CD8+ T cells are a critical component of the adaptive immune response against infections and tumors. A current paradigm in immunology is that naive CD8+ T cells require CD28 costimulation, whereas memory CD8+ T cells do not. We show here, however, that during viral infections of mice, costimulation is required in vivo for the reactivation of memory CD8+ T cells. In the absence of CD28 costimulation, secondary CD8+ T cell responses are greatly reduced and this impairs viral clearance. The failure of CD8+ T cells to expand in the absence of CD28 costimulation is CD4+ T cell help independent and is accompanied by a failure to down-regulate Bcl-2 and by cell cycle arrest. This requirement for CD28 costimulation was shown in both influenza A and HSV infections. Thus, contrary to current dogma, memory CD8+ T cells require CD28 costimulation to generate maximal secondary responses against pathogens. Importantly, this CD28 requirement was shown in the context of real infections were multiple other cytokines and costimulators may be up-regulated. Our findings have important implications for pathogens, such as HIV and measles virus, and tumors that evade the immune response by failing to provide CD28 costimulation. These findings also raise questions about the efficacy of CD8+ T cell-based vaccines against such pathogens and tumors.

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The precise role of each of the seven individual CD11c؉ dendritic cell subsets (DCs) identified to date in the response to viral infections is not known. DCs serve as critical links between the innate and adaptive immune responses against many pathogens, including herpes simplex virus type 1 (HSV-1). The role of DCs as mediators of resistance to HSV-1 infection was investigated using CD11c-diphtheria toxin (DT) receptor-green fluorescent protein transgenic mice, in which DCs can be transiently depleted in vivo by treatment with low doses of DT. We show that ablation of DCs led to enhanced susceptibility to HSV-1 infection in the highly resistant C57BL/6 mouse strain. Specifically, we showed that the depletion of DCs led to increased viral spread into the nervous system, resulting in an increased rate of morbidity and mortality. Furthermore, we showed that ablation of DCs impaired the optimal activation of NK cells and CD4؉ and CD8 ؉ T cells in response to HSV-1. These data demonstrated that DCs were essential not only in the optimal activation of the acquired T-cell response to HSV-1 but also that DCs were crucial for innate resistance to HSV-1 infection.Based on serological evidence, it is estimated that 60 to 80% of the adult population is infected with herpes simplex virus type 1 (HSV-1) (46). Most infected individuals remain asymptomatic. Of symptomatic individuals, clinical presentation ranges from mild illness, such as the development of orofacial vesicular lesions, all the way to life-threatening systemic complications, such as hepatitis and encephalitis (39). The outcome of infection is known to be influenced by both specific and nonspecific genetically linked host defense mechanisms (32). The immune system is particularly important in controlling HSV-1 infection in both the periphery and the nervous system, although the events that initiate this immunity in humans are not very well understood. Underlying immunosuppression does not appear to explain the distinct outcomes of host-virus interaction. Rather, the observed range in clinical outcomes appears to reflect differences in intrinsic resistance to infection.Studies using inbred strains of mice have revealed critical insights into the immunological basis for resistance to . Specifically, a range in innate resistance to systemic, lethal HSV-1 infection exists and has subsequently led to the grouping of inbred mice into resistant, moderately susceptible, and susceptible categories based upon the levels of virus required to cause death. In all cases examined, mice of the C57 background (C57BL/6 and C57BL/10) are most resistant and best able to effectively control HSV-1 infection (35).Human and murine studies suggest that the innate immune response, specifically the ability to produce elevated levels of type I interferon (IFN) at early time points, provides a threshold of resistance to acute HSV-1 infection (44).

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