Interleukin-6 delays neutrophil apoptosis via a mechanism involving platelet-activating factor

Background: Interleukin (1L)-6, an integral mediator of the physiologic acute phase response to injury, has been associated with adverse postinjury complications when present in excessive concentrations. The precise role of IL-6 is unclear, but may involve exacerbation of polymorphonuclear neutrophil leukocytes (PMN)-mediated hyperinflammation. We have shown that IL-6 delays PMN apoptosis, thereby inhibiting the resolution of inflammation. More recently we have found that IL-6 stimulates PMNs to generate platelet-activating factor (PAF). Given the evidence for PAF involvement in postinjury hyperinflammation, we hypothesized that IL-6 delayed apoptosis via a mechanism involving PAF. Methods: PMNs were isolated from healthy human donors using plasma-Percoll gradients and were cultured in enriched RPMI 1640 media at 2 x 10(7) PMNs/mL for 24 hours (37 degrees C, 5% CO2). Subgroups were treated with IL-6 (0.1-10 ng/mL) or PAF (0.1-10 ng/mL) or pretreated with the PAF receptor antagonist WEB 2170 (20 mu M) before IL-6 or PAF. Morphologic assessment and quantitation of apoptosis was performed with acridine orange/ethidium bromide stain. Results: Both IL-6 and PAF suppressed PMN apoptosis. Pretreating PMNs with WEB 2170 abrogated the effects of IL-6 as well as PAF. Conclusion: Interleukin-6 delays PMN apoptosis via a mechanism involving PAF. These observations may help elucidate the mechanisms of IL-6 and PAF in mediating postinjury hyperinflammation and secondary organ dysfunction, ultimately leading to effective therapeutic targets in patients at risk for multiple organ failure.