A Gammaherpesvirus Bcl-2 Ortholog Blocks B Cell Receptor-Mediated Apoptosis and Promotes the Survival of Developing B Cells In Vivo

Gammaherpesviruses such as Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV, HHV-8) establish lifelong latency in their hosts and are associated with the development of several types of malignancies, including a subset of B cell lymphomas. These viruses are thought to co-opt the process of B cell differentiation to latently infect a fraction of circulating memory B cells, resulting in the establishment of a stable latency setpoint. However, little is known about how this infected memory B cell compartment is maintained throughout the life of the host. We have previously demonstrated that immature and transitional B cells are long-term latency reservoirs for murine gammaherpesvirus 68 (MHV68), suggesting that infection of developing B cells contributes to the maintenance of lifelong latency. During hematopoiesis, immature and transitional B cells are subject to B cell receptor (BCR)-mediated negative selection, which results in the clonal deletion of autoreactive B cells. Interestingly, numerous gammaherpesviruses encode homologs of the anti-apoptotic protein Bcl-2, suggesting that virus inhibition of apoptosis could subvert clonal deletion. To test this, we quantified latency establishment in mice inoculated with MHV68 vBcl-2 mutants. vBcl-2 mutant viruses displayed a marked decrease in the frequency of immature and transitional B cells harboring viral genome, but this attenuation could be rescued by increased host Bcl-2 expression. Conversely, vBcl-2 mutant virus latency in early B cells and mature B cells, which are not targets of negative selection, was remarkably similar to wild-type virus. Finally, in vivo depletion of developing B cells during chronic infection resulted in decreased mature B cell latency, demonstrating a key role for developing B cells in the maintenance of lifelong latency. Collectively, these findings support a model in which gammaherpesvirus latency in circulating mature B cells is sustained in part through the recurrent infection and vBcl-2-mediated survival of developing B cells.
Author Summary Gammaherpesviruses such as Epstein-Barr virus and Kaposi's sarcoma herpesvirus are widespread pathogens that establish lifelong infections in a dormant state termed latency. Although most gammaherpesvirus infections are asymptomatic, infection of some individuals leads to the development of B cell lymphoma or other cancers. It is well known that during latency these viruses reside in mature B cells of the immune system; however, little is known about how this reservoir is maintained for life. Using murine gammaherpesvirus 68 infection of mice as a model to study gammaherpesvirus infections inside a living host, we have previously demonstrated that gammaherpesviruses can infect early precursors of B cells. In normal situations, the differentiation of such precursors into mature B cells is a tightly regulated process that leads to the death of cells that react inappropriately to host tissues. Here though, we demonstrate that a gammaherpesvirus protein called vBcl-2 can block the death of infected precursor B cells, and that vBcl-2 is critical for infection of these cells. Finally, we show that depleting precursor B cells reduces mature B cell latency. Together, these data suggest that vBcl-2 proteins play a key role in lifelong gammaherpesvirus latency and may be a potent target for future drug development.