Abstract
There is now overwhelming evidence that tumor-induced antigen (Ag)-specific T cell tolerance represents a critical problem in tumor immunology. Early studies of CD8 T cell tolerance equated peripheral tolerance with either ignorance or clonal deletion, although more recent evidence has suggested that this may be only partly accurate. While murine modeling outwardly supports the contention that high-affinity tumor-specific CD8 T cell responses are centrally deleted, cognate CD8 T cells displaying an Ag-experienced phenotype can nonetheless be detected in regional draining lymph nodes (dLN) or in non-lymphoid sites where the Ag is present. However, these CD8 T cells are typically deficient in one or more effector functions, including cytokine production, cytotoxicity, or proliferative capacity. To better define the state of Ag-specific CD8 T cell responsiveness in the face of progressive tumor, we adoptively transferred hemagglutinin (HA) Ag-specific Clone 4 (CLN4) CD8 T cells into animals bearing a genetically modified B cell lymphoma expressing HA as a model tumor antigen (A20HA). Analysis of the fate and function of these transferred antigen-specific CD8 T cells revealed that they encountered antigen in vivo, were capable of mounting an initial response to A20HA but this response was not sustained. Indeed, while a prominent CTL activation was observed in the spleen and draining lymph nodes of tumor bearing mice within 14 days of T cell transfer, responses (HA-specific proliferation, IFN-γ production and cytotoxicity) began to wane by day 21 after T cell transfer, and in particular their ability to produce IFN-γ. A similar pattern of transient activation followed by loss of CD8 T cell function has been also observed in an in vivo model of high-dose peptide induced antigen-specific CD8 T cell tolerance. Given our recent demonstration that the disruption of Stat3 signaling in APCs overcomes CD4 T cell tolerance we determined next whether Stat3 deficient APCs may be inherently better at cross-presenting tumor-Ags and elicit therefore a more productive and sustained CD8 T cell response. In an in vitro system in which tumor cells expressing a model tumor antigen (EL4mOVA) were cultured with APCs genetically devoid of Stat3 signaling and anti-OVA CD8 T-cells (OT-I), we found that these T cells displayed an enhanced function relative to antigen-specific CD8 T-cells that encountered antigen on APCs with an intact Stat3 signaling. Currently, we are investigating whether CD8 T-cell tolerance to tumor antigens occurred -or not- in tumor bearing mice with a genetic disruption of Stat3 signaling in APCs. Furthermore, given the emerging role of other members of the STAT family in regulation of APC function, we are exploring whether targeted disruption of Stat1, 4 and 6 can alter the ability of the CD8 T-cell to sustain a protective response or, more importantly to recover function once tolerance is induced.
CD8+ T Cell Tolerance to a Tumor-associated Antigen Is Maintained at the Level of Expansion Rather than Effector Function