AbstractMurine gammaherpesvirus 68 (MHV-68) is a natural pathogen of rodents closely related to the human γherpesviruses Kaposi’s sarcoma-associated herpesvirus and EBV. Following intranasal infection, the virus replicates in the lung epithelium prior to establishing latent infection in lymphoid tissue. Infection of mice deficient in IFN-γR signaling (IFN-γR−/−) results in a multiple organ fibrosis, in which the spleen is severely affected. We show here that by Day 12 postinfection, prior to development of fibrosis in the spleens of IFN-γR−/− mice, different subsets of splenic macrophages (Mϕs) are morphologically activated and enter latently infected germinal centers (GCs). Mϕs coexpressing arginase I (ARG1), a marker of alternative activation of Mϕs, and murine Mϕ markers F4/80, ER-TR9, and MOMA-1 are found in GCs of IFN-γR−/− mice but not of wild-type mice. Quantitative RT-PCR of spleen RNA confirms induction of ARG1 and in addition, shows up-regulation of found in inflammatory zone 1/resistin-like molecule-α, tissue inhibitor of metalloproteinase-1, matrix metalloproteinase-12, fibronectin, and factor XIIIA in IFN-γR−/− mice. In contrast, inducible NO synthase, associated with classical Mϕ activation, is up-regulated following infection of wild-type mice but not IFN-γR−/− mice. Concomitant with the aaMϕs, transcription of the Th2 cytokines IL-13, IL-21, and IL-5 is up-regulated. Thus, in the absence of IFN-γR signaling, MHV-68 initiates a Th2 immune response, leading to alternative activation of macrophages and induction of fibrosis. This system provides an important model for studying the pathogenesis of fibrosis initiated by a latent herpesvirus infection.