Altered TCR signaling in anergy

Abstract T cell anergy is a cell-intrinsic program of non-responsiveness to self-antigens, and is an important component of peripheral tolerance. As the induction and maintenance of anergy is critical to preventing breakdown of tolerance and initiation of autoimmunity, it is important to understand the mechanisms and molecules that regulate anergy. Initially, T cell anergy was induced by TCR stimulation in the absence of costimulation and a variety of stimuli have been used in vitro to induce anergy. This early work demonstrated the critical importance of activation of the Ras/Map kinase pathway upon TCR/CD28 stimulation for T cell activation, IL-2 production and proliferation. However, the mechanism is not clear as to how the Ras/Map kinase pathway is downregulated in anergic T cells. Recently, endogenously anergic T cells in WT mice have been identified and are characterized by high expression of CD73 and FR4. We have found that these naturally anergic FR4+CD73+ CD4+ T cells in vivo from WT mice express lower levels of Runx1 protein as compared to other CD4+ T cell subsets. TCR/CD28-induced activation of CD4+ T cells with conditional deletion of Runx1 resulted in lower IL-2 expression and reduced proliferation as compared to activation of WT CD4+ T cells. In addition, there was a higher frequency of naturally anergic FR4+CD73+ CD4+ T cells in CD4- cre Runx1 conditional knockout (cKO) mice or tamoxifen-treated ER-cre Runx1 cKO mice in mixed bone marrow chimeras. Thus, deletion of Runx1 in mature CD4+ T cells predisposes them to the development of T cell anergy. To understand the mechanism(s) responsible for hypo-responsiveness of Runx1-deficient T cells upon TCR/CD28 stimulation, RNAseq analysis was performed on WT and Runx1-deficient CD4+ T cells. Runx1-deficient CD4+ T cells upregulated expression of Dab2IP, an adaptor molecule that has been shown in non-hematopoietic cells to be a negative regulator of Ras signaling. Dab2IP contains a Ras-GAP domain, which leads to the inactivation of Ras through accelerating hydrolysis of GTP to GDP. To determine whether increased Dab2IP expression could inhibit T cell activation and promote anergy induction, we generated a novel cre-dependent, dox-regulatable Dab2IP transgenic mouse. Our initial results demonstrate that similar to CD4+ T cells conditionally deleted for Runx1, Dab2IP Tg CD4+ T cells have increased frequencies of naturally anergic FR4+CD73+ CD4+ T cells in their memory cell pool. Our hypothesis is that increased expression of Dab2IP interferes with normal TCR/CD28 signaling, preventing T cell activation, and predisposing T cells to the development of anergy.