Negative regulation of macrophage activation in response to IFN-gamma and lipopolysaccharide by the STK/RON receptor tyrosine kinase

IFN-gamma primes macrophages for antimicrobial activity, increased killing of intracellular pathogens, and Ag processing and presentation to lymphocyt es by cooperating with a second signal (provided by LPS or endogenous TNF-a lpha) to promote increased proinflammatory cytokine production, NO producti on, and MHC class II expression, Macrophage-stimulating protein (MSP) suppr esses NO production by activated peritoneal macrophages in vitro. Furthermo re, targeted deletion of the receptor for MSP, stem cell-derived tyrosine k inase receptor (STK/RON), resulted in increased production of NO by activat ed macrophages both in vitro and in vivo. Here we demonstrate that expressi on of STK in RAW264.7 cells resulted in suppression of NO production follow ing IFN-gamma(+/-) LPS stimulation in the presence of MSP, reflecting a dec rease in the levels of inducible NO synthase (iNOS) mRNA and protein, which was confirmed by decreased trans-activation of an iNOS reporter. The iNOS expression is regulated by the coordinate activity of the inducible transcr iption factors STAT-I, IFN response factor-1, and NF-kappa B, The presence of the STK receptor did not significantly alter the expression of the IFN-g amma receptor, STAT1 phosphorylation, or the up-regulation of IFN response factor-1 expression following IFN-gamma stimulation. However, nuclear trans location of NF-kappa B following stimulation of RAW cells with IFN-gamma an d LPS was reduced in the presence of the MSP/STK signaling pathway. These r esults suggest that the negative regulation of macrophage responses by MSP/ STK occurs at least in part via inhibition of costimulatory signals, result ing in NF-kappa B activation, that cooperate with IFN-gamma to promote acti vation.