How specific MHC class I and class II combinations affect disease resistance against infectious salmon anaemia in Atlantic salmon (Salmo salar).

The aim was to evaluate the performance of selected individual MHC class I and class II alpha (A) alleles, and combinations of these on disease resistance against infectious salmon anaemia (ISA). The material consisting of 1966 fish from seven families, contained five MHC class I alleles and four MHC class II A alleles. Which representing given class II A and class II beta (B) haplotypes, totalling 19 MHC class I and class II A genotypes. The fish were challenged with infectious salmon anaemia virus (ISAV), the virus causing ISA. Dead fish were collected daily during the challenge experiment and the survivors were collected at termination. All fish were genotyped for MHC class I and class II A. The total mortality in the material was 85.14%. For MHC class I, UBA0201 and UBA0301 were significantly the most resistant alleles, while UBA0601 for class I and DAA0301 for class II A were the significantly most susceptible alleles. The analysis of combined MHC class I and class II A genotypes detected that fish with the genotype UBA*0201/0301;DAA0201/0201 were the most resistant fish with a hazard ratio (HR) at 0.750, while the fish with the genotypes UBA0601/0801;DAA0501/0501 and UBA0201/0301;DAA0301/*0501 were the most susceptible fish with HR of 1.334 and 1.425. In addition, Cox regression analysis within family detected combined MHC class I and class II A genotypes that contributed significantly to resistance and susceptibility. The study confirmed the expectation of performance of individual MHC class I and class II A alleles, and also detected an effect of MHC class I and class II A in combinations.