SELECTIVE EFFECTS OF SERUM ON BACTERIAL LPS-INDUCED IL-6 AND NITRIC-OXIDE PRODUCTION IN MURINE PERITONEAL-MACROPHAGES

In lipopolysaccharide (LPS)-dependent activation of human monocytes, t he primary function of serum has been thought to provide a source of L PS-binding protein (LBP) for complex formation with LPS preparatory to CD14 binding and initiation of signal transduction. In this report we have investigated the contribution of serum factors in the mouse macr ophage response to LPS. Our results show that the production of interl eukin-6 (IL-6) by in vitro LPS-stimulated mouse peritoneal macrophages is suppressed in the presence of serum. In contrast, optimal producti on of nitric oxide (NO) by LPS-stimulated macrophages requires the pre sence of serum. Detailed kinetic studies indicate that these observati ons are not exclusively the result of differences in the time course o f NO secretion. These findings contrast with equivalent studies carrie d out using human PBMC, where, under identical conditions of in vitro culture, the presence of serum markedly potentiates LPS-dependent IL-6 production. We have confirmed by RT-PCR, using specific primers for I L-6 and inducible nitric oxide synthase (iNOS), that the effects obser ved are manifest primarily at the level of mRNA expression. Enhancemen t of NO responses and suppression of IL-6 responses are both dependent upon serum concentration. These potentiating and inhibiting effects o f serum are less apparent with a second microbial stimulus (heat-kille d Staphylococcus aureus). Collectively, these results indicate that se rum effects on mouse macrophages are multifactorial and, depending upo n the particular macrophage response to be measured, can be either enh ancing or suppressing. These findings would not, therefore, support th e concept of an obligatory role for LBP (and by inference CD14) in the in vitro mouse macrophage response to LPS.