Target cells for EBV

EBV can infect a variety of cell types under different circumstances, including T-cells, NK -cells, smooth muscle cells and possibly follicular dendritic cells (Rickinson and Kieff, 2001). However, B-lymphocytes and epithelial cells are its two major targets. B-cells are the primary reservoir of virus in persistently infected individuals and it is likely, although not certain, that the first cell infected in vivo is an epithelial cell. There has been some controversy over whether EBV normally infects epithelial cells during the courses of a primary infection or whether the virus infects epithelial cells only in the context of oncogenesis (nasopharyngeal carcinoma) or extreme immune dysfunction (oral hairy leukoplakia). Detection of EBV in epithelial tissue of healthy donors or patients with acute infectious mononucleosis has not always been consistent (Anagnostopoulos et al., 1995: Frangou et al., 2005; Lemon et al., 1977; Niedobitek et al., 1989; Pegtel et al., 2004; Sixbey et al., 1984; Venables et al., 1989). However, the preponderance of findings and the characteristics of virus shed in saliva of healthy carriers support a role for epithelial cells in amplification of virus during primary and persistent infection (Jiang et al., 2006). The clear association of the virus with significant epithelial pathology and its ability to infect epithelial cells in vitro either as free virus (Tugizov et al., 2003; Turk et al., 2006) or as B-cell associated virus (Imai et al., 1998, Shannon-Rowe et al., 2006) also reinforce the position that failures to detect virus may represent technical difficulties rather than reality.

B cells have been most extensively studied because the immortalizing effects of virus are so striking in this cell type, because B-cells are easier to isolate and because, after EBV infection, they are extremely easy to grow as latently infected cells. They retain episomes well in culture and are the best source of cell-free virus. Reactivation occurs spontaneously in a small subpopulation of some semipermissive B cell lines and can be induced in larger numbers of cells by agents such as phorbol esters, sodium butyrate or by cross-linking of immunoglobulin on the cell surface (Kieff and Rickinson, 2001). Epithelial cells typically lose episomes in vitro, but recent derivation of recombinant viruses that carry drug resistant markers has allowed selection of cells that can support episomal maintenance. The degree of lytic replication that occurs in epithelial cells varies from cell line to cell line. In many cell lines virus remains latent unless reactivated by treatment with inducing agents, but there is a recent report of extensive lytic replication in two polarized cell lines (Tugizov et al., 2003).

Target cells for KSHV

The in vivo host cell range of KSHV is not yet fully characterized, but appears to be broad in that KSHV viral DNA and transcripts have been detected in B-cells from the peripheral blood, B-cells in primary effusion lymphomas (PEL) or body-cavity based B-cell lymphomas (BCBL) and multicentric Castleman’s disease (MCD), flat endothelial cells lining the vascular spaces of Kaposi’s sarcoma (KS) lesions, typical KS spindle cells, CD 45+/CD68+ monocytes in KS lesions, keratinocytes, and epithelial cells (Antman and Chang, 2000; Dourmishev et al., 2003; Ganem, 1998; Sarid et al., 1999; Schulz et al., 2002). KSHV DNA is present in a latent form in the vascular endothelial and spindle cells of KS tissues and expresses the latency associated LANA 1 (ORF 73), cyclin D (ORF72), vFLIP (ORF 71) and K12 genes. However, virus DNA is lost within a few passages of cells cultured from the KS lesions. This is reminiscent of the loss of EBV episomes from nasopharyngeal carcinoma cells in tissue culture and the reason for it is not currently known. KSHV DNA is found in the CD 19+ peripheral blood B cells of KSHV seropositive individuals and the detection of both lytic and latent forms in B- cells of KS patients suggests that CD 19+ B-cells may be a primary reservoir for persistent infection (Antman and Chang, 2000; Dourmishev et al., 2003; Schulz et al., 2002).

Cell lines with B-cell characteristics such as BC -1, HBL -6, JSC, BCBL -1 and BC -3 have been established from PEL tumors (Dourmishev et al., 2003). The BC -1, HBL -6 and JSC cells carry KSHV and EBV genomes, while BCBL -1 and BC -3 cells carry only KSHV genome. KSHV exists in a latent state in these BCBL cells and expresses the latency associated ORF 73, ORF 72, K13, K12, K15 and ORF 10.5 genes. In parallel with EBV, spontaneous reactivation and expression of lytic cycle proteins occurs in 1% to 5% of BCBL cells. The lytic cycle can also be induced in about 20% to 30% of cells by phorbol esters, sodium butyrate and the lytic cycle switch protein known as RTA encoded by KSHV ORF 50 (Antman and Chang, 2000; Dourmishev et al., 2003; Liao et al., 2003; Schulz et al., 2002)

KSHV also has a much broader in vitro tropism than EBV. The virus can infect human B, lymphocytes, endothelial, epithelial, and fibroblast cells, as well as a variety of animal cells such as owl monkey kidney cells, baby hamster kidney fibroblast cells, Chinese hamster ovary (CHO) cells, and mouse fibroblasts (Akula et al., 2001b; Bechtel et al., 2003; Blackbourn et al., 2000; Ciufo et al., 2001; Dezube et al., 2002; Lagunoff et al., 2002; Moses et al., 1999; Naranatt et al., 2003; Renne et al., 1998; Vieira et al., 2001). Unlike EBV, infection of primary B-cells by KSHV does not result in immortalization. However, like EBV in vitro infection by KSHV is characterized by the expression of latency associated genes and the absence of productive lytic replication. After activation with phorbol esters or ORF 50 protein, lytic replication is supported by many cells including primary human microvascular dermal endothelial cells (HMVEC-d), human umbilical vein endothelial cells (HUVEC), human foreskin fibroblast cells (HFF), human endothelial cells immortalized by telomerase (TIME) or the E6/E7 proteins of human papilloma virus, monkey kidney cells and mouse fibroblasts (Bechtel et al., 2003).

Detection of KSHV latency associated nuclear antigen (LANA-1) encoded by ORF 73 after 2 days post-infection has led to the notion that the establishment of latency is the default pathway of infection (Bechtel et al., 2003; Lagunoff et al., 2002; Schulz et al., 2002; Tomescu et al., 2003). However, in vitro KSHV latent infection in primary fibroblasts, endothelial cells, or non-adherent B-cells is unstable, and viral DNA is not efficiently maintained (Tomescu et al., 2003). Even during primary infection of endothelial and other cells, the proportion of KSHV -infected cells decreases over time (Grundhoff and Ganem, 2004). Loss of genomes begins approximately 4 days after infection and by 3 weeks fewer than 15% of the cells remain positive for LANA 1 (Tomescu et al., 2003). Whether the wild-type KSHV from the saliva of infected individuals or KSHV isolates from Africa have similar properties and tropisms remains unknown at the present time.

Virion structure

The virions of EBV and KSHV are at least superficially very similar to those of other herpesviruses. Each consists of a linear double stranded genome, an icosahedral capsid surrounded by an amorphous layer of tegument proteins and a lipid envelope carrying multiple unique glycoprotein species.

The KSHV genome is made up of a unique long region (LUR) of about 145kb flanked by varying numbers of terminal repeats. A large region of the KSHV genome is conserved among herpesviruses, and is colinear with both the gamma 1 EBV and the gamma 2 herpesvirus saimiri (HVS) (Neipel et al., 1997; Russo et al., 1996). It encodes more than 90 open reading frames (ORFs), designated ORFs 4 to 75 for their homology to those of HVS (Neipel et al. 1997; Russo et al., 1996). Divergent regions in-between the conserved gene blocks contain more than 20 KSHV unique genes which are designated with the prefix K.

The EBV genome is slightly larger than that of KSHV at approximately 180 kb. However, it includes a large, internal, tandemly reiterated 300 kb repeat known as IR 1, a number of smaller internal repeats (IR2-IR4) and 500 bp terminal repeats arranged in tandem, all of which can vary in number in different strains of viruses. The more than 90 ORFs of EBV are named for the fragment within the original BamH 1 restriction map of the B95–8 genome (Baer et al., 1984) in which they originate and their direction of transcription. The BALF 4 ORF, for example, is the fourth leftward reading ORF originating in the BamH 1 A fragment.

Cryoelectron microscopy analyses reveal that KSHV capsid has the same T = 16 triangulation number, capsid architecture, structural organization and size as herpes simplex virus (HSV) and the human cytomegalovirus (HCMV) (Lo et al., 2003; Trus et al., 2001; Wu et al., 2000) (Fig. 23.1). As in other herpesviruses, the capsids have a typical icosahedral shell and are composed of four structural proteins encoded by KSHV ORFs 25, 26, 62 and 65. The major capsid protein encoded by ORF 25 forms the hexameric and pentameric capsomers. The capsid floor between the hexons and pentons is formed by the heterotrimeric complexes composed of one molecule of the ORF 62 protein and two molecules of the ORF 26 protein. Both these proteins show significant amino acid sequence homology to the capsid proteins of alpha- and betaherpesviruses. The fourth protein, ORF 65, is a small basic and highly antigenic protein, lacks significant sequence homology with its structural counterparts from the other subfamilies; however, similar to the small basic capsid protein VP 26 of HSV, KSHV ORF 65 decorates the surface of the capsids. Lytic replication of KSHV leads to the formation of at least three capsid species, A, B, and C, and the A capsids are empty, B capsids contain an inner array of a fifth structural protein, ORF 17.5, and C capsids contain the viral genome (Lo et al., 2003; Trus et al., 2001; Wu et al., 2000).

Fig. 23.1

3D structure of the KSHV capsid at 24-Å resolution by electron cryomicroscopy. The capsid is shown as shaded surface color-coded according to particle radius. The three structural components of the capsid are indicated, including 12 pentons (“5”), (more...)

