TLR proteins are a family of type I transmembrane receptors characterized by an NH2-terminal extracellular leucine-rich repeat (LRR) domain, which mediate the recognition of their respective PAMPs, and a COOH-terminal intracellular tail containing a conserved region called the Toll/interleukin 1 (IL-1) receptor (TIR) homology domain. The TIR domain is the defining motif of the TLR/IL-1 superfamily, and it is likely to be one of the earliest signaling domains to have evolved (17) . TLRs can recognize a diverse range of PAMPs, generate inflammatory signals to coordinate innate immune responses and modulate adaptive immune responses. The list of TLR ligands is growing. However, the ligand for TLR10 and mouse TLR8 remains unknown at present. Activation of TLRs initiates two major pathways: the MyD88-dependent pathway, which is used by all TLRs except TLR3, resulting in the activation of nuclear factor (NF)-κB and activator protein-1 (AP-1); and the TRIF-dependent pathway, which is initiated by TLR3 and TLR4, resulting in the activation of type I interferons (IFNs) (13, 18, 19) . Expression of numerous proinflammatory cytokines, such as tumor necrosis factor (TNF)-α, IL-6, IL-12 and IFNs, is one of the major outcomes of the activation of the pathways (15) . TLR signaling is summarized and shown in Figure 2 .