Herpesviruses

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Herpesviruses

Jeffrey I. Cohen, MD

Overview

Eight herpesviruses have been isolated from humans: two of these, Epstein–Barr virus (EBV) and Kaposi sarcoma-associated herpesvirus (KSHV), are associated with human tumors. EBV has been detected in lesions from patients with post-transplant lymphoproliferative disease, nasopharyngeal and some types of gastric carcinoma, Burkitt lymphoma, Hodgkin lymphoma, and certain other lymphoid tumors. KSHV is associated with Kaposi sarcoma, primary effusion lymphoma, and Castleman disease. Each of these viruses encodes proteins important for establishment of latency, transforming cells, and evading the immune system.


Properties of herpesviruses

Herpesviruses are enveloped DNA viruses that have the capacity to establish latent infection as well as to undergo lytic infection. The ability to establish latent infection in vivo and to reactivate from latency ensures a source of virus to infect previously uninfected individuals. Most adults latently harbor herpes simplex 1, varicella-zoster virus, human herpesviruses 6 and 7, and Epstein–Barr virus (EBV). Several features of herpesvirus replication are important for the maintenance of latency and for oncogenicity; EBV will serve as an example to illustrate the principles of herpesvirus infection relevant to oncogenicity.

First, viral DNA must be maintained in the cell. The EBV genome is usually maintained in latently infected B cells as a multicopy circular episome in the host cell. Second, a cell transformed by a virus must avoid immune clearance. Replication of EBV requires up to 100 viral proteins; however, latent infection of B cells with EBV results in expression of 12 or fewer genes.1–3 This limited repertoire of gene products prevents frequent virus replication, avoiding death of the infected cell, and restricts the ability of the immune system to recognize and destroy cells latently infected with the virus. Third, specific viral proteins interact with other cell proteins or directly transactivate other cell genes to provide additional functions necessary for cell proliferation and immortalization. Several EBV proteins interact with cellular proteins to activate transcription of viral and cellular genes or to engage signal transduction pathways in the cell.

EBV: an oncogenic human herpesvirus

EBV gene expression in transformed lymphocytes

Infection of primary B cells with EBV in vitro results in transformation of the cells, which can then proliferate indefinitely. Eight EBV proteins and several nontranslated RNAs are expressed in latently infected B lymphocytes that have been growth transformed by EBV in vitro (Table 1). The EBV nuclear proteins EBNA-1, EBNA-2, EBNA-LP, EBNA-3A, EBNA-3B, and EBNA-3C comprise the EBV nuclear antigen complex. EBNA-1 binds to the oriP sequence (origin of viral DNA replication) on EBV DNA and allows the virus genome to be maintained as an episome in transformed B cells.4 EBNA-1 also transactivates its own expression. EBNA-1 transcripts are initiated from one of four different promoters. The Cp and Wp promoters are used to express EBNA-1 in lymphoblastoid cell lines in vitro; the Qp promoter is used in tissues from Burkitt lymphoma, nasopharyngeal carcinoma, and Hodgkin lymphoma; and the Fp promoter is used during lytic replication.5 EBNA-1 inhibits its own protein degradation by proteasomes6 and limits its own translation,7 both of which may reduce its presentation to CD8+ cytotoxic T cells; however, EBNA-1 remains a target for CD4+ cells.8–11 EBNA-1 inhibits apoptosis induced by expression of p53.12

Table 1 Selected EBV genes and their cellular homologs and activities

Gene Expression class Cellular homolog Activity
EBNA-1 Latent, lytic None Episome maintenance, transactivates viral genes, inhibits apoptosis
EBNA-2 Latent Notch Transactivates viral and cellular genes, inhibits apoptosis
EBNA-3A, B, C Latent None Regulates EBNA-2 activity, transactivates cellular genes
EBNA-LP Latent None Increases EBNA-2 activity
LMP1 Latent, lytic CD40 Transactivates cellular genes, inhibits apoptosis
LMP2 Latent None Prevents EBV reactivation, transactivates Akt
EBERs Latent None Upregulates cellular genes
BARF-1 Lytic CSF-1R Inhibits IFN-α
BCFR1 Lytic IL-10 Inhibits IFN-γ and IL-12
BNLF2a Lytic None Blocks antigen-specific CD8 T-cell recognition
BHRF1 Lytic Bcl-2 Inhibits apoptosis
BALF1 Lytic Bcl-2 Regulates BHRF1 activity
BGFL5 Lytic None Blocks synthesis of MHC class I and II
BILF1 Lytic GPCR Removes MHC class I from cell surface
BLLF3 Lytic None Upregulates IL-10, TNF-α, and IL-1β
BPLFI Lytic None Inhibits toll-like receptor signaling
BZLF1 Lytic None Inhibits IFN-γ effects, inhibits function of p53, inhibits TNF-α, initiates lytic infection