No similar detailed analyses of the EBV capsid have yet been done, although transmission electron microscopy reveals a structural organization and size similar to other herpesviruses. Very little is known about the individual capsid proteins and assignments of ORFs as encoding capsid proteins have been made until recently only by homology with known HSV capsid proteins. Even less has been hypothesized about the composition of the tegument as few EBV ORFs have homology to any of the known HSV tegument proteins. The only EBV protein, which has been consistently described as a tegument protein is p140, which is thought to be encoded by the BNRF 1 ORF (Hummel and Kieff, 1982). This ORF is described as having homology with N-formylglycinamide ribotide amidotransferases of Eschericia coli and Drosophila melanogaster (Russo et al., 1996), but the significance of the homology is unknown. The first proteomic analysis of the proteins in the virion particle promises to stimulate new research in this understudied area (Johannsen et al., 2004).

Virus structural proteins involved in entry

In contrast, considerable effort has been expended on analysis of the biochemistry and function of the envelope proteins. These are the proteins that are critically needed to mediate attachment, entry, assembly and egress of virus and the majority of them are glycoproteins. The different tropisms of herpesviruses are potentially due to unique complements of these glycoproteins as well as subtle differences among those that are conserved.

The precise number of EBV membrane proteins that are exclusively expressed during the lytic cycle is not yet known, but there is evidence for at least twelve (Table 23.1). Five of these are glycoproteins that have homologues in all herpesvirus studied to date and are now commonly referred to by the naming system originally developed for the prototype alphaherpesvirus, herpes simplex virus. They are gB gH, gL, gM, and gN (Hutt-Fletcher, 2002). The remaining seven proteins are named for their apparent mass or referred to by the name of the open reading frame (ORF) that encodes them. One, p38, the product of the BFRF 1 gene (Farina et al., 2000) is also conserved among all the herpesvirus families and is the homolog of the most widely studied alphaherpesvirus UL 34. It is not glycosylated and is not found in the virion particle (Farina, 2004; Johannsen et al., 2004). It interacts with the BFLF 2 gene product, the homolog of alphaherpesvirus UL 31 (Gonella et al., 2005; Lake and Hutt-Fletcher, 2004) and like its homologs in other herpesviruses its major role is probably to facilitate exit of the newly formed EBV nucleocapsid from the nucleus (Farina et al., 2005). The remaining six are glycoproteins that are unique to the gammaherpesviruses. Glycoproteins gp150 (Kurilla et al., 1995; Nolan and Morgan, 1995), gp78 (Mackett et al., 1990) and the product of the BILF 1 gene, which is glycoprotein with an apparent mass of approximately 50 kDa (Paulson et al., 2005), have counterparts only in lymphocryptoviruses (Rivailler et al., 2002). Glycoproteins gp350/220 (Hummel et al., 1984), gp42 (Li et al., 1995) and the BMRF 2 protein (Tugizov et al., 2003) have counterparts in lymphocryptoviruses (Rivailler et al., 2002) and in certain of the rhadinoviruses (Russo et al., 1996; Telford et al., 1995; Virgin et al., 1997). Glycoproteins gp350/220, gp42 and the BMRF 2 protein, together with the conserved glycoproteins gB, gH and gL, are all involved in virus entry as described below. It also is possible that complexes of gN and gM may play a role in this process although their major role is probably in envelopment and egress of newly made virus as described in Chapter 25 (Lake and Hutt-Fletcher, 2000). The phenotype of a virus that lacks gp150 is little changed from wild type virus, except that its ability to infect epithelial cells is very slightly enhanced (Borza and Hutt-Fletcher, 1998). The BILF 1 gene product functions as a constitutively active G protein-coupled receptor. It is not found in the virion (Johannsen et al., 2004), but may play a role in modulating intracellular signaling pathways (Beisser et al., 2005). Nothing is known about possible functions of gp78.

Table 23.1

Genes encoding membrane proteins that are expressed only in the lytic cycle.

KSHV also expresses the five conserved herpesvirus glycoproteins. (Table 23.1). Open reading frames 8, 22, 47, 39 and 53 encode glycoproteins gB, gH, gL, gM and gN, respectively (Akula et al., 2001a; Baghian et al., 2000; Naranatt et al., 2002; Neipel et al., 1997; Russo et al., 1996). HHV -8 also encodes for additional glycoproteins such as gpK8.1A, gpK8.1B, K1, K14 and K15 that are expressed during lytic replication (Birkmann et al., 2001; Chandran et al., 1998; Neipel et al., 1997; Russo et al., 1996). Studies have shown that gB, gH/ gL, gM/gN and gpK8.1A are virion-envelope associated glycoproteins (Akula et al., 2001a; Baghian et al., 2000; Koyano et al., 2003; Naranatt et al., 2002; Neipel et al., 1997; Russo et al., 1996; Zhu et al., 1999).

Virus attachment

Entry of enveloped viruses into cells involves at least two events, attachment to the cell surface and penetration through the cell membrane. Herpesviruses, not unexpectedly given the large number of different proteins found in a herpesvirus envelope, use multiple different proteins to complete the entry process. Increasingly, in recent years it has also become apparent not only that multiple proteins are involved in entry, but also that the complement of proteins that is used can vary considerably when different cells are the target of infection and different molecules are available for interaction with the virus.

Penetration

Penetration of any enveloped virus into a cell involves fusion of the virion envelope with the membrane of the cell and can occur either at the cell surface or after endocytosis. Endocytosis affords a convenient and often rapid system of transit across the plasma membrane and through the cytoplasm for delivery of viral cargo to the vicinity of the nucleus (Sieczkarski and Whittaker, 2002; Whittaker, 2003). The best understood paradigm for virus cell fusion is provided by the RNA viruses such as the human immunodeficiency virus, which fuses at the cell surface, and influenza virus, which fuses with the endocytic vesicle. The virus glycoproteins that mediate fusion are made as single type 1 membrane proteins, but are cleaved during processing to create two species which reassociate in a metastable state (Colman and Lawrence, 2003). The fragment that retains the transmembrane domain includes a hydrophobic sequence or “fusion peptide” that can be triggered by conformational changes to insert into an opposing cell membrane and initiate formation of a fusion pore. The conformational change in the human immunodeficiency virus is triggered by interaction with coreceptors, the conformational change in the influenza virus fusion protein is triggered by exposure to the low pH of the endosome. However, no clear-cut paradigm has yet been identified for any herpesvirus. Fusion appears to require cooperation between several unique protein species, none of which include readily identifiable “fusion peptides” and the site at which fusion occurs varies from virus to virus and even cell type to cell type.

Penetration by EBV

Penetration of EBV into B cells and epithelial cells is significantly different both in terms of the virus and cell proteins involved and in terms of the routes that are used. Attachment of EBV to the B cell surface via CR 2 stimulates endocytosis of virus into thin-walled non-clathrin coated vesicles (Nemerow and Cooper, 1984; Tanner et al., 1987) and fusion occurs in a low pH environment. Exposure to low pH is not an essential requirement, but endocytosis does appear to be necessary as virus fails to fuse with B cells treated with the endocytosis inhibitors chlorpromazine and sodium azide. In contrast, fusion with an epithelial cell occurs at neutral pH and is resistant to treatment with either chlorpromazine or sodium azide (Miller and Hutt-Fletcher, 1992).

Fusion of the EBV envelope with the B cell requires at least three and probably four virus glycoproteins, gB, gH, gL and gp42. EBV gB is a 857-residue protein that shares some structural, although little sequence homology with its counterparts in other herpes viruses (Gong et al., 1987). The positions of many of the cysteine residues in gB are conserved. Like gB of HHV -8 it undergoes cleavage to produce two polypeptides of approximately 56 and 80 kDa that are linked by disulfide bonds (Johannsen et al., 2004; C. M. Lake and L. M. Hutt-Fletcher, unpublished data). Although some strains of EBV carry very little gB in the virion (Gong and Kieff, 1990) and, as discussed in Chapter , EBV gB plays a very important role in virus assembly (Lee and Longnecker, 1997), recent work in which different combinations of virus proteins were expressed and examined for their abilities to fuse cell membranes indicates that EBV gB is also an essential part of the fusion machinery (Haan et al., 2001).

The remaining three proteins that are known to be required for fusion with B cells, gH, gL and gp42, form a non-covalently linked complex in virus (Li et al., 1995). Liposomes that contain all virus envelope proteins, except gH, gL and gp42, bind to receptor positive cells but fail to fuse (Haddad and Hutt-Fletcher, 1989) and recombinant virus that lacks all three can bind to but cannot penetrate B cells (Molesworth et al., 2000). Glycoprotein gH, the largest of the three is a 708 residue type 1 membrane protein with five potential N-linked glycosylation sites and it carries about 10 kDa of N-linked sugar (Baer et al., 1984; Heineman et al., 1988; Oba and Hutt-Fletcher, 1988). Although members of the gH family of proteins share little sequence homology, if aligned at a conserved N-linked glycosylation site at the carboxyterminus they show a colinearity of cysteine residues that suggests a conservation of secondary structure (Klupp and Mettenleiter, 1991). Each member is also dependent on the smaller membrane protein, gL, for folding and transport through the cell. The EBV gL is a 137-residue glycoprotein of approximately 25 kDa that remains anchored in the envelope by an uncleaved signal sequence (Li et al., 1995; Yaswen et al., 1993). Because of the dependence of gH on gL and because EBV in which expression of the gH gene is interrupted phenotypically lacks gL as well (Molesworth et al., 2000), there are only few instances in which the functions of the two proteins can be separated.

