Effects of serum on C3b-stimulated release of prostaglandins and thromboxane B2 from human monocytes.

Previous studies have demonstrated that C3b stimulates the release of prostaglandin E and thromboxane B2 from human monocytes in serum-free cultures. We have examined the influence of serum on this phenomenon, and have found that addition of normal human serum or fetal calf serum to such cultures at concentrations of 0.1-5.0% results in enhanced release of both eicosanoids in C3b-stimulated cultures, while release in control cultures is unaffected or, in many cases, inhibited. Addition of human serum albumin or transferrin to serum-free cultures is not sufficient to mimic the serum effect. Fetal calf and normal human sera increase the efficacy of C3b in stimulating release of both prostaglandin E and thromboxane B2 at all but the lowest doses tested, but fail to significantly alter the kinetics or release or the acquired unresponsiveness of monocytes to C3b that occurs following culture periods of greater than 24 h. By preincubation of purified C3b in normal human serum, it can be shown that such serum degrades C3b stimulatory activity, but at a rate that is slow enough to permit expression of its activity when added at the beginning of the culture period. In contrast, the stimulatory activity of E. coli lipopolysaccharide is unaffected by pretreatment with serum. Thus, in contrast to the inhibitory effect of serum on the activity of C3 fragments in in vitro assays of certain mononuclear cell functions such as lymphocyte transformation and antibody production, serum enhances the ability of C3b to stimulate monocyte prostaglandin release.