Effects of chemotactic peptide f-Met-Leu-Phe (FMLP) on C3b receptor (CR1) expression and phagocytosis of microspheres by human neutrophils.

FMLP caused maximal upregulation of CR1 on neutrophils at a concentration of 10(-8) M but caused maximal enhancement of CR1-dependent phagocytosis of C3b.IgG-coated microspheres only at a concentration of 10(-6) M. There were positive correlations between FMLP-mediated upregulation of CR1 and FMLP-mediated enhancement of phagocytosis (correlation coefficient = 0.73, slope = 2.2) and between FMLP-mediated upregulation of CR1 and FMLP-mediated increase in total cell-associated microspheres (correlation coefficient = 0.88, slope = 1.3). The phagocytic capacity of both untreated and 10(-6) M FMLP-treated neutrophils was completely inhibited by fluid phase C3b and partially inhibited by aggregated IgG. The data suggest that CR1 upregulation is required but is not sufficient for maximal phagocytosis by the leukocytes. The data also suggest that FMLP at the higher concentrations may impart a phagocytic function to CR1, activate other phagocytic receptors, elicit phagocytosis-inducing mediators or may elicit a separate mechanism of phagocytosis. During the study, it was observed that there was considerable individual variation among different neutrophil preparations with respect to CR1 expression and binding and phagocytic capacity.