Role of complement receptor type three and serum opsonins in the neutrophil response to yeast

Previous studies have suggested that neutrophil complement receptor type three (CR3) has two binding sites: (1) a site for fixed iC3b that does not trigger ingestion or a superoxide (O2-) burst, and (2) a function-triggering site for the beta-glucan component of yeast (Saccharomyces cerevisiae) cell walls. In the present study it was found that yeast (Y) coated with C3b (YC3b) or iC3b (YC3bi), prepared with purified complement in an IgG-free system, were avidly ingested ans stimulated a vigorous O2- burst, whereas sheep erythrocytes (E) bearing C3b or iC3b, were not ingested and did not give an O2- burst. YC3b and YC3bi contained an amount of fixed C3 that was approximately equal to serum-opsonized Y (OY), and produced O2- bursts comparable to OY. Experiments utilizing rabbit F(ab')2 anticomplement receptor type one (anti-CR1) to block fixed C3b binding to CR1, and monoclonal anti-CR3 (MN-41 or OKM1) to block fixed iC3b and Y cell wall binding to CR3, indicated that the O2- burst response to OY was primarily due to fixed iC3b and Y cell wall binding to CR3. Fixed C3b (that represented 33% of the fixed C3 on OY) and IgG anti-Y antibodies that bound to CR1 and Fc receptors, respectively, were found to contribute little to the response. Although YC3b did bind avidly to neutrophil CR1, the results suggested that the O2- burst response to YC3b was triggered after the initial YC3b binding by the secondary attachment of Y cell wall components to CR3. When neutrophils were treated with anti-CR3, 90% of neutrophils bound YC3b (via CR1), but phagocytosis and an O2- burst were completely absent. Similar findings were made with OKM1-treated neutrophils and YC3bi. Responses of OKM1-treated neutrophils were inhibited because only the iC3b-binding site of CR3 was ligated by the YC3bi. Thus, fixed C3b or iC3b on Y mediate avid binding of Y to neutrophils via CR1 or the iC3b-binding site of CR3, respectively, but ingestion and an O2- burst response are only triggered when glucans in the Y cell wall secondarily bind to neutrophils via the beta-glucan binding site of CR3.