TCR-MEDIATED ADHESION OF T-CELL HYBRIDOMAS TO PLANAR BILAYERS CONTAINING PURIFIED MHC CLASS-II PEPTIDE COMPLEXES AND RECEPTOR SHEDDING DURING DETACHMENT

T cell recognition of foreign Ag/MHC class Il complexes is sensitive d own to similar to 100 complexes per cell or similar to 0.2 complexes/m u m(2). To better understand the physical basis of the recognition sta ge of Ag presentation, we examined adhesion of the lysozyme-specific T cell hybridoma, 3A9, to artificial bilayers containing covalent MHC c lass II/peptide complexes or adhesion molecules. Adhesion of 3A9 cells required a superphysiologic density of the MHC class II/peptide compl ex and was partly dependent on CD4; cells adhered but did not crawl, N o adhesion was observed to bilayers containing MHC class II molecules without the lysozyme peptide. Activated 3A9 cells adhered and crawled on bilayers containing ICAM-1, The physical strength of contacts was t ested with fluid shear, 3A9 cells adherent to bilayers containing MHC class II/peptide complexes shed their contact, which remained on the s ubstrate and contained TCR. In contrast, 3A9 cells peeled from the ICA M-1 bilayer, and held firmly on LFA-1 bilayers in a manner dependent o n filamentous actin. When ICAM-1 and the MHC/peptide complexes were co mbined, the 3A9 cells adhered tightly and spread, but did not crawl, o n the bilayers and TCR clustered at the center of the contact area. Ph ysiologically, the TCR is unlikely to directly initiate adhesion. TCR clusters formed with the assistance of adhesion mechanisms may have to be shed to allow de-adhesion, and this may contribute to TCR down-reg ulation.