The monocyte/macrophage has been identified as an effector cell infiltrating the glomerulus in human and experimental nephritis. To clarify the role of the macrophage in this context, an in vitro system was developed in which mouse mesangial cell cultures were maintained. Macrophage supernatants were obtained from peritoneal macrophages harvested from either resident or endotoxin-stimulated C57BL/6J male mice cultured for 24 hr. Incubation of mesangial cell cultures with macrophage supernatants resulted in depression of mesangial cell metabolism as indicated by incorporation of (3H)-thymidine; the effect was more marked when supernatants of endotoxin-treated mice were used. The molecular mechanisms by which suppression was obtained was clarified by experiments fractionating macrophage supernatants by G-100 column chromatography. By this means, two fractions were obtained with different molecular and physiologic properties. One fraction, a molecular size of 14,600 to 29,000 daltons, was shown to mediate the suppressive effect by stimulating endogenous mesangial cell PGE synthesis; additionally, a novel molecular species was identified, which was biologically active at higher concentrations of macrophage supernatants, had a larger molecular size (29,000 to 68,000 daltons), exerted its suppressive effect by an independent mechanism, and accounted for the inability of indomethacin pretreatment of mesangial cells to abrogate completely the suppressive effect of macrophage supernatant at higher concentrations.

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