Background: The discovery of circulating autoantibodies to neutrophil cytoplasm antigens (ANCA) has implicated autoimmune mechanisms in the pathogenesis of systemic vasculitis although it is uncertain whether these play a primary role or arise secondary to neutrophil injury.
Experimental design: We examined the pathophysiologic effect of ANCA on intracellular cytosolic-free calcium concentration (Ca2+)i in neutrophils by preincubating cells with ANCA-positive IgG or F(ab')2 preparations followed by stimulation with calcium ionophore (A23187). The mechanism by which ANCA may affect (Ca2+)i was studied by analyzing the production of platelet-activating factor (PAF), an intracellular messenger which is the mediator through which A23187 acts.
Results: ANCA-positive F(ab')2 induced a small but insignificant increase in (Ca2+)i. Preincubation of neutrophils with ANCA-positive IgG or F(ab')2 reduced the calcium mobilization induced by subsequent stimulation with A23187 compared with experiments performed with antibody preparations from ANCA-negative disease or healthy controls. The suppression of A23187-induced calcium mobilization was dose-dependent and correlated with the serum ANCA levels at clinical presentation. ANCA did not bind PAF but reduced the binding of PAF to neutrophils, suggesting that ANCA may prevent the expression of PAF receptor on activated neutrophils. The preincubation of neutrophils with increasing concentrations of F(ab')2 preparation resulted in reduced membrane and supernatant concentrations of PAF, an effect that was more prominent with ANCA-positive F(ab')2.
Conclusions: Our data suggest ANCA may potentially mediate a pathophysiologic effect on neutrophils through interference with the signal transduction pathways utilized in neutrophils' activation. One of the possible mechanisms underlying the suppressive effect of ANCA on A23187-induced calcium mobilization may be mediated through the reduction of PAF synthesis.