Glycoprotein gp42, the third member of the complex required for penetration of a B cell, is not dependent on gHgL for processing and does not have counterparts in most herpesviruses (Li et al., 1995). The gHgL complex in both human cytomegalovirus and human herpes virus 6 also include a third component known, respectively, as gO and gQ (Huber and Compton, 1998; Mori et al., 2003), but these latter proteins, have no obvious homology to gp42. Only in the gammaherpesviruses are similar proteins predicted (Rivailler et al., 2002; Telford et al., 1995). EBV gp42 is a 223 residue type 2 membrane glycoprotein that has weak similarity to a C-type lectin (Spriggs et al., 1996). The predicted signal sequence and transmembrane anchor lie between residues 7 and 28, the region of the molecule that is responsible for the interaction with gHgL lies between residues 40 and 58 (Wang et al., 1998; Ressing et al., 2005) and the carboxyterminal domain of the protein interacts with the variable region of the β chain of HLA class Ⅱ (Spriggs et al., 1996).

Several lines of evidence indicate that the interaction between gp42 and HLA class Ⅱ is essential to B-cell infection. A monoclonal antibody to gp42 that blocks the interaction with HLA class Ⅱ inhibits virus cell fusion (Miller and Hutt-Fletcher, 1988) and a monoclonal antibody to HLA class Ⅱ that blocks gp42 binding neutralizes virus infection (Li et al., 1997). A soluble form of gp42 in which the transmembrane domain is replaced with the Fc domain of human immunoglobulin competes with gp42 in virus for binding to HLA class Ⅱ and blocks infection and B-cells that lack HLA class Ⅱ cannot be infected unless HLA class Ⅱ expression is restored (Li et al., 1997). Finally, a recombinant virus that lacks gp42 fails to infect B-cells unless cells and bound virus are fused with polyethylene glycol (Wang and Hutt-Fletcher, 1998), or a soluble form of gp42 which lacks a transmembrane domain but which retains the ability to bind to gH and gL is added in trans (Wang et al., 1998). Binding shows some allelic specificity (Haan and Longnecker, 2000) and the crystal structure of HLA class Ⅱ bound to gp42 reveals that gp42 binds peripherally to the variable domain of the β-chain (Mullen et al., 2002). A current minimalist model of B cell penetration would then suggest that, following attachment of virus via gp350 and CR 2, gp42 interacts with HLA class Ⅱ and that this event leads to triggering of the fusion machinery, gHgL and gB.

Fusion of virus with epithelial cells requires only gB and gHgL (McShane and Longnecker, 2004) and residues required for fusion can be distinguished from those required for fusion with B cells (Omerovic et al., 2005; Wu et al., 2005). However, epithelial cells do not express HLA class Ⅱ constitutively, and recombinant virus that lacks gp42 infects epithelial cells as well as wild-type virus. Not only is gp42 dispensable for infection of epithelial cells, its presence is also inhibitory. Stoichiometric analysis of wild-type virus demonstrates the presence of much larger amounts of gHgL than gp42 in the virion, implying that some complexes naturally lack or are low in gp42. Saturation of the complexes by addition of soluble gp42 in trans blocks epithelial infection. In addition infection of epithelial cells, but not B cells, can be blocked by antibodies that interact with gHgL or gH alone (Wang et al., 1998). These findings have been interpreted to mean that there is a coreceptor on epithelial cells that can substitute for HLA class Ⅱ and with which gHgL interacts in the absence of gp42. Soluble forms of gHgL bind saturably to epithelial cells. Scatchard analysis indicates the presence of as many as 20 0000 high affinity receptors per cell with a K_D of approximately 5 × 10⁻⁹ M (L. Chesnokova, A. Morgan and L. Hutt-Fletcher, unpublished data). Whether or not this receptor is the same as that which can be used to attach virus to epithelial cells is not yet known. However, a minimal model of penetration of an epithelial cell suggests that following attachment an interaction of gHgL alone with a coreceptor triggers the activity of the fusion machinery.

The observation that gp42 is essential for B-cell infection but dispensable for epithelial infection suggested that changes in the stoichiometry of the gHgLgp42 complex would influence tropism of EBV for the two cell types and comparisons of virus made in HLA class Ⅱ positive B-cells and HLA class Ⅱ negative epithelial cells support the hypothesis that such changes might occur in vivo (Borza and Hutt-Fletcher, 2002). Virus made in HLA class Ⅱ -negative epithelial cells can be as much as two logs more infectious for B-lymphocytes than the same amount of virus produced by an HLA class Ⅱ -positive B-cell. Virus originating from either cell type binds equally well to CR 2 on the B-cell surface, but virus made in the B-cell enters less efficiently. This appears to reflect the fact that in a class Ⅱ -positive virus-producing cell some complexes containing gp42 interact with class Ⅱ during biosynthesis and are targeted to the class Ⅱ trafficking pathway where they are vulnerable to degradation. The resulting loss of three-part complexes from virus reduces the efficiency of class Ⅱ -dependent entry. Such a loss does not occur in a class Ⅱ -negative epithelial cell where virus has a relative increase in gp42 and an increased efficiency for class Ⅱ -dependent entry and induction of HLA class Ⅱ expression can reverse the phenotype. The levels of gp42 in virus also impact infection of epithelial cells via the class Ⅱ -independent pathway. B cell virus is on average fivefold better at infecting epithelial cells than epithelial virus. These findings suggest that gp42 may function as a switch of virus tropism that might be relevant to spread of virus between tissues in vivo (Borza and Hutt-Fletcher, 2002) (Fig. 23.2). However, since the effects on B-cell infection are by far the most striking, the biological effects may primarily be to drive into the B-cell pool any virus that initiates infection by replicating in an epithelial cell.

Fig. 23.2

Glycoprotein gp42 can function as a switch of EBV tropism. (a) EBV makes both three part gHgLgp42 complexes and two part gHgL complexes. When virus is made in a B-cell, some of the three-part gHgLgp42 complexes bind to HLA class Ⅱ within the cell (more...)

The events that occur concurrent with, and following, fusion of the virus and cell membrane that are necessary to facilitate transport, uncoating and delivery of the genome to the nucleus are largely unknown and currently can only be guessed at, based on what little is understood for other herpesviruses. There are, however, hints that additional envelope proteins may be involved in efficient delivery of infectious virus. Loss of the complex of gM and gN not only severely compromises envelopment and egress of EBV, as described in Chapter , but also leads to a defect in infection that cannot be rescued by treating bound virus with exogenous mediators of fusion. One possible explanation for this finding is that there is a defect in dissociation of envelope and tegument necessary for movement of virus toward the nucleus (Lake and Hutt-Fletcher, 2000).

Cell surface signaling during entry

The interactions of eukaryotic cells with their extracellular environments are largely mediated by ligand-induced signaling molecules exposed at the cell surface. The ensuing multitudes of biological processes are mediated by highly inter-linked networks of signaling pathways. Ligand mimicry is an opportunistic mechanism by which microbes, including viruses, subvert host signaling molecules for their benefit (Virji, 1996). By evolving to use cell surface molecules for attachment or entry into a cell, viruses have also evolved to take advantage of the events triggered by signaling to facilitate intracellular transport, to manipulate cell defense mechanisms, or to induce the pattern of cellular gene expression that is most conducive to establishment of latent or productive infection. Understanding virus induced signaling and its consequence is emerging as an important area of virology.

Signaling by KSHV during the early stages of infection

The integrins with which KSHV gB interacts are part of a large family of heterodimeric receptors containing noncovalently associated transmembrane α and β glycoprotein subunits (Giancotti, 2000; Giancotti and Ruoslahti, 1999; Sastry and Burridge, 2000). There are 17 α and 9 β subunits, generating more than 24 known combinations of αβ cell surface receptors. Each cell expresses several combinations of αβ integrins, and each αβ combination has its own binding specificity and signaling properties (Giancotti, 2000; Giancotti and Ruoslahti, 1999; Sastry and Burridge, 2000). Integrin interactions with ECM proteins provide robust signals for host-cell gene expression and mediate a variety of cell functions such as activation of cytoskeleton elements, endocytosis, attachment, cell cycle progression, cell growth, apoptosis, and differentiation (Giancotti, 2000; Giancotti and Ruoslahti, 1999; Sastry and Burridge, 2000). FAK is a non-receptor protein-tyrosine kinase that localizes in focal adhesions with vinculin, and FAK activation is the first step necessary for the outside–in signaling of integrins (Calderwood et al., 2000; Giancotti, 2000; Sastry and Burridge, 2000). Within 5 minutes of infection, KSHV induces the integrin-mediated activation of FAK in endothelial and fibroblast cells, and co-localizes with the focal adhesion component vinculin (Akula et al., 2002). Soluble gB, but not soluble gpK8.1A, induces FAK, which also colocalizes with pakillin (Wang et al., 2003). FAK activation is inhibited by the pre-incubation of virus or gB with soluble α3β1 integrin or anti-gB antibodies, and is not activated by a soluble form of gB in which the RGD sequence had been mutated (Akula et al., 2002; Wang et al., 2003). The ability of anti-integrin antibodies and soluble integrin to neutralize virus infection without affecting virus entry suggests that integrin and the associated signaling pathways have a role to play in KSHV entry and infection of target cells.

Studies with FAK knockout mouse fibroblasts Du3 (FAK^−/−) and parental Du17 (FAK^+/+) cells confirm that FAK plays a key role in HHV -8 infection (Naranatt et al., 2003). Since activation of FAK is central to many paradigms of outside-in signaling by integrins, actin assembly, and endocytosis, KSHV may be taking advantage of these signaling pathways both to promote entry and to produce a cellular state that facilitates infection.

