ABSTRACT
The livestock industry plays important role in the economy through the production of milk, meat and skin. Every year there are huge losses in these industries due to bacterial and viral diseases (in particular, mastitis and foot-and-mouth diseases). Toll-like receptors are important pattern-recognition receptors which are activated after exposure to diverse classes of pathogens to activate immune response. TLRs consist of several LRR repeats, divided into three domains: the ectodomain, which recognizes the pathogen; the transmembrane domain, for localization; and the TIR domain, which recruits different downstream signaling adaptors. The total of ten TLRs have evolved and are characterized based on different pathogen load in livestock. Depending on their localization in the cell, they are divided into two groups. The first group TLRs (TLRs 1, 2, 4, 5 and 6) are found on the membrane, and the second group (TLRs 3, 7, 8 and 9) on intracellular endosomes. After pathogen exposure, all TLRs recruit the MyD88 signaling pathway, except TLRs 3 and 4, which recruit TRIF signaling. Mutation in these receptors is associated with several diseases in livestock. Phylogenetic analysis of TLR groups results endosomally expressing TLRs 3, 7, 8 and 9 (viral, bacterial CpG) at same clade, and gram-negative-recognizing TLR4 is close to gram-positive TLR2. Flagellin-recognizing TLR5 lies between TLRs 4 and 3, like other species.
