Macrophage Suppression By a Low Molecular Weight Fraction of Murine Spleen Cell Culture Supernatant

Macrophage suppression has been shown to be mediated by a unique, low molecular weight fraction of murine serum. The present investigation involves the in vitro production of this macrophage modulator (suppressor) by Concanavalin A-stimulated spleen cells. Spleen cell culture supernatant containing macrophage suppressor factor (MSF) caused a significant decrease in in vitro phagocytosis of Listeria monocytogenes by non-elicited peritoneal macrophages. The molecular weight of MSF was determined by ultrafiltration to be less than 10,000, and the modulating activity of MSF was not altered by heating at 100°C for 30 minutes or freezing at -70°C for six months. MSF is resistant to treatment with Pronase E, but is, however, sensitive to acid hydrolysis. Activity of MSF in spleen cell culture supernatants from normal mice does not differ from supernatants from mice immunized with L. monocytogenes. It was therefore concluded that MSF is not affected by antigenic stimulation and is apparently produced constituitively. An important consequence of the presence of this factor could be macrophage modulation and, thus, regulation of the macrophage function of the cellular branch of the immune system