KSHV induced the phosphorylation of FAK in FAK-positive Du17 mouse embryonic fibroblasts early during infection. The absence of FAK in Du3 (FAK^-/-) cells resulted in about 70% reduction in the internalization of KSHV DNA, suggesting that FAK plays a role in KSHV entry. Expression of FAK in Du3 (FAK^-/-) cells via an adenovirus vector augmented the internalization of viral DNA. Expression of the FAK dominant-negative mutant FAK -related non kinase (FRNK) in Du17 cells significantly reduced the entry of virus. Reduced quantity of virus entry in Du3 cells, delivery of viral DNA to the infected cell nuclei (Krishnan et al., 2006), and expression of KSHV genes suggested that in the absence of FAK, another molecule(s) may be partially compensating for FAK function. Infection of Du3 cells induced the phosphorylation of the FAK -related proline-rich tyrosine kinase (Pyk2) molecule, which has been shown to complement some of the functions of FAK. Expression of an autophosphorylation site mutant of Pyk2 in which Y402 is mutated to F (F402 Pyk2) reduced viral entry in Du3 cells, suggesting that Pyk2 facilitates viral entry moderately in the absence of FAK. These results suggest a critical role for KSHV infection-induced FAK in the internalization of viral DNA into target cells (Krishnan et al., 2006). One of the important downstream effectors of FAK that is activated directly or through Src kinase via Ras is PI 3-K, a member of a family of lipid kinases (Giancotti and Ruoslahti, 1999; Sastry and Burridge, 2000) that act as second messengers for a number of cell functions including the activation of Rho-GTPases and anti-apoptotic pathway Akt molecule (Giancotti and Ruoslahti, 1999; Sastry and Burridge, 2000). KSHV induces PI 3-K within 5 min of infection which decreased after 15 min (Naranatt et al., 2003). The response can be inhibited by pre-incubating KSHV with integrin and by the PI 3-K inhibitors wortmannin and LY 294002. Another hallmark of integrin interaction with ligands is the reorganization and remodeling of actin cytoskeleton. This is controlled by the Rho family of small GTP ases, such as Rho, Rac, and Cdc42, and the morphological changes induced by Rho, Rac and Cdc42 activation are downstream effects of PI 3-K activation (Hall and Nobes, 2000). Immediately following KSHV infection, target cells exhibit morphological changes and cytoskeletal rearrangements such as filopodia, lamellipodia and stress fibers. This together with the phosphorylation of PI 3-K by KSHV at early time infection suggests the induction of RH o-GTPases and the associated signal pathways (Naranatt et al., 2003).

FAK represents a point of convergence from activated integrins and initiates a cascade of intracellular signals that eventually activate the mitogen activated protein kinase (MAPK) pathways (Giancotti and Ruoslahti, 1999; Sastry and Burridge, 2000). MAPK pathways exist in all eukaryotes and control fundamental cellular processes such as proliferation, differentiation, survival and apoptosis (Giancotti and Ruoslahti, 1999; Sastry and Burridge, 2000). As early as 5 minutes postinfection, KSHV activates the MEK (MAPK/ERK kinase) and extracellular-signal-regulated kinase (ERK) (Naranatt et al., 2003) (Fig. 23.3). Focal adhesion components PI 3-K and protein kinase C-ζ (PKC-ζ) are recruited as upstream mediators of the HHV -8 induced ERK pathway.

Fig. 23.3

Model for early events of KSHV infection of target cells. Early events of KSHV infection are depicted in overlapping dynamic phases. In phase 1, KSHV binds to the cell surface via its interactions with heparan sulfate proteoglycans (HSPGs) and integrins (more...)

Antibodies to KSHV -gB that neutralize infection and soluble α3β1 integrin inhibit the virus-induced ERK signaling pathways. Early kinetics of the cellular signaling pathway and its activation by UV -inactivated KSHV suggest a role for virus binding or entry, but not viral gene expression, in this induction. Studies with human α3 integrin transfected CHO cells, and FAK negative mouse DU 3 cells suggest that the α3β1 integrin and FAK play critical roles in the KSHV mediated signal induction (Naranatt et al., 2003). Inhibitors specific for PI 3-K, PKC -ζ, MEK and ERK significantly reduce virus infectivity without affecting virus binding to the target cells. Examination of entry of viral DNA supports a role for PI 3-K in KSHV entry and a role for PKC -ζ, MEK and ERK at a stage after entry (Naranatt et al., 2003). PI 3-K is involved in the activation of Rho-GTPases. These in turn are critical for the activation of Rac, Rho, Cdc42 and Rab5 which are involved in the modulation of actin dynamics, formation of endocytic vesicles and the fission of endocytic vesicles. Furthermore, viral capsid movement in the cytoplasm probably depends upon the microfilaments and microtubules, which are controlled by the RhoGTPases. Since KSHV induces the RhoGTPases, it is reasonable to speculate that these inductions serve a vital role in the infectious process. The interaction of KSHV with cells induces the polymerization of cortical actin filaments (Naranatt et al., 2003). Further detailed analyses are essential to decipher the link between these pathways and their potential roles not only in KSHV entry into target cells but also the release and movement of capsids in the cytoplasm and delivery of viral DNA into the nucleus.

Besides playing an important role in the entry of viral nucleic acid into the nucleus of the infected cells, KSHV interactions with HS, integrins and other host cell surface molecules may also dictate the outcome of an infection by creating an appropriate intracellular environment facilitating infection. For example, there are many obstacles that viruses have to overcome during the early and late stages of infection of target cells in the host. They include external threats to infected cells from the innate and adaptive immune systems as well as internal obstacles such as transcriptional blocks and cellular apoptosis that may be triggered by virus binding and entry. To establish a successful infection, herpesviruses must have developed many ways to manipulate and overcome these obstacles early during infection. In this respect, stimulation of PI 3-K by KSHV, which may influence the Akt induced anti-apoptotic pathway, and modulation of interferon response factors by the virion associated tegument protein ORF 45 protein (Zhu and Yuan, 2003) are of considerable interest.

Cytoplasmic trafficking, delivery of viral genome into the nucleus

After release into the cytoplasm, the EBV and KSHV capsid/tegument must traffic through the cell in order for viral DNA to be delivered into the nucleus. HSV -1 utilizes dynein motors and microtubules for this purpose and the activation by KSHV of RhoGTPases, which are important to control of microtubules, is consistent with a similar mechanism of transport for this virus.

Similar to HSV, KSHV utilizes the dynein motors in the cytoplasmic trafficking and delivery of viral DNA to the nucleus (Narranat et al., 2005). Microtubules play important roles in KSHV infection since depolymerization of microtubules even though did not affect KSHV binding and internalization, it inhibited the nuclear delivery of viral DNA and infection (Narranat et al., 2005). The interesting aspect is that KSHV induced the acetylation of microtubules, an essential step for the microtubule aggregation, which are mediated by the host cell pre-existing signals induced by KSHV binding an entry steps. The inactivation of Rho GTP ases by Clostridium difficile toxin B significantly reduced the microtubular acetylation and the delivery of viral DNA to the nucleus (Narranat et al., 2005). Activation of Rho GTP ases by Escherichia coli cytotoxic necrotizing factor significantly augmented the nuclear delivery of viral DNA. Activation of RhoA-GTP-dependent diaphanous 2 was observed, with no significant activation in the Rac- and Cdc42-dependent PAK 1/2 and stathmin molecules. The nuclear delivery of viral DNA increased in cells expressing a constitutively active RhoA mutant and decreased in cells expressing a dominant-negative mutant of RhoA. Like in HSV-1, KSHV capsids colocalized with the microtubules, and the colocalization was abolished by the destabilization of microtubules with nocodazole and by the PI-3K inhibitor affecting the Rho GTP ases. These results suggest that KSHV induces Rho GTP ases, modulates microtubules and promotes the trafficking of viral capsids and the establishment of infection (Narranat et al., 2005). These studies demonstrated for the first time modulation of the microtubule dynamics by virus-induced host cell signaling pathways to aid in the trafficking of viral DNA to the infected cell nucleus. These data strongly suggest that KSHV manipulates the host cell signaling pathway to create an appropriate intracellular environment that is conducive to the establishment of a successful infection.

No studies of EBV have directly addressed this issue, although the observation that at least in epithelial cells the EBV BMRF 2 protein interacts with integrins is provocative (Tugizov et al., 2003). In primary B cells transcription from incoming EBV DNA can be detected within 10–12 h post infection and since circularization of the genome requires host protein synthesis (Sinclair and Farrell, 1995), whereas initiation of transcription from the viral genome does not (Hurley and Thorley-Lawson, 1988), it can be inferred that transcription initiates from the incoming linear genome. Transcription is unaffected by inhibitors of tyrosine kinases and PI 3-K, but synthesis of virus proteins is reduced implying that stimulation of kinases is not required for transport of virus to the nucleus, as it may be for KSHV, but for a later stage in the infection process (Sinclair and Farrell, 1995). Circular episomes have been detected by 16 h post infection and their formation may require that the cell move from G0 to G1 (Alfieri et al., 1991; Hurley and Thorley-Lawson, 1988). De novo protein synthesis is probably necessary for circularization and a cellular protein complex that includes Sp1 and binds to the recombination junctions within the terminal repeats of the genome may be involved in the process (Sun et al., 1997). Early studies have suggested that amplification of the first formed episome does not occur in primary B cells until more than one week after infection (Hurley and Thorley-Lawson, 1988). No similar studies have yet been reported for KSHV, although the virus is found in episomal form in PEL cells (Ballestas et al., 1999). Clearly much more needs to be known about the events between entry and early transcription from the incoming genome for both KSHV and EBV.

