Identification and characterization of a functional, alternatively spliced Toll‐like receptor 7 (TLR7) and genomic disruption of TLR8 in chickens

Based upon the recognition of antiviral compounds and single stranded viral RNA the Toll‐like receptors TLR7 and TLR8 are suggested to play a significant role in initiating antiviral immune responses. Here we report the molecular characterization of the chicken TLR7/8 loci which revealed an intact TLR7 gene and fragments of a TLR8‐like gene with a 6‐kilobase insertion containing chicken repeat 1 (CR1) retroviral‐like insertion elements. The chicken TLR7 gene encodes a 1047‐amino‐acid protein with 62% identity to human TLR7 and a conserved pattern of predicted leucine‐rich repeats. Highest levels of chicken TLR7 mRNA were detected in immune‐related tissues and cells, especially the spleen, caecal, tonsil and splenic B cells. Alternative spliced forms of TLR7 mRNA were identified in chicken, mouse and human and expressed in similar tissues and cell types to the major form of chicken TLR7. The chicken TLR7+ HD11 cell line and fresh splenocytes produced elevated levels of interleukin‐1β (IL‐1β) mRNA after exposure to the agonists R848 and loxoribine. Interestingly, none of the TLR7 agonists stimulated increased type I interferon (IFN) mRNA whereas poly(I:C) (a TLR3 agonist) up‐regulated both chicken IFN‐α and chicken IFN‐β mRNA. In contrast, TLR7 agonists, particularly R848 and poly(U) stimulated up‐regulation of chicken IL‐1β, and chicken IL‐8 mRNAs more effectively than poly(I:C). Stimulation of chicken TLR7 with R848 was chloroquine sensitive, suggesting signalling within an endosomal compartment, as for mammalian TLR7. The deletion of TLR8 in galliforms, accompanied with the differential response after exposure to TLR7 agonists, offers insight into the evolution of vertebrate TLR function.

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