Age-related alterations of somatic hypermutation and CDR3 lengths in human Vκ4-expressing B lymphocytes

Abstract

The lower avidity and/or affinity of antibodies generated by an aged immune system could be attributed to two major changes in the antibody repertoire: a shift in germline gene usage and a decrease in the rate of immunoglobulin hypermutation. In an attempt to identify the mechanisms involved in the observed humoral immune deficiency in the elderly, we studied whether differences in the somatic diversity of a particular Vκ region occurred with ageing. By using the polymerase chain reaction and sequencing, we analysed and compared Vκ4-Jκ rearrangements isolated from young (mean age 21 years) and aged (mean age 83 years) healthy adults. Mutations in the Vκ4 gene compared with the germline sequence were determined as well as the length and structure of the CDR3 sequence. We analysed in detail various mechanisms contributing to CDR3 and Vκ variability in rearrangements involving the Vκ4 gene. Our data revealed that, despite strong individual variations, significantly lower levels of somatic mutation were found in the aged group, both for complementarity-determining regions (CDRs) and framework regions (FRs) encoding Vκ4 sequences. This decrease mostly affected mutations responsible for replacements and thus resulted in a lowered somatic diversification of the encoded Vκ4 proteins in aged individuals. Moreover, comparison of the CDR3 regions of the Vκ4-Cκ cDNA revealed changes in light-chain junctional diversity that correlated with age. Altogether these data suggest an impaired light-chain somatic diversity in connection with human senescence.