EBNA-2 transactivates expression of EBV LMP113 and LMP2,14 and the cellular genes CD21, CD23, c-myc, and c-fgr.15, 16 EBNA-2 is targeted to the LMP1, LMP2, Cp EBNA, and CD23 promoters by the GTGGGAA-binding protein Jκ, and thereby activates these promoters.17 EBNA-2 is a functional homolog of the Notch receptor, which uses Jκ to regulate gene expression during development.18 EBNA-2 also interacts with the DNA-binding protein PU-1 to transactivate the LMP1 promoter19 and with AUF to transactivate the EBNA Cp promoter.20 The transactivation domain of EBNA-2 is essential for B-lymphocyte transformation.21 This domain interacts with transcription factors TFIIB and the TATA-binding protein-associated factor TAF40.22 EBNA-2 inhibits apoptosis mediated by Nur77.23

EBNA-LP interacts with and enhances the ability of EBNA-2 to transactivate LMP1 and LMP2.24 Although EBNA-LP binds to Rb and p53 in vitro,25 the significance of these interactions is uncertain. Deletion of the carboxy terminus of EBNA-LP markedly reduces the ability of the virus to transform B lymphocytes.26

EBNA-3A, EBNA-3B, and EBNA-3C are distantly related to each other. The EBNA-3 proteins bind to Jκ preventing it from binding DNA, thereby inhibiting transactivation by EBNA-2.27 EBNA-3C upregulates expression of LMP1 and CD21. EBNA-3C binds to Nm23-HI, a human metastasis suppressor protein, and inhibits the protein’s ability to suppress migration of Burkitt lymphoma cells.28 EBNA-3C degrades Rb and enhances kinase activity by disrupting p27.29, 30 EBNA-3A and EBNA-3C are essential for B-lymphocyte transformation in vitro, while EBNA-3B is dispensable.31, 32

LMP1 functions as a transforming oncogene in nude mice.33 Expression of LMP1 in EBV-negative Burkitt lymphoma cells results in B-cell clumping and increased villous projections. Upregulation of bcl-2, bfl-1, and A20, and inhibition of Bax, by LMP1 in B cells protects the cells from apoptosis.34, 35 Expression of LMP1 in epithelial cells inhibits differentiation of the cells.36

LMP1 is a functional homolog of CD40, a member of the tumor necrosis factor receptor (TNFR) family.37 The carboxy terminus of LMP1 interacts with the TNFR-associated factors (TRAFs) 1, 2, 3, and 5, TRADD, RIP, and JAK3 in vitro.38 LMP1 functions as a constitutively active form of CD40 resulting in activation of NF-κB, stress-activated protein kinases, STATs, adhesion molecules, the B7 co-stimulatory molecule, JNK, and B-cell proliferation.39 LMP1 upregulates expression of intracellular adhesion molecules, Fas, CD40, and MMP-940 in B cells and EGF in epithelial cells.41 LMP1 inhibits phosphorylation of Tyk2 resulting in inhibition of interferon (IFN)-α signaling.42 LMP1 is essential for transformation of B lymphocytes by EBV.43 Analysis of EBV-containing human lymphomas shows that LMP1 localizes with TRAF-1, TRAF-3, and that activated NF-κB is present.44

LMP2 is dispensable for transformation of B cells,45 but induces a transforming phenotype in epithelial cells and promotes their motility.46, 47 LMP2 prevents lytic reactivation of EBV-infected primary B cells in response to activation of the B-cell receptor complex by cross-linking of surface immunoglobulin. LMP2 associates with the src family and syk protein-tyrosine kinases that are coupled to the B-cell receptor complex.48 Binding of LMP2 to these proteins results in their constitutive phosphorylation, which inhibits their ability to mediate signaling for virus reactivation.48, 49 B cells from transgenic mice expressing LMP2 survive even without normal B-cell receptor signaling activity.50 LMP2 activates β-catenin and Ras/PI3K/Akt signaling pathways in epithelial cells resulting in transformation of the cells.51, 52 LMP2 also activates mTOR and increases c-myc expression.53

