Institute for Clinical and Medical Virology, University of Erlangen-Nürberg, Germany, and the Munich mix 2 peptide ELISA from the Max von Pettenkofer Institute, Gezentrum, Munich, Germany. For sensitivity in this survey, the Rome lytic IFA and the Liverpool LANA IFA also performed the best, but several other assays proved almost as sensitive. The ORF-K8.1 and ORF-K12 ELISAs, while shown to be very specific, did not prove to be as sensitive as other assays.

TRANSMISSION

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It is probable that the viral load in the blood of HHV-8-infected subjects may vary considerably depending upon the state of infection, as demonstrated by Sitas et al. (269), who found that HHV-8 peripheral blood viral load and the IgG antibody to HHV-8 were well correlated. Ariyoshi et al. (13) also found that in Gambia, West Africa, KS is less frequent in HIV-2-positive individuals in spite of evidence of HHV-8 infection. In HIV patients with no diagnosis of KS, HHV-8 DNA has been found in 13% (18 of 135) of the PBMCs of the patients studied (198). Furthermore, HHV-8 DNA in PBMCs and lower CD4⁺ cell counts correlated with elevated plasma levels of HIV mRNA, giving good evidence for the relationship between immunosuppression, HIV replication, and HHV-8 expression (84, 198). After finding a high copy number (9,000 copies) of HHV-8 DNA in the PBMCs of 45% of KS patients studied, several researchers (87, 156) raised the question of why there has been no evidence of blood-borne viral transmission.

HHV-8 DNA was detected in the saliva of 75% of HIV-1 patients tested plus in the saliva of one HIV-negative patient (150), where concentrations ranged from 10^2.4 to 10⁶/ml, but no HHV-8 DNA was found in the saliva of healthy donor controls. This suggests that HHV-8 is transmitted in the saliva like other human herpesviruses (EBV, cytomegalovirus, HHV-6, HHV-7, and human herpes virus simplex type 1) (28, 47, 157, 177, 178). In a study, LaDuca and colleagues (46, 156) quantitated HHV-8 DNA (33,000 copies per µg) in the saliva of 37% of the KS patients studied. HHV-8 DNA was also found in PBMCs of 46% (9,000 copies per µg), plasma of 7% (40 copies per µg), semen of 12% (300 copies per µg), normal skin of 23%, and KS skin lesions of 92% (64,000 copies per µg) of the same KS patients. The finding of high copy numbers of HHV-8 DNA in the saliva makes it a likely route for transmission of the virus.

HHV-8 DNA was found in 8% of 184 semen samples of HIV-infected individuals by Pellett et al. (228), indicating that semen can carry the virus, albeit in small amounts. This was confirmed by other reports (74, 129, 202), except that of Lin et al. (173), in which a high prevalence of HHV-8 was detected. In a note to Lancet, Lin et al. (174) retracted the previous report by stating that the high incidence of HHV-8 DNA reported previously was probably due to contamination.

HHV-8 was detected in the genital tract of women who were HHV-8 seropositive (293). Sexual transmission of HHV-8 was also reported in young men who have sex with other men (76). High-risk sexual behavior and drug use were factors in the incidence of HHV-8 infection in a study of a group of 1,295 women from the United States (40). The independent association of HHV-8 infection with injection drug use suggested that HHV-8 can be transmitted via needle sharing, albeit less efficiently than EBV, hepatitis C virus, or HIV-1. HHV-8 seropositivity increased with the frequency of injection drug use (P < 0.001), e.g., in women who used drugs daily; in women who used cocaine, the HHV-8 seropositivity was three times higher.

If HHV-8 can be transmitted via needle sharing, it should also be able to be transmitted via blood transfusion and through blood derivative products. To date, no studies have been published regarding the potential risk of HHV-8 transmission or its association with disease development. The lack of detectable HHV-8 in the semen of non-KS HIV-infected individuals is highly suggestive that it may be a rare event in transmitting HHV-8 by the genital route in HIV-1 and HHV-8 antibody-positive patients (74).

The pattern of familial aggregation between mother and child and among siblings in French Guiana plus the variation with age in the seroprevalence of HHV-8 suggests transmission from mother to child and between siblings in areas of Africa where HHV-8 is endemic (185, 230). Similar evidence of horizontal transmission has been found in the central African country of Cameroon (113). Additional evidence for horizontal transmission from mother to child has been reported in South Africa by Bourboulia et al. (34) and by Andreoni et al. (9) in Egyptian children. In another report, high correlations of seropositivity were found between mother and child and among siblings but not between spouses (230).

