Selective effects of serum on bacterial LPS-induced IL-6 and nitric oxide production in murine peritoneal macrophages

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Abstract
In lipopolysaccharide (LPS)-dependent activation of human monocytes, the primary function of serum has been thought to provide a source of LPS-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 macrophage response to LPS. Our results show that the production of interleukin-6 (IL-6) by in vitro LPS-stimulated mouse peritoneal macrophages is suppressed in the presence of serum. In contrast, optimal production of nitric oxide (NO) by LPS-stimulated macrophages requires the presence of serum. Detailed kinetic studies indicate that these observations are not exclusively the result of differences in the time course of NO secretion. These findings contrast with equivalent studies carried 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 IL-6 and inducible nitric oxide synthase (iNOS), that the effects observed are manifest primarily at the level of mRNA expression. Enhancement of NO responses and suppression of IL-6 responses are both dependent upon serum concentration. These potentiating and inhibiting effects of serum are less apparent with a second microbial stimulus (heat-killed Staphylococcus aureus). Collectively, these results indicate that serum effects on mouse macrophages are multifactorial and, depending upon the particular macrophage response to be measured, can be either enhancing or suppressing. These findings would not, therefore, support the concept of an obligatory role for LBP (and by inference CD14) in the in vitro mouse macrophage response to LPS.