To date, little is known about how trout IgM, the primary antibody of fish, varies in titer, specificity, disulfide cross-linking, and affinity following immunization with a pathogen. Work using defined antigens has demonstrated that the disulfide cross-linking structure of IgM becomes increasingly more polymerized during an immune response, coinciding with an increase in affinity, but it is unknown if this has relevance to aquatic pathogens. Understanding how IgM varies following vaccination with an aquatic pathogen is of considerable importance as effector functions allocated to multiple antibody isotypes in mammals are essentially relegated to this single molecule. To gain insights into the dynamism of IgM, rainbow trout were immunized with Streptococcus iniae and individual serum titers, their specificity and affinity to S. iniae, and the disulfide cross-linking pattern of both total-serum and specific Ig were analyzed over a period of 37 weeks. We found that in vaccinated animals titer increased by a factor of ≈100 from starting levels, affinity increased 10-fold, and diversity of S. iniae proteins recognized by trout antibody increased at least 5-fold. Most intriguing, though less cross-linked IgM predominated early in response, by week 5, the fully tetramerized antibody comprised 50% of total specific protein. We propose that this is a mechanism to optimize efficacy of carrying out effector functions and recognizing a wide array of epitopes with higher affinity.
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