Genes of the major histocompatibility complex (MHC) are involved in the primary recognition of foreign pathogens in the acquired immune response and are associated with resistance to a variety of diseases in vertebrate organisms. In tetrapods, including mammals, the MH genes are tightly linked in a single chromosomal region, whereas in teleosts, MH remained unlinked (Stet, Kruiswijk & Dixon 2003). Traditionally, the MH gene family is divided into three classes, namely classes I, II and III. MH class II molecules are dimers, including one α chain and one β chain, which are encoded by separate class II A and B genes, respectively. MH class II genes are highly polymorphic with multiple loci in teleosts. There are two to three loci in the MH class II A gene (Murray et al. 2000; Kruiswijk et al. 2004; Scharsack, Kalbe & Schaschl 2007; Harstad et al. 2008) and two to six loci in the MH class II B gene (Sultmann et al. 1994; Erp, Egberts & Stet 1996; Kruiswijk et al. 2004; Reusch, Schaschl & Wegner 2004; Zhang & Chen 2006; Harstad et al. 2008; Zhang et al. 2010). There were a large number of alleles at each locus in MH II genes (Yang et al. 2006; Michel, Bernatchez & Behrmann-Godel 2009; Xu et al. 2009; Croisetiere, Bernatchez & Belhumeur 2010; Du et al. 2011). In cyprinid fish species, some researchers identified six MH II B loci (Dare-DAB, -DBB, -DCB, -DDB, -DEB and –DFB) in the zebrafish, Danio rerio Hamilton (Ono et al. 1992; Sultmann et al. 1994) and at least five loci in the common carp, Cyprinus carpio L. (Hashimoto, Nakanishi & Kurosawa 1990). Then, Zhang et al. (2010) identified 34 alleles of the DAB gene in the grass carp, Ctenopharyngodon idella (Valenciennes), collected from the Yangtze River, and identified at least five loci in the MH II B gene. In the common carp, four different Cyca-DAB alleles have been described, namely Cyca-DAB1*01 and Cyca-DAB2*01 (denoted here as Cyca-DAB1-like genes) (Ono et al. 1993) as well as Cyca-DAB3*01 and Cyca-DAB4*01 (denoted here as Cyca-DAB3-like genes) (Erp et al. 1996). Some researchers considered that there were at least two paralogous groups of MH class II B genes (DAB1 and DAB3) in cyprinid fish species (Dixon et al. 1996; Ottova et al. 2005; Simkova, Ottova & Morand 2006).
High polymorphism in the DAB genes was suitable for studies of association with disease resistance. Several studies reported that Cyca-DAB1-like genes were associated with increased immune responsiveness in common carp after infection with cyprinid herpesvirus-3 (CyHV-3), the bacterium Aeromonas hydrophila, the ectoparasite Argulus japonicus and the blood parasite Trypanoplasma borreli (Rakus et al. 2009a; Rakus et al. 2009b). Analysis of carp challenged with A. hydrophila indicated that the lowest cumulative mortality was observed in the group of fish carrying both a heterozygous Cyca-DAB1 gene and a homozygous Cyca-DAB2 gene (Rakus et al. 2008).
The grass carp is among the most extensively aquacultured freshwater fish in China. However, infection of grass carp with the parasitic ciliate Ichthyophthirius multifiliis has caused significant economic losses. The association of the MH II-DAB gene with I. multifiliis infection of grass carp has yet to be elucidated. In this study, we examined the expression of the MH II-DAB gene in seven major immuno-competent tissues of grass carp, in a period of 10 days after infection with I. multifiliis by real-time quantitative polymerase chain reaction (RT-PCR).