GPI-anchored receptors do not have intracellular signaling domains. Instead, the mammalian Toll-like receptors (TLRs) transduce signals for cellular activation due to LPS binding. It has recently been shown that binding of microbial factors to TLRs to activate signal transduction occurs not on the cell surface, but rather in the phagosome of cells that have internalized the microbe. This interaction is probably due to the release of the microbial surface factor from the cell in the environment of the phagosome, where the liberated factor can bind to its cognate TLRs. TLRs initiate cellular activation through a series of signaltransducing molecules. | Chapter 114. Molecular Mechanisms of Microbial Pathogenesis Part 8 GPI-anchored receptors do not have intracellular signaling domains. Instead the mammalian Toll-like receptors TLRs transduce signals for cellular activation due to LPS binding. It has recently been shown that binding of microbial factors to TLRs to activate signal transduction occurs not on the cell surface but rather in the phagosome of cells that have internalized the microbe. This interaction is probably due to the release of the microbial surface factor from the cell in the environment of the phagosome where the liberated factor can bind to its cognate TLRs. TLRs initiate cellular activation through a series of signaltransducing molecules Fig. 114-3 that lead to nuclear translocation of the transcription factor nuclear factor kB NF-kB a master-switch for production of important inflammatory cytokines such as tumor necrosis factor a TNF-a and interleukin IL 1. Inflammation can be initiated not only with LPS and peptidoglycan but also with viral particles and other microbial products such as polysaccharides enzymes and toxins. Bacterial flagella activate inflammation by binding of a conserved sequence to TLR5. Some pathogens including Campylobacter jejuni Helicobacter pylori and Bartonella bacilliformis make flagella that lack this sequence and thus do not bind to TLR5. The result is a lack of efficient host response to infection. Bacteria also produce a high proportion of DNA molecules with unmethylated CpG residues that activate inflammation through TLR9. TLR3 recognizes double-strand RNA a pattern-recognition molecule produced by many viruses during their replicative cycle. TLR1 and TLR6 associate with TLR2 to promote recognition of acylated microbial proteins and peptides. The myeloid differentiation factor 88 MyD88 molecule is a generalized adaptor protein that binds to the cytoplasmic domains of all known TLRs and also to receptors that are part of the IL-1 receptor IL-1Rc family. Numerous .