Summary

The importance of EBV and KSHV as oncogenic viruses has appropriately focused efforts on understanding the ways in which they influence cell survival and growth. An unintended consequence has been that information about productive replication of both viruses has been less forthcoming. Recent progress, particularly in understanding the early events that initiate entry, has been promising, but many important gaps remain to be filled. The determinants of tropism remain incompletely defined for both EBV and KSHV and although the perennial problem of how herpesviruses mediate virus cell fusion is not unique to either EBV or KSHV, it is still largely unsolved. The effect of entry on cell signaling pathways is proving to be an extremely fertile area of study, not just for understanding how viruses establish a suitable intracellular milieu for gene expression and DNA replication, but also for understanding how viruses are transported to the nucleus after the cell membrane has crossed. Progress with KSHV has been particularly interesting in this regard. It already appears likely that infection of endothelial cells by KSHV and infection of B cells by EBV may follow different pathways. However, whether this reflects fundamental differences in virus strategy, or adaptation to different host cells will require study of the viruses in more than one of the cell types that each infects. As important as a comprehensive understanding of lytic replication is to understanding the pathogenesis of disease as a whole, for EBV and KSHV it remains a distant goal.

References

• Akula S. M., Pramod N. P., Wang F. Z., Chandran B. Human herpesvirus 8 envelope-associated glycoprotein B interacts with heparan sulfate-like moieties. Virology. 2001a;284(2):235–249. [PubMed: 11384223]
• Akula S. M., Wang F. Z., Vieira J., Chandran B. Human herpesvirus 8 interaction with target cells involves heparan sulfate. Virology. 2001b;282(2):245–255. [PubMed: 11289807]
• Akula S. M., Pramod N. P., Wang F. Z., Chandran B. Integrin a3b1 (CD49c/29) is a cellular receptor for Kaposi’s sarcoma-associated herpesvirus (KSHV/HHV8) entry into target cells. Cell. 2002;108:407–419. [PubMed: 11853674]
• Akula S. M., Naranatt P. P., Walia N. S., Wang F. Z., Fegley B., Chandran B. Kaposi’s sarcoma-associated herpesvirus (human herpesvirus 8) infection of human fibroblast cells occurs through endocytosis. J. Virol. 2003;77(14):7978–7990. [PMC free article: PMC161913] [PubMed: 12829837]
• Alfieri C., Birkenbach M., Kieff E. Early events in Epstein–Barr virus infection of human B lymphocytes. Virology. 1991;181:595–608. [PubMed: 1849678]
• Anagnostopoulos I., Hummel M., Kreschel C., Stein H. Morphology, immunophenotype and distribution of latently and/or productively Epstein–Barr virus-infected cells in acute infectious mononucleosis: implications for the interindividual individual infection route of Epstein–Barr virus. Blood. 1995;5:744–750. [PubMed: 7530505]
• Antman K., Chang Y. Kaposi’s sarcoma. N. Engl. J. Med. 2000;342(14):1027–1038. [PubMed: 10749966]
• Baer R., Bankier A. T., Biggin M. D., et al. DNA sequence and expression of the B95–8 Epstein–Barr virus genome. Nature. 1984;310:207–211. [PubMed: 6087149]
• Baghian A., Luftig M., Black J. B., et al. Glycoprotein B of human herpesvirus 8 is a component of the virion in a cleaved form composed of amino- and carboxyl-terminal fragments. Virology. 2000;269(1):18–25. [PubMed: 10725194]
• Ballestas M. E., Chatis P. A., Kaye K. M. Efficient persistence of extrachromosomal KSHV DNA mediated by latency-associated nuclear antigen. Science. 1999;282:641–644. [PubMed: 10213686]
• Bayliss G. J., Wolf H. Epstein–Barr virus induced cell fusion. Nature. 1980;287:164–165. [PubMed: 6253791]
• Bechtel J. T., Liang Y., Hvidding J., Ganem D. Host range of Kaposi’s sarcoma-associated herpesvirus in cultured cells. J. Virol. 2003;77(11):6474–6481. [PMC free article: PMC155009] [PubMed: 12743304]
• Beisel C., Tanner J., Matsuo T., et al. Two major outer envelope glycoproteins of Epstein–Barr virus are encoded by the same gene. J. Virol. 1985;54:665–674. [PMC free article: PMC254850] [PubMed: 2987520]
• Beisser P. S., Versijl D., Gruijthuijsen Y. K., et al. The Epstein-Barr virus BILF 1 gene encodes a G protein-coupled receptor that inhibits phosphorylation of RNA-dependent protein kinase. J. Virol. 2005;79:441–449. [PMC free article: PMC538699] [PubMed: 15596837]
• Birkmann A., Mahr K., Ensser A., et al. Cell surface heparan sulfate is a receptor for human herpesvirus 8 and interacts with envelope glycoprotein K8.1. J. Virol. 2001;75(23):11583–11593. [PMC free article: PMC114745] [PubMed: 11689640]
• Blackbourn D. J., Lennette E., Klencke B., et al. The restricted cellular host range of human herpesvirus 8. AIDS. 2000;14(9):1123–1133. [PubMed: 10894276]
• Borza C., Fletcher L. M. and Hutt-1998Epstein–Barr virus recombinant lacking expression of glycoprotein gp150 infects B cells normally but is enhanced for infection of the epithelial line SVKCR 2 J. Virol. 727577–7582. [PMC free article: PMC110006] [PubMed: 9696856]
• Borza C. M., Fletcher L. M. and Hutt-2002Alternate replication in B cells and epithelial cells switches tropism of Epstein–Barr virus Nature Med. 8594–599. [PubMed: 12042810]
• Borza C. M., Morgan A. J., Turk S. M., Fletcher L. M., and Hutt-2004Use of gHgL for attachment of Epstein–Barr virus to epithelial cells compromises infection J. Virol. 785007–5014. [PMC free article: PMC400351] [PubMed: 15113881]
• Calderwood D. A., Shattil S. J., Ginsberg M. H. Integrins and actin filaments: reciprocal regulation of cell adhesion and signaling. J. Biol. Chem. 2000;275(30):22607–22610. [PubMed: 10801899]
• Chandran B., Bloomer C., Chan S. R., Zhu L., Goldstein E., Horvat R. Human herpesvirus-8 ORF K8.1 gene encodes immunogenic glycoproteins generated by spliced transcripts. Virology. 1998;249(1):140–149. [PubMed: 9740785]
• Ciufo D. M., Cannon J. S., Poole L. J., et al. Spindle cell conversion by Kaposi’s sarcoma-associated herpesvirus: formation of colonies and plaques with mixed lytic and latent gene expression in infected primary dermal microvascular endothelial cell cultures. J. Virol. 2001;75(12):5614–5626. [PMC free article: PMC114274] [PubMed: 11356969]
• Colman P. M., Lawrence M. C. The structural biology of type 1 viral membrane fusion. Nat. Rev. Mol. Cell Biol. 2003;4:309–319. [PubMed: 12671653]
• Addario M. D., Libermann T. A., Xu J., Ahmad A., Menezes J.D’2001Epstein–Barr virus and its glycoprotein-350 upregulate IL -6 in human B-lymphocytes via CD 21, involving activation of NF -kB and different signaling pathways J. Mol. Biol. 308501–504. [PubMed: 11327783]
• Dezube B. J., Zambela M., Sage D. R., Wang J. F., Fingeroth J. D. Characterization of Kaposi sarcoma-associated herpesvirus/human herpesvirus-8 infection of human vascular endothelial cells: early events. Blood. 2002;100(3):888–896. [PubMed: 12130499]
• Dourmishev L. A., Dourmishev A. L., Palmeri D., Schwartz R. A., Lukac D. M. Molecular genetics of Kaposi’s sarcoma-associated herpesvirus (human herpesvirus-8) epidemiology and pathogenesis. Microbiol. Mol. Biol. Rev. 2003;67(2):175–212. [PMC free article: PMC156467] [PubMed: 12794189]
• Farina A., Santarello R., Gonnella R., et al. The BFRF 1 gene of Epstein–Barr virus encodes a novel protein. J. Virol. 2000;74:3235–3244. [PMC free article: PMC111824] [PubMed: 10708440]
• Farina A., Cardinale G., Santarella R., et al. Intracellular localization of the Epstein–Barr virus BFRF 1 gene product in lymphoid cell lines and oral hairy leukoplakis lesions. J. Med. Virol. 2004;72:102–111. [PubMed: 14635017]
• Farina A., Feederle R., Raffa S., et al. BFRF1 of Epstein–Barr virus is essential fro efficient primary envelopment and egress. J. Virol. 2005;79:3703–3712. [PMC free article: PMC1075683] [PubMed: 15731264]
• Feral C. C., Nishiya N., Fenczik C. A., Stuhlmann H., Slepak M., Ginsberg M. H. CD98hc (SLC3A2) mediates integrin signaling. Proc. Natl Acad. Sci. USA. 2005;102:355–360. [PMC free article: PMC544283] [PubMed: 15625115]
• Fenczik C. A., Zent R., Dellos M., et al. Distinct domains of CD 98hc regulate integrins and amino acid transport. J. Biol. Chem. 2001;276:8746–8752. [PubMed: 11121428]
