Synergy between viral and bacterial TLR signaling, which leads to amplification of the inflammatory response, has been reported previously28. Additionally, Tian X. et al. found that Poly I:C enhanced susceptibility to secondary pulmonary infections by bacteria29. Co-stimulation with LPS and Poly I:C markedly enhances the immune response, although the mechanism for this combined effect remains poorly understood. Alexopoulou L. et al.16 found that when LPS was injected intraperitoneally, TLR3 expression in lung tissue was dramatically up-regulated, which suggests that the expression of TLR3 is inducible. Pan et al.30 provided direct evidence that LPS induces TLR3 expression via a TLR4-MyD88-IRAK-TRAF6-NF-κB-dependent signaling pathway in human peripheral blood monocytes and monocytic cell lines, such as THP-1 cells. Furthermore, we observed that LPS could induce TLR3 expression in a dose- and time-dependent manner in AMΦ. The stimulation of TLR4 by LPS induces the release of critical proinflammatory cytokines that are necessary to activate a potent immune response. Indeed, LPS/TLR4 signaling has been intensively studied in recent years313233. In our study, the role of TLR4 signaling in regulating TLR3 expression was clearly shown using TLR4−/− mice. LPS challenge in WT mice induced the up-regulation of TLR3, whereas this response was impaired in TLR4−/− mice. The major adaptor molecules that bind to the intracellular domain of TLR4 are MyD88 and TRIF3435, and our results showed that MyD88 mediated the TLR4-TLR3 cross talk, as LPS challenge of MyD88−/− mice failed to induce TLR3 expression. Furthermore, the activation of NF-κB was associated with an increase in TLR3 expression after LPS challenge and a reduced expression of TLR3 in AMΦ in which NF-κB was inhibited by IKK-NBD. Therefore, these results demonstrate an important role for TLR4-MyD88-NF-κB in mediating TLR3 expression.