Immune complex stimulation of neutrophil apoptosis: investigating the involvement of oxidative and nonoxidative pathways

Neutrophils are involved in the pathogenesis of various inflammatory diseases. One of the mechanisms by which neutrophilic inflammation is generated is immune complex (IC) deposition in tissue. As the clearance of apoptotic neutrophils from inflamed sites is considered a crucial determinant for the resolution of inflammation, we investigated the effects of IC-induced neutrophil activation on apoptosis and the mechanisms regulating neutrophil survival. Our results show that IC stimulated apoptosis efficiently. The percentage of apoptotic neutrophils was reduced by the anti-FcγRII mAb IV.3, but not by anti-FcγRIII mAb 3G8. The spontaneous apoptosis was completely inhibited by the antioxidant compound catalase, which in turn prevented only partially the apoptosis in presence of IC. The oxidative metabolism triggered by IC was inhibited only blocking both FcγRII and FcγRIII. Neutrophils from patients with chronic granulomatous disease, congenitally incapable of producing oxidants, showed low level of spontaneous apoptosis, but underwent a nearly 3-fold increment in the apoptosis rate when incubated with IC. In conclusion, neutrophil apoptosis appears to be a process governed by multiple pathways, some of which are strictly ROS-dependent, others acting in a nonoxidative manner. In particular, the herein shown FcγRII-dependent, ROS-independent, signal-inducing neutrophil apoptosis may uncover new pharmacological targets for the promotion of cell removal from sites of inflammation, thereby favoring the resolution of the inflammatory process.