Toll-like receptors (TLRs) are pattern recognition receptors that are vital for the recognition of pathogen-associated molecular patterns. TLR5 is responsible for the recognition of bacterial flagellin to induce the NF-κB activation and innate immune responses. In this study， we cloned and identified the TLR5 gene from the King pigeon (Columba livia) designated as PiTLR5. Full-length PiTLR5 cDNA (2583 bp) encoded an 860-amino acid protein containing a signal peptide sequence， 10 leucine-rich repeat domains， a leucine-rich repeat C-terminal domain， a transmembrane domain， and an intracellular Toll-interleukin-1 receptor domain. Pigeon TLR5 mRNA expression was quantified by performing quantitative rea... More
Toll-like receptors (TLRs) are pattern recognition receptors that are vital for the recognition of pathogen-associated molecular patterns. TLR5 is responsible for the recognition of bacterial flagellin to induce the NF-κB activation and innate immune responses. In this study， we cloned and identified the TLR5 gene from the King pigeon (Columba livia) designated as PiTLR5. Full-length PiTLR5 cDNA (2583 bp) encoded an 860-amino acid protein containing a signal peptide sequence， 10 leucine-rich repeat domains， a leucine-rich repeat C-terminal domain， a transmembrane domain， and an intracellular Toll-interleukin-1 receptor domain. Pigeon TLR5 mRNA expression was quantified by performing quantitative real-time PCR (qRT-PCR)， which showed that PiTLR5 was broadly expressed in all examined tissues， with the highest expression in the liver， peripheral blood mononuclear cells， and spleen. PiTLR5-mediated innate immune responses were measured by determining its effects on NF-κB activation and cytokine expression. The results showed that HEK293T cells transfected with PiTLR5 robustly activated the NF-κB response to flagellin， but not other TLR stimuli， and induced significant upregulation of IL-1β， IL-8， TNF-α， and IFN-γ， indicating that PiTLR5 is a functional TLR5 homolog. Additionally， following flagellin stimulation of pigeon splenic lymphocytes， the levels of TLR5， NF-κB， IL-6， IL-8， CCL5， and IFN-γ mRNA， assessed using qRT-PCR， were significantly upregulated. Besides， TLR5 knockdown resulted in the significantly downregulated expression of NF-κB and related cytokines/chemokines. Triggering pigeon TLR5 contributes to significant upregulation of inflammatory cytokines and chemokines， suggesting that pigeon TLR5 plays an important role in the innate immune responses.