SEROEPIDEMIOLOGY

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Many studies (reviewed in reference 58) have been done to measure the prevalence of antibodies to HHV-8. A variety of techniques have been used in these studies, but to date there is no gold standard against which to measure the efficacy of these techniques, so some may overestimate the number of positives, and some may underestimate the number. Nonetheless, certain trends are clear (264). Unlike most human herpesviruses, HHV-8 is not spread universally among all human populations. The highest seroprevalences in healthy populations are found in sub-Saharan Africa, where approximately 40% are positive for antibodies to HHV-8 (1, 264). In black South African blood donors and patients with cancers other than KS, the seroprevalence is 32%; in KS patients, it rises to 83% (269). In countries surrounding the Mediterranean Sea, the seroprevalence averages 10%, although population pockets where the prevalence is higher can be found (7, 10, 58, 66, 264); this may be why classic KS is most often diagnosed in men of Italian or Jewish extraction.

In northern Europe, southeast Asia, and the Caribbean, seroprevalence falls into the 2 to 4% range (1). In Japan, the seroprevalence of HHV-8 antibodies in 1,000 blood donors was measured at 0.2% and rose to 11% in HIV-1-positive patients (n = 276) (102). In northern Europe, the prevalence of HHV-8 and the incidence of KS are so low that HHV-8 antibodies can be used to track the risk for KS. In Amsterdam, HHV-8 antibodies were most often found in HIV-infected men who had recently seroconverted and could be used to predict risk for KS (119). In the French-speaking part of Switzerland, no cases of classic KS were recorded before 1983; thereafter, most KS was found in HIV patients, but the assays used were unable to predict the incidence of KS by testing for HHV-8 antibody (232). The exception for northern Europe is Sweden, where 20% of blood donors were reported to test positive for HHV-8 antibodies; however, the same report shows a wide variety of results depending on the assay used (86).

In the United States, prevalences have been measured in the 5 to 20% range. In populations infected with HIV-1, the prevalence of HHV-8 can rise from 20 to 50% above that in the local healthy population. No rise in prevalence in HIV-1 patients is seen in the Caribbean or southeast Asia, areas where no AIDS-associated KS has been reported (1).

In patients with one of the diseases associated with HHV-8, such as KS, primary effusion lymphoma, and multicentric Castleman's disease, prevalence rates can rise to 100% (58, 260). In contrast, HIV-associated non-Hodgkin's lymphoma, a disease which is not associated with HHV-8, showed no correlation with HHV-8 antibodies (110). In areas of Africa where juvenile KS is endemic, such as Uganda and Zambia, the seroprevalence of HHV-8 is the same as in the Gambia and the Ivory Coast where endemic juvenile KS does not occur (118), suggesting that other factors may be involved in disease expression.

The seroprevalence in children tends to reflect that in the adult population, but in somewhat lower percentages (3). Increase in seroprevalence with age has been found in many studies (7, 246, 269). In central African countries with high seroprevalences, such as Cameroon (112), French Guiana (230), and Uganda (190), seropositivity increased with age and usually reached adult prevalence before puberty. For example, in Cameroon, the overall prevalence for children and young adolescents studied was 27.5% and rose to 48% in those above 15, similar to the rate of 54.4% in pregnant women. Similar results were found in Italy, with seroprevalences in children reflecting those of the surrounding adult population but in somewhat lower percentages that rose with age (292). In Italy, only 2 of 57 infants studied carried antibody, but in older children, the rate rose to 4.4% (292). This clustering is strongly indicative of nonsexual horizontal transmission before puberty.

KAPOSI'S SARCOMA

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In 1872, Moriz Kaposi reported several cases of a multifocal pigmented sarcoma in elderly Viennese men (140). These patients had cutaneous lesions, typically on the lower extremities (Fig. 5A). All the patients described by Kaposi eventually died of what is now known as "classic" KS, which predominantly affects older men of Mediterranean and eastern European descent. Since Kaposi's initial description, three more forms of KS have been identified. The second type, endemic-African KS, which is more aggressive than classic KS, may involve lymph nodes in addition to skin and often affects HIV-negative hosts and children (298). A third, iatrogenic form of KS occurs after solid-organ transplantation in patients on immunosuppressive medications. Like classic KS, this form is more common in those of Mediterranean ancestry (62, 97). A fourth and very aggressive type of KS was described in the early 1980s in otherwise healthy homosexual men (100) (Fig. 5B). AIDS-KS not only involves skin (Fig. 5B) and lymph nodes but often disseminates to the lung, gastrointestinal tract, liver, and spleen. Early in the epidemic, lifetime incidence of KS in gay men was around 50% (145), but this incidence has declined in the 1980s and then more precipitously with the introduction of effective HIV therapy in the late 1990s (117, 163).

