Regulatory T cells are critical for the generation and maintenance of peripheral tolerance. Conditional deletion of the transcriptional repressor NKAP in Tregs using Foxp3-YFP-cre NKAP conditional knockout mice causes aggressive autoimmunity characterized by thymic atrophy, lymphadenopathy, peripheral T cell activation, generation of autoantibodies, immune infiltration into several organs, and crusty skin at 3 weeks of age, similar to that of "scurfy" Foxp3-mutant mice. While Treg development in the thymus proceeds normally in the absence of NKAP, there is a severe loss of thymically-derived Tregs in the periphery. NKAP-deficient Tregs have a recent thymic emigrant phenotype, and are attacked by complement in a cell-intrinsic manner in the periphery. Previously, we demonstrated that NKAP is required for conventional T cell maturation as it prevents complement-mediated attack in the periphery. We now show that Tregs undergo a similar maturation process as conventional T cells, requiring NKAP to acquire complement resistance after thymic egress.

The transcription factor Runx1 has essential roles throughout hematopoiesis. Here, we demonstrate that Runx1 is critical for T cell maturation. Peripheral naïve CD4+ T cells from CD4-cre Runx1 cKO mice are phenotypically and functionally immature as shown by decreased production of TNF-α upon TCR stimulation. The loss of peripheral CD4+ T cells in CD4-cre Runx1 cKO mice is not due to defects in homeostasis or decreased expression of IL-7Rα, as transgenic expression of IL-7Rα does not rescue the loss of CD4+ T cells. Rather, immature Runx1-deficient CD4+ T cells are eliminated in the periphery by the activation and fixation of the classical complement pathway. In the thymus, there is a severe block in all aspects of intrathymic T cell maturation, although both positive and negative selection are unaltered. Thus, loss of Runx1 leads to the earliest characterized block in post-positive selection intrathymic maturation of CD4 T cells.

Recent thymic emigrants that fail postpositive selection maturation are targeted by complement proteins. T cells likely acquire complement resistance during maturation in the thymus, a complement-privileged organ. To test this, thymocytes and fresh serum were separately obtained and incubated together in vitro to assess complement deposition. Complement binding decreased with development and maturation. Complement binding decreased from the double-positive thymocyte to the single-positive stage, and within single-positive thymocytes, complement binding gradually decreased with increasing intrathymic maturation. Binding of the central complement protein C3 to wild-type immature thymocytes required the lectin but not the classical pathway. Specifically, MBL2 but not MBL1 was required, demonstrating a unique function for MBL2. Previous studies demonstrated that the loss of NKAP, a transcriptional regulator of T cell maturation, caused peripheral T cell lymphopenia and enhanced complement susceptibility. To determine whether complement causes NKAP-deficient T cell disappearance, both the lectin and classical pathways were genetically ablated. This blocked C3 deposition on NKAP-deficient T cells but failed to restore normal cellularity, indicating that complement contributes to clearance but is not the primary cause of peripheral T cell lymphopenia. Rather, the accumulation of lipid peroxides in NKAP-deficient T cells was observed. Lipid peroxidation is a salient feature of ferroptosis, an iron-dependent nonapoptotic cell death. Thus, wild-type thymocytes naturally acquire the ability to protect themselves from complement targeting by MBL2 with maturation. However, NKAP-deficient immature peripheral T cells remain scarce in complement-deficient mice likely due to ferroptosis.

NKAP and HDAC3 are critical for T cell maturation. NKAP and HDAC3 physically associate, and a point mutation in NKAP, NKAP(Y352A), abrogates this interaction. To evaluate the significance of NKAP and HDAC3 association in T cell maturation, transgenic mice were engineered for cre-mediated endogenous NKAP gene deletion coupled to induction of NKAP(Y352A) or a wild type (WT) control transgene, NKAP(WT), in double positive thymocytes or regulatory T cells (Tregs). T cell maturation was normal in mice with endogenous NKAP deletion coupled to NKAP(WT) induction. However, severe defects occurred in T cell and Treg maturation and in iNKT cell development when NKAP(Y352A) was induced, recapitulating NKAP deficiency. Conventional T cells expressing NKAP(Y352A) failed to enter the long-term T cell pool, did not produce cytokines, and remained complement susceptible, whereas Tregs expressing NKAP(Y352A) were eliminated as recent thymic emigrants leading to lethal autoimmunity. Overall, these results demonstrate the significance of NKAP-HDAC3 association in T cells. ImmunoHorizons, 2019, 3: 352-367.