Long‐term thymic reconstitution by peripheral CD4 and CD8 single‐positive lymphocytes

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Abstract
Significant immigration of peripheral T cells into SCID thymus was observed following reconstitution with normal Peyer's patch, mesenteric lymph node or peripheral lymph node cells. Immunohistologic and flow cytometric analyses reveal that T cells from these tissues are found in the thymus for as long as 177 days and can account for up to 67% of intrathymic cells. The returning cells express the CD3/Tcell receptor α/β complex, indicative of mature cells, and are equally divided among helper (CD4+ CD8 ) and cytotoxic (CD4−/CD8+) phenotypes. The immigration of peripheral T cells is not accompanied by the appearance of immature, double‐positive (CD4+CD8+) thymocytes as seen in similar reconstitutions using bone marrow. Taken together, these results suggest that peripheral T cells from a variety of lymphoid organs may regularly re‐enter the thymus and, thus, possibly play a role in normal thymic development.