The four clinical-epidemiological forms of KS have indistinguishable histologic features. KS is composed of a variable mixture of ectatic, irregularly shaped, round capillary and slit-like endothelium-lined vascular spaces and spindle-shaped cells accompanied by a variable inflammatory mononuclear cell infiltrate. Red blood cells and hemosiderin pigment are frequently present, often extravasated between the spindle cells. Small granules of intracytoplasmic or extracellular hyalin material may be identified. Sometimes the earliest patch and plaque stage lesions are difficult to distinguish from granulation tissue. The spindle cells eventually become the predominant cell population, forming fascicles that compress the vascular slits, and the lesions become progressively nodular (63, 99).

The histogenesis of the KS spindle cell has not been easy to trace. Although KS cells stain for certain endothelial cell markers such as CD34⁺ and factor VIII, some studies show that they express proteins similar to dendritic cells, macrophages, or smooth muscle cells (275). It is debated, therefore, whether KS cells represent a heterogeneous population of cells or, instead, arise from a pluripotent mesenchymal precursor cell. More recent cell surface marker studies suggest that spindle cells may be of lymphatic endothelial cell origin (82). It has also been questioned whether KS represents a clonal, neoplastic process or a polyclonal inflammatory lesion.

In early KS lesions, there are few spindle cells compared to the surrounding inflammatory cells. Furthermore, KS cells in culture are dependent on exogenous growth factors and, when implanted into nude mice, can induce an inflammatory and angiogenic reaction but do not induce tumors as would fully transformed cells (255). Moreover, regression of KS can happen spontaneously or when immunosuppression is corrected. Such characteristics, along with the multifocality of KS lesions, argue that the process is primarily one of dysregulated inflammation. Confusing the picture, however, X chromosome inactivation studies within single lesions as well as comparisons of multiple lesions from a single patient support a clonal origin in a subset of advanced cases (233).

More recent studies have shown varying monoclonality, oligoclonality, and polyclonality from lesions of various patients (114). Furthermore, three neoplastic cell lines have been established from KS lesions (18, 179). A likely possibility is that KS starts as a hyperplastic polyclonal lesion that later gives rise to a clonal cell population only under specific circumstances, such as immunosuppression and selective pressures. KS may be similar to posttransplantation lymphoproliferative disorders, which are EBV-driven B-cell proliferations, which may progress from a polyclonal hyperplasia to monoclonal tumors with no evident genetic abnormalities, to frank malignant lymphomas with oncogene and tumor suppressor gene alterations (148). Figure 4 shows HHV-8 sequences detected by PCR in PBMCs, KS lesion skin, and plasma from KS patients with an HHV-8 primer sequence.

PRIMARY EFFUSION LYMPHOMAS

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The first reported cases of malignant lymphoma occurring as body cavity effusions were described as AIDS-associated lymphohematopoietic neoplasms displaying an indeterminate immunophenotype (149). In this report of three cases, two of which were lymphomatous effusions, a B-cell lineage and the presence of EBV were demonstrated by using DNA-based assays. Subsequent studies recognized these lymphomatous effusions as occurring relatively frequently in HIV-infected individuals (120, 288; D. Karcher, F. Dawkins, C. T. Garret, and R. S. Schulof, abstract, Lab. Investig. 66:80, 1992). However, these were thought of as unusual AIDS-related lymphomas, and it only became clear that they represent a quite distinct clinicopathological entity with the recognition of the presence of KSHV within them (50).

The initial study documenting this association revealed KSHV DNA in all AIDS-related lymphomas presenting in body cavities as lymphomatous effusion; therefore, they were initially called body cavity-based lymphomas and, subsequently, PELs. This virus was not found in other AIDS- and non-AIDS-related non-Hodgkin's lymphoma disease or lymphoid leukemia (50). These lymphomas contained very large amounts of viral DNA, ranging between 40 and 80 copies per cell; therefore, this virus was easily identifiable by Southern blot analysis, in contrast to KS tissues, which may contain less than 1 copy per cell, making it necessary to perform PCR for its detection in some cases.

