Analysis of the effects of lipopolysaccharide on macrophages: differential phagocytic responses of C3H/HeN and C3H/HeJ macrophages in vitro

The phenomenon of lipopolysaccharide (LPS)-induced in vitro macrophage cytotoxicity has been reported by a number of investigators but has often been difficult to reproduce and to quantitate. In this report, we have examined the effect of LPS on the ability of macrophages to ingest 51Cr-labeled, opsonized sheep erythrocytes as a method for examining the direct toxic effects of LPS on macrophages in vitro. By using this assy, we can clearly discriminate between LPS responder C3H/HeN macrophages and LPS nonresponder C3H/HeJ macrophages and demonstrate that LPS induces a profound inhibition of Fc-mediated phagocytosis in LPS responsive macrophages. Furthermore, low concentrations of LPS stimulate phagocytosis in macrophages derived for C3H/HeJ mice. The lipid A moiety of the LPS is responsible for the observed enhancement or inhibition of Fc-mediated phagocytosis. This assay was more sensitive than LPS-induced cytotoxicity, since inhibition of phagocytosis was detectable in cultures of LPS-sensitive macrophages even when cytotoxicity, assessed by trypan blue exclusion, was not. Thus, this assay represents an extremely sensitive method for analyzing the direct effects of LPS on macrophages.