A variety of adhesion molecules regulate the traffic and tissue localization of lymphocytes in vivo by mediating their binding to vascular endothelial cells. The homing receptor gp90MEL-14 (gp90), also known as LECAM-1 or L-selectin, mediates the adhesion of lymphocytes to specialized high endothelial venules in lymph nodes (LN) and is the primary molecule regulating lymphocyte recirculation and homing to LN, whereas other adhesion molecules have a major role in the localization of lymphocytes in inflammatory sites. We used four-color flow cytometric analysis to examine the regulation of adhesion receptor expression on LN CD8 T cells responding to skin allografts in vivo. In normal mice, greater than 95% of LN CD8 T cells are gp90+, being either gp90+Pgp1- (Population (Pop.) 1 or gp90+Pgp-1+ (Pop.2). Allografting induces the down-regulation of gp90 and up-regulation of Pgp-1 on a subset of cells, resulting in the appearance of CD8+gp90-Pgp-1hi (Pop. 3) cells. Pop. 3 cells also express high levels of LFA-1, ICAM-1, and ICAM-2, and a subset of them are VLA-4 alpha-positive. Purified Pop. 3 cells have potent cytolytic activity directed against donor alloantigen, whereas no such activity is present in Pop. 1 or 2 cells. Correlating with this is the high granzyme activity in Pop. 3 cells. In addition, Pop. 3 lymphocytes, but not Pop. 1 or 2, secrete a large amount of IFN-gamma in response to Ag. Finally, the CD8 T cells that infiltrate sponge matrix allografts are markedly enriched for the Pop. 3 subset. These results show that, during the immune response to alloantigen in vivo, a small subset of CD8 T cells down-regulates the LN homing receptor while increasing the expression of other adhesion molecules, as they differentiate into highly active cytolytic T lymphocytes. Thus, the differential regulation of LN homing receptors and receptors for peripheral vascular endothelium provides a mechanism that would redirect the traffic of activated effector cells away from lymphoid tissue and to sites of Ag deposition, where they would participate in the inflammatory response.

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