TIR8 expression in tissues of adult and embryo chicken

The orphan receptor TIR8/SIGIRR, belongs to the TIR superfamily and plays an important role in the inflammatory responses. TIR8 does not activate NF-kB and IRF3 and negatively modulates the inflammatory responses. It acts as antagonist for the TIR family members and triggers a negative pathway of the TIR receptor system, crucial for dampening inflammation in the gastrointestinal tract (GI) and in other districts. TIR8 was well characterized in mouse, humans and in other mammals, but it is poorly known in birds. We investigated TIR8 distribution in adult and embryo chicken samples at both mRNA and protein levels. The pattern of expression of chicken TIR8 in adult was ubiquitary and similar to mammals (high levels in kidney and GI), but it resulted unique in other tissues (pancreas, female reproductive tract, heterophils). Different protein isoforms of TIR8 were detected, suggesting the occurrence of different post-translational modifications or alternative splicing. Immunohistochemistry revealed TIR8 immunolabeling in the intestine, thymus and oesophageal ganglia. These results demonstrate that TIR8, although evolutionarily conserved, show species-specific peculiarities. We also analyzed TIR8 expression in embryos of two chicken breeds (layer- and broiler-type) at different time-points of incubation. TIR8 was detected since the first stages of chicken development (day 1), but it reached a remarkable expression level at day 10. This receptor was ubiquitously expressed, but the highest expression levels were found in liver and kidney. This pattern was comparable to those observed in adult chickens and in mammals examined to date. No expression differences were observed between the two different chicken breeds despite their marked immunological discrepancies. These are the first findings concerning TIR8 expression in developmental stages and they contribute to better understand the reproductive physiology and transovarian pathogen transmission, together with the recent findings of TLR expression in ovary and embryos of different species.