Memory CD8+ T cell responses have been considered to be independent of CD80/CD86-CD28 costimulation. However, recall responses are often severely blunted in CD28−/− mice. Whether this impairment represents a requirement for CD28 costimulation for proper memory CD8+ T cell development or a requirement during the recall response is unknown. Furthermore, how CD28 costimulation affects the phenotype and function of memory CD8+ T cells has not been characterized in detail. In this study, we investigate these questions by studying the role of the CD28 costimulatory pathway in memory CD8+ T cell responses to acute and persistent DNA virus infections. Memory CD8+ T cells against vaccinia virus (VV) infection which develop without CD28 costimulation exhibit lower expression of differentiation markers CD27 and CD122 (IL-15Rβ). These memory CD8+ T cells also fail to produce IL-2. Our data indicate that for an optimal recall response, CD28 costimulation is required both for T cell priming and also during the recall response. Similar requirements were observed for memory CD8+ T cell responses during persistent infection with murine gammaherpesvirus 68 (MHV-68) infection, indicating CD28 may play the same role in both acute and persistent infections. Finally, we show deficits in the recall response are restored by IL-2 signaling during recall, but not during priming. The data presented show that CD28 costimulation not only controls the magnitude of the primary response but also affects development of memory CD8+ T cells and is required during the recall response in addition to initial T cell priming.

CD4 help is crucial for memory CD8+ T cell development, yet the mechanisms of CD4 help and why (CD4) helpless memory CD8+ T cells elicit poor recall responses are currently not well understood. In this study we investigated these questions using an in vivo acute virus infection model. We show herein that CD4 help during priming is required for memory CD8+ T cell differentiation, and that stimulation of CD40 during priming rescues the helpless defects in the absence of CD4+ T cells. The defective recall response by helpless memory cells did not correlate with the amount of cell death and was independent of TRAIL. However, helpless memory cells excessively up-regulated the inhibitory receptor PD-1 (programmed cell death-1), and PD-1 blockade enhanced the recall response of helpless memory cells. Furthermore, providing IL-2 signaling in vivo during the recall response reduced PD-1 expression and rescued the recall response of helpless memory cells. Our study identifies molecular pathways involved in CD4 help for memory CD8+ T cell generation that are independent of TRAIL, and it provides therapeutic implications that helpless memory cell function can be restored at multiple stages through various immunological interventions.

The interactions between CD80 and CD86 on antigen-presenting cells and CD28 on T cells serve as an important costimulatory signal in the activation of T cells. Although the simplistic two-signal hypothesis has been challenged in recent years by the identification of different costimulators, this classical pathway has been shown to significantly impact antiviral humoral and cellular immune responses. How the CD80/CD86-CD28 pathway affects the control of chronic or latent infections has been less well characterized. In this study, we investigated its role in antiviral immune responses against murine gammaherpesvirus 68 (MHV-68) and immune surveillance using CD80/CD86 ؊/؊ mice. In the absence of CD80/CD86, primary antiviral CD8 ؉ T-cell responses and the induction of neutralizing antibodies were severely impaired. During long-term immune surveillance, the virus-specific CD8 ؉ T cells were impaired in IFN-␥ production and secondary expansion and exhibited an altered phenotype. Surprisingly, a low level of viral reactivation in the lung was observed, and this effect was independent of CD28 and CTLA-4. Thus, CD80 and CD86, signaling through CD28 and possibly another unidentified receptor, are required for optimal immune surveillance and antiviral immune responses to murine gammaherpesvirus.

