Summary During persistent murine cytomegalovirus (MCMV) infection the T cell response is maintained at extremely high levels for the life of the host. These cells closely resemble human CMV-specific cells which comprise a major component of the peripheral T cell compartment in most people. Despite a phenotype that suggests extensive antigen-driven differentiation, MCMV-specific T cells remain functional and respond vigorously to viral challenge. We hypothesized that a low rate of antigen-driven proliferation would account for the maintenance of this population. Instead, we found that most of these cells divide only sporadically in chronically infected hosts and have a short half-life in circulation. The overall population is supported, at least in part, by memory cells primed early in infection as well as recruitment of naïve T cells at late times. These data show that memory inflation is maintained by a continuous replacement of short-lived, functional cells during chronic MCMV infection.

Central tolerance is established through negative selection of self-reactive thymocytes and the induction of T-regulatory cells (T R s). The role of thymic dendritic cells (TDCs) in these processes has not been clearly determined. In this study, we demonstrate that in vivo , TDCs not only play a role in negative selection but in the induction of T R s. TDCs include two conventional dendritic cell (DC) subtypes, CD8 lo Sirpα hi/+ (CD8 lo Sirpα + ) and CD8 hi Sirpα lo/− (CD8 lo Sirpα − ), which have different origins. We found that the CD8 hi Sirpα + DCs represent a conventional DC subset that originates from the blood and migrates into the thymus. Moreover, we show that the CD8 lo Sirpα + DCs demonstrate a superior capacity to induce T R s in vitro . Finally, using a thymic transplantation system, we demonstrate that the DCs in the periphery can migrate into the thymus, where they efficiently induce T R generation and negative selection.

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Cytotoxic lymphocytes express a large family of granule serine proteases, including one member, granzyme (Grz)M, with a unique protease activity, restricted expression, and distinct gene locus. Although a number of Grzs, including GrzM, have been shown to mediate target cell apoptosis in the presence of perforin, the biological activity of Grz has been restricted to control of a number of viral pathogens, including two natural mouse pathogens, ectromelia, and murine CMV (MCMV). In this article, we describe the first reported gene targeting of GrzM in mice. GrzM-deficient mice display normal NK cell/T cell development and homeostasis and intact NK cell-mediated cytotoxicity of tumor targets as measured by membrane damage and DNA fragmentation. GrzM-deficient mice demonstrated increased susceptibility to MCMV infection typified by the presence of more viral inclusions and transiently higher viral burden in the visceral organs of GrzM-deficient mice compared with wild-type (WT) mice. The cytotoxicity of NK cells from MCMV-infected GrzM-deficient mice remained unchanged and, like WT control mice, GrzM-deficient mice eventually effectively cleared MCMV infection from the visceral organs. In contrast, GrzM-deficient mice were as resistant as WT control mice to mouse pox ectromelia infection, as well as challenge with a number of NK cell-sensitive tumors. These data confirm a role for GrzM in the host response to MCMV infection, but suggest that GrzM is not critical for NK cell-mediated cytotoxicity.