CD8αα(+) Intraepithelial Lymphocytes Arise from Two Major Thymic Precursors

TCRαβ(+) CD8αα(+) intestinal intraepithelial lymphocytes (CD8αα IELs) are an abundant population of thymus-derived T cells that protect the gut barrier surface. We sought to better define the thymic IEL precursor (IELp) by analysis of their maturation, localization, and emigration. We defined two precursors among TCRβ(+)CD4(–)CD8(–) thymocytes using TAK1 dependence and rigorous lineage-exclusion criteria. Those IELp populations include a nascent PD-1(+) population and a T-bet(+) population that accumulated with age. Both gave rise to intestinal CD8αα IELs upon adoptive transfer. PD-1(+) cells contained more strongly self-reactive clones and were largely classical MHC restricted. They localized to the cortex, and efficiently emigrated in an S1PR1-dependent manner. T-bet(+) IELp localized to the medulla, included non-classical MHC-restricted cells and expressed NK1.1, CD103 and CXCR3. The two IELp populations further differed in TCR V(α) and V(β) usage. These data provide an important foundation for understanding the biology of CD8αα IELs.