BACKGROUND Studies have shown that survival factors including cytokines and growth factors delay apoptosis of human neutrophils via induction of the phosphatidylinositol-3 kinase (PI 3-K)/Akt pathway. In the present study, we explored whether complement fragment C5a has a modulatory effect on neutrophil apoptosis through this signaling pathway. METHODS Human neutrophils were isolated and treated with C5a for up to 24 hours, with or without wortmannin, a PI 3-K inhibitor, and staurosporine, a caspase-9 activator. Apoptosis was quantified by flow cytometry, using propidium iodide nuclear staining, and confirmed by the detection of DNA fragmentation on gel electrophoresis. PI 3-K downstream signaling events were evaluated by measuring the expression of cytosolic total and phosphorylated Akt and Bad proteins by Western blot analyses, and caspase-9 activity. RESULTS C5a inhibited neutrophil apoptosis in a dose- and time-dependent manner. The anti-apoptotic effects of C5a were markedly abrogated in the presence of wortmannin. Brief stimulation of neutrophils with C5a induced phosphorylation of Akt and Bad proteins through a PI 3-K-dependent pathway. Caspase-9 activity was minimal in C5a-treated cells, but markedly increased following PI 3-K inhibition by wortmannin. Finally, C5a reduced caspase-9 activity in staurosporine-treated cells. CONCLUSIONS This study demonstrates that C5a inhibits neutrophil apoptosis via a PI 3-K signaling pathway. This effect may be an important mechanism that improves cell survival and function in the inflammatory milieu.