REGULATION OF HUMAN MONOCYTE APOPTOSIS BY THE CD14 MOLECULE

Bacterial products such as LPS have been shown to activate monocytes a nd to increase CD14 expression, while antiinflammatory cytokines, i.e. , IL-4, down-regulate CD14. Furthermore, activation of monocytes incre ases survival, whereas deactivation evokes apoptosis (programmed cell death, PCD), This correlation among activation, CD14 expression, acid the lifespan of the cells prompted us to investigate the role of CD14 in monocyte apoptosis, The effects of LPS and IL-4 on the expression o f CD14, indicated by binding of Leu M3 Ab, and PCD of monocytes were s tudied simultaneously and in a kinetic fashion by multiparameter flow cytometry, Monocyte PCD was determined by binding of FITC-conjugal ed annexin V, which indicates apoptotic cell death in early stages, and w as confirmed using well-established detection methods, i.e., DNA elect rophoresis, electron microscopy, or colorimetric DNA staining. The pre sent study shows that the LPS-induced increase in CD14 expression resc ued monocytes from apoptosis, where eas IL-4 treatment first down-regu lated CD14 expression and consecutively evoked apoptosis. CD14(-)/anne xin V-monocytes were not apoptotic as confirmed by DNA electrophoresis , whereas CD14(-)/annexin V+ monocytes showed clear apoptotic features , Kinetic studies ruled out that monocytes first bound annexin V and l ater lost the CD14 Ag, Other molecules, such as HLA-A, -B, and -C Ags, were not down-regulated during apoptosis, Enzymatic removal of membra ne-bound CD14 by phosphatidylinositol-specific phospholipase C evoked PCD similarly to IL-4, These results suggest that regulation of CD14 r eceptor expression is an early effector mechanism mediating life ol de ath of monocytes, Down-regulation or removal of the receptor triggers apoptosis, whereas up-regulation promotes survival.