The elderly have reduced humoral immunity, as manifested by increased susceptibility to infections and impaired vaccine responses. To investigate the effects of aging on B-cell receptor (BCR) repertoire evolution during an immunological challenge, we used a phylogenetic distance metric to analyze Ig heavy-chain transcript sequences in both young and elderly individuals before and after influenza vaccination. We determined that BCR repertoires become increasingly specialized over a span of decades, but less plastic. In 50% of the elderly individuals, a large space in the repertoire was occupied by a small number of recall lineages that did not decline during vaccine response and contained hypermutated IgD + B cells. Relative to their younger counterparts, older subjects demonstrated a contracted naive repertoire and diminished intralineage diversification, signifying a reduced substrate for mounting novel responses and decreased fine-tuning of BCR specificities by somatic hypermutation. Furthermore, a larger proportion of the repertoire exhibited premature stop codons in some elderly subjects, indicating that aging may negatively affect the ability of B cells to discriminate between functional and nonfunctional receptors. Finally, we observed a decreased incidence of radical mutations compared with conservative mutations in elderly subjects' vaccine responses, which suggests that accumulating original antigenic sin may be limiting the accessible space for paratope evolution. Our findings shed light on the complex interplay of environmental and gerontological factors affecting immune senescence, and provide direct molecular characterization of the effects of senescence on the immune repertoire.aging | antibody repertoire | influenza vaccine | CMV | UniFrac T he deterioration of immune function with age, a process referred to as immunosenescence, is well recognized. Notable changes contributing to immunosenescence include, among others, decreased proliferation of lymphocytes, reduced T-cell receptor repertoire, and defects in antibody production (1, 2). This phenomenon contributes to an age-related increase in susceptibility to viral and bacterial infections and decreased response to vaccination (3-5). Indeed, individuals over the age of 65 y are less than half as protected by standard influenza vaccines as younger individuals (6), and pneumonia and influenza represent the fourth most common cause of death among aging individuals (4).Antibody-mediated immunity is the result of an evolutionary arms race between the pathogens to which an individual is exposed and antibody-producing B cells. A tremendous diversity of potential antibody affinities is generated by the mechanisms of V(D)J recombination, random junctional insertions/deletions, and somatic hypermutation (7). Preferential proliferation of activated B cells upon encounter with a cognate antigen then exerts a selective pressure for B-cell receptors (BCRs) with a high binding affinity to the antigen (7). The clonal history of B cells circulating in the blood c...