Activation-induced apoptosis in human macrophages: Developmental regulation of a novel cell death pathway by macrophage colony-stimulating factor and interferon γ

Activated macrophages (Mφs) are important participants in host defense, but their uncontrolled activation leads rapidly to septic shock and death. One mechanism for regulating other dangerous cells in the immune system is programmed cell death, or apoptosis. Monocytes are known to undergo spontaneous apoptosis upon leaving the circulation unless provided with specific survival signals, but mature tissue Mφs are more robust cells, and it was not clear that they could be similarly regulated by apoptosis. We now show that during differentiation monocytes rapidly lose their sensitivity to apoptosis triggered by passive cytokine withdrawal, but they may retain a novel pathway which initiates apoptosis after activation with specific stimuli (zymosan and phorbol esters). Sensitivity to activation-induced apoptosis was developmentally determined, being downregulated by the maturation-promoting cytokine macrophage colony-stimulating factor but stably upregulated by even transient exposure to the proinflammatory cytokine interferon γ (IFN-γ). Apoptosis began within 2-4 h of activation, occurred in >95% of susceptible cells, and in mixed cocultures selectively affected only those Mφs with a history of IFN-γ priming. Consistent with a possible role for protein kinase C in the signaling pathway leading to cell death, the kinase inhibitor staurosporine was protective against both phorbol ester- and zymosan-induced apoptosis. Our studies describe a novel form of activation- induced Mφ apoptosis which is developmentally regulated by two physiologically relevant cytokines. We speculate that apoptosis may serve to restrict the destructive potential of inflammatory Mφs.