The specific association between KSHV and PEL has been confirmed by multiple investigators (42, 112, 143, 220, 225; A. Carbone, U. Tirelli, A. Gloghini, C. Pastore, E. Vaccher, and G. Gaidano, letter to the editor, Eur. J. Cancer 32A:555-556, 1996). In addition, cases of PEL occurring in HIV-negative men as well as in women have been identified, and these PEL cases also contained KSHV (42, 210, 253; R. G. Nador, E. Cesarman, D. M. Knowles, and J. W. Said, letter to the editor, N. Engl. J. Med. 333:943, 1995). Lymphomas lacking KSHV can, however, involve body cavities as lymphomatous effusions, even in the absence of a tumor mass. Effusions are particularly common in Burkitt's lymphomas but can also be seen in non-Hodgkin's lymphomas; therefore, certain criteria should be used for the diagnosis of PEL (210). In our experience these criteria include (i) presentation as a lymphomatous effusion in the pleural, peritoneal and/or pericardial cavity without a contiguous tumor mass (86%), frequently remaining localized to the body cavity of origin (81%); (ii) morphology bridging large-cell immunoblastic lymphoma and anaplastic large-cell lymphoma (100%) (Fig. 5G); (iii) expression of CD45 and one or more activation-associated antigens (95%) in the frequent absence of B-cell-associated antigens (95%) and immunoglobulin expression (76%); (iv) B-cell origin as demonstrated by the presence of clonal immunoglobulin gene rearrangements (97%); (v) coinfection with Epstein-Barr virus (86%); (vi) lack of c-myc gene rearrangements (97%); and (vii) lack of bcl-2, ras, and p53 gene alterations (87%).

Small series and isolated cases reported by other investigators have shown a similar set of characteristics (220, 225; A. Carbone, U. Tirelli, A. Gloghini, C. Pastore, E. Vaccher, and G. Gaidano, letter to the editor, Eur. J. Cancer 32A:555-556, 1996; D. S. Karcher and S. Alkan, letter to the editor, N. Engl. J. Med. 333:797-798, 1995). AIDS-related lymphomas displaying several of these features should be evaluated for the presence of KSHV/HHV-8 to confirm the diagnosis of PEL. Patients with this type of lymphoma have a very poor clinical outcome, with a median survival of 5 months. PELs are extremely rare tumors, estimated to account for about 3% of AIDS-related lymphomas and 0.4% of all AIDS-unrelated diffuse large-cell non-Hodgkin's lymphomas (42).

An additional complication to the understanding of PELs is the finding that lymphomas containing KSHV can present as solid tissue masses, usually extranodally, similar to other AIDS-related non-Hodgkin's lymphomas. While some of these lymphomas subsequently develop an effusion, others apparently do not. In fact, we have seen several cases that have presented as solid extranodal lymphomas and were diagnosed as diffuse large-cell, immunoblastic, or anaplastic large-cell lymphomas, in which the presence of KSHV in practically all the lymphoma cells could be demonstrated by in situ hybridization and immunohistochemistry (Fig. 5H). In support of the notion that these lymphomas fall in the spectrum of PEL are the observations that they usually lack expression of B-cell antigens and immunoglobulin, have an immunoblastic or anaplastic morphology, and are frequently coinfected with EBV. In addition, a recent epidemiologic study found a statistically significant association between the presence of KS and immunoblastic lymphoma in patients with AIDS (88). Therefore, it appears that KSHV-associated lymphomas represent a distinct pathobiologic category which is frequently, but not exclusively, associated with a lymphomatous effusion, comprising approximately 5% or less of all AIDS-related lymphomas.

Cases of AIDS-related lymphoma which have been positive for KSHV by PCR but not by Southern blot analysis have been identified by us, as well as by others (112, 220), suggesting low viral copy numbers in the tumor tissue. In situ hybridization studies of these cases have shown that the virus is present only in scattered atypical cells and some reactive-appearing cells in these cases (R. G. Nador et al., unpublished data). The cases we identified were not reminiscent of PEL, since they exhibited a polymorphic morphology and expression of B-cell-associated antigens CD19 and CD20. While KSHV may be playing an indirect role in these lymphomas as well, cases containing low copy numbers of KSHV should not be considered part of the spectrum of PEL. Further studies are necessary to determine whether KSHV is acting as an antigenic stimulus or using paracrine mechanisms to stimulate a proliferation of B cells and therefore is involved in the pathogenesis of these lesions, or whether its presence in these lymphomas is merely the result of disseminated viremia in KSHV-positive patients.

