Lymphocyte Activation and Regulation of Three Adhesion Molecules with Supposed Function in Homing: LECAM‐1 (MEL‐14 Antigen), LPAM‐1/2 (α₄‐Integrin) and CD44 (Pgp‐1)

Directed migration of lymphocytes from blood into lymph nodes and gut‐associated lymphatic tissue, also referred to as homing, is subject to change following activation. Lymphocyte migration into lymphoid organs in vivo and binding to high endothelial venules in vitro is largely suppressed after short‐term stimulation with phorbol esters. The observed functional alterations were correlated with changes in the expression of three putative homing receptors. LECAM‐1 (MEL‐14 antigen), LPAM‐1/2 (α₄‐integrin) and the murine CD44 (Pgp‐1, H‐CAM, Hermes‐antigen equivalent) upon different modes of cellular activation. Expression of LECAM‐1 (gp90 MEL‐14), a lymphocyte adhesion molecule implicated in targeting extravasation into lymph nodes, was found to be lost almost completely within minutes after protein kinase C activation LECAM‐1 re‐expression occurred within less than 24 h. Rapid loss of LECAM‐1 was also observed after calcium ionophores whereas anti‐CD3 or concanavalin A elicited a gradual and heterogeneous loss of LECAM‐1 becoming delectable after several hours only. A number of cytokines tested were not able to induce alterations in LECAM‐1 expression. In contrast, expression of LPAM‐1/2 (α₄‐integrin) and CD44 (Pgp‐1, H‐CAM), two adhesion molecules supposed to direct extravasation into Peyer's patches, remained stable for hours after every stimulus tested; CD44 expression gradually increased 24 h after mitogenic activation, whereas a small reduction only was observed for the expression of the α₄‐chain under certain conditions. Thus, reduced extravasation of lymphocytes into Peyer's patches after activation is not due to a decline in the surface density of LPAM‐1/2 α‐chain or CD44 whereas alterations in migration into lymph nodes parallel the expression of LECAM‐1.