Low-dose lipopolysaccharide (LPS) pretreatment of mouse macrophages modulates LPS-dependent interleukin-6 production in vitro

Lipopolysaccharide (LPS) can induce mouse macrophages to produce a number of cytokines and other inflammatory mediators. Our laboratory previously reported that LPS-dependent macrophage-derived tumor necrosis factor alpha (TNF-alpha) production could be significantly potentiated by pretreatment with LPS at substimulatory LPS priming doses. The observed potentiation was shown to be coincident with a down-regulation of LPS-dependent nitric oxide (NO) production (X. Zhang and D. C. Morrison, J. Exp. Med. 177: 511-516, 1993). In order to determine whether these LPS reprogramming effects in mouse macrophages were selective for these two macrophage-derived mediators, we have examined the effects of LPS pretreatment on LPS-dependent interleukin 6 (IL-6) production. Thioglycolate-elicited mouse peritoneal macrophages were pretreated with various subthreshold stimulatory concentrations of LPS for 6 h, washed three times, and then stimulated with an effective stimulatory concentration of smooth LPS for 18 h. In confirmation of earlier studies, pretreatment of mouse macrophages with substimulatory doses of LPS inhibited the subsequent LPS-dependent NO production. This down-regulation was accompanied by a coordinate up-regulation of LPS-dependent IL-6 production, similar to what was shown earlier for TNF-alpha production. These priming effects with the substimulatory dose of smooth LPS are shown to be independent of doses of LPS used for subsequent activation and are not restricted to specific LPS stimulation. Moreover, the enhancement of the IL-6 response by LPS pretreatment is still observed in the presence of neutralizing antibody to TNF-alpha. These findings, therefore, provide further support for the conclusion that LPS-dependent macrophage reprogramming is likely to involve common regulatory pathways that control the secretion of both IL-6 and TNF-alpha.