Tissue‐specific migration pathways by phenotypically distinct subpopulations of memory T cells

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Abstract
A proportion of T cells recirculate in a tissue‐selective manner. Recent studies which showed that the skin‐tropic subset of T cells was of memory/activated type, led us to examine whether the preferential homing of T cells to the gut also involved memory T cells, and if so whether these memory T cells were phenotypically distinct from other memory T cells. Lymphocytes migrating through the gut and the skin of sheep was collected by cannulating the lymphatic ducts draining these tissues. Both naive and memory T cells were found to recirculate through the gut, although only memory T cells migrated through the skin. However, when T cells from the gut were labeled with fluorescein isothiocyanate and assessed for their migration back to the gut, it was the memory population which showed a tropism for the gut. Gut‐tropic memory T cells migrated poorly through the skin, indicating that these cells were distinct from skin‐tropic memory T cells. This was confirmed by phenotypic analysis. Gut memory T cells expressed very low levels of the α6 and β1 integrins, in contrast to skin memory T cells which expressed high levels. There was no evidence for heterogeneity within the naive T cell population, which migrated preferentially to lymph nodes. This migration pattern could be explained in part by the high expression of the L‐selectin (lymph node homing receptor, LAM‐1) on naive T cells, in contrast to memory T cells from gut or skin which were mostly L‐selectin negative. These results in sheep indicate that subsets of α/β memory T cells show tissue‐selective migration patterns, which probably develop in a particular environment following encounter with antigen.