Independence of Neutrophil Respiratory Burst Oxidant Generation from the Early Cytosolic Calcium Response After Stimulation with Unopsonized Candida Albicans Hyphae

We previously noted differences between neutrophil responses to unopsonized Candida albicans hyphae and responses to other particulate stimuli such as opsonized hyphae or zymosan; these differences include delayed rises in cytosolic calcium [( Ca2+]i), 1,4,5-inositol trisphosphate, and superoxide release and the total absence of early membrane depolarization. Respiratory burst stimulation is required for killing of C. albicans hyphae. Since an early rise in [Ca2+]i may act as a second messenger for burst activation by most agonists, we chelated (Ca2+)i and extracellular Ca2+ [( Ca2+)e] to compare requirements for superoxide responses to hyphae and other stimuli. Intracellular chelation, which ablated early [Ca2+]i rises, eliminated the fMet-Leu-Phe-induced respiratory burst and profoundly reduced that response to opsonized zymosan (by 96.7%), but chelation of both (Ca2+)i and (Ca2+)e only partially inhibited responses to opsonized and unopsonized hyphae (60.5 and 23.3%, respectively; the latter exceeded absolute responses evoked by opsonized zymosan, a 12-fold-more-potent stimulus for unchelated cells). Simultaneous (Ca2+)i and (Ca2+)e chelation further decreased superoxide responses to opsonized zymosan and hyphae (99.4 and 90.4%, respectively) but not to unopsonized hyphae (26.7% inhibition). Though both ingestible (zymosan) and uningestible (hyphae) opsonized particulate stimuli elicited reduced but significant respiratory bursts without early [Ca2+]i rises, the greater superoxide responses and sensitivity to chelation with opsonized zymosan suggest important differences in initiation and/or regulation of responses to these particulate stimuli. In contrast, the respiratory burst elicited by unopsonized hyphae appeared largely Ca2+ independent. If different events mediate neutrophil activation by opsonized and unopsonized hyphae, candidacidal activity in vivo may vary under divergent conditions with specific localized sites of infection.