The purpose of this study was to determine if interleukin (IL)-10 inhibits lipopolysaccharide (LPS)-induced IL-6 production in microglia by inhibiting activation of nuclear factor-κB (NF-κB). N13 microglia (a murine microglial cell line) and primary microglia from neonatal mice were cultured in the presence or absence of LPS and increasing amounts of murine IL-10 for 24 h. As predicted, LPS treatment increased supernatant IL-6 concentration in both N13 and primary microglia cultures. Pretreatment with IL-10, however, decreased LPS-induced IL-6 secretion in a dose-dependent manner in both culture systems. Likewise, ribonuclease protection assays showed that LPS increased steady-state IL-6 mRNA levels, but that pretreatment with IL-10 blocked the LPS-induced increase in IL-6 mRNA. Because NF-κB is the predominant transcription factor responsible for IL-6 transcription in response to inflammatory stimuli, it was hypothesized that IL-10 inhibited IL-6 production by preventing nuclear translocation of NF-κB. Consistent with this idea, LPS increased nuclear translocation of NF-κB as assessed by gel mobility shift assay. Supershift assays and immunocytochemical staining showed that both the p50 and p65 subunits of NF-κB translocated from the cytoplasm to the nucleus upon LPS stimulation. Pretreatment with IL-10, however, inhibited LPS-induced activation of NF-κB. Furthermore, inhibition of NF-κB activity with tosyl-Phe-chloromethlyketone (a serine protease inhibitor that prevents degradation of the NF-κB-IκB complex), completely blocked LPS-induced IL-6 production. These data suggest that IL-10 inhibited IL-6 production in microglia by decreasing the activity of NF-κB and, therefore, extend what little is known of the intricate relationship between anti-inflammatory and inflammatory cytokines in the central nervous system. (C) 2000 Elsevier Science B.V.