Hidden effects of polymyxin B: Its role in activating TLR2-mediated inflammatory responses

Polymyxins are cyclic cationic lipopeptide antibiotics that disrupt both the outer and inner membranes of Gram-negative bacteria. They also bind to bacterial lipopolysaccharides (LPSs), thereby inhibiting LPS recognition by Toll-like receptor 4 (TLR4) and preventing macrophage activation. However, the immunomodulatory effects of polymyxin B during infection, in which multiple host- and bacteria-derived factors coexist, remain unclear. In this study, we examined the impact of polymyxin B, a representative polymyxin antibiotic, on mouse macrophage responses in the presence of interferon γ (IFN-γ), with or without LPS stimulation. Polymyxin B induced nitric oxide (NO) production when macrophages were exposed to high concentrations of IFN-γ. In the presence of lower concentrations of IFN-γ, polymyxin B alone cannot induce NO production, but together with LPS can cause the induction of NO production. These NO productions were not largely affected by TLR4 deficiency but were attenuated when TLR2 was deficient. Furthermore, polymyxin B-immobilized agarose beads precipitated TLR2 from cell extracts, indicating an association between polymyxin B and TLR2. This association was not inhibited by LPS, although polymyxin B and LPS synergistically induced NO production. Collectively, these findings demonstrate that polymyxin B (together with LPS) associates with TLR2 and functions as a TLR2 agonist, suggesting that polymyxin B preparations can induce macrophage activation under conditions in which IFN-γ and LPS are present.