Upon virus infection, upregulation of TLR4 or crosstalk between TLR signaling pathways35 might alter the sensitivity of TLR4 to γ-PGA. Alterations in the expression profile of TLRs including TLR4 following norovirus infection was further investigated. TLR1-9 mRNA expression levels were not altered by more than 2-fold in MNV-1-infected RAW264.7 cells (Fig. 2f). Neither norovirus infection nor γ-PGA treatment enhanced TLR4 expression. Immunoblotting analyses detected pIRF-3, indicating that both these stimuli activated TLR4 signaling as evidenced by the activation of IRF-3 (Fig. 2g). Similarly, neither TLR3, a sensor for norovirus dsRNA24 nor TLR2, which could be a potential innate immune sensor for not-yet-identified norovirus-derived or induced PAMPs, was upregulated by norovirus infection or γ-PGA. These results suggest that the antiviral activity of γ-PGA is not dependent on the upregulation of TLRs.