Differential Toll-like receptor (TLR) mRNA expression patterns during chicken embryological development

Toll-like receptors (TLRs), important components of innate immune response, play a pivotal role in early recognition of pathogen as well as in the initiation of robust and specific adaptive immune response. In the present study, the expression profile of chicken TLRs (TLR2A, TLR3, TLR4, TLR5, TLR7, TLR15, and TLR21) in various chicken embryonic tissues during embryo development was examined by real-time PCR assay. All the TLR mRNAs were expressed in whole embryonic tissue as early as 3rd embryonic day (ED). Four of the seven TLRs (TLR2, TLR3, TLR4, and TLR7) mRNA expressions were significantly (P < 0.01) higher at 12ED relative to expression at 3 ED, whereas TLR15 mRNA expression was significantly (P < 0.01) higher on 7ED and TLR5 and 21 were highly expressed on 18 ED. Among all the TLRs investigated TLR4 mRNA was the highest expressed and TLR15 mRNA expression was the lowest in all tissues during chicken embryo development. Tissue wise analysis of mRNA expression of TLRs showed that liver expressed significantly (P < 0.01) higher levels of most of the genes (TLR2, TLR4, and TLR21). However no significant difference was found in TLR15 mRNA expression among the tissues during development. Our results suggest the innate preparedness of chicken embryos and also a possible role for TLRs in the regulation of chicken embryo development that needs to be further evaluated.