The two EBV-encoded RNAs, EBER-1 and EBER-2, are the most abundant EBV RNAs in latently infected B cells; however, they are not required for latent or lytic EBV infection, but may contribute to B-cell transformation.54, 55 The EBERs upregulate expression of bcl-2 and IL-1056 and interact with the double-stranded RNA-activated protein kinase, and IFN-inducible oligoadenylate synthetase.57, 58

EBV encodes at least 44 miRNAs from the BART and BHRF1 regions of the genome; both BART and BHRF1 microRNAs downregulate cellular genes and inhibit apoptosis and enhance B-cell proliferation.59 EBV also encodes cellular RNAs that enhance transformation.60 Both EBV microRNAs and LMP1 are secreted from infected cells in exosomes, which may enhance tumorigenicty.61, 62

EBV genes expressed during productive infection

Infection of epithelial cells with EBV results in productive infection, with replication of virus and lysis of infected cells. Immediate-early genes encode regulators of virus gene expression, including the BZLF1 and BRLF1 proteins, which act as switches to initiate lytic infection. The BZLF1 protein inhibits TNF-α signaling63 and helps the virus evade T-cell responses.64 BZLF1 protein also inhibits the expression of the IFN-γ receptor65 and inhibits the function of p53.66 Early genes encode proteins involved in viral DNA synthesis, while late genes encode structural proteins.

Three viral genes expressed during productive infection are functional homologs of cellular genes and are important for the survival of EBV-infected B cells.67, 68 The EBV BCRF-1 protein is homologous to IL-10 and has IL-10 activity.69 BCRF-1 is a B-cell growth factor and inhibits IFN-γ release from activated peripheral blood mononuclear cells and secretion of IL-12 from macrophages.

The EBV BARF-1 protein acts as a soluble receptor for CSF-1.70 BARF-1 inhibits IFN-α secretion by human monocytes. The EBV BNLF2a protein interacts with the TAP complex to block antigen-specific CD8 T-cell recognition.71 The EBV BHRF1 protein is homologous to bcl-2 and protects cells from apoptosis.72 EBV BALF1 is also homologous to bcl-2 and antagonizes the antiapoptotic effect of BHRF1.73

Clinical aspects

EBV infection is usually spread by saliva. The virus infects B cells directly, or oropharyngeal epithelial cells and then spreads to subepithelial B cells.74 During primary infection, up to a few percent of peripheral blood B lymphocytes are infected with EBV and have the capacity to proliferate indefinitely in vitro. Natural killer (NK) cells, CD4 T cells, and HLA- and EBNA- or LMP-restricted cytotoxic T cells control the latently infected B lymphocytes.8 T- and B-cell interactions release lymphokines and cytokines, giving rise to many of the clinical manifestations of acute infectious mononucleosis. After recovery, the fraction of B cells latently infected with EBV in the peripheral blood remains at 1 in 105 to 1 in 106. These lymphocytes are the primary site of EBV persistence and a source of virus for persistent infection of epithelial surfaces.

B-cell tumors that occur early after EBV infection are usually lymphoproliferative processes, in which latent virus infection in B cells is the principal cause of proliferation. In contrast, nasopharyngeal carcinoma occurs long after primary EBV infection and viral gene expression is less important to the growth of the malignant cells.

Lymphoproliferative disease

EBV is associated with B-cell lymphoproliferative disease in patients with congenital or acquired immunodeficiency. X-linked lymphoproliferative syndrome is an inherited immunodeficiency of males; most patients die of a fatal lymphoproliferative disorder or fulminant hepatitis, but some survive with hypogammaglobulinemia or develop EBV-positive lymphomas. The gene mutated in X-linked lymphoproliferative syndrome has been identified as SAP,75 which encodes an SH2-containing protein that interacts with SLAM on B and T cells, and with 2B4 on NK and T cells. Anti-CD20 antibody (rituximab) has been effective in treating some patients with X-linked lymphoproliferative disease and acute EBV infection.76 Mutations in additional cellular proteins including XIAP, ITK, CD27, MagT1, CTP synthase 1, and PI3K 110δ predispose to severe EBV infections.

