| Summary: | Chemokines triggered by Toll-like receptors (TLRs) are small chemoattractant
proteins, which mainly regulate leukocyte trafficking in inflammatory reactions via
interaction with G protein-coupled receptors. Forty-two chemokines and 19 cognate
receptors have been found in the human genome. Prior to this study, only 11 chicken
chemokines and 7 receptors had been reported. The objectives of this study were to
identify systematically chicken chemokines and their cognate receptor genes in the
chicken genome and to annotate these genes and ligand-receptor binding by a
comparative genomics approach. Twenty-three chemokine and 14 chemokine receptor
genes were identified in the chicken genome. The number of coding exons in these genes
and the syntenies are highly conserved between human, mouse, and chicken although the
amino acid sequence homologies are generally low between mammalian and chicken
chemokines. Chicken genes were named with the systematic nomenclature used in
humans and mice based on phylogeny, synteny, and sequence homology. The independent nomenclature of chicken chemokines and chemokine receptors suggests that
the chicken may have ligand-receptor pairings similar to mammals.
The TLR family represents evolutionarily conserved components of the patternrecognizing
receptors (PRRs) of the innate immune system that recognize specific
pathogen-associated molecular patterns (PAMPs) through their ectodomains (ECDs).
TLR's ECDs contain 19 to 25 tandem copies of leucine-rich repeat (LRR) motifs. TLRs
play important roles in the activation of pro-inflammatory cytokines, chemokines and
modulation of antigen-specific adaptive immune responses. To date, nine TLRs have
been reported in chicken, along with a non-functional TLR8. Two non-mammalian
TLRs, TLR21 and TLR22, have been identified in pufferfish and zebrafish. The
objectives of this study were to determine if there is the existence of chicken genes
homologous to fish-specific TLRs, and if possible ligands of these receptors exist. After
searching the chicken genome sequence and EST database, a novel chicken TLR
homologous to fish TLR21 was identified. Phylogenetic analysis indicated that the
identified chicken TLR is the orthologue of TLR21 in fish. Bioinformatic analysis of
potential PAMP binding sites within LRR insertions showed that CpG DNA is the
putative ligand of this receptor.
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