The toll‐like receptor 7 (tlr7) gene was identified and characterized from grass carp Ctenopharyngodon idella (designated as citlr7), and the messenger (m)RNA expression profiles were examined in vivo and in vitro. The citlr7 genomic
sequence consists of 4276 nucleotides (nts), including two exons and one intron. The full length of citlr7 complementary (c)DNA sequence is 3354 nts with the longest open reading frame (ORF) of 3156 nts encoding a peptide of 1051 amino acids. citlr7 mRNA expression was high in
spleen, skin and heart, and low in hepatopancreas, muscle, head kidney and trunk kidney in healthy fish. The expression of citlr7 was rapidly and significantly up‐regulated at 6 h after grass carp reovirus (GCRV) injection (72·91 fold, P < 0·05), and recovered
to the original level at 24 h post‐injection in the spleen. The citlr7 transcript was rapidly and significantly down‐regulated at the 6 h time point (0·32 fold, P < 0·05) and retrieved the normal level at 72 h post‐injection in the hepatopancreas.
The citlr7 transcripts were rapidly and significantly inhibited at 2 h post‐GCRV infection in the C. idella kidney (CIK) cell line (0·62 fold, P < 0·05), and were rapidly and significantly elevated by the stimulation of the synthetic double‐stranded
RNA polyriboinosinic–polyribocytidylic acid sodium salt [poly(I:C)] in CIK in a dose and time‐dependent manner (P < 0·05). The results imply that citlr7 is involved in the responses to double‐stranded RNA and virus infection.