Molecular characterization of Toll-like receptor type-3 in mallard duck and its response to Newcastle disease virus infection

Toll-like receptors (TLRs), type I transmembrane pattern recognition receptors (PRRs), are composed of the extracellular domain that is implicated in the recognition of microbial products and initiates the innate and adaptive immune response. Previous reports on TLRs in birds showed significant levels of inter- and intraspecific genetic variation. Little is known about the structure and function of the avian immune system, especially waterfowl species. This work aimed to identify and clone Anas platyrhynchos (mallard duck) TLR-3 (dTLR-3) and its expression level following challenge with velogenic Newcastle disease virus (NDV) as a model for waterfowl species. The mallard duck TLR-3 full-length cDNA sequence had been cloned, which consisted of 2457 nucleotides. The translated amino acid sequence showed identity degree as 97% with Muscovy duck, 95% with geese, 89% with helmeted guineafowls, 88% with the chickens TLR-3 gene, 82% with turkey TLR-3, and 79% with zebra finch, while it showed 54% with human one; the analysis data suggested that the new sequence is probably homologous to vertebrates’ TLR-3. The predicted protein encoded by the duck dTLR-3 mRNA sequence is composed of 819 amino acids. Analysis of the deduced amino acid sequence indicated that dTLR-3 has typical structural features and contains the main components of proteins in the TLR family. The dTLR-3 expressed in almost all examined tissues of mallard duck following quantitative real-time polymerase chain reaction (qPCR) analysis and using B-actin as a housekeeping gene. To check the functionality of the receptor and its role in viral infection, we evaluate the expression level in different tissues and its changes following NDV infection. The results showed significant (P < 0.05) upregulated in the brain at 24 h (1.84-fold), reached a peak at 48 h (4.82-fold), and recovered to normal levels at 72 h post-infection. These results indicate a complete and functional dTLR-3 that is orthologous to other vertebrate receptors with its potential role in early response against viral infection in mallard duck species.