| Chapter 93A Human Herpes-virus 8 HAMIDA B. QAVI and DHARAM V. ABLASHI Table Of Contents |
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GROWTH PROPERTIES AND VIRUS STRUCTURE REPLICATION, LATENCY, AND REACTIVATION VIRAL DNA REPLICATION AND GENE EXPRESSION EPIDEMIOLOGY AND PATHOGENESIS REFERENCES |
| Taxonomically, Herpesviridae are divided into three subfamilies: Alphaherpesvirinae, Betaherpesvirinae, and Gammaherpesvirinae. There are eight human herpesviruses (HHV), including three that have been more recently discovered, HHV-6, HHV-7, and HHV-8.1–3 HHV include herpes simplex viruses (HSV) 1 and 2 and varicella zoster virus (VZV) (HHV-3), which belong to the alphaherpesvirus subfamily. This subfamily is characterized by its neurotropism and limited gene expression during latency. The beta subfamily of HHV is composed of cytomegalovirus (HHV-5) and HHV-6 and HHV-7. These viruses are distinguished from other herpesviruses by their limited host range and their cell type specificity. The newest member of the HHV family is HHV-8 (Kaposi's sarcoma [KS] herpesvirus), a member of the gammaherpesvirus subfamily. This subfamily also includes Epstein-Barr virus (EBV) (HHV-4) and is characterized by productive and latent infection of lymphocytes. Probably the most important characteristic of all herpesviruses is their ability to establish latent infections. |
| GROWTH PROPERTIES AND VIRUS STRUCTURE |
| HHV-8 is a member of the gammaherpesvirus subfamily, showing greatest nucleotide
homology to members of the genus Rhadinovirus. The existence
of this virus was initially recognized by using representational difference
analysis of nucleic acids from tissues of a KS patient. The deoxyribonucleic
acid (DNA) fragments obtained by this procedure had regions
of homology to late genes of other gammaherpesviruses.1 Since the initial discovery, HHV-8 has been found in all forms of KS: classic, endemic, and
acquired immunodeficiency syndrome (AIDS) associated. Because
of its association with KS, it is also known as Kaposi's
sarcoma associated herpesvirus (KSHV). Similar to other herpesviruses, strain
variations in HHV-8 have also been found. The strain variations
are mainly due to variations in the gene products of KSHV open
reading frame (ORF)-K1 and ORF-K15. Both of these seem to code for membrane-signaling
proteins and may play a key role in the biology and disease
manifestation associated with HHV-8.4 Three distinct strains of HHV-8 have been identified, which appear to
be restricted in their geographic distribution: B and C are the most common
in Africa, whereas A predominates in the Mediterranean region. Geographic
strain variations in gene sequences have also been identified
in virus isolated from Japan, Kuwait, Europe, Russia, Australia, South
America, United States of America, and Saudi Arabia.5–9 HHV-8 does not readily infect most cell types. Thus far, no primary cell or cell line is permissive to HHV-8 infection and none can produce infectious virus. However, several investigators have shown the transmission of HHV-8 DNA from BC-1 cell line into Raji (EBV positive B-cell line), BJAB (EBV negative B-cell line), Molt-3, and owl monkey kidney (OMK) cell lines and cord blood mononuclear cells.10,11 HHV-8 from KS lesions has also been propagated in embryonic kidney 293 cells. In all these cells, active infection was not noticed and the viral DNA was not detected after a few passages.12 About 38 additional cell lines, including B cells, endothelial cells, and cells of fibroblastic origin were used as targets for transmission, but all failed. Primary cultures of 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.13 Microvascular endothelial cells transformed with papilloma virus type 16 genes, E6 and E7, were also permissive to HHV-8 from BC-1 cell line.14 Untransformed primary fetal microvascular endothelial cells have also been successfully infected with HHV-8 from a primary effusion lymphoma (PEL) cell line.15,16 HHV-8 exhibits typical herpesvirus morphology of 100 to 150 nm-enveloped particles. The capsids are icosahedral with a diameter of 90 to 110 nm, which consists of 162 hexagonal capsomeres. The tegument is a protein-containing region between capsid and the envelope. Electron micrographs clearly indicated that the core is a complex of DNA and protein. About 35% of the virion is usually intact, and 65% is defective.17 The double-stranded linear DNA genome of the virus has been estimated to be 165 to 170 kilobase (kb).11 The genome of HHV-8 is similar to herpesvirus saimiri (HVS) found in squirrel monkeys. It has a single contiguous region, 140 to 145 kb, containing all the coding regions. The genome has repeats of 803 bp in length that are 85% guanine and cytosine. Each DNA molecule harbors about 35 to 45 repeat units.18,19 |
