TNFα-induced suppression of PMN apoptosis is mediated through interleukin-8 production

Dysregulated neutrophil (polymorphonuclear PMN) apoptosis is thought to contribute to the onset of adult respiratory distress syndrome (ARDS) in critically ill patients. Tumor necrosis factor-alpha (TNF alpha), which is present in elevated levels in the bronchoalveolar lavage fluid in patients with ARDS, is thought to play a central role in regulating PMN function in the lungs. Studies have shown that short-term culture with TNF alpha increases apoptosis yet extended culture with TNF alpha suppresses apoptosis. However, it is unclear whether this latter effect of TNF alpha is directly or indirectly mediated through production of anti-apoptotic cytokines such as interleukin (IL)-8. To investigate the role of IL-8 in TNF alpha -induced apoptosis PMN were exposed to TNF alpha (100 ng/mL) in the presence or absence of antibodies to IL-8, and the extent of apoptosis was assessed. An enzyme-linked immunoassay was used to measure levels of the anti-apoptotic cytokine IL-8, induced by TNF alpha -stimulation. Because TNF alpha may mediate its effect through various cell-signaling pathways, we next assessed the effect of kinase inhibition on the ability of TNF alpha to effect apoptosis and IL-8 production. Treatment with TNF alpha had a biphasic effect: at 4-8 h, apoptosis was increased but was markedly suppressed at 24 h (P < 0.05). PMN cultured for 24 h with TNF<alpha> also showed markedly increased levels of IL-8. Neutralization of IL-8 inhibited the ability of TNF alpha to suppress apoptosis (P < 0.05). Incubation of TNF<alpha> + p38-mitogen-activated protein kinase (MAPK) inhibitor SB202190 increased apoptosis (P < 0.01) and decreased IL-8 production to PMN control. To a lesser extent, incubation of TNF<alpha> with inhibitors to NF-KB (SN50) and PI3K (LY294002) also increased apoptosis and decreased IL-8 production (P < 0.05). These data illustrate a novel mechanism by which TNF<alpha> can indirectly elicit an anti-apoptotic effect via p38-MAPK induced release of the anti-apoptotic chemokine IL-8. The exploitation of such a pathway represents a potential target for regulation of PMN-mediated acute lung injury.