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

UNLABELLED: MicroRNA-155 (miR-155) has been shown to play significant roles in the immune response, including in the formation of germinal centers (GC) and the development and maturation of T follicular helper (Tfh) cells. There is in vitro evidence to support a critical role for cellular miR-155 and viral miR-155 homologs in the establishment of gammaherpesvirus latency in B cells. We sought to determine the contribution of miR-155 to the establishment and maintenance of latency in vivo using murine gammaherpesvirus (MHV-68) infection. MHV-68-infected mice deficient in miR-155 exhibited decreases in GC B cells and Tfh cells. However, the frequencies of spleen cells harboring latent MHV-68 genomes were the same in both miR-155-deficient and wild-type (WT) mice. Similar latent loads were also observed in mixed bone marrow chimeric mice, where B cell-extrinsic effects of miR-155 deficiency were normalized. Interestingly, we observed markedly lower efficiency of reactivation from latency in miR-155-deficient cells, indicating an important role for miR-155 in this process. These in vivo data complement previous in vitro studies and lead to the conclusion that miR-155 is not necessary for the establishment or maintenance of gammaherpesvirus latency but that it does affect reactivation efficiency. IMPORTANCE: Gammaherpesvirus infection leads to severe disease in immunosuppressed populations. miR-155 has been shown to play important roles in many pathological processes, including tumorigenesis and diseases caused by an overly aggressive immune response. Our work provides valuable in vivo data showing that miR-155 is dispensable for gammaherpesvirus latency but that it is critical for reactivation from latency, which is a crucial step in the viral life cycle.