Novel role of ICAM3 and LFA-1 in the clearance of apoptotic neutrophils by human macrophages.

Apoptotic cells express eat-me signals which are recognized by several receptors mainly on professional phagocytes of the mononuclear phagocyte system. This "engulfment synapse" can define a safe and effective clearance of apoptotic cells in order to maintain tissue homeostasis in the entire body. We show that the expression of four genes related to apoptotic cell clearance is strongly up-regulated in human macrophages 30 min after administration of apoptotic neutrophils. Out of these the significant role of the up-regulated intercellular adhesion molecule 3 (ICAM3) in phagocytosis of apoptotic neutrophils could be demonstrated in macrophages by gene silencing as well as treatment with blocking antibodies. Blocking ICAM3 on the surface of apoptotic neutrophils also resulted in their decreased uptake which confirmed its role as an eat-me signal expressed by apoptotic cells. In macrophages but not in neutrophils silencing and blocking integrin alphaL and beta2 components of lymphocyte function-associated antigen 1 (LFA-1), which can strongly bind ICAM3, resulted in a decreased phagocytosis of apoptotic cells indicating its possible role to recognize ICAM3 on the surface of apoptotic neutrophils. Finally, we report that engulfing portals formed in macrophages during phagocytosis are characterized by accumulation of ICAM3, integrin alphaL and beta2 which show co-localization on the surface of phagocytes. Furthermore, their simultaneous knock-down in macrophages resulted in a marked deficiency in phagocytosis and a slight decrease in the anti-inflammatory effect of apoptotic neutrophils. We propose that ICAM3 and LFA-1 act as recognition receptors in the phagocytosis portals of macrophages for engulfment of apoptotic neutrophils.

The glucocorticoid dexamethasone programs human dendritic cells for enhanced phagocytosis of apoptotic neutrophils and inflammatory response.

GCs are powerful anti-inflammatory compounds inhibiting inflammatory cell recruitment and production of proinflammatory cytokines. We have recently found that DCs, the key players of T cell priming and polarization, respond to allogeneic apoptotic neutrophils with proinflammatory cytokine release and Th1 cell activation. Here, we show that monocyte-derived human DCs develop their capacity to engulf apoptotic cells by up-regulating a set of apoptophagocytic genes. This gene expression pattern was reprogrammed when differentiation took place in the presence of the synthetic GC Dex, which increased the expression of phagocytosis receptors MERTK and CD14, the bridging molecule C1QA, DNASE2, and ADORA3. The increased phagocytosis was attenuated by the addition of ADORA3 antagonist and could not be observed when bone marrow-derived DCs of ADORA3 KO mice were treated with Dex. The GC-treated human DCs loaded with allogeneic apoptotic neutrophils secreted, in response to LPS and IFN-γ, the inflammatory cytokine TNF-α. Furthermore, the Dex-treated DCs could activate autologous T lymphocytes toward Th1 effector cells, and this was enhanced by their exposure to allogeneic apoptotic neutrophils.