A study profiling chemokine expression in LCLs
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 LB Agar Miller mass 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. The histologic finding that EBV-infected B cells in infectious mononucleosis tend to avoid the GCs and instead accumulate under the epithelium of certain mucosal tissues34, 129 could be explained by the above-mentioned expression patterns of chemokine receptors on EBV-infected B cells. CXCR4 and CXCR5 respond to their ligands, which are highly expressed in the center of lymphoid follicles and thereby direct B cells to GC formation. CCL20 and CCL28, on the other hand, are normally expressed by the epithelial cells of mucosal tissues such as salivary glands and the tonsils.130, 131, 132 As such, downregulation of CXCR4 and CXCR5 and upregulation of CCR6 and CCR10 could ensure migration toward the tonsillary epithelial cells. EBV also induces the expression of the mRNA levels of CCR9, which is responsible for B cell homing to mucosal tissues, as well as C5AR1, the receptor for the complement factor C5a. Alterations of the expression level of the aforementioned molecules may cause the EBV-infected cells to be retained in the interfollicular region of the tonsils. The manipulation of the chemokine system by EBV seems to be in favor of the virus infection, but it is likely that some of the regulation also reflects cell protection against the intruding virus and antiviral immune responses. As such, it was found that EBV-infected cells as well as EBV negative cells expressing LMP1 upregulate the expression of CCL17 and CCL22, known to preferentially attract Th2 cells and regulatory T cells via CCR4. This expression pattern would benefit the virus and skew the immune response away from a Th1 and CTL response. In contrast, EBV-infected cells were found to upregulate CCL3, CCL4, and CCL5, which are known to attract Th1 cells and activated CTL via CCR5. Expression of CCL5 was also found to be elevated in LMP1 expressing cells as well as Burkitt\'s lymphoma cell lines. Thus, the chemokine system may work in favor of both the virus and the immune system.