Effect of IL-5, glucocorticoid, and Fas ligation on Bcl-2 homologue expression and caspase activation in circulating human eosinophils

IL-5 is a potent eosinophil viability-enhancing factor that has been strong ly implicated in the pathogenesis of IgE-mediated inflammation in vivo. Rec ently published data have suggested that IL-5 (and related cytokines) may a ct by altering the expression of the anti-apoptotic regulator Bcl-2 or its homologues, but this is controversial. The behaviour of the recently descri bed pro-apoptotic cysteine proteases (caspases) in eosinophils after IL-5 t reatment has not been explored. We examined the effect of IL-5 on the expre ssion of four major Bcl-2 homologues, as well as on the expression/activati on of key members of the caspase cell death cascade in cultured circulating human eosinophils. The effect of relevant inducers of eosinophil apoptosis (glucocorticoid and Fas ligation) on these regulatory proteins was also ex amined. We observed baseline expression of the anti-apoptotic Mcl-1 and pro -apoptotic Bax proteins in immunoblots of eosinophil lysates, but not Bcl-x , Bcl-2. IL-5 treatment had the effect of maintaining this basal level of e xpression over time without altering the balance of Bcl-2 homologues. The ( upstream) caspase 8 and (downstream) caspase 3 proenzymes were detected in eosinophils at baseline, and were processed during spontaneous and stimulat ed eosinophil death. IL-5 completely blocked caspase processing in spontane ous and dexamethasone-induced cell death, and significantly slowed processi ng during Fas ligation. Our data do not support the theory that IL-5 acts b y altering the balance of anti-apoptotic and pro-apoptotic Bcl-2 homologues , but suggest that it may act by regulating activation of the caspase cell death cascade.