Genetic alloforms of rainbow trout (Oncorhynchus mykiss) complement component C3 and resistance to viral haemorrhagic septicaemia under experimental conditions

Viral haemorrhagic septicaemia (VHS) is a severe viral disease in farmed rainbow trout (Jensen, 1965), especially of fry and fingerlings, and a commercial vaccine is not yet available (de Kinkelin, 1988). Breeding of trout strains with a higher natural resistance is an alternative to vaccination. Such breeding programmes would gain much if immunological markers linked to disease resistance were available (Fjalestad et al., 1993). The complement system plays an important role in the immunodefence against micro-organisms. The third complement component C3 is a key component in the activation of both the classical, antibody dependent, pathway and the alternative, antibody independent, pathway in mammals. In rainbow trout, a protein homologous to human C3 was described by Nonaka et al. (1984), and the gene encoding for rainbow trout C3 was cloned by Lambris et al. (1993). This indicates that rainbow trout possess a complement system comparable to mammals. The existence of codominantly expressed alloforms of the rainbow trout C3 protein can be demonstrated using high voltage gel electrophoresis and immunoblotting (Jensen & Koch, 1991). Briefly, serum samples were applied to a 1% agarose gel, and electrophoresed for 2-5 h at 20 V cm-1. The proteins were blotted onto nitrocellulose paper, incubated overnight with a monoclonal mouse anti-trout C3 antibody, and subsequently with a peroxidase conjugated rabbit anti-mouse antibody (Dako, Denmark). Staining was carried out with H2O2 and tetramethylbenzidine (TMB) (Koch et al., 1985). Three different alleles could be distinguished, according to electrophoretic mobility: slow (s), fast-1 (f1) and fast-2 (f2). Five different phenotypes were demonstrated: f1, f2, f1/f2, s/f1 and s/f2. In a previous study, the distribution of C3 phenotypes was analysed in three fish farms: a non-VHS-infected fish farm, which produced fry; a VHS-infected fish farm, which received fry from the first fish farm; and a non-VHS-infected fish farm, which also received fry from the first fish farm (Slierendrecht et al., 1993). The infected fish farm was characterised by a significantly higher frequency of the f1 allele in the surviving trout population, when compared to the frequencies in the uninfected fish farms. As other selective forces could not be excluded, another experiment was designed, in which individually tagged rainbow trout with known C3 phenotypes were challenged with VHS-virus under aquaria conditions.