Multifaceted roles for STAT3 in gammaherpesvirus latency revealed through in vivo B cell knockout models

Cancers associated with the oncogenic gammaherpesviruses, EpsteinBarr virus and Kaposi sarcoma herpesvirus, are notable for their constitutive activation of the transcription factor signal transducer and activator of transcription 3 (STAT3). To better understand the role of STAT3 during gammaherpesvirus latency and the B cell response to infection, we used the model pathogen murine gammaherpesvirus 68 (MHV68). Genetic deletion of STAT3 in B cells of CD19^cre/+Stat3^f/f mice reduced peak MHV68 latency approximately sevenfold. However, infected CD19^cre/+Stat3^f/f mice exhibited disordered germinal centers and heightened virusspecific CD8 T cell responses compared to wildtype (WT) littermates. To circumvent the systemic immune alterations observed in the B cellSTAT3 knockout mice and more directly evaluate intrinsic roles for STAT3, we generated mixed bone marrow chimeric mice consisting of WT and STAT3 knockout B cells. We discovered a dramatic reduction in latency in STAT3 knockout B cells compared to their WT B cell counterparts in the same lymphoid organ. RNA sequencing of sorted germinal center B cells revealed that MHV68 infection shifts the gene signature toward proliferation and away from type I and type II IFN responses. Loss of STAT3 largely reversed the virusdriven transcriptional shift without impacting the viral gene expression program. STAT3 promoted B cell processes of the germinal center, including IL21stimulated downregulation of surface CD23 on B cells infected with MHV68 or EBV. Together, our data provide mechanistic insights into the role of STAT3 as a latency determinant in B cells for oncogenic gammaherpesviruses. IMPORTANCE There are no directed therapies to the latency program of the human gammaherpesviruses, EpsteinBarr virus and Kaposi sarcoma herpesvirus. Activated host factor signal transducer and activator of transcription 3 (STAT3) is a hallmark of cancers caused by these viruses. We applied the murine gammaherpesvirus pathogen system to explore STAT3 function upon primary B cell infection in the host. Since STAT3 deletion in all CD19+ B cells of infected mice led to altered B and T cell responses, we generated chimeric mice with both normal and STAT3deleted B cells. B cells lacking STAT3 failed to support virus latency compared to normal B cells from the same infected animal. Loss of STAT3 impaired B cell proliferation and differentiation and led to a striking upregulation of interferonstimulated genes. These findings expand our understanding of STAT3dependent processes that are key to its function as a proviral latency determinant for oncogenic gammaherpesviruses in B cells and may provide novel therapeutic targets.