The transcription factor Runx1 has essential roles throughout hematopoiesis. Here, we demonstrate that Runx1 is critical for T cell maturation. Peripheral naive 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.
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