germline amino acid diversity in b cell receptors is a good predictor of somatic selection pressures.

The diversity of the immune repertoire is important for the adaptive immunesystem's ability to detect pathogens. Much of this diversity is generated in twosteps, first through the recombination of germline gene segments and secondthrough hypermutation during an immune response. While both steps are to someextent based on the germline level repertoire of genes, the final structure andselection of specific receptors is at the somatic level. How germline diversityand selection relate to somatic diversity and selection has not been clear.To investigate how germline diversity relates to somatic diversityand selection, we considered the published repertoire of Ig heavy chain Vgenes taken from the blood of 12 individuals, post-vaccination againstinfluenza, sequenced by 454 high-throughput sequencing. We here show that whenwe consider individual amino acid positions in the heavy chain V gene sequence,there exists a strong correlation between the diversity of the germlinerepertoire at a position and the number of B cell clones that change amino acidat that position. At the same time, we find that the diversity of amino acidsused in the mutated positions is greater than in the germline, albeit stillcorrelated to germline diversity. From these findings, we proposethat while germline diversity and germline amino acid usage at a givenposition do not fully specify the amino acid mutant needed to promotesurvival of specific clones, germline diversity at a given position is agood indicator for the potential to survive after somatic mutation at thatposition. We would therefore suggest that germline diversity at eachspecific position is the better a priori model for the effects of somaticmutation and selection, than simply the division into complementaritydetermining and framework regions.