MHCII-mediated antigen presentation by thymic stromal cells is required to enforce central T-cell tolerance

Abstract Central T-cell tolerance relies on elimination of self-reactive CD4+ T-cell clones in the thymus by either deletion or conversion to regulatory T cells (tTregs). Developing thymocytes are selected by exposure to tissue-specific antigens (TSAs) expressed by thymic medullary epithelial cells (mTECs) under the control of Autoimmune Regulator (Aire). TSA-derived peptides-MHCII complexes can be presented either directly by mTECs or indirectly by thymic DCs. Dissecting the contributions of these antigen presenting cell (APC) subsets to thymic tolerance has been hindered by the lack of appropriate genetic tools to selectively target presentation by mTECs. To address this, we created a mouse model in which MHCII can be inducibly ablated in Aire-expressing mTECs (iAire-CreERT2;MHCIIfl/fl). We show that treatment of these mice with tamoxifen leads to a complete loss of MHCII expression by Aire+ and post-Aire mTECs without affecting other thymic APCs or perturbing thymic homeostasis. In a TCR-restricted mouse model of negative selection, ablation of MHCII on mTECs led to a failure to delete self-reactive T cells, allowing escape and activation in the periphery. Surprisingly, we also found a defect in conversion of self-reactive CD4+ T cells to FoxP3+ tTregs. Similarly, in the diverse repertoire, loss of direct antigen presentation by mTECs resulted in tTreg reduction. In contrast, using CD11c-Cre;MHCIIfl/fl mice, in which MHCII is absent on thymic DCs but not mTECs, we found no disruption to negative selection or Treg generation. In sum, our data support a distinct and non-redundant role for direct antigen presentation by mTECs in shaping thymic T cell selection.