Identification of Toll-like receptors (TLRs)and their ligands, and tumor necrosis factor-tumor necrosis factor receptor (TNF-TNFR) pairs have provided the first logical, hypothesis-based strategies to molecularly concoct adjuvants to elicit potent cell-mediated immunity via activation of innate and adaptive immunity. However, isolated activation of one immune pathway in the absence of others can be toxic, ineffective, and detrimental to long-term, protective immunity. Effective engineered vaccines must include agents that trigger multiple immunologic pathways. Here, we report that combinatorial use of CD40 and TLR agonists as a cancer vaccine, compared with monotherapy, elicits high frequencies of selfreactive CD8 ؉ T cells, potent tumorspecific CD8 ؉ memory, CD8 ؉ T cells that efficiently infiltrate the tumor-burdened target organ; therapeutic efficacy; heightened ratios of CD8 ؉ T cells to FoxP3 ؉ cells at the tumor site; and reduced hepatotoxicity. These findings provide intelligent strategies for the formulation of multifactorial vaccines to achieve maximal efficacy in cancer vaccine trials in hu IntroductionThe molecular identification of Toll-like receptors (TLRs) and their ligands, as well as tumor necrosis factor (TNF)-tumor necrosis factor receptor (TNFR) pairs that control adaptive immunity, has provided the first logical, hypothesis-based strategies to molecularly concoct adjuvants that elicit potent cell-mediated immunity. Paralleling TLRs in mobilizing the innate immune response, CD40 and its ligand represent the primary ligand-receptor pair essential for development of the adaptive immune response. Individually, TLR agonists 1 and CD40 agonists 2-4 have entered clinical trials as adjuvants for eliciting protective immune responses to cancer. Inherent in these monotherapeutic approaches are limited induction of immunity, lack of clinical efficacy and, in some cases, hepatotoxicity. 3,4 TLRs are widely expressed on both hematopoietic and nonhematopoietic cells and elicit proinflammatory responses upon receptor engagement. Indeed, use of TLR agonists as solitary adjuvants triggers dendritic cell (DC) maturation, leukocyte migration, and release of chemokines and cytokines, and enhances immunity. 5,6 Studies in which TLR agonists have been scrutinized for their ability to induce cross-presentation and antigen-specific CD8 ϩ responses in vivo 7 show some level of activity that is minimal compared with that observed when combined with a CD40 agonist. 8,9 TLR agonists as unitary adjuvants in murine tumor models have demonstrated marginal efficacy, as reviewed, 10 but have proven effective when combined with other vaccine modalities. [11][12][13] Finally, clinical use of a TLR9 agonist in lung cancer trials has been recently suspended due to lack of clinical response. 1Studies from animal models underscore the utility of anti-CD40 (␣CD40) as a unitary adjuvant. 14,15 We previously demonstrated that the magnitude of immune responses elicited by TLR or CD40 agonists alone is minimal compared with the magnit...

In herpesvirus infections, the virus persists for life but is contained through T-cell-mediated immune surveillance. How this immune surveillance operates is poorly understood. Recent studies of other persistent infections have indicated that virus persistence is associated with functional deficits in the CD8 ؉ T-cell response. To test whether this is the case in a herpesvirus infection, we used a mutant murine gammaherpesvirus that is defective in its ability to persist in the host. By comparing the immune response to this virus with a revertant virus that can persist, we were able to dissect the changes in the antiviral CD8 ؉ T-cell response that are induced by virus persistence. Surprisingly, persistently infected mice controlled a secondary challenge infection more rapidly than nonpersistently infected mice, indicating enhanced rather than diminished effector functions. Consistent with this, virus-specific CD8 T cells from these mice exhibited faster upregulation of the cytotoxic mediator granzyme B. Another unexpected finding was that CD8 ؉ T cells from neither infection responded efficiently to homeostatic cytokines. The unresponsiveness of the memory cells from the nonpersistently infected mice appears to be linked to the prolonged replication of virus within the lungs. Other changes seen in different chronic infection models were also observed, such as changes in Bcl-2 levels, interleukin-2 production, and the immunodominance hierarchy. These data show persistence of gammaherpesvirus type 68 alters the properties of CD8 ؉ T cells and illustrates that immune surveillance does not require CD8 T cells with the same attributes as "classical" memory CD8 ؉ T cells.