Mechanisms of innate immune responses mediated by Toll-like receptors

The innate immune response is thought to be a rapid and nonclonal host defense. The recent discovery of Toll-like receptors (TLRs) and analyses of their physiological roles have established the notion that TLRs play a central role in innate immunity. Accumulating evidence suggests that individual TLRs recognize distinct ligands derived from bacterial components to generate specific cellular immune responses. In this review, we delineate the relationships between TLRs and microbial components, the TLR-mediated signaling pathways mainly based on cytoplasmic adaptor molecules containing Toll/interleukin-1R domains, the mechanism of TLR-mediated gene expression, and the involvement of TLRs in septic shock, including up-to-date observations.