Association between polymorphism 140T of MHC class IIβ and resistance to lactococcosis in rainbow trout

Genes within the major histocompatibility complex (MHC) are important for both innate and adaptative immunity in mammals; however, less is known regarding their contribution in teleost fish. We examined the involvement of MHCIIB gene in resistance to Lactococcus garvieae, the causative agent of lactococcosis, after rainbow trout exposure to naturally contaminated water. The polymorphism 140t and the haplotype 25, including the same polymorphism, were found strongly associated with resistance to lactococcosis. This mutation and those present in haplotype 25 all code for phenylalanine, characterized by an aromatic side chain common in polyproline-binding sites and reported in staphylococci and streptococci surface proteins. This could explain the affinity between MHC class IIβ mutated chain and L. garvieae. A study is ongoing to confirm the role of this mutation so to use it in the future for molecular marker-assisted selective breeding in rainbow trout.