It is well known that tumors can be “seen” by the immune system, but immune responses differ depending on the nature and context of the target antigen: tumor antigens that are self-proteins are generally weakly immunogenic due to pre-existing self-tolerance, whereas tumor antigens that are truly tumor-specific (viral and mutated proteins) are potentially highly immunogenic because the immune system has not been previously exposed to these antigens. Therefore, tumor-specific T cells that recognize these antigens as non-self should be able to eliminate cancer cells expressing such tumor-specific neo-antigens. We developed a spontaneous Tamoxifen-inducible cancer mouse model to investigate the fate of naïve tumor-specific T cells encountering a tumor-specific neo-antigen during the pre-malignant phase of tumor development. We found, contrary to our expectation, that tumor-specific T cells were rendered tolerant and unresponsive to the cancer cell as early as 8 days post-tumor initiation and displayed very similar phenotypic and functional characteristics compared to self-tolerant T cells. Interestingly, this unresponsiveness was not due to global tumor-induced immune suppression in the microenvironment but instead the result of continuous antigen encounter, as tumor-infiltrating control T cells specific for an antigen not expressed by the cancer cell remained functional in the tumor site. Transcriptional profiling uncovered the molecular program underlying this early cancer antigen-induced T cell dysfunction. Thus, an antigenic stimulus in a non-inflammatory context, whether by a self-antigen or tumor-specific antigen, induces a distinct state of “tolerance-specific” functional unresponsiveness. While many immunotherapies aim to reverse the immune suppression mediated by the tumor microenvironment, we demonstrate that there is another level of antigen-specific, cell-intrinsic dysfunction that must also be overcome.

The blood-stage of infection with the parasite Plasmodium induces clinical malaria. Murine infection with Plasmodium chabaudi provides a physiologically relevant model of the immune response to infection. Blood-stage parasites express Merozoite-Surface Protein (MSP) 1, which undergoes a series of proteolytic cleavage events. The remaining c-terminal MSP1-19 region is necessary for erythrocyte endocytosis and serves as an immunogenic target. Control of parasitemia and resolution of clinical malaria has been shown to be T cell dependent, both through secretion of signaling molecules and through activation of B cells. To understand how T cell populations develop in a manner specific to the parasite and the contribution they make to the overall immune response, we have developed a novel system to identify MSP1-19-specific CD4+ T cells using a peptide: major histocompatibility class II (pMHCII) tetramer of our own design and magnetic bead-based enrichment techniques. Enriched fractions are stained with fluorescent antibodies against phenotypic surface markers and analyzed by flow cytometry to identify previously described CD4+ cellular subsets. T follicular helper (Tfh) cells that are required for B cell antibody production are formed around day 8 post-infection near the peak of parasitemia. Surprisingly, the number of Tfh cells decreases between days 12-15 post-infection and begins to return by day 20, persisting during chronic, low-level infection. We hypothesize that Tfh cell death is driven by high levels of antigen presentation by B cells. To address this hypothesis, we will assess markers of cell death in both TCR signaling reporters and various transgenic mice that lack important CD4+ T cell signaling molecules. Characterization of T cell depletion could elucidate mechanisms of immune evasion critical to parasite survival and persistence of infection, possibly implicating new therapeutic targets.