Differential effects of serum on lipopolysaccharide receptor-directed macrophage activation for nitric oxide...

In this manuscript, the effects of fetal bovine serum (FBS) on activation of mouse macrophages through the p73 lipopolysaccharide (LPS) receptor have been evaluated. In confirmation of earlier published studies, FBS will significantly potentiate the ability of LPS to activate macrophages to produce nitric oxide (NO). Evidence that this potentiating effect of FBS is mediated primarily by an interaction with LPS is provided by data showing that the stimulating effects of a hamster IgM monoclonal antibody to the p73 LPS receptor are significantly suppressed under identical conditions of FBS addition. The presence of FBS enhances both the kinetics of LPS-induced NO production and delays the induction of MAb5D3-induced NO production. The data establish that the ability of FBS to reduce MAb5D3-initiated NO production can only be manifest during the first 4-6 h following activation with antibody. Similarly, the ability of polymyxin B (which does not affect MAb5D3 activity) to inhibit LPS-dependent macrophage activation is also only effective during the first 4-6 h of stimulation, suggesting a parallel kinetic profile in the activation of the inducible NO synthase by these related activators. These studies provide data which suggest a dual role for serum factors in LPS-dependent macrophage activation, a direct effect of FBS on LPS which potentiates its immunostimulatory activity, and a secondary down-regulation effect which is manifest at the level of the macrophage.