• Fingeroth J. D., Weis J. J., Tedder T. F., Strominger J. L., Biro P. A., Fearon D. T. Epstein–Barr virus receptor of human B lymphocytes is the C3d complement CR 2. Proc. Natl Acad. Sci. USA. 1984;81:4510–4516. [PMC free article: PMC345620] [PubMed: 6087328]
• Fingeroth J. D., Diamond M. E., Sage D. R., Hayman J., Yates J. L. CD-21 dependent infection of an epithelial cell line, 293, by Epstein–Barr virus. J. Virol. 1999;73:2115–2125. [PMC free article: PMC104456] [PubMed: 9971794]
• Frade R., Barel M., Henricksson B., Klein G., Ehlin-1985gp140 the C3d receptor of human B lymphocytes is also the Epstein–Barr virus receptor. Proc. Natl Acad. Sci. USA 821490–1493. [PMC free article: PMC397288] [PubMed: 2983347]
• Frangou P., Buettner M., Niedobitek G. Epstein–Barr virus (EBV) infection in epithelial cells in vivo; rare detection of EBV replication in tongue mucosa but not in salivary glands. J. Infect. Dis. 2005;191:238–242. [PubMed: 15609234]
• Gan Y., Chodosh J., Morgan A., Sixbey J. W. Epithelial cell polarization is a determinant in the infectious outcome of immunoglobulin A-mediated entry by Epstein–Barr virus. J. Virol. 1997;71:519–526. [PMC free article: PMC191081] [PubMed: 8985380]
• Ganem D. Human herpesvirus 8 and its role in the genesis of Kaposi’s sarcoma. Curr. Clin. Top. Infect. Dis. 1998;18:237–251. [PubMed: 9779358]
• Giancotti F. G. 2000Complexity and specificity of integrin signalling Nat. Cell Biol. 2(1)E13–E14. [PubMed: 10620816]
• Giancotti F. G., Ruoslahti E. Integrin signaling. Science. 1999;285(5430):1028–1032. [PubMed: 10446041]
• Gong M., Kieff E. Intracellular trafficking of two major Epstein–Barr virus glycoproteins, gp350/220 and gp110. J. Virol. 1990;64:1507–1516. [PMC free article: PMC249284] [PubMed: 2157039]
• Gong M., Ooka T., Matsuo T., Kieff E. Epstein-Barr virus glycoprotein homologous to herpes simplex virus gB. J. Virol. 1987;61:499–508. [PMC free article: PMC253974] [PubMed: 3027378]
• Gonnella R., Farina A., Santarella R., et al. Characterization and intracellular localization of the Epstein-Barr virus protein BFLF 2: interactions with BFRF 1 and with the nuclear lamina. J. Virol. 2005;49:3713–3237. [PMC free article: PMC1075684] [PubMed: 15731265]
• Gordon J. S., Walker L., Guy G., Brown G., Rowe M., Rickinson A. Control of human B-lymphocyte transformation. Ⅱ. Transforming Epstein-Barr virus exploits three distinct viral signals to undermine three separate control points in B cell growth. Immunology. 1986;58:591–595. [PMC free article: PMC1453114] [PubMed: 2426189]
• Grundhoff A., Ganem D. Inefficient establishment of KSHV latency suggests an additional role for continued lytic replication in Kaposi sarcoma pathogenesis. J. Clin. Invest. 2004;113:124–136. [PMC free article: PMC300762] [PubMed: 14702116]
• Haan K. M., Longnecker R. Coreceptor restriction within the HLA -DQ locus for Epstein-Barr virus infection. Proc. Natl Acad. Sci. USA. 2000;97:9252–9257. [PMC free article: PMC16854] [PubMed: 10908662]
• Haan K. M., Lee S. K., Longnecker R. Different functional domains in the cytoplasmic tail of glycoprotein gB are involved in Epstein–Barr virus induced membrane fusion. Virology. 2001;290:106–114. [PubMed: 11882994]
• Haddad R. S., Fletcher L. M. and Hutt-1989Depletion of glycoprotein gp85 from virosomes made with Epstein–Barr virus proteins abolishes their ability to fuse with virus receptor-bearing cells J. Virol. 634998–5005. [PMC free article: PMC251159] [PubMed: 2555536]
• Hall A., Nobes C. D. Rho GTP ases: molecular switches that control the organization and dynamics of the actin cytoskeleton. Phil. Trans. R. Soc. Lond. B Biol. Sci. 2000;355(1399):965–970. [PMC free article: PMC1692798] [PubMed: 11128990]
• Heineman T., Gong M., Sample J., Kieff E. Identification of the Epstein-Barr virus gp85 gene. J. Virol. 1988;62:1101–1107. [PMC free article: PMC253115] [PubMed: 2831372]
• Huber M. T., Compton T. The human cytomegalovirus UL 74 gene encodes the third component of the glycoprotein H-glycoprotein L-containing envelope complex. J. Virol. 1998;72:8191–8197. [PMC free article: PMC110166] [PubMed: 9733861]
• Hummel M., Kieff E. Epstein-Barr virus RNA. VIII. Viral RNA in permissively infected B95–8 cells. J. Virol. 1982;43:262–272. [PMC free article: PMC256117] [PubMed: 6180174]
• Hummel M., Lawson D., Kieff E., Thorley-1984An Epstein-Barr virus DNA fragment encodes messages for the two major envelope glycoproteins (gp350/300 and gp220/200) J. Virol. 49413–417. [PMC free article: PMC255481] [PubMed: 6319741]
• Hurley E. A., Lawson D. A. and Thorley-1988B cell activation and the establishment of Epstein–Barr virus latency J. Exp. Med. 1682059–2075. [PMC free article: PMC2189139] [PubMed: 2848918]
• Holzenburg, BognerHutt-Fletcher, L. M. (2002). Epstein–Barr virus glycoproteins and their roles in virus entry and egress. In Structure–function Relationships of Human Pathogenic Viruses, ed. New York: Kluwer Academic/Plenum;
• Imai S., Nishikawa J., Takada K. Cell-to-cell contact as an efficient mode of Epstein-Barr virus infection of diverse human epithelial cells. J. Virol. 1998;72:4371–4378. [PMC free article: PMC109667] [PubMed: 9557727]
• Inoue N., Winter J., Lal R. B., Offermann M. K., Koyano S. Characterization of entry mechanisms of human herpesvirus 8 by using an Rta-dependent reporter cell line. J. Virol. 2003;77:8143–8152. [PMC free article: PMC161930] [PubMed: 12829853]
• Jiang R., Scott R. S., Hutt-Fletcher L. M. Epstein–Barr virus shed in saliva is high in B cell tropic gp42. J. Virol. 2006;80:7281–7283. [PMC free article: PMC1489022] [PubMed: 16809335]
• Johannsen E., Luftig M., Weicksel S., et al. Proteins of purified Epstein–Barr virus. Proc. Natl Acade. Sci. USA. 2004;101:16286–16291. [PMC free article: PMC528973] [PubMed: 15534216]
• Kaleeba J. A., Berger E. A. R and 2006Kaposi’s Sarcoma–associated herpesvirus fusion-entry receptor: cystine transporter xCT Science 3111921–1924. [PubMed: 16574866]
• Knipe, HowleyKieff, E. and Rickinson, A. B. (2001). Epstein–Barr virus and its replication. In Fields Virology ed. , Vol. 2, pp. 2511–2573. 2 vols. Philadelphia: Lippincott Williams and Wilkins;
• Klupp B., Mettenleiter T. C. Sequence and expression of the glycoprotein gH gene of pseudorabies virus. Virology. 1991;182:732–741. [PubMed: 1850925]
• Koyano S., Mar E. C., Stamey F. R., Inoue N. Glycoproteins M and N of human herpesvirus 8 form a complex and inhibit cell fusion. J. Gen. Virol. 2003;84:1485–1491. [PubMed: 12771417]
• Krishnan H. H., Walia N. S., Streblow D. N., Naranatt P. P., Chandran B. Focal adhesion kinase (FAK) is critical for Kaposi's Sarcoma-associated herpesvirus (KSHV/HHV-8) entry into the target cells. J. Virol. 2006;80:1167–1180. [PMC free article: PMC1346954] [PubMed: 16414994]
• Kurilla M. G., Heineman T., Davenport L. C., Kieff E., Fletcher L. M., and Hutt-1995A novel Epstein–Barr virus glycoprotein gp150 expressed from the BDLF 3 open reading frame Virology 209108–121. [PubMed: 7747461]
• Lagunoff M., Bechtel J., Venetsanakos E., et al. De novo infection and serial transmission of Kaposi’s sarcoma-associated herpesvirus in cultured endothelial cells. J. Virol. 2002;76(5):2440–2448. [PMC free article: PMC153827] [PubMed: 11836422]
• Lake C. M., Fletcher L. M. and Hutt-2000Epstein–Barr virus that lacks glycoprotein gN is impaired in assembly and infection J. Virol. 7411162–11172. [PMC free article: PMC113204] [PubMed: 11070013]
• Lake C. M., Fletcher L. M. and Hutt-2004The Epstein–Barr virus BFRF 1 and BFLF 2 proteins interact and coexpression alters their cellular localization Virology 32099–106. [PubMed: 15003866]
• Lambris J. D., Ganu V. S., Hirani S., Eberhard H. J., and Muller-1985Mapping of the C3d receptor (CR2) binding site and a neoantigenic site in the C3d domain of the third component of complement Proc. Natl Acad. Sci. USA 824235–4239. [PMC free article: PMC397971] [PubMed: 2408276]
• Lee S. K., Longnecker R. The Epstein–Barr virus glycoprotein 110 carboxy-terminal tail domain is essential for lytic virus replication. J. Virol. 1997;71:4092–4097. [PMC free article: PMC191563] [PubMed: 9094688]
