The Duck Toll Like Receptor 7: Genomic Organization, Expression and Function

TLR7 and TLR8 are triggered by antiviral compounds and single-stranded RNA, and are implicated in the immune response to viruses such as influenza. Chickens have a functional TLR7 gene and a genomic disruption in TLR8. PCR evidence suggested that the TLR8 disruption existed in galliform birds, but not anseriform birds. We investigated this due to the different susceptibility of ducks and chickens to influenza. We sequenced a 39 kb duck genomic clone spanning the TLR7/8 locus. Similar to chicken, we detect only fragments of the TLR8 gene in the region downstream of TLR7. Duck TLR7 shares only 85% amino acid identity to chicken TLR7, differing primarily in the ligand-binding LRR domains. TLR7 transcripts were most abundantly expressed in duck spleen, bursa and lung. Duck splenocytes respond to TLR7 agonists with upregulation of mRNA encoding proinflammatory cytokines and IFN alpha, with imiquimod eliciting the greatest response. TLR7 expression in lung, and upregulation of IFN alpha by TLR7 agonists, not typically seen in chickens, could contribute significantly to the antiviral defense of ducks. These differences in TLR7 function, not genomic organization, may contribute to the differential susceptibility of avian species to viral infection.