The thymus chapter in the life of gut-specific intra epithelial lymphocytes

The intestinal intraepithelial lymphocytes (IEL) represent multi-lineage T cell populations. In addition to a major gamma delta TCR(+) T cell subset, many IEL express alpha beta TCRs and they can be separated into up sublineages. Some TCR alpha beta(+)IEL have characteristics in common with conventional TCR alpha beta(+)T cells whereas others share an unconventional phenotype with their TCR gamma delta(+) counterparts. Because the latter are enriched for autoreactive TCRs and can be generated in the absence of a thymus, it has long been postulated that some IEL subsets develop locally in the intestine. Several new data however, indicate that under physiological conditions, IEL require a thymic education that directs lineage commitment and functional differentiation. This review will discuss the contributions of the thymus in shaping the various intestinal IEL sublineages.