In mammals, TLR7 and TLR8 belong to TLR9 subfamily and senses RNA in endosomal compartments, initiating the MyD88 signaling cascade.
Although functional assays showed that chicken cells respond to known mammalian TLR7 and TLR9 ligands in vitro and in ovo [Philbin et al 2005, Jenkins KA et al 2009], bioinformatics analysis of the chicken genome have identified a disrupted ortholog of TLR8 and no direct ortholog of TLR9. Only TLR7 gene was found intact[Philbin et al 2005, Temperley ND et al 2008].The chicken TLR7 (chTLR7) amino acid sequence shows 62% identity to its human counterpart. chTLR7 mRNA was detected in immune-related tissues - spleen, caecal tonsil [Iqbal et al 2005, Phiblin et al 2005]. Chicken leukocytes, heterophils, monocytes and the chicken macrophage cell line HD11 have been also shown to express chTLR7 [Philbin et al 2005].