EBV lymphoproliferative disease occurs in patients who are immunosuppressed as a result of transplantation or AIDS.77–79 Risk factors for development of lymphoproliferative disease include EBV-seronegativity prior to transplant and receipt of T-cell-depleted bone marrow or antilymphocyte antibody. Lymphoproliferative lesions are most commonly seen in the lymph nodes, liver, lungs, kidney, bone marrow, or small intestine. Tumors in transplant patients are usually classified as lymphomas; some patients have hyperplastic lesions. The proliferating lymphocytes in these tumors generally do not have chromosomal translocations.

AIDS-related lymphomas may be systemic (nodal or extranodal) lymphomas or primary central nervous system lymphomas. Although most B-cell tumors in transplant recipients and central nervous system lymphomas in AIDS patients contain EBV, about 50% of other lymphomas in AIDS patients contain EBV. Tumors in patients with AIDS are usually either immunoblastic lymphomas or Burkitt lymphomas; most of the latter have c-myc translocations.

Tissues from transplant recipients or AIDS patients with EBV lymphoproliferative disease show expression of EBERs, EBNA-1, EBNA-2, and LMP1 (Table 2). The EBV viral load in the peripheral blood has been used to predict development of disease and to follow patients after therapy. The expression of EBV genes, which are targets for cytotoxic T cells, has important implications for therapy. Infusion of EBV-specific cytotoxic T cells, nonirradiated donor leukocytes, or HLA-matched allogeneic cytotoxic T cells have been effective in many cases for treatment of EBV lymphoproliferative disease.80–86 Anti-CD20 antibody (rituximab) has induced remissions in some patients and has been used in some studies as pre-emptive therapy when EBV viral DNA in the blood is rising in transplant recipients at risk of lymphoproliferative disease,87 although other studies suggest that pre-emptive therapy may be unnecessary.88

Table 2 Diseases associated with EBV latent gene expression

Disease BARTs EBERs EBNA-1 EBNA-2 LMP1 LMP2
Burkitt lymphoma + + +
Nasopharyngeal carcinoma + + + + +
Hodgkin lymphoma + + + + +
Peripheral T-cell lymphoma + + + + +
Lymphoproliferative disease + + + + + +

Burkitt lymphoma

Seroepidemiologic studies show a strong association between Burkitt lymphoma and EBV in Africa.89, 90 More than 90% of African Burkitt lymphomas are associated with EBV, whereas only approximately 20% of Burkitt lymphomas in the United States are associated with the virus. African patients with Burkitt lymphoma often have high levels of antibody to EBV antigens, and the virus can be recovered from the tissue.

Burkitt lymphomas contain chromosomal translocations that result in c-myc dysregulation. The most common chromosomal translocation, t(8;14), places a portion of the c-myc oncogene adjacent to an immunoglobulin heavy chain gene. Less common translocations involve the c-myc oncogene and the κ or λ immunoglobulin light chain genes t(2;8) and t(8;22), respectively. These translocations result in high constitutive expression of c-myc. New approaches to treating Burkitt lymphoma based on inhibition of c-myc are in development.91

EBV-associated endemic Burkitt lymphoma is thought to develop in steps. First, EBV infection may expand the pool of differentiating and proliferating B cells. Second, chronic endemic malaria may cause T-cell suppression and B-cell proliferation. Third, enhanced proliferation of differentiating B cells may favor the chance occurrence of a reciprocal c-myc (t[8;14] or t[8;22]) translocation placing c-myc partially under the control of immunoglobulin-related transcriptional enhancers, with development of a monoclonal tumor.

Nasopharyngeal carcinoma

The nonkeratinizing nasopharyngeal carcinomas are uniformly associated with EBV. Patients with nasopharyngeal carcinoma have high levels of antibodies to EBV antigens. A prospective study of Taiwanese men showed that those with IgA antibodies to EBV viral capsid antigen (VCA) and anti-EBV deoxyribonuclease antibodies had an increased risk for developing nasopharyngeal carcinoma when compared to men without these antibodies.92 These antibodies are useful in screening patients for early detection of nasopharyngeal carcinoma and are prognostic for patients after treatment. Another study showed that quantifying the level of EBV DNA in plasma of patients with advanced nasopharyngeal carcinoma was useful for monitoring patients and predicting outcomes.93 Nasopharyngeal carcinoma tissue contains EBV genomes in every cell. These tumors are monoclonal with regard to EBV infection, indicating that EBV infection precedes malignant cell outgrowth at the cellular level. Unlike Burkitt lymphoma, the association of EBV with nasopharyngeal carcinoma is uniform and universal. Infusions of EBV-specific cytotoxic T cells resulted in remissions in some patients with refractory nasopharyngeal carcinoma.94

