UC San Diego

The gastrointestinal tract is the central organ for uptake of fluids and nutrients, while also forming the main protective barrier between the sterile environment of the body and the outside world. The intestine is a major T cell organ, which is absolutely required for the discrimination between non-self from self. Recently, the ability of the immune system to further distinguish between non-pathogenic and pathogenic self has been appreciated. Agonist selection in the thymus is a pathway that drives differentiation of self-specific TCRab+ T cells, under conditions that delete conventionally selected T cells. Agonist selected T cells are specialized T cells with regulatory features and include CD8aa⁺ IEL. The precursors that undergo agonist selection in the thymus exist yet their phenotypea are unknown. Using in vitro and in vivo model systems, we show that two thymic precursor subsets, the immature triple positives (TP) and mature DN TCRab⁺ thymocytes give rise to TCRab⁺ CD8aa⁺ IEL. The immature TP thymocytes represent a rapidly proliferating pre-selected subset whose appearance is induced by CD3&egr; signaling but is independent of MHC or TCRa expression. In vitro immature TP thymocytes cultured with agonist peptide and the survival cytokine IL-15 generate TCRab⁺ CD8aa⁺ thymocytes. In vivo intrathymically injected TP give rise to TCRab⁺ CD8aa⁺ IEL. DN TCRab⁺ thymocytes, represent a mature subset whose phenotype resembles that seen with TCRab⁺ CD8aa⁺ IEL. In vitro culture of this population with IL-15 allows for the induction of CD8aa⁺ to generate TCRab⁺ CD8aa⁺ thymocytes. In vivo intravenous injected DN TCRab⁺ thymocytes gave rise to almost exclusively TCRab⁺+ CD8aa⁺ IEL. These data suggest that immature TP thymocytes that undergo agonist selection down-regulate all co-receptors and mature into DN TCRab⁺ thymocytes, which migrate to the intestine and become TCRab⁺ CD8aa⁺ IEL. The development/survival of TCRab⁺ CD8aa⁺ IEL was observed when (b2m) expression was restricted to radioresistant host cells. Further, a b 2m- dependent molecule other then Kb and Db on radioresistant host cells was sufficient for the development/survival of TCRab⁺ CD8aa⁺ IEL. Collectively, these data propose that agonist selected TCRab⁺ CD8aa⁺ IEL proceed through unconventional developmental intermediates and utilize non -classical class I molecules (presumably b 2m-dependent)