In the present study, full length of canine and feline Toll-like receptor 4 (TLR4) cDNAs were sequenced, and the expression of canine and feline TLR4 mRNAs in dog and cat tissues were investigated. The full-length cDNA of TLR4 of dog and cat was 2709bp encoding 637 amino acids and 3113bp encoding 833 amino acids, respectively. The similarity of canine and feline TLR4 were 83.6% at the nucleotide sequence level and 77.6% at the amino acid sequence level. At the amino acid sequence level, canine and feline TLR4 showed sequence similarities of approximately 62-78% with those of Homo sapiens, Mus musculus, Bos taurus and Equus caballus, respectively. Southern hybridization analyses with TLR4 cDNA probes gave one distinct band in BamHI, EcoRI and HindIII digests of genomic DNA from dogs and cats, respectively, indicating the likely presence of a single TLR4 gene in each species. By RT-PCR analysis, mRNA of canine TLR4 was expressed highly in peripheral blood leukocytes (PBL), moderately in spleen, stomach and small intestine, at low levels in liver, with no expression in kidney, large intestine and skin. On the other hand, mRNA of feline TLR4 was expressed highly in lung, bladder and PBL, moderately in kidney, liver, spleen and large intestine and at low levels in pancreas and small intestine.
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