Murine Gammaherpesvirus 68 Reactivation from B Cells Requires IRF4 but Not XBP-1

Abstract

Gammaherpesviruses display tropism for B cells and, like all known herpesviruses, exhibit distinct lytic and latent life cycles. One well-established observation among members of the gammaherpesvirus family is the link between viral reactivation from latently infected B cells and plasma cell differentiation. Importantly, a number of studies have identified a potential role for a CREB/ATF family member, X-box binding protein 1 (XBP-1), in trans-activating the immediate early BZLF-1 or BRLF1/gene 50 promoters of Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV), respectively. XBP-1 is required for the unfolded protein response and has been identified as a critical transcription factor in plasma cells. Here, we demonstrate that XBP-1 is capable of trans-activating the murine gammaherpesvirus 68 (MHV68) RTA promoter in vitro, consistent with previous observations for EBV and KSHV. However, we show that in vivo there does not appear to be a requirement for XBP-1 expression in B cells for virus reactivation. The MHV68 M2 gene product under some experimental conditions plays an important role in virus reactivation from B cells. M2 has been shown to drive B cell differentiation to plasma cells, as well as interleukin-10 (IL-10) production, both of which are dependent on M2 induction of interferon regulatory factor 4 (IRF4) expression. IRF4 is required for plasma cell differentiation, and consistent with a role for plasma cells in MHV68 reactivation from B cells, we show that IRF4 expression in B cells is required for efficient reactivation of MHV68 from splenocytes. Thus, the latter analyses are consistent with previous studies linking plasma cell differentiation to MHV68 reactivation from B cells. The apparent independence of MHV68 reactivation from XBP-1 expression in plasma cells may reflect redundancy among CREB/ATF family members or the involvement of other plasma cell-specific transcription factors. Regardless, these findings underscore the importance of in vivo studies in assessing the relevance of observations made in tissue culture models. IMPORTANCEAll known herpesviruses establish a chronic infection of their respective host, persisting for the life of the individual. A critical feature of these viruses is their ability to reactivate from a quiescent form of infection (latency) and generate progeny virus. In the case of gammaherpesviruses, which are associated with the development of lymphoproliferative disorders, including lymphomas, reactivation from latently infected B lymphocytes occurs upon terminal differentiation of these cells to plasma cellsthe cell type that produces antibodies. A number of studies have linked a plasma cell transcription factor, XBP-1, to the induction of gammaherpesvirus reactivation, and we show here that indeed in tissue culture models this cellular transcription factor can trigger expression of the murine gammaherpesvirus gene involved in driving virus reactivation. However, surprisingly, when we examined the role of XBP-1 in the setting of infe...

Identification of Novel Kaposi's Sarcoma-Associated HerpesvirusOrf50Transcripts: Discovery of New RTA Isoforms with Variable Transactivation Potential

“…While the external cellular environment is important, so is the internal environment within each individual infected cell, where transcription factors such as XBP-1 binding viral promoters can alter the viral life cycle (69)(70)(71)(72). One of the most important functions of RTA is the ability to initiate a downstream cascade of viral gene expression.…”

MicroRNA miR-155 Is Necessary for Efficient Gammaherpesvirus Reactivation from Latency, but Not for Establishment of Latency

“…In MHV-68 infection XBP-1 was found to transactivate Rta in vitro; however, it was dispensable for reactivation after in vivo infection. In contrast, IRF4 was found to be essential for reactivation in vivo following MHV-68 infection due to its requirement in the generation of plasma cells (73). In this study, we found a cell-intrinsic defect in plasma cell differentiation in the miR-155 KO mice, which led to a 2-fold difference in the proportion and absolute number of plasma cells compared to the levels in the WT controls.…”

Herpesviruses and the Unfolded Protein Response

“…The physiologic signals that control the switch from latency to lytic replication are unclear, but in cell culture viral reactivation has often been linked to exposure to stress-inducing stimuli. ER stress can trigger reactivation of EBV, KSHV and murine gammaherpesvirus 68 (MHV68) [119][120][121][122]. Lytic reactivation in response to ER stress is primarily due to XBP1s (Figure 2).…”

“…Control of MHV68 latent/lytic switch by XBP1s is only partially conserved; XBP1s transactivates the MHV68 RTA gene in vitro, but in vivo the plasma cell differentiation factor interferon regulatory factor 4 (IRF4) serves in this role [120]. The ER-localized MHV68 M1 protein is induced by RTA and IRF4 and may also play a role in reactivation from latency [136,137].…”