Pathology due to the immune system's response to viral infections often represents a delicate balance between inhibition of viral pathogenesis and regulation of protective immunity. In susceptible C57BL/6 (B6) mice, the murine retroviral isolate LP-BM5 induces splenomegaly, hypergammaglobulinemia, profound B- and T-cell immunodeficiency, and increased susceptibility to opportunistic pathogens and terminal B-cell lymphomas. Here, we report that B6.PD-1 (programmed death-1) and B6.IL-10 knockout mice are substantially more susceptible to LP-BM5-induced disease than wild-type B6 mice. LP-BM5-infected B6.PD-1(-/-) mice developed more severe splenomegaly, hypergammaglobulinemia, and immunodeficiency than infected B6 mice: PD-1(-/-) mice are more susceptible to lower doses of LP-BM5 and show more exaggerated disease early postinfection. LP-BM5-infected B6.IL-10(-/-) mice also develop exaggerated LP-BM5-induced disease, compared to B6 mice, without a significant change in the retroviral load. By reciprocal reconstitution experiments, comparing wild-type versus PD-1(-/-) sources of the requisite cells for LP-BM5 pathogenesis-CD4 T and B cells, PD-1(+) B cells appear to be crucial in the normal limitation of LP-BM5-induced disease in B6 mice. Also, infected B6 mice have increased CD11b(+) spleen cells that express interleukin-10 (IL-10). However, PD-1(-/-) mice, though showing an even greater expansion of CD11b(+) cells after LP-BM5 inoculation, did not show an equivalent increase in IL-10-producing cells. Thus, it appears that PD-1/PD-L interactions and IL-10 are primarily important in moderating the effects of LP-BM5-induced disease in B6 mice.
