Sockeye salmon immunoglobulin V usage and pathogen loads differ between spawning sites.

The Immunological Imprinting Hypothesis proposes that juvenile anadromous fish respond to the pathogen fingerprint specific to their natal site by producing protective long lived plasma cells (LLPCs) that constitutively produce antibodies against those pathogens. Hence, fish returning to their natal streams have immunological protection from pathogens at that specific location. Here, we tested the hypothesis through analysis of antibody composition and usage in sockeye salmon populations in Alaska. Spleen and anterior kidney were sampled from salmon from six sites, and relative usage levels of six different Immunoglobulin V gene families determined using RT-qPCR. Additionally, prevalence and pathogen loads were measured in each fish for Renibacterium salmoninarum, Flavobacterium psychrophilum, and Infectious Hematopoietic Necrosis Virus. Results revealed differences in V usage, pathogen loads, and infection rates between spawning sites, while probability of infection was dependent on location for each pathogen analyzed. Further, several negative correlations between specific V usage patterns and pathogen loads were uncovered. Greater understanding of site-dependent V usage in spawning fish potentially suggests a method of natural immunization against common fish pathogens and thus protection of both farmed and wild populations.