Hodgkin lymphoma

Persons with a history of infectious mononucleosis are at a greater risk of developing Hodgkin lymphoma.95 Patients with Hodgkin lymphoma generally have higher titers of antibody to EBV VCA than does the general population. Tissues from ∼40% to 60% of patients with Hodgkin lymphoma contain EBV genomes. Cases of Hodgkin lymphoma in patients with HIV or from developing countries are more likely to contain EBV genomes than persons without HIV or from developed countries.96 The EBV genome is monoclonal and present in Reed–Sternberg cells. EBV is more often associated with aggressive subtypes (especially mixed cellularity) of Hodgkin lymphoma. Tumors from patients with EBV+ Hodgkin lymphoma and some from patients with lymphoproliferative disease arise from postgerminal center cells.97 Infusion of cytotoxic T cells generated from 11 patients with relapsed Hodgkin lymphoma and measurable disease resulted in complete remissions in two patients, partial remission in one patient, stable disease in five patients, and no response in three patients.98 Infusion of arginine butyrate and ganciclovir to induce EBV thymidine kinase expression with phosphorylation of ganciclovir and induction of apoptosis resulted in antitumor responses in some patients with EBV B-cell malignancies.99

Other tumors associated with EBV

EBV has also been detected in non-Hodgkin’s lymphomas. EBV-positive diffuse large B-cell lymphoma has a poorer prognosis than EBV-negative lymphoma.100 Treatment of patients with EBV+ non-Hodgkin’s lymphoma in remission with autologous antigen-presenting cells transduced with LMP2 resulted in increased frequencies of LMP2-specific cytotoxic T cells and tumor responses in several patients with relapsed disease.101 EBV DNA and latency proteins have been detected in tissues from patients with peripheral T-cell lymphomas.102 EBV DNA has also been detected in central nervous system lymphomas from patients with no underlying immunodeficiency, T cells in patients with virus-associated hemophagocytic syndrome, nasal T-cell lymphoma, carcinoma of the palatine tonsil, laryngeal carcinoma, and angioimmunoblastic T-cell lymphoma. EBV DNA and nuclear antigens have been detected in thymic carcinomas and in B-cell lesions from patients with lymphomatoid granulomatosis.

EBV DNA has been found in leiomyosarcomas in AIDS patients,103 and viral RNA and EBNA-2 have been detected in smooth muscle tumors in organ transplant recipients.104 About 7% of primary gastric carcinomas are EBV+, especially in undifferentiated lymphoepithelioma-like carcinomas.

KSHV and malignancies

In 1994, Chang et al.105 detected sequences of a new human herpesvirus, Kaposi-sarcoma-associated herpesvirus (KSHV) in Kaposi sarcoma tissues from patients with AIDS. KSHV is present in B cells in asymptomatic persons. B-cell lines derived from primary effusion lymphomas maintain KSHV in a latent state and can be induced to undergo lytic virus replication by the addition of phorbol ester or butyrate. Infection of dermal microvascular endothelial cells in vitro with KSHV results in transformation of the cells with maintenance of long-term infection. The cells become spindle-shaped and show loss of contact inhibition with anchorage-independent growth.106 Although KSHV transforms bone-marrow-derived endothelial cells, virus is present in only a small fraction of the cells.107

Viral proteins

Several KSHV proteins are important for transformation, establishing latency, and modulating the immune response to the virus.108–110 KSHV encodes a large number of cellular homologs (Table 3) that have been grouped into different classes, depending on when they are expressed in primary effusion lymphoma cell lines.111 Expression of the KSHV K1 gene in rodent fibroblasts results in transformation of the cells.112 The K1 protein induces tyrosine phosphorylation in cells113 and activates the Akt signaling pathway.114 The K1 protein inhibits apoptosis115 and results in constitutive calcium-dependent signal activation in B cells.116

