A NEW CD43 MONOCLONAL-ANTIBODY INDUCES HOMOTYPIC AGGREGATION OF HUMAN-LEUKOCYTES THROUGH A CD11A CD18-DEPENDENT AND CD11A/CD18-INDEPENDENT MECHANISM/

We describe a monoclonal antibody (mAb), designated 1.C1, that causes rapid and vigorous aggregation among normal leucocytes and among T and myeloid/monocytic cell lines. As shown by competitive binding and seq uential immunoprecipitation experiments, the antigen recognized by mAb 1.C1 is a 115,000 MW sialoglycoprotein, that corresponds to the human CD43 antigen, also known as leukosialin or sialophorin. The aggregati on process starts within minutes and reaches maximum level 6-18 hr aft er addition of the antibody. It is dependent on active cell metabolism (inhibited at low temperatures and by a mixture of the metabolic pois ons azide and 2-deoxy-D-glucose), a fluid plasma membrane (inhibited b y pretreatment of the cells with paraformaldehyde) and an intact cytos keleton (inhibited by cytochalasin B). Two reference CD43 antibodies ( MEM-59 and DF-T1), both binding the same or closely related sialic aci d-dependent epitope as mAb 1.C1, are also capable of inducing cell clu mp formation. CD11a/CD18 mAb block the 1.C1-induced adhesion of restin g peripheral blood leucocytes, but not of haematopoietic cell line cel ls. In addition, mAb 1.C1 induces homotypic aggregation of K-562 cells , which do not express members of the beta2 integrin subfamily on thei r surface. These data suggest that triggering of the CD43 antigen prom otes homotypic cell adhesion that is mediated by both CD11a/CD18-depen dent and -independent pathways.