mTORC1 Inhibition Protects Human Regulatory T Cells From Granzyme-B-Induced Apoptosis

Regulatory T cells (T_regs) have shown great promise as a means of cellular therapy in a multitude of allo- and auto-immune diseases-due in part to their immunosuppressive potency. Nevertheless, the clinical efficacy of human T_regs in patients has been limited by their poor in vivo homeostasis. To avert apoptosis, T_regs require stable antigenic (CD3ζ/T-cell-receptor-mediated), co-stimulatory (CD28-driven), and cytokine (IL-2-dependent) signaling. Notably, this sequence of signals supports an activated T_reg phenotype that includes a high expression of granzymes, particularly granzyme B (GrB). Previously, we have shown that aside from the functional effects of GrB in lysing target cells to modulate allo-immunity, GrB can leak out of the intracellular lysosomal granules of host T_regs, initiating pro-apoptotic pathways. Here, we assessed the role of inhibiting mechanistic target of rapamycin complex 1 (mTORC1), a recently favored drug target in the transplant field, in regulating human T_reg apoptosis via GrB. Using ex vivo models of human T_reg culture and a humanized mouse model of human skin allotransplantation, we found that by inhibiting mTORC1 using rapamycin, intracytoplasmic expression and functionality of GrB diminished in host T_regs; lowering human T_reg apoptosis by in part decreasing the phosphorylation of S6K and c-Jun. These findings support the already clinically validated effects of mTORC1 inhibition in patients, most notably their stabilization of T_reg bioactivity and in vivo homeostasis.