Table 3 Selected KSHV genes and their cellular homologs and activities

Gene Expression class Cellular homolog Activity
K1 II ITAM motif Transformation, activates signaling pathways
K2 II IL-6 B-cell growth factor, angiogenesis, hematopoiesis
K3 III None Reduces surface MHC class I
K4 II MIP-1α Chemokine receptor antagonist; angiogenesis; chemotaxis
K4.1 II MIP-1α Chemokine receptor agonist; angiogenesis; chemotaxis
K5 II None Inhibits NK-cell activity; reduces surface MHC class I; increases monocyte proliferation
K6 II MIP-1α Chemokine receptor agonist; angiogenesis; chemotaxis
K8 III None Inhibits p53
K9 II IRF Represses IFN activity, transformation, inhibits p53
K10.5 (LANA-2) II IRF Represses IFN activity; inhibits apoptosis; inhibits p53
K11.1 I IRF Represses IFN activity
K12 (Kaposin A) II None Transformation
K12 (Kaposin B) II None Increases cytokine mRNA stability
K14 II OX-2 Induces proinflammatory cytokines
K15 (LAMP) III None Binds TRAFs, inhibits B-cell receptor signaling
ORF4 II CR2 Complement-binding protein
ORF16 II/III Bcl-2 Inhibits apoptosis
ORF45 III None Inhibits IRF7
ORF50 III None Increases CD21 and CD23 expression, degrades IRF7
ORF63 III NLR proteins Inhibits NLRP1
ORF71 I FLIP Inhibits apoptosis; activates NF-κB
ORF72 I Cyclin D Cell-cycle progression, inhibits Rb
ORF73 (LANA-1) I None Episome maintenance, inhibits p53 and Rb
ORF74 II GPCR Angiogenesis, transformation, and proliferation

Expression class I = latent gene, expressed in uninduced primary effusion lymphoma cells, not induced by phorbol ester; class II = expressed in uninduced cells, induced by TPA; class III = lytic gene, expressed only after induction by TPA (includes many structural proteins and DNA replication enzymes). FLIP = FLICE inhibitory protein; GPCR = G-protein-coupled receptor; IRF = interferon regulatory factor; ITAM = immunoreceptor tyrosine-based activation motif; MHC = major histocompatibility complex; MIP = macrophage inflammatory protein; NK = natural killer; Rb = retinoblastoma protein; TRAFs = TNFR-associated factors.

The KSHV K2 gene encodes an IL-6 homolog (vIL-6). IL-6 is a B-cell growth factor and acts as an autocrine growth factor for lymphoid tumors resulting in proliferation.117 vIL-6 prevents death of IL-6-dependent B9 cells in vitro,118 promotes hematopoiesis, and induces VEGF to promote angiogenesis.119 The K3 [modulator of immune recognition 1 (MIR1)] and K5 (MIR2) proteins induce rapid endocytosis of MHC (major histocompatibility complex) class I molecules and IFN-γ receptor 1 from the surface of cells by ubiquitination of these proteins.120–122 The K5 protein also downregulates ICAM-1 and B7.2, which results in inhibition of NK-cell-mediated cytotoxicity,123 and removes CD31 from the surface of endothelial cells.124

The KSHV K4, K4.1, and K6 genes encode three chemokines: the viral macrophage inflammatory proteins (MIP)-II, -III, and -I, respectively. vMIP-I inhibits replication of HIV strains dependent on CCR5.118 vMIP-I and vMIP-III are chemokine receptor agonists for CCR8125 and CCR4,126 respectively, while vMIP-II is a broad-spectrum chemokine receptor antagonist. vMIP-II is a chemoattractant for eosinophils,127 binds to both CC and CXC chemokines, and blocks calcium mobilization induced by chemokines.128

The KSHV K9, K11.1, and K10.5 proteins are referred to as viral IFN regulatory factor (vIRF)-1, -2, and -3, respectively. Each of these proteins inhibits virus-mediated activation of the IFN-α promoter.129 vIRF1 inhibits MHC-1 transcription and surface expression.130 vIRF1 and vIRF3 inhibit p53-mediated apoptosis131, 132 and transform NIH 3T3 cells.133 vIRF3 (also called LANA2) protects cells from p53-induced apoptosis.134, 135 KSHV ORF45 blocks the activity of IRF7 and inhibits the activation of IFN-α and -β.136

