Tumor Tolerance-Promoting Function of Regulatory T Cells Is Optimized by CD28, but Strictly Dependent on Calcineurin

Regulatory T cells (Treg) restrain immune responses against malignant tumors, but their global depletion in cancer patients will likely be limited by systemic autoimmune toxicity. Instead, approaches to "tune" their activities may allow for preferential targeting of tumor-reactive Treg. Although Ag recognition regulates Treg function, the roles of individual TCR-dependent signaling pathways in enabling Treg to promote tumor tolerance are not well characterized. In this study, we examined in mouse tumor models the role of calcineurin, a key mediator of TCR signaling, and the role of the costimulatory receptor CD28 in the differentiation of resting central Treg into effector Treg endowed with tumor tropism. We find that calcineurin, although largely dispensable for suppressive activity in vitro, is essential for upregulation of ICOS and CTLA-4 in Treg, as well as for expression of chemokine receptors driving their accumulation in tumors. In contrast, CD28 is not critical, but optimizes the formation of tumor-homing Treg and their fitness in tumor tissue. Accordingly, although deletion of either CnB or CD28 strongly impairs Treg-mediated tumor tolerance, lack of CnB has an even more pronounced impact than lack of CD28. Hence, our studies reveal distinct roles for what has classically been defined as signal 1 and signal 2 of conventional T cell activation in the context of Treg-mediated tumor tolerance.