IL-32γ Delays Spontaneous Apoptosis of Human Neutrophils through MCL-1, Regulated Primarily by the p38 MAPK Pathway

IL-32 gamma is a multifunctional cytokine involved in various inflammatory and auto-immune diseases in which neutrophils can affect the evolution of these diseases. To persist at inflammatory sites, neutrophils require inhibition of their rapid and constitutive apoptosis, an inhibitory effect that phlogogenic cytokines support. To date, the effects of IL-32 gamma on neutrophils remain unknown. We demonstrate that IL-32 gamma delays, in a dose-dependent manner, the spontaneous apoptosis of human blood neutrophils by activating mainly p38 MAPK through rapid p38 phosphorylation. PI3-K and ERK1/2 MAPK are also involved, but to a lesser extent. Most of cytokines that induce retardation of neutrophil apoptosis activate the expression of MCL-1 at both mRNA and protein levels. IL-32 gamma added to human blood neutrophils in vitro is associated with sustained levels of MCL-1 protein. This effect in neutrophils corresponds to a decrease of MCL-1 protein degradation without any effect on MCL-1 mRNA levels. The sustained levels of MCL-1 induced by IL-32 gamma are only abrogated by the p38 beta MAPK inhibitor SB202190. Additionally, IL-32 gamma induces a reduction in caspase 3 activity in neutrophils. In conclusion, IL-32 gamma affects human blood neutrophils in vitro by increasing their survival, suggesting that this cytokine could have profound effects on the deleterious functions of neutrophils in several diseases.