Abstract
Background: In the host innate immune system, various pattern recognition receptors (PRRs) recognize conserved pathogen-associated molecular patterns (PAMPs) and represent an efficient first line of defense against invading pathogens. Toll-like receptors (TLRs) are a major class of PRRs, which are able to recognize a wide range of PAMPs and play a central role in initiating innate immune responses. TLR21 is one of the non-mammalian TLRs identified in some bird and fish species. Results: In the present study, we reported the cloning and identification of a TLR21 cDNA from the head kidney of common carp (Cyprinus carpio L.), named CcTLR21. The full-length CcTLR21 cDNA was 3557 bp long, including an open reading frame (ORF) of 2895 bp, which encoded a putative protein of 964 amino acids. The putative CcTLR21 protein was found to comprise a signal peptide, 14 LRR domains in the extracellular region and a TIR domain in the cytoplasmic region, which fits with the characteristic TLR domain architecture. The phylogenetic analysis showed that CcTLR21 possessed high amino acid identities with the TLR21s in other freshwater teleosts. A Real-time PCR assay showed that CcTLR21 mRNA was expressed in almost all tissues examined in healthy common carp, while the levels obviously varied among different tissues. During the embryonic and early larval developmental stages of common carp, the CcTLR21 showed two peaks of expression, with the first at 1 dpf and the second at 10 dpf. When challenged with poly(I:C) (a viral model) or Aeromonas hydrophila, the expression level of CcTLR21 was up-regulated in a variety of common carp tissues. Conclusions: Our findings indicate that CcTLR21 plays a significant role in innate immune defense during larvae ontogeny and in responses to viral or bacterial pathogens.