Regulation of intrinsic activation thresholds of T cells

Abstract

T cells are activated when their T cell receptor (TCR) senses peptide-MHC (pMHC) molecules from pathogens and tumors. A network of signaling molecules downstream of the TCR drives the extent and nature of subsequent cellular responses. The activation threshold (AT) of these signaling pathways is a critical checkpoint for T cell responses. Here, we examined mechanisms by which the AT of a T cell is first determined and how it changes during the course of responding to antigen. The initial AT of a T cell is set during development by calibrating to how strongly it senses pMHC in the thymus. This calibration affects the surface levels of a receptor CD5, whose subsequent role is poorly defined. We found that CD5, independent of the TCR, sets basal levels of IκBα in T cells. Since IκBα critically modulates the transcription factor NFκB, which regulates multiple T cell functions including cell-survival, we hypothesized that variations in basal AT of T cells stem from varying NFκB depots maintained by CD5. Indeed, blocking NFκB abolished differences in cell-survival of thymocytes with different CD5 levels. The initial heterogeneities are further modified when peripheral T cells encounter antigen. Resulting memory T cells acquired higher CD5 levels and continuously required CD5 expression to maintain higher IκBα expression. If the stimulating antigen was not cleared efficiently, peripheral T cells further ‘tuned’ their AT in the opposite direction and resulted in loss of sensitivity to antigen (as seen in exhausted T cells). Importantly, this AT tuning involved additional regulation of TCR-proximal kinases such as Zap70 and was reversible in vivo, but not in vitro. We also found that rather than just the duration of antigen exposure, AT-tuning was potentially influenced by the rate at which antigen changes in vivo. This can help us understand how different persistent pathogens or tumors affect T cell responses, potentially based on the rates at which they replicate in the host. Finally, we characterized compounds that can target a T cell’s AT, identified in a high-throughput pharmacological screen, to potentially isolate drugs for altering T cell function during these physiological contexts.