| REPLICATION, LATENCY, AND REACTIVATION |
| When the envelope glycoproteins of HHV-8 bind proteoglycans on the surface
of the host cells, penetration occurs by fusion of viral envelope
with plasma membrane of the cell. Following insertion into the host genome, DNA
replication and capsid assembly occur in the nucleus of the
host cell.11,12 Infectious virus has been isolated from body cavity-based B-cell lymphoma cells.20 In some cell lines derived from these lymphomas, EBV is also present, and both viruses may be latent.21 HHV-8 in these cell lines can be reactivated to produce viral particles.20,22 Virus transcripts have also been detected in endothelial-derived spindle cells from tumors of KS.23 HHV-8 is transmissible to B cells and replicates in the peripheral blood mononuclear cells of KS patients.10,24 HHV-8 expresses unique polyadenylated (poly A) nuclear ribonucleic acid (RNA) that colocalizes with uracil (U) small nuclear RNA but not with polysomes, suggesting a regulatory role for the RNA.25 In the latently infected B-cell lymphoma cell lines and KS cells, the genome exists in a circular form, similar to that described for other herpesviruses during latency.26 In KS, transcription of viral genes is restricted; only two transcripts are detected: one for a possible membrane protein and the second the abundant, poly A-containing nuclear transcript.27 |
| VIRAL DNA REPLICATION AND GENE EXPRESSION |
| The exact nature of the steps involved in the DNA replication is unknown. The
whole viral genome has been sequenced. Of the approximately 95 genes
comprising the HHV-8 genome, nearly 25 encode novel proteins not
found in other HHVs. HHV-8 contains regions encoding homologues of several
cellular genes that have regulatory roles in the immune response.18,28 KSHV carries 11 ORFs that encode homologues to cellular proteins involved
in signal transduction, cell cycle regulation, transformation, and/or
inhibition of apoptosis.29–32 A homologue of bcl-2, an important inhibitor of apoptosis in lymphoid and other cell types, is produced by HHV-8 and can inhibit apoptosis induced by several stimuli, thereby leading to an extended survival of the infected cells.29,30 Homologues of the chemokines, macrophage inflammatory protein-1, macrophage inflammatory protein-11, and the cytokine IL-6 are also produced.33–35 HHV-8-associated malignancies such as KS have been found to express a unique protein called kaposin, coded by latent transcript (ORF-12). Kaposin gene product induces tumorigenic transformation in transfected rat cells. Kaposin has also been detected in PEL cell lines, which are persistently infected with HHV-8.31,32 The virus also encode a D-type cyclin that can associate with CDK6 and induce phosphorylation of retinoblastoma (Rb) protein and overcome Rb-mediated cell cycle arrest.36,37 LNA-1 (ORF-37), the most frequently identified protein in KS tumor cells, has also been tumorigenic. It transforms primary rat embryo cells that seem to act as a transcriptional cofactor and contributes to HHV-8 induced oncogenesis by the retinoblastoma E2F transcriptional regulatory pathway.38 The virus also encodes homologues of interferon regulatory factors, a neural cell adhesion molecule, and the IL-8 receptor.39–41 The roles of these molecules in modifying the environment in which the virus infected cell exists, including the impact on the immune response to the virus, remain to be elucidated. |
| EPIDEMIOLOGY AND PATHOGENESIS |
| Unlike most HHVs, 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 HHV-8 antibodies. In
the areas around the Mediterranean Sea, the seroprevalence
is about 10%. In North Europe, Southeast Asia, and the
Caribbean, the seroprevalence is 2% to 4%. In the United
States, prevalence has been measured in the 5% to 20% range. In
HIV-1 infected populations, the prevalence of HHV-8 can rise
to 20% to 50% above the healthy population.42 The seroepidemiology of HHV-8 in HIV-1 positive patients (the patient population in the United States in which KS is most common) suggests that HHV-8 is sexually transmitted.43 The geographic distribution of KS and of seropositivity to HHV-8 vary in a coordinated way. In one study, the incidence of seropositivity to HHV-8 was compared in Uganda, where the rate of KS is among the world's highest; Italy, which has a moderate rate of KS; and the United States, which before the occurrence of AIDS had a low rate of KS. It was found that in Uganda, 50% to 60% of HIV-1 negative individuals without KS had antibody to HHV-8. In Italy, only 4% of blood donors were seropositive; in the United States, none of the blood donor samples tested had antibody to HHV-8.44 