MONOCLONAL-ANTIBODIES TO LEUCOSIALIN (CD43) INDUCE HOMOTYPIC AGGREGATION OF THE HUMAN MAST-CELL LINE HMC-1 - CHARACTERIZATION OF LEUCOSIALIN ON HMC-1 CELLS

CD43 (leucosialin, sialophorin) is the major sialoprotein of nearly al l circulating leucocytes and has important biological activities in ce llular differentiation and activation. Recently, the expression of CD4 3 has also been demonstrated on mast cells and basophils by flow cytom etry. In order to further characterize mast cell/basophil leucosialin we have investigated CD43 on the human mast cell line HMC-1, the human basophilic precursor cell line KU-812, and the human promonocytic cel l line U-937. The apparent molecular weights (MW) were 123,000 (HMC-1 and KU-812) and 144,000 (U-937) by Western blot analysis. Expression o f CD43 on HMC-1 was down-regulated after stimulation with phorbol myri state acetate (PMA). Three monoclonal antibodies (mAb) specific for hu man CD43 induced homotypic mast cell line (HMC-1) aggregation in a sem i-quantitative assay, a phenomenon that has not been described before with mast cells. Monoclonal antibodies specific for seven other surfac e antigens and an irrelevant mAb of the same isotype had no effect. Th e level of aggregation was dependent on anti-CD43 mAb concentration, t ime and temperature. Anti-leucosialin-induced aggregation of HMC-1 cel ls was completely inhibited by mAb against CD11a (LFA-1) and CD18 (bet a(2)-chain). Monoclonal antibody to CD54 (ICAM-1) partially inhibited anti-CD43-induced homotypic aggregation, while anti-CD11b (CR3), anti- CD11c (p 150, 95) and a control mAb had no inhibitory effect. We concl ude that mast cell line CD43 antigen expression is differentially regu lated during cell activation, and speculate that anti-CD43-induced hom otypic aggregation of HMC-1 cells is closely associated with modulatio n of beta(2)-integrins.