Recent thymic emigrants (CD4⁺) continuously migrate through lymphoid organs: Within the tissue they alter surface molecule expression

T-cell progenitors migrate from bone marrow (BM) into the thymus. After maturation they are released as recent thymic emigrants (RTE) into the periphery ensuring the diversification of the T-cell repertoire. Both the kinetics with which RTE migrate through the periphery and the surface molecules they express are still unclear. In 1- and 18-month-old Lewis rats CD4⁺ RTE were identified in blood, spleen, lymph node, and thoracic duct lymph by flow cytometry (CD45RC^- and CD90⁺), were differentiated from CD4⁺ naive (CD45RC⁺) and memory T cells (CD45RC^-CD90^-) and were characterized regarding the expression of surface molecules. Both in 1- and 18-month-old animals the percentage of RTE among the CD4⁺ population in blood was comparable to that in all other compartments. Surprisingly, RTE expressed α₄-integrin, LFA-1, and interleukin (IL)-2 receptor at a significantly higher level than naive T cells and more comparable to memory T cells. Within lymphoid tissues RTE, naive, and memory T cells significantly upregulated the expression of CD44 and ICAM-1, and downregulated the expression of L-selectin. These changes were reversed before the cells re-entered the blood. Thus, our data indicate that CD4⁺ RTE travel through the periphery of young and old rats like mature T cells, continuously modulating their surface molecule expression.