LONG-TERM THYMIC RECONSTITUTION BY PERIPHERAL CD4 AND CD8 SINGLE-POSITIVE LYMPHOCYTES

Significant immigration of peripheral T cells into SCID thymus was obs erved following reconstitution with normal Peyer's patch, mesenteric l ymph node or peripheral lymph node cells. Immunohistologic and flow cy tometric 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 in trathymic cells. The returning cells express the CD3/T cell receptor a lpha/beta complex, indicative of mature cells, and are equally divided among helper (CD4+CD8-) and cytotoxic (CD4-/CD8+) phenotypes. The imm igration of peripheral T cells is not accompanied by the appearance of immature, double-positive (CD4+CD8+) thymocytes as seen in similar re constitutions using bone marrow. Taken together, these results suggest that peripheral T cells from a variety of lymphoid organs may regular ly re-enter the thymus and, thus, possibly play a role in normal thymi c development.