[Antiinfective host defense mechanism: toll-like receptors and innate immunity]

The innate immune system has evolved as the first line of defense against invading microorganisms. The recent discovery of the toll-like receptors(TLRs) has rapidly expanded our knowledge of molecular events that initiate host-pathogen interactions. The TLRs, which are expressed on the surface of cells, involved in innate immune recognition, including macrophages and dendritic cells, have a crucial role in the detection of microbial infection. Signals initiated by the interaction of TLRs with pathogen-associated molecular patterns (PAMPs) induce activation of the inflammatory and antimicrobial innate immune response. Ten members of the TLR family have been identified, and they appear to recognize PAMPs, including lipopolysaccharide, peptidoglycan, and bacterial DNA. There has been considerable interest in how adaptive immune responses are controlled by the innate immune system. Recent studies have suggested that TLRs may control the induction of Th1 responses and that a separate system of recognition regulates the Th2 response. Thus TLR signaling represents a key component in the innate immune response to microbial infection.