B cell-activating factor regulates the survival of B lymphocytes infected with human cytomegalovirus

Background: Previous studies have suggested that B lymphocytes can be polyclonally activated by human cytomegalovirus (HCMV), and individuals infected by HCMV exhibit characteristic features of an autoimmunity disease. B cell-activating factor (BAFF) plays important roles in the survival and differentiation of B cells; however, few studies have examined the potential role of BAFF on B cells infected by HCMV.

Methods: HCMV virus strain (HCMV AD-169) was concentrated by normal methods and used to infect microbead-purified tonsil CD19+ B cells. Cells and supernatants were collected at the 1st, 3rd, 5th, and 7th day of co-culture, respectively. Cellular phenotypes, including expression of BAFF and its cognate receptors (BAFF-R, TACI, and BCMA) were detected by flow cytometry (FCM); cells apoptosis rates were also examined by FCM; and IgG titers in supernatants was detected by ELISA. In parallel, neutralizing anti-BAFF-R antibody was applied to observe the effect of BAFF/BAFF-R signaling on apoptosis and the IgG secretion ability of B cells stimulated by HCMV.

Results: LogTCID₅₀ of 3rd and 4th generation of HCMV was -3.54 and -3.28, respectively. FCM results showed that the purity of CD19⁺ B cells was >98%. BAFF-R was highly expressed and upregulated on HCMV-infected B cells (93.5%-99.3%), compared with B cells prior to HCMV infection and uninfected group; while BAFF-R expression gradually decreased with time and to the lowest level at 5th day (81%) in the control medium-only group. In contrast, expression of TACI and BCMA gradually increased during culture in both HCMV-infected and medium-only control B cells. Furthermore, the apoptosis rate of HCMV-infected and medium-only control B cells did not vary significantly during culture, but IgG secretion ability of HCMV-infected B cells significantly increased over time while no changes were observed with the medium-only control. Importantly, the apoptosis rate of B cells significantly increased when BAFF/BAFF-R signal was blocked prior to HCMV infection (P<0.05), although no significant changes of IgG levels were observed (P>0.05).

Conclusions: BAFF-R was consistently expressed on B cells infected by HCMV. Enhancement of BAFF/BAFF-R signaling decreased the apoptosis rate and extended the survival of B cells.