In persistent viral infections, the host's immune system is challenged by the constant exposure to antigen, potentially causing continuous activation of CD8 ؉ T cells with subsequent immunopathology. Here we demonstrate, for experimental chronic lymphocytic choriomeningitis virus and human HIV infection, that upon prolonged in vivo exposure to antigen, TCR-triggered Ca 2؉ flux, degranulation, and cytotoxicity are maintained on a cellular level, whereas cytokine production is severely impaired because of a selective defect in activation-induced NFAT nuclear translocation. During chronic infection, this differential regulation of pathways leading to diverse effector functions may allow CD8 ؉ T cells to sustain some degree of local viral control by direct cytotoxicity while limiting systemic immune pathology by silencing cytokine production.dysfunction ͉ cytotoxic T lymphocyte ͉ persistent infection ͉ cytokine production

Chronic viral infections are often characterized by CD8 T-cell responses with poor cytokine secretion potential and limited expansion of the CD8 T-cell pool, collectively referred to as CD8 T-cell exhaustion. Exhaustion of lymphocytic choriomeningitis virus (LCMV)-specific CD8 T cells was shown to be partially regulated by the inhibitory receptor programmed death 1 (PD-1). Here, we demonstrate that exhausted LCMV-specific CD8 T cells also express the negative regulatory receptor lymphocyte activation gene 3 (LAG-3) which is mainly expressed on cells co-expressing the negative regulatory receptors PD-1 and Tim-3. Expression levels of LAG-3 on anti-viral CD8 T cells remain stable over short-term in vitro stimulations in presence of antigenic peptide. Nevertheless, in vitro and in vivo blockade of LAG-3 did not rescue cytokine production by virus-specific CD8 T cells and did not alter the virus titer in various organs. Likewise, chronic LCMV infection of LAG-3-/- mice led to a comparable degree of T-cell exhaustion as observed in C57BL/6 controls and to similar virus titers. Further, LAG-3 did not influence T-cell activation or cell division during chronic LCMV infection. These data suggest that even though LAG-3 is continuously up-regulated on LCMV-specific exhausted CD8 T cells, it alone does not significantly contribute to T-cell exhaustion.

One Sentence SummaryWe demonstrate that continued presence of CXCR5 +/+ TFH cells throughout chronic viral infection is dispensable for maintenance of overall virus-specific antibody titers but is vital for the generation of virus-neutralizing antibodies and eventual control of the infection.

CD8 + T cells play a crucial role in controlling intracellular pathogens. The level of memory CD8 + T cells developing after vaccination or infection influences the degree of T cell-mediated protection after secondary infection. We used defined animal models and infections/immunizations by replicating or non-replicating antigens to define on a molecular and cellular level in vivo the parameters that identify and shape long-lived CD8 + T cell memory. We show that the timing of antigen exposure during vaccination is key for the induction of long-lived T cell memory. Brief antigen exposure induced high numbers of effector cells but limited development of long-lived CD8 + memory T cells. In contrast, prolonged antigen exposure for up to 9 days induced similar numbers of effector T cells but additionally resulted in high levels of memory CD8 + T cells. Unexpectedly CD127 (IL-7Ra) expression on CD8 + T cells during the acute priming phase was a necessary but not sufficient requirement for entering the pool of long-lived antigen-independent memory CD8 + T cells. However, we provide strong evidence for the interpretation that programming of long-lived memory T cells was driven by low levels of transcription factor eomesodermin and protease inhibitor Spi2A as well as reduced phosphorylation of c-JUN.

The requirements for the generation of fully competent long-lived memory CD8 T cells and in particular the role and the mechanisms of help from CD4 T cells remain ill-defined. Memory CD8 T cells generated in the absence of CD4 T cell help often have an impaired recall proliferation and are thus unable to confer protection against certain pathogens. However, the timing and the mechanisms involved in the delivery of help are still unclear and differ between various experimental systems. In this study, we investigated the role of CD4 T help in generating memory CD8 T cells in a defined heterologous prime-boost system, consisting of priming with replication incompetent virus-like particles and challenge with recombinant vaccinia virus, both sharing only a common lymphocytic choriomeningitis virus-derived CD8 T cell epitope. We show in this system that delivery of help is only essential during the challenge phase for recall proliferation of memory CD8 T cells. Furthermore, we show that generation of proliferation-competent memory CD8 T cells is independent of CD40 and CCR5 and that in vivo IL-2 supplementation neither during priming nor during challenge was able to rescue recall proliferation of “unhelped” memory CD8 T cells.