We further investigated the impact of norovirus infection on TLR4-mediated induction of IFN-β by γ-PGA. Our results suggest lack of significant alteration of TLR4 expression in norovirus-infected mice. Thus, responsiveness of TLR4 to γ-PGA was unlikely to be elevated upon norovirus infection. However, norovirus infection led to a ~2-fold increase in the expression of most TLRs, except TLR4 and TLR6 (Fig. 6a). TLR1 level was increased ~5-fold. Upregulated TLR expression could be in part due to the infiltration of leukocytes to the infected ileum. We envision that TLR4 signaling activation by γ-PGA might be potentiated through crosstalks with other upregulated TLRs. Among the upregulated TLRs, the endosomal TLRs including TLR3, TLR7, TLR8, and TLR9 may influence the sensitivity of TLR4 to γ-PGA early during norovirus infection. Currently, their involvement (except TLR3) in innate immune response to norovirus infection remains unknown24,55. Although norovirus infection did not affect the expression of TLR4 in the ileum, TLR responses to colon-resident microbiota and/or PAMPs derived from them might influence the downstream cascade of TLR4 signaling. Thus, induction of IFN-β by γ-PGA is likely to be modulated by commensal microbiome in the large intestine adjacent to the ileum.