Tissue-specific emergence of regulatory and intraepithelial T cells from a clonal T cell precursor.

Peripheral Foxp3 regulatory T cells (pT) maintain immune homeostasis by controlling potentially harmful effector T cell responses toward dietary and microbial antigens. Although the identity of the T cell receptor (TCR) can impose commitment and functional specialization of T cells, less is known about how TCR identity governs pT development from conventional CD4 T cells. To investigate the extent to which TCR identity dictates pT fate, we used somatic cell nuclear transfer to generate a transnuclear (TN) mouse carrying a monoclonal TCR from a pT (pT TN mice). We found that the pT TCR did not inevitably predispose T cells to become pT but instead allowed for differentiation of noninflammatory CD4CD8αα intraepithelial lymphocytes (CD4) in the small intestine. Only when we limited the number of T cell precursors that carried the TN pT TCR did we observe substantial pT development in the mesenteric lymph nodes and small intestine lamina propria of mixed bone marrow chimeras. Small clonal sizes and therefore decreased intraclonal competition were required for pT development. Despite bearing the same TCR, small intestine CD4 developed independently of precursor frequency. Both pT and CD4 development strictly depended on the resident microbiota. A single clonal CD4 T cell precursor can thus give rise to two functionally distinct and anatomically segregated T cell subsets in a microbiota-dependent manner. Therefore, plasticity of the CD4 T cell compartment depends not only on the microbiota but also on specialized environmental cues provided by different tissues.