Regulation and functional involvement of distinct determinants of leucocyte function-associated antigen 1 (LFA-1) in T-cell activation in vitro.

The expression of leucocyte function-associated antigen 1 (LFA-1) was studied by immunofluorescence method on human peripheral blood mononuclear cells (PBMC) stimulated by the phytohaemagglutinin lectin (PHA). Monoclonal antibodies (MoAb) Mas 191c, IOT18 (directed against the beta-chain, 95 kDa, CD18) and IOT16, SPVL7, MHM24 (identificating the alpha-chain, 180 kDa, CD11a) were used, defining the 'CD11a/CD18' antibody panel. By means of cross-linking or competitive experiments, we showed that these antibodies recognized at least four distinct and spatially distant domains on the LFA-1 molecule. Immunofluorescence analysis revealed that the up-regulation of LFA-1 expression was a late event, similar to the expression kinetics of the HLA DR and CD38 molecules, and followed the appearance of CD25 and CD71 molecules. Moreover, it was established that the LFA-1 up-regulation required mRNA and protein synthesis. Functional activity comparison of the different anti LFA-1 MoAb showed that the CD11a MoAb significantly inhibited the proliferation of lymphocytes stimulated by the phytohaemagglutinin to various extents, as the LFA-1 alpha determinant identified. By contrast, the CD18 MoAb did not influence strongly this cell process. We observed only a dim inhibitory effect with the CD18 MoAb recognizing an epitope common or very close to an LFA-1 alpha determinant. These results suggested that the LFA-1 antigen was important, at a molecular level, in the regulation of T-cell activation.