ALPHA(3)BETA(1) AND ALPHA(6)BETA(1) INTEGRINS MEDIATE LAMININ MEROSINBINDING AND FUNCTION AS COSTIMULATORY MOLECULES FOR HUMAN THYMOCYTE PROLIFERATION

Integrins comprise a superfamily of ap heterodimers that serve as cell signaling as well as adhesion molecules. We demonstrate that the alph a(3) beta(1) and alpha(6) beta(1) integrins are laminin/merosin recept ors expressed in human thymocytes. By reverse transcriptase-PCR analys is, we determined that the alpha(3A)beta(1), but not the alpha(3B)beta (1), cytoplasmic structural variant of alpha(3) beta(1) is expressed i n thymocytes. In contrast, both alpha(6A)beta(1) and alpha(6B)beta(1) cytoplasmic structural variants of alpha(6) beta(1) are expressed. A s mall percentage (10 to 15%) of human thymocytes bind to immobilized la minin, and even fewer (3 to 5%) bind to merosin, the laminin isoform n ormally present in the thymus. This binding, however, can be increased to 39 to 41% after activation of thymocytes with Mn2+ (or PMA). Bindi ng to either laminin or merosin is completely inhibited by anti-beta(1 ) mAb or by a mixture of anti-alpha(3) and anti-alpha(6) mAbs, indicat ing that both alpha(3) beta(1) and alpha(6) beta(1) participate in thy mocyte adhesion to the laminin family of extracellular matrix proteins . The protein kinase C inhibitors, calphostin C and staurosporine, inh ibit Mn2+-enhanced thymocyte binding, suggesting that protein kinase C activity is crucial for the binding. Furthermore, the data indicate t hat at least two divalent cation binding sites serve to regulate integ rin binding activity. Finally, we show that both immobilized laminin a nd merosin have costimulatory function for anti-CD3-induced thymocyte proliferation, and both anti-alpha(3) and anti-alpha(6) mAbs can block this proliferative response. The cooperative function of alpha(3) bet a(1) and alpha(6) beta(1) evidenced in the laminin/merosin binding and proliferation assays suggests that thymocyte-merosin interactions may play an important role in thymic T cell development.