Molecular characterization and dynamic expressions of three Nile tilapia (Oreochromis niloticus) complement genes after Gyrodactylus cichlidarum (Monogenea) infection

The complement system has important roles in both innate and adaptive host defense against invading microorganisms in vertebrates. The complement factor B (Bf), complements 2 (C2) and 4 (C4) can initiate and activate the complement system. In the present study, three complement genes, named as OnBf, OnC2 and OnC4, were identified from Oreochromis niloticus, their distributions in host tissues and dynamic expressions after Gyrodactylus cichlidarum infection were investigated. The full-length cDNA of OnBf comprises an open reading frame (ORF) of 2055 bp, encoding a polypeptide of 684 amino acids which consists of two CCP modules at the N-domain, one VWA domain, and one Tryp_SPc domain at the C-terminal. The open reading frame (ORF) of OnC2 cDNA of 2385 bp encodes a protein of 794 amino acids with three (CCP) modules at the N-terminal, a less complex region, a VWA region, and a Tryp_SPc domain at the C-terminal. The full-length cDNA of OnC4 is comprised of an open reading frame (ORF) of 5082 bp encoding a polypeptide of 1693 amino acids which orderly includes A2M_N_2, ANATO, A2M, A2M_recep, C345C and three less complex regions. Quantitative real-time PCR (qPCR) analysis revealed that OnBf, OnC2 and OnC4 had higher levels of expression in the liver, stomach and spleen than in the tissues examined. After G. cichlidarum infection, the expression levels of OnBf, OnC2, and OnC4 in the skin, liver, spleen and kidney were up-regulated at 2, 4, 6, 8, 10, 12 and 14 days, in general accordance with dynamics of prevalence and mean intensity, each showing an initial increase and subsequent decrease. These results might suggest that G. cichlidarum infection had induced a host immune response via OnBf, OnC2, and OnC4.