Effect of C1q on the processing of immune complexes by human neutrophils

We prepared immune complexes (IC) composed of human anti-tetanus toxoid IgG and tetanus toxoid, and examined the effect of C1q on the processing of IC by human neutrophils. Treating IC with increasing amounts of C1q enhanced the binding and phagocytosis of IC by neutrophils, unless the amounts of C1q added were less than those required to saturate the C1q binding sites of IC. With the increase of unbound excess C1q, the IC processing by neutrophils decreased. Superoxide anions generated during the processing of IC-C1q were entrapped in phagosomes and were not released from neutrophils. The C1q-dependent inhibition of IC processing by neutrophils was not observed when C1q-treated neutrophils were washed and allowed to react with IC, suggesting that the inhibition by excess C1q is due to the hindrance of IC-C1q binding to neutrophils by loosely bound C1q on neutrophils. The C1q-treated, washed neutrophils still showed enhanced responses to IC, suggesting that free C1q as well as the IC-C1q complex can prime neutrophils to enhance Fc receptor (FcR)-mediated cellular responses. Thus, C1q may have two effects on the processing of IC by neutrophils; firstly, it enhances FcR-mediated cellular responses, and secondly, it prevents superoxide anion-induced tissue damage by trapping superoxide anions in phagosomes.