956 role of calcium in increased complement receptor expression during neutrophil activation

We have recently shown that human neutrophils increase expression of C3bi as well as C3b receptors (R) in response to f-met-leu-phe or other stimuli (J Clin Invest 74; 1566, ′84). Using monoclonal antibodies and flow cytometry, we found that ionophore A23187 and 1.2 mM Ca2+ increased C3bR 330% and C3biR 650%. We therefore examined the role of Ca2+ in response to chemo-attractants. Addition of Ca2+ or EDTA to divalent cation-free media had no effect on C3bR expression while 1.2 mM Ca2+ slightly enhanced and 5 mM EDTA markedly inhibited C3biR expression, suggesting that the two receptors increase by different mechanisms. Mg2+ EGTA effects were identical to EDTA. TMB-8, which inhibits release of Ca from intracellular stores, completely blocked increased expression of both receptors at 200-250 uM. The calmodulin inhibitors chlorpromazine (50 uM) and trifluoperazine (10 uM) also blocked increased expression of both receptors while the inactive metabolite chlorpromazine sulfoxide had no effect. These results suggest that release of intracellular Ca2+ is necessary and sufficient for maximal C3bR expression but that optimal expression of C3biR also requires an influx of extracellular Ca2+. Regardless of the source of the Ca2+, formation of an active Ca2+ - calmodulin complex appears to be necessary for increased complement receptor expression during neutrophil activation.