Macrophage activation in response to microbial infection depends on Toll-like receptors (TLRs), which are a family of pattern recognition receptors (PRRs) and key regulators of both innate and adaptive immunity4. TLRs are among the most well-studied PRRs because of their ability to detect a variety of pathogen-associated molecular patterns (PAMPs), such as lipids, proteins, lipoproteins, and nucleic acids56. To date, 10 human and 12 murine TLRs have been identified, and each TLR has a specific set of ligands. Specifically, TLR4 recognizes the lipopolysaccharide (LPS) of gram-negative bacteria78, while TLR3 recognizes viral double-stranded RNA (dsRNA), which is a common intermediate of viral replication and a potent indicator of infection. TLR3 also responds to the synthetic analog polyriboinosinic:polyribocytidylic acid (poly (I:C)) to induce type I interferon (IFN) and inflammatory cytokine/chemokine production910. TLR3 is expressed on immune cells, including myeloid dendritic cells (DCs) and macrophages, as well as non-immune cells such as fibroblasts, epithelial cells, and neurons11. Macrophages express TLR3 both on their cell surface and in the early endosome. Although TLR3 on the cell surface participates in dsRNA recognition, TLR3-mediated signaling is initiated from the endosomal compartment1213. In addition, RNA released from damaged cells or mRNA can also be recognized by TLR31415. Several studies in TLR3-deficient mice have demonstrated that TLR3 participates in the generation of protective immunity against some viral infections. Numerous studies have documented the importance of TLR cross talk, and multiple signaling pathways contribute to synergistic TLR ligand-dependent cytokine expression. In 2001, Alexopoulou L. et al.16 reported in Nature that following an intraperitoneal injection of LPS into mice, dramatic up-regulation of the expression of TLR3 mRNA was observed in all tissues except the thymus, suggesting that the expression of TLR3 is inducible. Of note, a particularly high expression level of TLR mRNA was observed in lung tissue. These results raised the possibility that bacterial infection can induce sensitivity to viral infection. This observation prompted us to further investigate the mechanism of TLR4-TLR3 cross talk in AMΦ.