The KSHV K12 locus encodes several proteins termed kaposins. Kaposin A induces transformation of cells.137 Kaposin B increases expression of cytokines by activating MAP-kinase-associated protein kinase 2 and inhibiting the degradation of cytokine mRNA.138, 139 The KSHV K14 protein, a homolog of the cellular OX2 protein, stimulates monocytes to produce proinflammatory cytokines such as TNF-α, IL-1β, and IL-6.140 KSHV ORF4 protein inhibits the complement system.141 KSHV ORF16 encodes a homolog of bcl-2 and inhibits bax-induced apoptosis.142 The ORF71 gene encodes a homolog of cellular FLIP that blocks apoptosis. ORF71 binds to Atg3 and protects cells from autophagy. KSHV ORF71 activates NF-κB, promotes tumor growth, and is required for survival of KSHV-infected lymphoma cells.143–147 KSHV ORF72 encodes a cyclin D homolog that binds to and activates cdk6 and phosphorylates p27, and stimulates cell-cycle progression in normally quiescent fibroblasts.148, 149 The viral cyclin protein phosphorylates and thereby inactivates Rb.150

KSHV ORF73 encodes LANA1 that localizes with viral DNA episomes and tethers them to chromosomes during cell division.151 KSHV LANA1 is required for persistence of the episome in dividing cells and transactivates its own promoter. In addition, LANA1 inhibits the activity of both p53 and Rb,152, 153 upregulates expression and stabilizes β-catenin,154 upregulates and activates survivin,155 induces nuclear accumulation of HIF,156 and activates c-myc.157 Expression of LANA in transgenic mice results in development of lymphomas.158 LANA inhibits TGF-β signaling.159 KSHV ORF74 encodes a G-protein-coupled receptor that is homologous to the cellular IL-8 receptor; unlike the latter protein, however, the KSHV receptor is constitutively active and induces cellular proliferation.160 ORF74 protein induces angiogenesis,161 activates the Akt signaling pathway,162 and induces proliferation and vascular permeability of endothelial cells.163–165 ORF74 activates NF-κB and JNK, and upregulates IL-1, IL-8, TNF-α, and FGF, and inhibits viral lytic gene expression.166

KSHV K15 encodes the latency-associated membrane protein (LAMP) and interacts with TRAFs 1, 2, and 3.167 K15 suppresses tyrosine phosphorylation and intracellular calcium mobilization, inhibiting B-cell receptor signaling.168 KSHV encodes several microRNAs located between K12 and ORF71 or within K12 that are expressed during latency and detected in patient plasma.169 These microRNAs are important for NF-κB activation and blocking cell-cycle arrest of latently infected cells.109, 170 KSHV microRNAs also target cellular genes.171

Clinical aspects

Seroprevalence rates for KSHV vary from <5% in normal blood donors in the United States or United Kingdom to 30–35% in HIV-positive homosexual men.172 Antibody to KSHV is more common in African and Mediterranean populations. At least 85% of patients with Kaposi sarcoma have antibodies to KSHV.173 The prevalence of Kaposi sarcoma is lower in women than in men, and HIV-seropositive women have a much lower incidence of antibody to KSHV than do seropositive men. KSHV seropositivity in HIV-positive homosexual men is predictive of subsequent development of Kaposi sarcoma.174 Levels of KSHV DNA are higher in patients with active Kaposi sarcoma or multicentric Castleman disease, than in those in remission, and are also elevated in patients with primary effusion lymphoma.175 The virus is not thought to be pathogenic in most healthy individuals and can persist in a latent phase for life; however, in immunocompromised persons, it is strongly associated with Kaposi sarcoma. Thus, although infection with KSHV appears to be required for development of Kaposi sarcoma, it is probably not sufficient by itself and other cofactors, such as HIV and impaired cellular immunity, are important. KSHV is thought to be sexually transmitted in homosexual men172 and has been associated with sexual transmission and intravenous drug use in women.176 In endemic populations (e.g., Africa), KSHV may be transmitted vertically from mother to child and between siblings. KSHV has been transmitted by renal allografts.177, 178

