The stimulation of Toll-like receptor 4 (TLR4) by lipopolysaccharide (LPS) induces the release of proinflammatory cytokines that activate immune responses. The Tlr4Lps-del spontaneous mutation corresponds to a 74723 bp deletion that completely removes the Tlr4 coding sequence. No mRNA or protein is expressed. Homozygous mutants exhibit a defective response to LPS stimulation. The functionally similar Tlr4Lps-d mutation found in C3H/HeJ mice (#000659) is a point mutation that causes an amino acid substitution.
Tlr4 -deficient mice display significantly reduced expression of proinflammatory genes compared to controls 24 h after reperfusion triggered by retinal ischemic injury. These include transcriptional factor p65 (Rela), tumor necrosis factor (Thf), interleukin 6 (Il6), chemokine (C-C motif) ligand 2 (Ccl2), chemokine (C-C motif) ligand 5 (Ccl5), chemokine (C-X-C motif) ligand 10 (Cxcl10), cytochrome b-245, beta polypeptide (Cybb), nitric oxide synthase 2 (Nos2), and intercellular adhesion molecule 1 (Icam1) (Dvoriantchikova et al., 2010).
The effect of TLR4 on tumor progression appears to depend on which cells TLR4 resides and the particular ligand involved. For example, enhanced prostrate tumor progression as a result of TLR4 response to a tumor-derived ligand such as Peroxiredoxin is not seen in the Tlr4 deficient mutant mice (Riddell et al., 2011). Conversely, tumor inhibition seen in wild type mice when B16 melanoma cells were stimulated in vitro with LPS was not seen in mutant mice (Nunez et al., 2012).
TLR4 signaling may also act as innate neuroprotective mechanism through clearance of alpha-synuclein as TLR4 ablation impairs the phagocytic response of microglia to alpha-synuclein and enhances neurodegeneration (Stefanova et al. 2011).