• Lemon S. M., Hutt L. M., Shaw J. E., Li J.-L. H., Pagano J. S. Replication of EBV in epithelial cells during infectious mononucleosis. Nature. 1977;268:268–270. [PubMed: 196210]
• Li Q. X., Young L. S., Niedobitek G., et al. Epstein–Barr virus infection and replication in a human epithelial system. Nature. 1992;356:347–350. [PubMed: 1312681]
• Li Q. X., Turk S. M., Fletcher L. M., and Hutt-1995The Epstein–Barr virus (EBV) BZLF 2 gene product associates with the gH and gL homologs of EBV and carries an epitope critical to infection of B cells but not of epithelial cells J. Virol. 693987–3994. [PMC free article: PMC189130] [PubMed: 7539502]
• Li Q. X., Spriggs M. K., Kovats S., et al. Epstein–Barr virus uses HLA class Ⅱ as a cofactor for infection of B lymphocytes. J. Virol. 1997;71(6):4657–4662. [PMC free article: PMC191687] [PubMed: 9151859]
• Liao W., Tang Y., Kuo Y. L., Liu B. Y., Xu C. J., Giam C. Z. Kaposi’s sarcoma-associated herpesvirus/human herpesvirus 8 transcriptional activator Rta is an oligomeric DNA -binding protein that interacts with tandem arrays of phased A/T-trinucleotide motifs. J. Virol. 2003;77(17):9399–9411. [PMC free article: PMC187432] [PubMed: 12915555]
• Lo P., Yu X., Atanasov I., Chandran B., Zhou Z. H. Three-dimensional localization of pORF65 in Kaposi’s sarcoma-associated herpesvirus capsid. J. Virol. 2003;77(7):4291–4297. [PMC free article: PMC150664] [PubMed: 12634386]
• McShane M. P., Longnecker R. Cell-surface expression of a mutated Epstein–Barr virus glycoprotein B allows fusion independent of other viral glycoproteins. Proc. Natl Acad. Sci. USA. 2004;101:17474–17479. [PMC free article: PMC536015] [PubMed: 15583133]
• Mackett M., Conway M. J., Arrand J. R., Haddad R. S., Fletcher L. M., and Hutt-1990Characterization and expression of a glycoprotein encoded by the Epstein–Barr virus BamHI 1 fragment J. Virol. 642545–2552. [PMC free article: PMC249430] [PubMed: 2159529]
• Martin D. R., Yuryev A., Kalli K. R., Fearon D. T., Ahearn J. M. Determination of the structural basis for selective binding of Epstein-Barr virus to human complement receptor type 2. J. Exp. Med. 1991;174:1299–1311. [PMC free article: PMC2119041] [PubMed: 1660522]
• Martin D. R., Marlowe R. L., Ahearn J. M. Determination of the role for CD 21 during Epstein–Barr virus infection of B lymphoblastoid cells. J. Virol. 1994;68:4716–4726. [PMC free article: PMC236411] [PubMed: 7913508]
• Miller N., Fletcher L. M. and Hutt-1988A monoclonal antibody to glycoprotein gp85 inhibits fusion but not attachment of Epstein–Barr virus J. Virol. 622366–2372. [PMC free article: PMC253393] [PubMed: 2836619]
• Miller N., Fletcher L. M. and Hutt-1992Epstein–Barr virus enters B cells and epithelial cells by different routes J. Virol. 66(6):3409–3414. [PMC free article: PMC241121] [PubMed: 1316456]
• Molesworth S. J., Lake C. M., Borza C. M., Turk S. M., Fletcher L. M., and Hutt-2000Epstein–Barr virus gH is essential for penetration of B cell but also plays a role in attachment of virus to epithelial cells J. Virol. 746324–6332. [PMC free article: PMC112138] [PubMed: 10864642]
• Moore M. D., DiScipio R. G., Cooper N. R., Nemerow G. R. Hydrodynamic, electron microscopic and ligand binding analysis of the Epstein–Barr virus/C3dg receptor (CR2). J. Biol. Chem. 1989;34:20576–20582. [PubMed: 2555366]
• Mori Y., Akkapaiboon P., Yang X., Yamanishi K. The human herpesvirus 6 U100 gene product is the third component of the gH–gL glycoprotein complex on the viral envelope. J. Virol. 2003;77:2452–2458. [PMC free article: PMC141122] [PubMed: 12551983]
• Moses A. V., Fish K. N., Ruhl R., et al. Long-term infection and transformation of dermal microvascular endothelial cells by human herpesvirus 8. J. Virol. 1999;73(8):6892–6902. [PMC free article: PMC112774] [PubMed: 10400787]
• Mullen M. M., Haan K. M., Longnecker R., Jardetzky T. S. Structure of the Epstein–Barr virus gp42 protein bound to the MHC class Ⅱ receptor HLA -DR1. Mol. Cell. 2002;9:375–385. [PubMed: 11864610]
• Naranatt P. P., Akula S. M., Chandran B. Characterization of gamma2-human herpesvirus-8 glycoproteins gH and gL. Arch. Virol. 2002;147(7):1349–1370. [PubMed: 12111412]
• Naranatt P. P., Akula S. M., Zien C. A., Krishnan H. H., Chandran B. Kaposi’s sarcoma-associated herpesvirus induces the phosphatidylinositol 3-kinase-PKC-zeta-MEK-ERK signaling pathway in target cells early during infection: implications for infectivity. J. Virol. 2003;77(2):1524–1539. [PMC free article: PMC140802] [PubMed: 12502866]
• Naranatt P. P., Krishnan H. H., Smith M. S., Chandran B. Kaposi’s sarcoma-associated herpesvirus (KSHV/HHV-8) modulates the microtubule dynamics via RhoA-GTP-Diaphenous-2 signaling and utilizes the dynein motors to deliver its DNA to the nucleus. J. Virol. 2005;79:1191–1206. [PMC free article: PMC538527] [PubMed: 15613346]
• Neipel F., Albrecht J. C., Fleckenstein B. Cell-homologous genes in the Kaposi’s sarcoma-associated rhadinovirus human herpesvirus 8: determinants of its pathogenicity? J. Virol. 1997;71(6):4187–4192. [PMC free article: PMC191632] [PubMed: 9151804]
• Nemerow G. R., Cooper N. R. Early events in the infection of human B lymphocytes by Epstein-Barr virus. Virology. 1984;132:186–198. [PubMed: 6320532]
• Nemerow G. R., Wolfert R., McNaughton M., Cooper N. R. Identification and characterization of the Epstein–Barr virus receptor on human B lymphocytes and its relationship to the C3d complement receptor (CR2). J. Virol. 1985;55:347–351. [PMC free article: PMC254939] [PubMed: 2410629]
• Nemerow G. R., Houghton R. A., Moore M. D., Cooper N. R. Identification of the epitope in the major envelope proteins of Epstein-Barr virus that mediates viral binding to the B lymphocyte EBV receptor (CR2). Cell. 1989;56:369–377. [PubMed: 2464439]
• Niedobitek G., Dutoit S., Herbst H., et al. , Hamilton-1989Identification of Epstein–Barr virus-infected cells in tonsils of acute infectious mononucleosis by in situ hybridization Hum. Pathol. 20796–799. [PubMed: 2545594]
• Nolan L. A., Morgan A. J. The Epstein–Barr virus open reading frame BDLF 3 codes for a 100–150 kDa glycoprotein. J. Gen. Virol. 1995;76:1381–1392. [PubMed: 7782767]
• Oba D. E., Fletcher L. M. and Hutt-1988Induction of antibodies to the Epstein–Barr virus glycoprotein gp85 with a synthetic peptide corresponding to a sequence in the BXLF 2 open reading frame J. Virol. 621108–1114. [PMC free article: PMC253116] [PubMed: 2831373]
• Oda T., Imai S., Chiba S., Takada K. Epstein–Barr virus lacking glycoprotein gp85 cannot infect B cells and epithelial cells. Virology. 2000;276:52–58. [PubMed: 11021994]
• Omerovic J., Lev L., Longnecker R. The amino terminus of Epstein–Barr virus glycoprotein gH is important for fusion with B cells and epithelial cells. J. Virol. 2005;79:12408–12415. [PMC free article: PMC1211543] [PubMed: 16160168]
• Paulsen S. J., Rosenkilde M. M., Eugen-Olsen J., Kledal T. N. Epstein–Barr virus-encoded BILF 1 is a constitutively active G protein-coupled receptor. J. Virol. 2005;79:536–546. [PMC free article: PMC538743] [PubMed: 15596846]
• Pegtel D. M., Middledorp J., Thorley-Lawson D. A. Epstein–Barr virus infection in ex-vivo tonsil epithelial cultures of asymptomatic carriers. J. Virol. 2004;78:12613–12624. [PMC free article: PMC525079] [PubMed: 15507648]
• Pertel P. E. Human herpesvirus 8 glycoprotein B (gB), gH, and gL can mediate cell fusion. J. Virol. 2002;76:4390–4400. [PMC free article: PMC155071] [PubMed: 11932406]
• Rappocciolo G., Jenkins F. L., Hensler H. R., et al. DC-SIGN is a receptor for human herpesvirus 8 on dendritic cells and macrophages. J. Immunol. 2006;176:1741–1749. [PubMed: 16424204]
• Renne R., Blackbourn D., Whitby D., Levy J., Ganem D. Limited transmission of Kaposi’s sarcoma-associated herpesvirus in cultured cells. J. Virol. 1998;72(6):5182–5188. [PMC free article: PMC110093] [PubMed: 9573290]
• Ressing M. E., Leeuwen, Verreck F. A. W., et al. Epstein–Barr virus gp42 is postranslationally modified to produce s-gp42 that mediates class Ⅱ immune evasion. J. Virol. 2005;79:841–852. [PMC free article: PMC538541] [PubMed: 15613312]
• Knipe, HowleyRickinson, A. B. and Kieff, E. (2001). Epstein–Barr virus. In fields virology ed. , Vol. 2, pp. 2575–2627. 2 vols. Philadelphia: Lippincott, Williams and Wilkins;