Additional cases of lymphoma that do not have the features of PEL but containing KSHV have been identified (161). Lymphomas arising from multicentric Castleman's disease with plasmablastic morphology, lacking EBV, and expressing lambda light chain have been reported (80). In addition, a case of Burkitt's lymphoma was found to contain KSHV by PCR and to occur in a child who had IgG antibody to KSHV lytic antigens (1:640). Therefore, the spectrum of KSHV-associated lymphomas may expand, but these observations await identification of additional similar cases and confirmation by other investigators.

Closer examination of PELs has provided information about the biology of this type of disease and its place in the spectrum of non-Hodgkin's lymphomas. Most cases have been B-cell lymphomas, as determined by the presence of clonal immunoglobulin gene rearrangements. However, two biphenotypic cases expressing CD3 have been identified, both of which contained B- and T-cell antigen receptor gene rearrangements (220, 252), as well as one case with a T-cell phenotype and genotype (164). PELs usually lack expression of B-cell-associated antigens, although some of them have been reported to express monotypic or mRNA (103, 104), and others have been shown to express surface or cytoplasmic immunoglobulin (210, 220). In our experience, faint but distinct cytoplasmic staining using antisera to or can be seen in a subset of cells in some PEL cell lines (unpublished observation).

Most PELs are thought to originate from post-germinal center B cells, since they have hypermutation of the immunoglobulin genes (89, 189). In addition to an immunoblastic morphology, PELs have a set of immunophenotypic features that suggest they are at a preterminal stage of B-cell differentiation. Loss of expression of B-cell antigens occurs in plasma cells, and this is a frequent finding in multiple myeloma as well as in immunoblastic lymphomas. Furthermore, most PELs express CD138/syndecan-1, an adhesion molecule that is selectively expressed by a subset of pre-B cells and by plasma cells, including myeloma plasma cells. The expression of CD138 by PELs seems to be quite specific, as it is not expressed by other lymphomatous effusions, primary or secondary, or by most other solid lymphomas (103).

The almost invariable presence of KSHV in lymphomas having the features described above suggests that this virus is necessary for the development of PELs. However, since PELs are so uncommon, even in populations in which the seroprevalence of KSHV is relatively high, it is evident that infection by this virus represents only one of several genetic events involved in their development. One other such factor appears to be EBV infection, as the vast majority of PELs, especially in immunocompromised hosts, contain both viral genomes. The specific role of each of these viruses and their interaction is still poorly understood, but analysis of the genes expressed by both of them has shed some light on their possible roles. Both herpesviruses can be lytic or latent, expressing distinct subsets of genes. PELs in vivo as well as in culture (see below) express mostly latent genes, but there is always a small proportion of cells in which EBV and KSHV lytic gene expression occurs. However, most cells have a latent pattern of gene expression.

It is known that EBV can establish different types of latency. Latency type I (restricted latent gene expression) is seen in Burkitt's lymphomas, while latency type III (full pattern of latent gene expression) is seen in lymphoblastoid cell lines and large-cell lymphomas in immunocompromised patients, particularly those with immunoblastic features. While the EBNA1 gene, necessary for EBV replication, is expressed in all latency types, other latent genes, including the EBV transforming genes LMP1 and EBNA 2, are only expressed in latencies type II (LMP1) and type III (LMP1 and all EBNAs). It is thought that expression of these transforming genes in Burkitt's lymphomas is not "necessary," since these carry a translocated c-myc oncogene, and the lack of expression of these immunogenic proteins further provides this type of lymphoma with an advantage to evade the immune system.

Analysis of the pattern of EBV gene expression in PELs revealed that only EBNA1 was expressed, corresponding to type I latency (128, 276). This was an unexpected finding, given the resemblance of PEL cells to immunoblastic lymphoma cells. This observation suggests that KSHV is playing a transforming role in PELs, as the major EBV oncogenes are not expressed. Also consistent with this hypothesis is the lack of structural alterations in the cellular transforming genes frequently involved in malignant lymphomas. In particular, c-myc gene rearrangements have been identified in only one case (E. Cesarman, unpublished observation); they are extremely uncommon in contrast to other lymphomatous effusions and EBV-associated lymphomas, where alterations in this gene are a frequent findin