Modulation of neutrophil expression of C3b receptors (CR1) by soluble monomeric human C3b

Upon incubation at 37°C with purified human C3b (500 μg/ml), polymorphonuclear neutrophils (PMN) were found to express up to 50% more C3b receptors (CR1) than PMN incubated with buffer alone. This up‐regulation of CR1, assessed by the binding of radiolabeled CR1‐specific monoclonal antibody, was dependent on the dose of C3b, occurred within 10–20 min and was stable for at least 90 min. PMN incubated with C3b also demonstrated enhanced CR1‐dependent binding functions, such as EC3b rosette formation and phagocytosis of EIgGC3b particles. C3b at a concentration of 500 μg/ml induced up to 90% increase in the attachment or the phagocytic index. However, CR1 remained unable to promote phagocytosis of EC3b intermediates. Fc receptor‐mediated functions were unaffected by the treatment with C3b. The active factor was characterized as monomeric C3b and, in particular, shown to be distinct from C5a. C3b purified by anion‐exchange fast protein liquid chromatography on a Mono Q column or eluted from a monoclonal anti‐C3b‐Sepharose retained its modulating activity, while native C3 or C3 fragments such as iC3b, C3c or C3d,g were ineffective.