We have demonstrated that Gas6 expression in macrophages was blocked by LPS, and that the down-regulation of Gas6 also contributed to the LPS inhibition of phagocytosis. This result is consistent with a previous observation that Gas6-deficient macrophages exhibit impaired phagocytosis of apoptotic cells.26 Gas6 has been reported to mediate specifically phagocytosis of apoptotic cells by phagocytes.27,28 Accordingly, selleckchem we demonstrated that LPS inhibition of phagocytosis is restricted to the uptake of apoptotic cells. One key signal for engulfment of apoptotic cells is an externalized phosphatidylserine (PS) on the apoptotic cell surface.29

Gas6 binds, through its gamma- carboxyglutamic (GLA) domains, to PS exposed on cell surfaces.30 As a common ligand, Gas6 activates the TAM receptors through its carboxy-terminal immunoglobulin-like domains. Of these, Mer is critical for initiating Selleck Romidepsin phagocytosis signalling.27,31

Notably, Gas6 is a potent inhibitor of the production of pro-inflammatory cytokines, including TNF-α.32 It is reasonable to speculate that Gas6 may also facilitate phagocytosis through suppressing TNF-α. We noted a significant latency of the maximal inhibitory effect of LPS on phagocytosis in comparison to TNF-α. The reduction in the Gas6 level was also delayed in comparison to the induction of TNF-α in the medium after treatment with LPS. Therefore, we speculate that LPS-induced TNF-α is responsible for the LPS inhibition of macrophage phagocytosis in the earlier time after LPS treatment, and that LPS

suppression of Gas6 production is responsible for the inhibition of phagocytosis at a later time after the challenge. LPS induces TNF-α production in macrophages by activating TLR4. However, we showed that Gas6 expression in macrophages was suppressed by LPS in a TLR4-independent manner, as LPS suppression of Gas6 expression and inhibition of phagocytosis also occurred in TLR4−/− macrophages. This finding suggests that TNF-α and Gas6 act independently of one another in regulating the phagocytosis of apoptotic cells by macrophages. Understanding the mechanism underlying the LPS inhibition of Gas6 expression may have clinical implications. In conclusion, this article demonstrated that Immune system LPS inhibits the engulfing of apoptotic neutrophils by mouse peritoneal macrophages through LPS-mediated induction of TNF-α in a TLR4-dependent manner and suppression of Gas6 in a TLR4-independent manner in macrophages. These findings provide new insights into the role of inflammatory modulators in regulating phagocytic removal of apoptotic cells, which may be helpful in developing therapeutic approaches to the resolution of inflammation. This work was supported by the Special Funds for Major State Basic Research Project of China (Grant No. 2007CB947504) and the National Natural Science Foundation of China (Grant No. 30971459). The authors indicated no potential conflicts of interest.