A comparison of the IgM and IgT repertoires reveals the structure of B cell communities in rainbow trout is a consequence of mechanistic and evolutionary forces

Abstract

In rainbow trout (Oncorhynchus mykiss), three classes of antibodies have been identified: IgM, IgD, and IgT, which differ in their abundance and effector functions. Though the process of VDJ recombination that creates these antibodies is often described as a stochastic process, recent findings in mammals suggest that there are biases in antibody construction. Because class switching is absent in teleosts, studying the IgM and IgT communities of rainbow trout provides the opportunity to see how evolution has differentially shaped the IgM and IgT repertoires. Even though it has not yet been demonstrated that biases exist in variable region construction for trout immunoglobulins, it seems reasonable that both natural and artificial selection have driven preferential gene-segment usage and pairing biases in rainbow trout. In this study, we sequenced the heavy chain variable regions of membrane IgM and IgT from multiple fish and predicted that given the more generalist role of the abundant IgM there would be less pressure to bias V gene segment usage and DJ pairings; rather, natural selection would have favored diversity and the ability to respond to a plethora of pathogens. Conversely, as IgT is substantially less abundant in the serum than IgM and specialized in its function, there would be selective pressure to make the most out of a little, thus favoring biased V segment usage and preferential DJ pairings. In support of our hypotheses, for IgT, over 70% of DJ pairs were biased and over 60% of antibodies were constructed with just two V gene segments. These biases were not prevalent in the IgM repertoire, where only 4% of DJ pairs were biased and no single V gene segment was utilized in more than 10% of antibodies. We found that these biases have profound influences in the richness and evenness of the repertoires, with the IgM repertoire investing more equitably in nearly double the number of VDJ combinations compared with IgT.