Stimulation of intracellular Ca2+ levels in human neutrophils by soluble immune complexes - Functional activation of Fc gamma RIIIb during priming

Soluble immune complexes bind to unprimed neutrophils and generate intracellular Ca2+ transients but fail to activate the NADPH oxidase. Following priming of the neutrophils with either tumor necrosis factor alpha of granulocyte-macrophage colony-stimulating factor, stimulation of the cells with the soluble immune complexes leads to an enhanced Ca2+ signal and significant secretion of reaction oxidants. The enhanced Ca2+ signal observed in primed neutrophils results from the influx of Ca2+ from the external environment and is partly sensitive to tyrosine kinase inhibitors. This is in contrast to the Ca2+ signal observed in unprimed neutrophils, which arises from the mobilization of intracellular stores. When the surface expression of Fc gamma RIIIb on primed neutrophils was decreased wither through incubation with Pronase of phosphoinositide-specific phospholipase C, the extra enhanced Ca2+ mobilization seen in primed cells was significantly lowered, while the initial rise in intracellular Ca2+ was unaffected. Depletion of Fc gamma RIIIb had no significant effect on the Ca2+ transients in unprimed neutrophils. Cross-linking Fc gamma RII, but not Fc gamma RIIIb, induced increase in intracellular Ca2+ in unprimed neutrophils, while cross-linking either of these receptors increased Ca2+ levels in primed neutrophils. The Fc gamma RII-dependent intracellular Ca2+ rise in primed cells was unaffected by incubation in Ca2+-free medium, whereas the Fc gamma RIIIb-dependent transient was significantly decreased when Ca2+ influx was prevented in Ca2+-free medium supplemented with EGTA. Cross-linking wither Fc gamma RII or Fc gamma RIIIb in primed or unprimed cells failed to stimulate substantial levels of inositol 1,4,5-trisphosphate production. These results indicate that following stimulation of primed neutrophils with soluble immune complexes the enhanced Ca2+ mobilization observed is the result of a functional activation of the glycosylphosphatidylinositol-linked Fc gamma RIIIb.