L-selectin-mediated lymphocyte aggregation: role of carbohydrates, activation and effects on cellular interactions.

L-selectin on lymphocytes reacts with glycosylated ligands on high endothelial venule walls in lymphoid organs. Through this carbohydrate-dependent interaction, rolling and initial attachment of lymphocytes to endothelium is mediated. Here we have studied an earlier described L-selectin-induced homotypic aggregation, to further elucidate the events that occur after engagement of L-selectin. It was found that the interaction of L-selectin with fucoidan, but not with other carbohydrates, or with monoclonal antibodies directed against the carbohydrate recognition domain of L-selectin, resulted in homotypic aggregation among both B- or T lymphocytes. Importantly, this aggregation was shown to be both lymphocyte function-associated antigen-1 (LFA-1) and calcium-independent. Furthermore, for aggregation metabolic energy was required, and signalling via protein tyrosine kinase appeared to be involved. Neither de novo protein synthesis, protein kinase C mediated signalling, Gi-protein mediated signal transduction, nor calcium mobilization were required for aggregation. During aggregation, L-selectin was not shed from the lymphocyte's cell surface. Finally, it was found that the lymphocyte binding capacity to high endothelial venules on cryostat sections was not altered upon triggering these lymphocytes via L-selectin. Interestingly, L-selectin-triggered cells showed increased binding to paracortical areas in peripheral lymph nodes. Our data suggest that signals via L-selectin, might lead to altered expression of cell surface molecules, important in interactions other than the first stage of lymphocyte rolling.