Identification of the molecular mechanism by which TLR ligation and IFN-gamma synergize to induce MIG.

Monokine Induced by Interferon-gamma (MIG), a CXC chemokine, is a potent inducer of T-cell chemotaxis and activation and has been implicated in the host response to viral infections and tumor immunity as well as in the pathogenesis of autoimmunity and transplant rejection. Although it is known that the Toll-Like Receptor-4 (TLR-4) ligand LPS synergizes with IFN-gamma to induce MIG expression in macrophages, the molecular mechanisms responsible for the synergy have yet to be elucidated. We determined that the marked synergy between LPS and IFN-gamma on MIG mRNA expression in mouse macrophages is a result of LPS-induced NF-kappaB and IFN-gamma-induced STAT. The synergy was not dependent on new protein synthesis, was independent of TNF-alpha, and occurred at the level of gene transcription. We identified 2 NF-kappaB sites located at -154 and -129 of the MIG promoter proximal to the gamma-responsive element that mediated this effect. Finally, we demonstrated that other TLR ligands (zymosan, double stranded RNA and CpG) synergized with IFN-gamma to induce MIG in an NF-kappaB dependent fashion. These data emphasize the ability of bacterial and viral products to activate/modify immune responses and promote adaptive T cell immunity through the NF-kappaB pathway.