The Role of Early Autocrine IL-2 Signals in Programming Antigen-Specific CD8 T cell Responses

Abstract

IL-2 is a potent cytokine in mediating antigen-specific CD8 T cell responses. Following antigen priming, IL-2 signals drive the differentiation of resting cytotoxic T cells into potent effector cells to elicit rapid expansion, and the expression of effector molecules critical for eradicating intracellular infections. Antigen-specific CD8 T cells more sensitive to early IL-2 signals undergo pronounced effector expansion as they differentiate into terminal effector cells that subsequently undergo substantial contraction following antigen clearance. Conversely, cells expressing lower levels of the high affinity IL-2 receptor, CD25, exhibit lesser expansion and preferentially form long-lived memory CD8 T cells, which provide protection from future infections. Interestingly, one of the hallmark features of these memory CD8 T cells is their retained function of autocrine IL-2 production. This suggests that autocrine IL-2 is intrinsically linked to the differentiation and maintenance of effector and memory cells. In our studies we have utilized a murine model of acute infection in which we studied the effects of autocrine IL-2 ablation on antigen-specific CD8 T cells. We were able to show that autocrine IL-2 signals were dispensable to antigen-specific CD8 T cells for mounting a primary immune response; the T cells showed no defect in effector expansion or function, and were capable of forming long-lived, polyfunctional memory CD8 T cells. However, upon antigenic rechallenge memory cells deficient in autocrine IL-2 signals underwent increased cell-death resulting in a compromised secondary response. We then used temporal ablation of autocrine IL-2 to determine that these signals were critical during the initial priming of antigen-specific CD8 T cells. Unlike acute infections, where antigen is cleared by the effector response, during chronic infections antigen persists in the host exhausting the antigen-specific T cells via constant stimulation. However, the initial priming events are similar in both acute and chronic IL-2 infections. Thus, we wanted to determine if autocrine IL-2 signals also program survival in CD8 T cells in a system where the cells are continuously stimulated with antigen. We discovered that in the absence of autocrine IL-2, antigen-specific CD8 T cells undergo lower expansion but are not affected in their ability to elicit effector function. Following establishment of exhaustion, CD8 T cells lacking autocrine IL-2 over-expressed inhibitory receptor PD-1, and had a pronounced defect in maintenance. Surprisingly, T cells lacking autocrine IL-2 underwent significantly better expansion following PDL1 checkpoint blockade therapy, expanding to levels similar to their WT counterparts. These data show for the first time a critical role for autocrine IL-2 during chronic infections. Furthermore, this research suggests that it is possible to tune exhausted CD8 T cells using IL-2 signals, to boost their responsiveness to checkpoint blockade therapy.