ACTIVATED T-LYMPHOCYTES EXPRESS OCCLUDIN, A COMPONENT OF TIGHT JUNCTIONS

T-lymphocytes routinely traffic from the lymphoid and vascular compart ments to the tissues during immune surveillance and inflammatory respo nses. This egress occurs without compromising endothelial barrier, whi ch is maintained by tight junctions (zonula occludens). We report that T-lymphocytes up-regulate the expression of occludin, a major compone nt of the tight junction in response to stimulation with phorbol ester (PMA) + calcium ionophore, CD3 antibody or T-cell receptor (TCR) anti body. Only activated T-lymphocytes express occludin; this adhesion mol ecule is nearly absent in resting T-lymphocytes. By immunofluorescence , occludin is seen in lymphocyte aggregates, but does not appear to me diate aggregation since only 50% of the cells in these clusters expres s occludin. Occludin is expressed between 8 and 24 h following stimula tion, and persists for at least 48 h. These data indicate that activat ed T cells produce occludin which may regulate lymphocyte adhesion and trafficking.