Memory Inflation during Chronic Viral Infection Is Maintained by Continuous Production of Short-Lived, Functional T Cells

SummaryDuring persistent murine cytomegalovirus (MCMV) infection, the T cell response is maintained at extremely high intensity for the life of the host. These cells closely resemble human CMV-specific cells, which compose 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 divided only sporadically in chronically infected hosts and had a short half-life in circulation. The overall population was supported, at least in part, by memory T cells primed early in infection, as well as by recruitment of naive T cells at late times. Thus, these data show that memory inflation is maintained by a continuous replacement of short-lived, functional cells during chronic MCMV infection

A Contribution of Mouse Dendritic Cell–Derived IL-2 for NK Cell Activation

Dendritic cells (DCs) play a predominant role in activation of natural killer (NK) cells that exert their functions against pathogen-infected and tumor cells. Here, we used a murine model to investigate the molecular mechanisms responsible for this process. Two soluble molecules produced by bacterially activated myeloid DCs are required for optimal priming of NK cells. Type I interferons (IFNs) promote the cytotoxic functions of NK cells. IL-2 is necessary both in vitro and in vivo for the efficient production of IFNγ, which has an important antimetastatic and antibacterial function. These findings provide new information about the mechanisms that mediate DC–NK cell interactions and define a novel and fundamental role for IL-2 in innate immunity

Activation of Natural Killer (NK) T Cells during Murine Cytomegalovirus Infection Enhances the Antiviral Response Mediated by NK Cells

NK1.1(+) T (NKT) cells are efficient regulators of early host responses which have been shown to play a role in tumor surveillance. The relevance of NKT cells in immune surveillance of viral infections, however, is not well understood. In this study, we investigated the functional relevance of NKT cells in controlling herpesvirus infections by using challenge with murine cytomegalovirus (MCMV) as the study model. This model has proven to be one of the best systems for evaluating the role of NK cells during virus infection. Using gene-targeted mice and α-galactosylceramide (α-GalCer) as an exogenous stimulator of NKT cells, we have analyzed the role of these cells in the immune surveillance of MCMV infection. Our studies in NKT-cell-deficient, T-cell receptor Jα281 gene-targeted mice have established that classical NKT cells do not play a critical role in the early clearance of MCMV infection. Importantly, however, activation of NKT cells by α-GalCer resulted in reduced viral replication in visceral organs. Depletion studies, coupled with analysis of gene-targeted mice lacking perforin and gamma interferon (IFN-γ), have revealed that the antiviral effects of α-GalCer involve NK cells and have clearly demonstrated that the antiviral activity of α-GalCer, unlike the antitumor one, is critically dependent on both perforin and IFN-γ

The roles of interferon-γ and perforin in antiviral immunity in mice that differ in genetically determined NK-cell-mediated antiviral activity

The design of effective antiviral immunotherapies depends on a detailed understanding of the cellular and molecular processes involved in generating and maintaining immune responses. Control of cytomegalovirus (CMV) infection requires the concerted activities of both innate and adaptive immune effectors. In the mouse, immunity to acute murine CMV (MCMV) infection depends on natural killer (NK) cells and/or CD8 T cells. The relative importance of NK and CD8 T cells varies in different mouse strains. In C57BL/6 mice, early viral infection is controlled by Ly49H NK cells, whereas in BALB/c mice, CD8 T cells exert the principal antiviral activities. Although the role of NK and CD8 T cells is defined, the molecular mechanisms they utilize to limit acute infection are poorly understood. Here, we define the specific roles of perforin (pfp) and interferon-γ (IFN-γ) in the context of NK- or T-cell-mediated immunity to MCMV during acute infection. We show that pfp is essential for both NK- and T-cell-mediated antiviral immunity during the early stages of infection. The relative importance of IFN-γ is more pronounced in Ly49H mice. Using BALB/c background mice congenic for Ly49H and lacking pfp, we show that Ly49H-regulated NK-cell control of MCMV infection is dependent on pfp-mediated cytolysis

Cross-talk between dendritic cells and natural killer cells in viral infection

Dendritic cells (DC), first characterized in 1973 by Steinman and Cohn, have been defined as the professional antigen presenting cells (APC), capable of activating naïve T cells much more efficiently than either B cells or macrophages. DC also capture and process antigen more efficiently than other APC, and offer MHC-antigen complexes to T cells at higher densities, and in the context of larger amounts of co-stimulatory molecules (i.e. CD40, CD80 and CD86) at the T cell-DC synapse. Although historically, the principal function of DC is the priming of naïve T cells, more recently they have also been shown to affect the functions of natural killer (NK) cells. Interactions between DC and NK cells may be critical in situations where immune surveillance requires efficient early activation of NK cells, as is the case during infections. This review aims to summarise the interactions that occur between DC and NK cells during viral infection

Functional analysis of granzyme M and its role in immunity to infection

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