LIGAND SPECIFICITY OF PURIFIED COMPLEMENT RECEPTOR TYPE-3 (CD11B CD18, ALPHA(M)BETA(2), MAC-1) - INDIRECT EFFECTS OF AN ARG-GLY-ASP (RGD) SEQUENCE

We have purified CR3 to homogeneity by affinity chromatography on C3bi -Sepharose and elution with EDTA. C3bi-coated erythrocytes bound to th is purified CR3, and binding was dependent on the concentration of bot h C3bi and CR3, as well as on temperature and the presence of divalent cations. Moreover, binding could be blocked by mAb against CR3 or C3b i and could be enhanced by the addition of integrin modulating factor- 1. We used the purified CR3 to test whether several putative ligands o f CR3 directly bound the receptor. The interaction of purified CR3 wit h fibrinogen, filamentous hemagglutinin of Bordetella pertussis, lipop hosphoglycan and glycoprotein 63 of Leishmania mexicana, and lipopolys accharide from Escherichia coli was confirmed. However the interaction of CR3 with zymosan or its major component, beta-glucan, was not obse rved in these assays. Previous studies showed that binding of C3bi to PMN could be blocked with Arg-Gly-Asp (RGD) containing peptides and we re interpreted to indicate that the RGD sequence in C3bi interacts dir ectly with CR3. We found, however, that RGD containing peptides were u nable to block the interaction of C3bi with purified CR3, yet retained the ability to block binding of C3bi to cells. We conclude that RGD-p eptides do not directly bind CR3, but instead indirectly effect CR3 fu nction. Inasmuch as the effect of RGD-peptides could be mimicked with antibodies against leukocyte response integrin, we suggest that RGD-pe ptides may bind to leukocyte response integrin on polymorphonuclear le ukocytes and influence CR3 activity via this receptor.