GRANULOCYTE ACTIVATION VIA A BINDING-SITE NEAR THE C-TERMINAL REGION OF COMPLEMENT RECEPTOR-TYPE-3 ALPHA-CHAIN (CD11B) POTENTIALLY INVOLVEDIN INTRAMEMBRANE COMPLEX-FORMATION WITH GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHORED FC-GAMMA-RIIIB (CD16) MOLECULES

We report that engagement of a particular epitope near the C-terminal region of complement receptor type 3 (CR3, CD11b/CD18) alpha-chain wit h CD11b mAb VIM12 induces granulocyte activation with a rise in cytoso tic-free Ca2+, actin polymerization, an up-regulation of CR3,cell surf ace expression and enhanced adhesiveness. Induction of enhanced adhesi veness and homotypic aggregation of human granulocytes represents an a ctive process. It is temperature and energy dependent, requires divale nt cations, and an intact cytoskeleton. The mAb VIM12-induced enhanced adhesiveness seems to be mediated, at least in part, by activated CR3 molecules. It can be significantly inhibited, although not completely abolished, with blocking mAbs against adhesiotopes of CR3. VIM12-indu ced adhesion could be blocked with the serine/threonine inhibitors oka daic acid and calyculin A and with dibuturyl-cAMP but not with the pro tein kinase inhibitors herbimycin A and staurosporine. We further pres ent evidence that the particular molecular region of CR3 recognized by mAb VIM12 might be involved in the reported intramembrane sugar-lecti n type interaction and complex formation between transmembrane CR3 and glycosylphosphatidylinositol (GPI)-anchored Fc gamma RIIIB (CD16) mol ecules on human granulocytes. Binding of mAb VIM12 to CR3 on granulocy tes enhances the release of GPI-anchored Fc gamma RIIIB molecules from granulocytes upon phosphoinositol-phospholipase C treatment. The suga r preparation N-acetyl-D-glucosamine, previously shown to dissociate C R3-Fc gamma RIIIB complex formation, inhibits mAb VIM12 binding. Engag ement of CR3 with mAb VIM12 may thus mimic a biologically relevant int ramembrane cooperation between two distinct receptor molecules on huma n granulocytes.