Avian toll-like receptors

Analysis of the genomes of two distantly related bird species, chicken and zebra finch (divergence of about 100 million years), indicate that there are ten avian toll-like receptors and that five of these, TLR2a, 2b, 3, 4, 5 and 7, are clear orthologs to TLRs found in mammals. Duplication of genes has led to TLR1La and 1Lb, TLR2a and 2b, and two TLR7s in the zebra finch. Avian TLR21 may be orthologous to TLR21 found in fish and amphibians, and avian TLR15, which is phylogenetically related to the TLR2 family, appears to be unique to avian species. While TLR2 is conserved between mammalian and avian species, the other TLR2 family members evolved independently. Dimerization between either of the two avian TLR2 species and TLR1La or 1Lb permits recognition of the same broad range of molecules as recognized by mammalian TLR2 dimerized with either TLR1, 6 and 10. Similarly, while TLR9 has been lost from the avian genome, DNA high in unmethylated CpG motifs is immunostimulatory through avian TLR21 which is absent in mammals. Thus, while some TLR members were commonly retained in both mammals and birds, others were separately lost or gained, or diverged independently; but broadly speaking the TLRs of the two classes of vertebrates evolved to recognize very similar spectra of microbial products. Components of downstream TLR signaling are also mostly conserved but with some losses in avian species; notably, TRAM is absent in avian genomes and, hence, the TRIF/TRAM-dependent signaling pathway utilized by mammals in LPS activation appears to be absent in birds.