The intestinal mucosal CD8 T cell response to infection with Listeria monocytogenes was measured using MHC class I tetramers and was compared with the response in peripheral blood, secondary lymphoid tissue, and liver. To assess the vaccination potential of Listeria and to analyze responses in C57BL/6 mouse strains, a recombinant Listeria expressing OVA (rLM-ova) was generated. The response peaked at 9 days postinfection with a much larger fraction of the intestinal mucosa and liver CD8 T cell pool OVA specific, as compared with the spleen. However, these differences were not linked to bacterial titers in each site. The higher responses in lamina propria and liver resulted in a larger CD8 memory population in these tissues. Furthermore, the level of memory induced was dependent on infectious dose and inversely correlated with the magnitude of the recall response after oral challenge. Recall responses in the tissues were most robust in the lamina propria and liver, and reactivated Ag-specific T cells produced IFN-γ. Infection of CD40- or MHC class II-deficient mice induced poor CD8 T cell responses in the intestinal mucosa, but only partially reduced responses in the spleen and liver. Overall, the results point to novel pathways of tissue-specific regulation of primary and memory antimicrobial CD8 T cell responses.

CD8 T cell cross-reactivity between viruses can play roles in protective heterologous immunity and damaging immunopathology. This cross-reactivity is sometimes predictable, such as between lymphocytic choriomeningitis virus (LCMV) and Pichinde virus, where cross-reactive epitopes share six out of eight amino acids. Here, however, we demonstrate more subtle and less predictable cross-reactivity between LCMV and the unrelated vaccinia virus (VV). Epitope-specific T cell receptor usage differed between individual LCMV-infected C57BL/6 mice, even though the mice had similar epitope-specific T cell hierarchies. LCMV-immune mice challenged with VV showed variations, albeit in a distinct hierarchy, in proliferative expansions of and down-regulation of IL-7Rα by T cells specific to different LCMV epitopes. T cell responses to a VV-encoded epitope that is cross-reactive with LCMV fluctuated greatly in VV-infected LCMV-immune mice. Adoptive transfers of splenocytes from individual LCMV-immune donors resulted in nearly identical VV-induced responses in each of several recipients, but responses differed depending on the donor. This indicates that the specificities of T cell responses that are not shared between individuals may influence cross-reactivity with other antigens and play roles in heterologous immunity upon encounter with another pathogen. This variability in cross-reactive T cell expansion that is unique to the individual may underlie variation in the pathogenesis of infectious diseases.

The main functions of memory T cells are to provide protection upon re-exposure to a pathogen and to prevent the re-emergence of low-grade persistent pathogens. Memory T cells achieve these functions through their high frequency and elevated activation state, which lead to rapid responses upon antigenic challenge. The significance and characteristics of memory CD8+ T cells in viral infections have been studied extensively. In many of these studies of T-cell memory, experimental viral immunologists go to great lengths to assure that their animal colonies are free of endogenous pathogens in order to design reproducible experiments. These experimental results are then thought to provide the basis for our understanding of human immune responses to viruses. Although these findings can be enlightening, humans are not immunologically naïve, and they often have memory T-cell populations that can cross-react with and respond to a new infectious agent or cross-react with allo-antigens and influence the success of tissue transplantation. These cross-reactive T cells can become activated and modulate the immune response and outcome of subsequent heterologous infections, a phenomenon we have termed heterologous immunity. These large memory populations are also accommodated into a finite immune system, requiring that the host makes room for each new population of memory cell. It appears that memory cells are part of a continually evolving interactive network, where with each new infection there is an alteration in the frequencies, distributions, and activities of memory cells generated in response to previous infections and allo-antigens.

Class II-restricted CD4 T cell-mediated killing of target cells has previously been documented in vitro but not in vivo. In this study, we demonstrate CD4-dependent MHC class II-restricted killing in lymphocytic choriomeningitis virus-infected mice in vivo using an in vivo cytotoxicity assay that features class II-expressing B cells as targets.

Here we demonstrate complex networks of CD8 T-cell cross-reactivities between influenza A virus (IAV) and Epstein- Barr virus (EBV) in humans and between lymphocytic choriomeningitis virus (LCMV) and vaccinia virus (VV) in mice. We also show directly that cross-reactive T-cells mediate protective heterologous immunity in mice. Subsets of T-cell populations reactive with one epitope cross-reacted with either of several other epitopes encoded by the same or the heterologous virus. Human T-cells specific to EBV-encoded BMLF1280-288 could be cross-reactive with two IAV or two other EBV epitopes. Mouse T-cells specific to the VV-encoded a11r198-205 could be cross-reactive with three different LCMV, one Pichinde virus, or one other VV epitope. Patterns of cross-reactivity differed among individuals, reflecting the private specificities of the host’s immune repertoire, and divergence in the abilities of T-cell populations to mediate protective immunity. Defining such cross-reactive networks between commonly encountered human pathogens may facilitate the design of vaccines.