PD-1 controls effectors but not the generation or function of natural or induced regulatory T-cells (P1011)

Abstract

Abstract During T cell activation, the balance of co-stimulatory and co-inhibitory interactions plays an important role in shaping the resulting T cell response. Co-inhibitory signals help maintain peripheral immune tolerance, and thus animals with defective co-inhibitory molecules, such as PD-1 KO mice, are predisposed to autoimmunity. Unlike PD-1 KO mice, which develop mild autoimmunity, adult Rag KO recipients of PD-1 KO hematopoietic stem cells (HSC) succumb to rapid and severe multi-organ autoimmune disease soon after T cells first emerge from the thymus, implicating PD-1 as a critical factor in the control of T cell self-reactivity during lymphopenia induced homeostatic proliferation (LIP). While we have found no reduction in the absolute number of FoxP3+ regulatory T cells (Treg) in diseased PD-1 KO vs. wild type (WT) HSC recipients, some studies have indirectly suggested that PD-1 deficiency may inhibit the development of induced Treg (iTreg) cells from conventional T cells in the periphery. We therefore investigated whether PD-1 deficiency results in an intrinsic inability of T cells to convert to iTreg in vivo, which could lead to disease in PD-1 KO HSC recipients. Surprisingly, our findings suggest that PD-1 deficiency does not inhibit the development of iTreg nor does it affect Treg suppressive function, and that in the context of LIP, co-inhibitor deficient effector T cells cause disease primarily by expanding beyond the ability of Treg to control them.