Most cases of KS in AIDS patients seen in the central east coast of the United States are essentially identical in a DNA sequence that was used for characterization in one study, suggesting that all of these U.S. cases may have a single recent origin.45 In patients with one of the diseases associated with HHV-8, such as KS, primary effusion lymphomas, and multicentric Castleman's disease, the seroprevalence rates rise to 100%.46 The transmission of HHV-8 most frequently occurs via saliva and sexual contacts.47,48 High correlations of seropositivity were also found between mother and child and among siblings.49 The virus has been cultured from a rare form of B-cell lymphoma, primary effusion lymphoma, multicentric Castleman's disease, and KS. The classic form of KS is a multifocal pigmented sarcoma found in older adult men. This type of sarcoma affects mostly older men of Mediterranean and Eastern European ancestry.50 Since its initial description, three more forms of KS have been identified. The second type, “endemic African” KS is more aggressive than the classic form and involves lymph nodes and skin. It often affects HIV-1 negative hosts and children. A third form of KS occurs after organ transplants in patients on immunosuppressive drugs. This form is also common in individuals of Mediterranean ancestry. The fourth form, which is one of the most aggressive types of KS, is often described in healthy homosexual men. It not only involves skin and lymph nodes but also often disseminates to the lungs, gastrointestinal track, liver, and spleen. Often this type of KS is also refers to as AIDS-associated KS.51–53 HHV-8 has been found in all forms of KS.54 Viral DNA has been specifically localized to tumor cells in KS and B-cell lymphoma by in situ PCR.55 In KS sarcoma patients, in situ hybridization has shown that viral DNA is restricted in its tissue distribution to cells of the lesions. Even in early lesions, the virus is present in spindle cells of KS.23 Using PCR analysis, HHV-8 DNA has also been detected in peripheral blood mononuclear cells from patients at the time of and years before development of KS, in lymphoid tissues, in bronchoalveolar lavage fluid from patients with pulmonary KS, in KS lesions from HIV-1 negative patients, and in saliva from people with KS.56–60 In situ hybridization studies have also demonstrated the virus in prostate tissue and semen of healthy men in Italy, but other studies failed to detect viral DNA in semen or urogenital or prostate tissue.56,60–62 The association between HHV-8 and KS is evident from numerous studies, but it remains unclear whether HHV-8 causes KS or KS creates an environment in patients that is conducive to HHV-8 infection or growth. In multicentric Castleman's disease, a disease characterized by a typical lymphoid proliferation, HHV-8 DNA is detectable, and increases in viral load correlates with increases in clinical measures of disease.63 The presence of the virus has been reported in non-Hodgkin's AIDS-associated body cavity-based lymphomas (primary effusion lymphomas) and some skin cancers and other tissues in HIV-1 infected individuals; however, it is not present in peripheral blood mononuclear cells of family members of sporadic KS patients. Although KS of eyelid or conjunctiva occurs in 20% to 25% of AIDS patients, detection of virus or viral DNA from these lesions has not yet been reported.64,65 Glucocorticoids are known to augment growth of KS tumors and have been shown to enhance replication of HHV-8 infected cells in culture. KS tumor cells produce transforming growth factor-β, an inhibitor of replication of some hematopoietic and endothelial cells that can act in an autocrine manner to reduce cell growth. Glucocorticoids do not inhibit transforming growth factor-β synthesis but block activation to an active form, thereby allowing increased cell proliferation.66 The sensitivity of HHV-8 to antiviral agents has been assessed in HHV-8-carrying B-cell lymphoma cells in culture. The virus appears to be insensitive to acyclovir but sensitive to antiviral agents used for anti-human cytomegalovirus (HCMV) therapy, including ganciclovir, foscarnet, and cidofovir.24,67 These results from cell cultures are consistent with findings from clinical use of antiviral drugs in AIDS patients. Both ganciclovir and foscarnet (but not acyclovir) reduced the risk of development of KS in this patient population.68 Antiviral therapy of HIV-1 has a beneficial effect on AIDS-associated KS patients. Topical treatment with 10% docosanol cream has reduced KS lesions by 20% as compared with untreated patients. The drug has inhibitory effects on several enveloped viruses in vitro including HSV-1, HSV-2, HCMV, HHV-6, and HIV-1.69 |