Our results demonstrate induction of type I IFN production primarily by the activation of TRIF-dependent pathway in mice following multiple peroral administration of γ-PGA. Previous studies have reported the primary activation of MyD88-dependent pathway by LPS in eliciting the secretion of numerous proinflammatory cytokines1–4,8. As a TLR4 ligand, γ-PGA is distinct from LPS in selectively increasing the serum IFN-β titers, but not of other inflammatory cytokines, namely IL-1α/β, IL-6, and TNF-α. This difference in the cytokine profile induced by γ-PGA and LPS suggests that a regulatory network different from that of LPS should control γ-PGA-mediated TLR4 activation. It remains unclear whether other non-LPS TLR4 ligands including endogenous TLR4 agonists such as heat shock proteins and cholesteryl ester hydroperoxides8 upon being sensed by the same receptor also behave differently than LPS in terms of the cytokine profile. Another unexpected finding was that the elevated serum IFN-β levels in mice after multiple oral administrations of γ-PGA (once daily for three days) neither increased nor decreased following two additional oral administrations. These results suggest the absence of tolerance phenomena with γ-PGA, which is normally seen with endotoxin, or LPS, wherein, innate immune cells pre-exposed to LPS become refractory to subsequent challenges47. Based on the results of our present study, γ-PGA seems to be a new class of TLR4 agonists that selectively activates the TRIF-dependent pathway.