Regulation of human neutrophil type 3 complement receptor (iC3b receptor) expression during phagocytosis of Staphylococcus aureus and Escherichia coli.

Human neutrophils (PMN) express a receptor for iC3b, a cleavage product of C3b. CR3 is an important receptor for phagocytosis of opsonized bacteria and its expression is enhanced by cell activation. We examined PMN CR3 expression during phagocytosis using flow cytometry and a CR3-specific monoclonal antibody. After 30 min phagocytosis of opsonized S. aureus and E. coli, CR3 expression increased to 151% and 221% of controls, respectively. Unopsonized S. aureus had no effect on CR3; however, unopsonized E. coli enhanced CR3 expression despite not being phagocytosed. Time-kinetic studies indicated a rapid initial fall in CR3 after addition of bacteria to PMN, followed by enhanced expression within 5-10 min. The initial fall in CR3 probably represented CR3 internalization rather than receptor destruction, as superoxide dismutase, catalase and protease inhibitors had no effect on this. Correlation of CR3 expression with the PMN oxidative response, measured with the intracellular fluorescent probe DCF-DA, demonstrated a dichotomy. Opsonized S. aureus and E. coli caused an oxidative response from PMN but unopsonized E. coli, which caused significant CR3 up-regulation, did not. CR3 up-regulation with unopsonized and opsonized E. coli was markedly inhibited by Polymyxin B, suggesting a role for endotoxin. These experiments indicate that CR3 expression can be regulated during phagocytosis, and the mechanisms responsible are distinct from those involved in the oxidative burst. CR3 up-regulation following exposure to bacteria in vivo may enhance neutrophil function at sites of infection.