br EBV BILF A Virus Encoded TM
EBV-BILF1—A Virus-Encoded 7TM Receptor with Immune Evasive Functions The EBV-encoded BILF1 receptor (EBV-BILF1) is thought to be implicated in the immune evasion strategy of EBV.56, 61, 62 This orphan 7TM receptor is expressed at significant levels during the early lytic phase of the virus infection, intermediate levels in LCLs and nasopharyngeal carcinoma derived C666-1 line and at low levels during the viral latent phase.63, 64 Several studies have shown that the pivotal role in immune evasion played by EBV-BILF1 occurs during the lytic replication rather than the latent phase.56, 61, 62, 63, 64 In addition to EBV, the BILF1 gene is present in the two so far characterized nonhuman primate LCVs, Old World rhesus LCV-1 (RhLCV-1), and New World callitrichine herpesvirus-3 (CalHV-3).62, 65 Furthermore, ungulate gammaherpesviruses belonging to the genera Macavirus (Porcine lymphotropic herpesviruses 1–3, Alcelaphine herpesvirus 1, and Ovine herpesvirus 2) and Percavirus (Equine herpesvirus 2) encode putative 7TM receptors at the homologous genomic position. Although these BILF1 receptor homologs remain uncharacterized, the strong conservation reflects a significance of this gene in the viral pathogenesis.
EBI2: An Endogenous 7TM Receptor Manipulated by EBV EBI2 was identified in 1993, when Kieff and colleagues used subtractive hybridization of DNA from Burkitt\'s lymphoma WZB117 to screen for upregulated genes in EBV-infected cells. They found, being more than 200-fold upregulated (hence the name: EBV-induced gene 2). Since 1993, this finding has been confirmed in four different studies describing EBI2 upregulation in both lytic and latent settings29, 88, 89, 90 (see Table 2). The name of EBI2 suggests that EBV directly induces expression of the receptor, but the fact is that we, after more than two decades, still do not know if EBI2 is induced by EBV or if it is induced as part of the immune response to the virus infection. As described in detail below, EBI2 has moved from being an orphan receptor with no known biological function to an oxysterol-induced receptor with important functions in the immune system within a few years; in addition, the molecular basis for activation and inhibition of EBI2 has been studied extensively.
Manipulation of the Host Immune System 7TM Receptors and Ligands by EBV—The Chemokine System Chemokines induce chemotaxis of leukocytes through interactions with 7TM chemokine receptors. Approximately, 40 chemokines and 18 functional chemokine receptors have been identified in humans. Based on the pattern of conserved cysteine residues in the N-terminal region they are classified into four subfamilies: CC, CXC, C, and CX3C. By controlling leukocyte migration, the chemokine system has important functions in coordinating the immune system, leukocyte homeostasis, lymphocyte activation, and the host immune response to infectious pathogens. It is therefore not surprising that many viruses have been shown to modulate the chemokine system, several of them by encoding chemokine receptors or ligands.60, 120 EBV has not yet been shown to encode either chemokine receptors or ligands, but the system is being markedly manipulated during EBV infection, as some chemokines and chemokine receptors are upregulated, while others are downregulated (Fig. 3). Hence, the chemokine system may play an important role in the tissue localization of EBV-infected B cells and thereby contribute to the pathogenesis of EBV-associated diseases. A study profiling chemokine expression in LCLs revealed high expression levels of CCR6, CCR7, and CCR10 and low expression of CXCR4 and CXCR5 in EBV immortalized cells. Accordingly, LCLs showed a markedly increase in migration in response to the ligands of CCR6 (CCL20), CC7 (CCL21), and CCR10 (CCL28), but showed only weak migration in response to the ligands of CXCR4 (CXCL12) and CXCR5 (CXCL13). Selective expression of EBNA-2, LMP1 or both in an EBV negative cell line showed that EBNA-2 directly induces expression of CCR6, while both EBNA-2 and LMP1 downregulate CXCR4. In contrast neither CCR10 or CXCR5 was directly induced by EBNA-2 or LMP1 and the regulation of these two chemokine receptors was more likely a consequence of the plasmablast state of the LCLs. CXCR4 has also been found to be upregulated in EBNA-3B-deficient LCLs suggesting that this viral protein is a negative regulator of CXCR4. In addition, BILF1 has been found to heterodimerize with CXCR4 and thereby inhibit CXCR4-mediated signaling. The importance of the chemokine pattern described above can be deduced from the fact that also human herpesviruses 6A, 6B, and 7 has been shown to induce CCR7 expression and reduce CXCR4 expression.125, 126 Interestingly, a study examining the lymphoproliferation in EBV-infected huSCID mice showed that the axis was important for lymphoma development as CXCR4 was highly expressed on the tumors and inhibition of the CXCR4/CXCL12 axis reduced tumor development. Consistently, the gene, which is expressed in latency III program and is important for growth transformation of infected B cells, upregulated the levels of both CXCL12 and CXCR4. The diverging expression of CXCR4 by the viral tumor suppressor EBNA-3B (downregulation) and the growth transforming EBNA-3C (upregulation) taken together with increased expression of CXCR4 in EBV-mediated huSCID lymphoma development clearly suggests that CXCR4 is important for the growth transformation of infected B cells. Another study on the effect of EBV on CXCR5 and CCR7 expression in tonsillary B cells showed that 2 days after EBV infection there were minor changes in the expression levels of CXCR5. By day 7, however, the expression levels of both CXCR5 and CCR7 go down and both of the aforementioned receptors were no longer expressed at the cell surface by day 14. Also, the chemotactic response to CXCL13 and CCL21 was reduced by day 2, when CXCR5 and CCR7 was still expressed, suggesting that the virus impairs chemokine-directed migration even in the presence of the receptors.