Fibronectin at the lymphocyte surface. Evidence for activation-dependent binding to VLA4 and VLA5 integrins.

The surface of in vitro cultured fixed and viable human T lymphocytes and certain T-cell lines was found to react with different monoclonal anti-fibronectin (FN) antibodies as revealed by ELISA, immunocytochemistry and FACS analysis. SDS-PAGE showed that anti-FN antibodies defined a high molecular weight lymphocyte component which could be iodinated using the lactoperoxidase method and which had gelatin binding capacity. FACS analysis showed that the reactivity of anti-FN antibodies with lymphocytes was most pronounced in activated cells and increased during the culture period. By contrast, FACS analysis revealed equal high expression of the VLA4 and VLA5 integrins on freshly purified as well as on mixed lymphocyte culture (MLC) activated cells. Freshly purified lymphocytes and lymphocytes cultured in vitro overnight did not bind 3H-labelled FN in solution whereas MLC-activated cells were capable of 'spontaneous' binding of such [3H]-FN. However, brief 12-o-tetradecanoylphorbol-13-acetate (TPA) exposure rendered freshly purified lymphocytes capable of binding soluble FN. These interactions of the lymphocytes with 3H-labelled FN in solution could be almost completely blocked by monoclonal anti-VLA4 and VLA5 antibodies. These results indicate that activated T cells express fibronectin at their surface under 'normal' culture conditions. Although both freshly purified and MLC-activated lymphocytes have equal expression of the integrins VLA4 and VLA5, only activated cells are capable of 'spontaneous' binding FN in solution via an integrin-mediated process, probably via an increase in the affinity of these receptors for FN.