The Fas-FasL death receptor and PI3K pathways independently regulate monocyte homeostasis.

Peripheral blood-derived monocytes spontaneously undergo apoptosis mediated by Fas-Fas ligand (FasL) interactions. Activation of monocytes by LPS or TNF-alpha prevents spontaneous monocyte apoptosis through an unknown mechanism. Here, we demonstrate that LPS and TNF-alpha up-regulate Flip and suppress spontaneous Fas-FasL mediated monocyte apoptosis and caspase 8 and 3 activation. Flip was responsible for this protection, since inhibition of Flip by antisense oligonucleotides in the presence of LPS or TNF-alpha restored monocyte sensitivity to spontaneous apoptosis. We also investigated whether the PI3K pathway contributes to the suppression of spontaneous monocyte apoptosis mediated by LPS and TNF-alpha. Monocytes treated with a reversible PI3K inhibitor (LY294002) displayed enhanced apoptosis, while LPS and TNF-alpha partially protected against apoptosis mediated by LY294002. However, direct suppression of Fas-FasL interactions by addition of neutralizing anti-FasL antibody did not further suppress LY294002-induced apoptosis in the presence of LPS or TNF-alpha. Collectively, these data demonstrate that LPS or TNF-alpha protect monocytes from death receptor-mediated apoptosis through the up-regulation of Flip, but not apoptosis initiated by inhibition of the PI3K pathway.