Activation of human monocytes induces differential resistance to apoptosis with rapid down regulation of caspase-8/FLICE

Cells of the monocyte/macrophage lineage play a central role in both innate and acquired immunity of the host. However, the acquisition of functional competence and the ability to respond to a variety of activating or modulating signals require maturation and differentiation of circulating monocytes and entail alterations in both biochemical and phenotypic profiles of the cells. The process of activation also confers survival signals essential for the functional integrity of monocytes enabling the cells to remain viable in microenvironments of immune or inflammatory lesions that are rich in cytotoxic inflammatory mediators and reactive free-radical species. However, the molecular mechanisms of activation-induced survival signals in monocytes remain obscure. To define the mechanistic basis of activation-induced resistance to apoptosis in human monocytes at the molecular level, we evaluated the modulation of expression profiles of genes associated with the cellular apoptotic pathways upon activation and demonstrate the following: (i) activation results in selective resistance to apoptosis particularly to that induced by signaling via death receptors and DNA damage; (ii) concurrent with activation, the most apical protease in the death receptor pathway, caspase-8/FLICE is rapidly down-regulated at the mRNA level representing a novel regulatory mechanism; and (iii) activation of monocytes also leads to dramatic induction of the Bfl-1 gene, an anti apoptotic member of the Bcl-2 family. Our findings thus provide a potential mechanistic basis for the activation-induced resistance to apoptosis in human monocytes.