Kaposi sarcoma

KSHV has been found in nearly all biopsies of classic Kaposi sarcoma, African endemic Kaposi sarcoma, Kaposi sarcoma in HIV-seronegative transplant recipients and homosexual men, and Kaposi sarcoma in patients with AIDS.179, 180 KSHV is present in the endothelial and spindle cells of the tumor, but not in normal endothelium.181 Most of the tumor cells are latently infected with the virus, but 1–5% of the spindle cells in HIV-positive Kaposi sarcoma show lytic KSHV infection. Kaposi sarcoma can be polyclonal, oligoclonal, or monoclonal. KSHV is also present in the peripheral blood mononuclear cells of approximately 50% of patients with Kaposi sarcoma, and its presence is predictive of development of the malignancy.172 KSHV has also been detected in the saliva of patients with Kaposi sarcoma, and, infrequently, in semen. Several KSHV proteins are expressed in Kaposi’s tissues (Table 4). Foscarnet and ganciclovir reportedly reduce the frequency of new Kaposi sarcoma lesions in some, but not all, studies.182, 183 Cidofovir had no effect on treatment of established lesions.184 In contrast, HIV protease inhibitors have been reported to induce regression of Kaposi sarcoma lesions.185 IL-12, in combination with liposomal doxorubicin, resulted in tumor responses in AIDS patients with Kaposi sarcoma receiving HAART; responses were maintained with IL-12 therapy.186 Sirolimus,187 imatinib,188 bevacizumab,189 and paclitaxel190 have all been reported to have activity against Kaposi sarcoma.

Table 4 Diseases associated with KSHV gene expression

Gene Kaposi sarcoma Primary effusion lymphoma Castleman disease
LANA (ORF73) + + +
K12 (Kaposin) + +
ORF72 (v-cyclin) + +
ORF71 (v-FLIP) + + +
ORF74 (GPCR) +
K10.5 (vIRF3) + +
K9 (vIRF1) +
K2 (vIL-6) ± +

Primary effusion lymphoma

KSHV has also been found in primary effusion lymphomas in patients with AIDS.180, 181, 191 These body-cavity-based lymphomas of B-cell lineage are located in the pleural, peritoneal, or pericardial space and usually contain EBV as well as KHSV genomes. Some KSHV-positive lymphomas have been found in patients without AIDS.

Multicentric Castleman disease

KSHV has also been detected in biopsies from some patients with multicentric Castleman disease, especially in the variant known as the plasma cell type.180, 181, 192–194 This disease is usually polyclonal and presents as generalized lymphadenopathy, fever, and hypergammaglobulinemia. Symptoms are thought to be due to increased levels of IL-6 and vIL-6.195 KSHV is detected more frequently in biopsies from HIV-positive patients than in biopsies from those patients without HIV. KSHV is present in the immunoblastic B cells of the mantle zone of the lesions. Zidovudine plus valganciclovir showed activity in some patients with KSHV-associated Castleman disease.196

Summary

  • Two herpesviruses, Epstein–Barr virus (EBV) and Kaposi sarcoma-associated herpesvirus (KSHV), are associated with human tumors.
  • EBV is associated with Burkitt lymphoma, Hodgkin and non-Hodgkin lymphoma, post-transplant lymphoproliferative disease, T-cell lymphoma, nasopharyngeal carcinoma, and certain types of gastric carcinoma.
  • EBV transformed B cells and EBV post-transplant lymphoproliferative lesions express EBNA-1, -2, -3s, and -LP and LMP1 and LMP2 and RNAs including EBERs and BARTs in culture. Burkitt lymphomas express only EBNA-1, EBERs, and BARTs. Hodgkin lymphomas and nasopharyngeal carcinomas express EBNA-1, LMP1, LMP2, EBERs, and BARTs.
  • EBV EBNA-1 is important for maintaining the viral episome during replication of the cells. EBNA-2 transactivates several virus and cellular promoters and is a functional homolog of the Notch receptor. EBNA-3’s regulate the activity of EBNA-2 and also transactivate viral genes. LMP1 is a functional homolog of CD40 and binds to TNFR-associated factors to upregulate NF-κB, STATs, JNK, and stress-activated protein kinases.
  • KSHV is associated with primary effusion lymphoma, Kaposi sarcoma, and Castleman disease.
  • Each of the three KSHV-associated tumors express LANA (ORF73) and v-FLIP (ORF71). Other KSHV proteins, including Kaposin (K12), v-cyclin (ORF72), a G-protein-coupled receptor (GPCR, ORF74), v-IRF3 (K10.5), v-IRF1 (K9), and v-IL-6 (K2) are expressed in some, both not all virus-associated malignancies.
  • LANA maintains the viral episome during replication and inhibits the activity of Rb and p53. KSHV v-FLIP is a homolog of cellular FLIP and inhibits apoptosis and activates NF-κB. Kaposin increases the activity of cytokines, v-cyclin inhibits Rb and helps virus-infected cells to progress through the cell cycle, and the viral GPCR contributes to proliferation of KSHV-infected cells. v-IRF1 and v-IRF3 inhibit the activity of interferon and p53, while v-IL-6 functions as a B-cell growth factor.

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