The effect of activating human monocytes in vitro with lipopolysaccharide (LPS) and muramyl dipeptide (MDP) on the production of cytostatic protein factor(s) (CF) has been investigated, and an antiserum against CF has been raised and tested. Upon incubation for 7 hr with LPS, in vitro differentiated human monocytes released CF. During LPS exposure, the presence of the protein synthesis inhibitor cycloheximide, at concentrations which reduced the overall protein synthesis by 60 and 80%, reduced the amount of CF released by only 20 and 40%, respectively. This indicates that the released CF was to a large extent already present in the monocytes before exposure to LPS. Compared to LPS, MDP induced only modest CF release. However, when lymphokine-activated monocytes were exposed to MDP, an increased CF release was observed. By immunizing a rabbit with CF purified by ion-exchange chromatography, chromatofocusing, and gel filtration, an antiserum was raised which neutralized the cytostatic activity released from monocytes exposed to LPS or lymphokines/LPS in sequence on the fourth day of culture. The cytostatic activity obtained by incubating freshly isolated monocytes with LPS was inhibited by the antiserum to a lesser extent, indicating the presence of other cytotoxins or cytotoxic cellular products in addition to CF in supernatants from freshly isolated monocytes. Various CF preparations were tested for IL-1 activity; no correlation between IL-1 activity and cytostatic activity was observed. Moreover, upon gel filtration the CF and IL-1 activities could be separated from each other and are consequently associated with different proteins.
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