Paracrine suppression of apoptosis by cytokine-stimulated neutrophils involves divergent regulation of NF-κB, Bcl-XL, and Bak

Dysregulated polymorphonuclear leukocyte (PMN) apoptosis and PMN-mediated organ damage have been associated with several medical conditions such as systemic inflammatory response syndrome (SIRS), acute respiratory distress syndrome (ARDS), and ischemia/reperfusion injury. IL-1 beta and IL-8 are two cytokines that are elevated under similar conditions. Therefore, we hypothesized that PMN exposed to these cytokines would secrete factors that could affect PMN apoptosis in a cell contact-independent manner. We have previously shown that media conditioned by IL-1 beta -stimulated PMN (CM-IL-1 beta) for 2 h suppressed spontaneous PMN apoptosis. Data presented here demonstrate that media conditioned by IL-8-stimulated PMN (CM-IL8) also have the ability to suppress spontaneous, as well as FasL- and TNF-alpha -induced apoptosis. In contrast, CM-IL1 beta was able to suppress FasL-induced, but not TNF-alpha -incluced, apoptosis. To elucidate the mechanisms these media use to elicit their effects, we examined the expression and function of several apoptosis-related proteins, Experimental results demonstrate that both CM-IL1 beta and CM-IL8 have the ability to delay caspase activation, but have no effect on the expression of their upstream activator, Fas, or its ligand, FasL. Examination of several Bcl-2 family members revealed a selective regulation by each media: CM-IL1 beta up-regulated Bcl-X-L, while CM-IL8 down-regulated Bak expression. Additionally, CM-IL1 beta, but not CM-IL8, promoted the activation of NF-kappaB, which has anti-apoptotic activity. Together, we can conclude that IL-1 beta- and IL-8-stimulated PMN have the ability to suppress PMN apoptosis in a paracrine manner, and that the extent and mechanism of suppression is specific for each.