• Rivailler P., Jiang H., Cho Y.-G., Quink C., Wang F. Complete nucleotide sequence of the rhesus lymphocryptovirus: genetic validation for an Epstein–Barr virus animal model. J. Virol. 2002;76:421–426. [PMC free article: PMC135707] [PubMed: 11739708]
• Russo J. J., Bohenzky R. A., Chien M. C., et al. Nucleotide sequence of the Kaposi sarcoma-associated herpesvirus (HHV8). Proc. Natl Acad. Sci. USA. 1996;93:14862–14867. [PMC free article: PMC26227] [PubMed: 8962146]
• Sarid R., Olsen S. J., Moore P. S. Kaposi’s sarcoma-associated herpesvirus: epidemiology, virology, and molecular biology. Adv. Virus Res. 1999;52:139–232. [PubMed: 10384236]
• Sastry S. K., Burridge K. Focal adhesions: a nexus for intracellular signaling and cytoskeletal dynamics. Exp. Cell Res. 2000;261(1):25–36. [PubMed: 11082272]
• Schulz T. F., Sheldon J., Greensill J. 2002Kaposi’s sarcoma associated herpesvirus (KSHV) or human herpesvirus 8 (HHV8) Virus Res. 82(1–2), 115–126. [PubMed: 11885938]
• Shannon-Rowe C. D., Neuhierl B., Baldwin G., Rickinson A. B., Delecluse H. -J. (2006Resting B cells as a transfer vehicle for Epstein–Barr virus infection of epithelial cells Proc. Natl Acad. Sci. USA 1037065–7070. [PMC free article: PMC1459019] [PubMed: 16606841]
• Sieczkarski S. B., Whittaker G. R. Dissecting virus entry via endocytosis. J. Gen. Virol. 2002;83(7):1535–1545. [PubMed: 12075072]
• Sinclair A. J., Farrell P. J. Host cell requirements for efficient infection of quiescent primary B lymphocytes by Epstein–Barr virus. J. Virol. 1995;69:5461–5468. [PMC free article: PMC189395] [PubMed: 7543582]
• Sixbey J. W., Yao Q.-Y. Immunoglobulin A-induced shift of Epstein–Barr virus tissue tropism. Science. 1992;255:1578–1580. [PubMed: 1312750]
• Sixbey J. W., Nedrud J. G., Traub N., Hanes R. A., Pagano J. S., Raab-1984Epstein–Barr virus replication in oropharyngeal epithelial cells N. Engl. J. Med. 3101225–1230. [PubMed: 6323983]
• Spriggs M. K., Armitage R. J., Comeau M. R., et al. The extracellular domain of the Epstein–Barr virus BZLF 2 protein binds the HLA -DR beta chain and inhibits antigen presentation. J. Virol. 1996;70:5557–5563. [PMC free article: PMC190515] [PubMed: 8764069]
• Sugano N., Chen W., Roberts M. L., Cooper N. R. Epstein–Barr virus binding to CD 21 activates the initial viral promoter via NF kB induction. J. Exp. Med. 1997;186:731–737. [PMC free article: PMC2199015] [PubMed: 9271588]
• Sun R., Spain T. A., Lin S.-F., Miller G. Sp1 binds to the precise locus of end processing within the terminal repeats of Epstein–Barr virus DNA. J. Virol. 1997;71:6136–6143. [PMC free article: PMC191874] [PubMed: 9223508]
• Tanner J., Weis J., Fearon D., Whang Y., Kieff E. Epstein–Barr virus gp350/220 binding to the B lymphocyte C3d receptor mediates adsorption, capping and endocytosis. Cell. 1987;50:203–213. [PubMed: 3036369]
• Tanner J., Whang Y., Sample J., Sears A., Keiff E. Soluble gp350/220 and deletion mutant glycoproteins block Epstein–Barr virus adsorption to lymphocytes. J. Virol. 1988;62:4452–4464. [PMC free article: PMC253554] [PubMed: 2460635]
• Tanner J. E., Tosato G. Regulation of B-cell growth and immunoglobulin gene transcription by interleukin-6. Blood. 1992;79:452–459. [PubMed: 1370387]
• Telford E. A., Watson M. S., Aird H. C., Perry J., Davison A. J. The DNA sequence of equine herpesvirus 2. J. Mol. Biol. 1995;249:520–528. [PubMed: 7783207]
• Tomescu C., Law W. K., Kedes D. H. Surface downregulation of major histocompatibility complex class Ⅰ, PE -CAM, and ICAM -1 following de novo infection of endothelial cells with Kaposi’s sarcoma-associated herpesvirus. J. Virol. 2003;77(17):9669–9684. [PMC free article: PMC187401] [PubMed: 12915579]
• Trus B. L., Heymann J. B., Nealon K., et al. Capsid structure of Kaposi’s sarcoma-associated herpesvirus, a gammaherpesvirus, compared to those of an alphaherpesvirus, herpes simplex virus type 1, and a betaherpesvirus, cytomegalovirus. J. Virol. 2001;75(6):2879–2890. [PMC free article: PMC115914] [PubMed: 11222713]
• Tugizov S. M., Berline J. W., Palefsky J. M. Epstein–Barr virus infection of polarized tongue and nasopharyngeal epithelial cells. Nature Med. 2003;9:307–314. [PubMed: 12592401]
• Turk S. M., Jiang R., Chesnokova L. S., Hutt-Fletcher L. M. Antibodies to gp350/220 enhance the ability of Epstein–Barr virus to infect epithelial cells. J. Virol. 2006;80:9628–9633. [PMC free article: PMC1617223] [PubMed: 16973566]
• Venables P. J. W., Teo C. G., Baboonian C., Griffin B. E., Hughes R. A. Persistence of Epstein–Barr virus in salivary gland biopsies from healthy individuals and patients with Sjogren’s syndrome. Clin. Exp. Immunol. 1989;75:359–364. [PMC free article: PMC1541970] [PubMed: 2539280]
• Vieira J., Hearn P., Kimball L., Chandran B., Corey L., O’2001Activation of Kaposi’s sarcoma-associated herpesvirus (human herpesvirus 8) lytic replication by human cytomegalovirus J. Virol. 75(3):1378–1386. [PMC free article: PMC114044] [PubMed: 11152511]
• Virgin H. W. I., Latreille P., Wamsley P., et al. Complete sequence and analysis of murine gammaherpesvirus 68. J. Virol. 1997;71:5894–5904. [PMC free article: PMC191845] [PubMed: 9223479]
• Virji M. Microbial utilization of human signalling molecules. Microbiology. 1996;142(12):3319–3336. [PubMed: 9004497]
• Wang F. Z., Akula S. M., Pramod N. P., Zeng L., Chandran B. Human herpesvirus 8 envelope glycoprotein K8.1A interaction with the target cells involves heparan sulfate. J. Virol. 2001;75(16):7517–7527. [PMC free article: PMC114987] [PubMed: 11462024]
• Wang F. Z., Akula S. M., Walia N., Zeng L., Chandran B., Sharma-2003Human herpesvirus 8 envelope glycoprotein B mediates cell adhesion via its RGD sequence J. Virol. 77(5):3131–3147. [PMC free article: PMC149745] [PubMed: 12584338]
• Wang X., Fletcher L. M. and Hutt-1998Epstein–Barr virus lacking glycoprotein gp42 can bind to B cells but is not able to infect J. Virol. 72158–163. [PMC free article: PMC109360] [PubMed: 9420211]
• Wang X., Kenyon W. J., Li Q. X., Mullberg J., Fletcher L. M., and Hutt-1998Epstein–Barr virus uses different complexes of glycoproteins gH and gL to infect B lymphocytes and epithelial cells J. Virol. 725552–5558. [PMC free article: PMC110204] [PubMed: 9621012]
• Weisman H. F., Bartow T., Leppo M. K., et al. Soluble human complement receptor type 1: in vivo inhibitor of complement suppressing post-ischemic myocardial inflammation and necrosis. Science. 1990;249:146–151. [PubMed: 2371562]
• Whittaker G. R. Virus nuclear import. Adv. Drug Deliv. Rev. 2003;55(6):733–747. [PubMed: 12788537]
• Wu L., Lo P., Yu X., Stoops J. K., Forghani B., Zhou Z. H. Three-dimensional structure of the human herpesvirus 8 capsid. J. Virol. 2000;74:9646–9654. [PMC free article: PMC112397] [PubMed: 11000237]
• Wu L., Borza C. M., Hutt-Fletcher L. M. Mutations of Epstein–Barr virus gH that are differentially able to support fusion with B cells or epithelial cells. J. Virol. 2005;79:10923–10930. [PMC free article: PMC1193614] [PubMed: 16103144]
• Yaswen L. R., Stephens E. B., Davenport L. C., Fletcher L. M., and Hutt-1993Epstein-Barr virus glycoprotein gp85 associates with the BKRF 2 gene product and is incompletely processed as a recombinant protein Virology 195387–396. [PubMed: 8393232]
• Young L. S., Dawson C. W., Brown K. W., Rickinson A. B. Identification of a human epithelial cell surface protein sharing an epitope with the C3d/Epstein-Barr virus receptor molecule of B lymphocytes. Int. J. Cancer. 1989;43:786–794. [PubMed: 2469656]
• Zhu F. X., Yuan Y. The ORF 45 protein of Kaposi’s sarcoma-associated herpesvirus is associated with purified virions. J. Virol. 2003;77(7):4221–4230. [PMC free article: PMC150667] [PubMed: 12634379]
• Zhu L., Puri V., Chandran B. Characterization of human herpesvirus-8 K8.1A/B glycoproteins by monoclonal antibodies. Virology. 1999;262(1):237–249. [PubMed: 10489357]
• Zhu L., Wang R., Sweat A., Goldstein E., Horvat R., Chandran B. Comparison of human sera reactivities in immunoblots with recombinant human herpesvirus (HHV)-8 proteins associated with the latent (ORF73) and lytic (ORFs 65, K8.1A, and K8.1B) replicative cycles and in immunofluorescence assays with HHV -8-infected BCBL -1 cells. Virology. 1999;256(2):381–392. [PubMed: 10191203]