Project Summary Antibodies specific for pathogenic threats can provide immediate protection from infectious disease but longevity of an antibody responses after vaccination or infection can be highly variable. Responses induced by some live vaccines can persist for a lifetime, whereas protein-based vaccines are in general shorter lasting. However, antibody durability is not necessarily linked to the use of live virus as long-lived antibody responses have been shown to be induced by the human papilloma virus (HPV) vaccine, a non-live viral-like particle- based platform. This suggests that distinct immunological cues can be engineered to result in the generation of longer-lived antibody responses. While memory lymphocytes also provide a system of protective efficacy, strategies to maximize robust levels of protective secreted antibodies that are stable over time is an important goal in modern immunology. Understanding the capabilities of the immune system in this context, and how available vaccines can elicit durable secreted antibody responses will be important to decipher. This is relevant to the ongoing SARS-CoV-2 pandemic and for vaccine strategies more broadly. Preliminary data suggest that antibodies induced by natural infection harbor robust long-term stability at modest levels and greater polyclonal neutralizing breadth across viral variants compared to infection-naïve vaccinees. In addition, differential antibody durability trajectories tend to favor COVID-19 convalescent subjects with dual memory B cell features of greater antibody somatic mutation and cross-coronavirus reactivity. These findings support a hypothesis that high somatical mutation and cross-reactivity in antigen-binding memory B cell repertoires early after recovery predicts antibody durability and that recalled immunity may confer greater longevity of differentiated plasma cells. This hypothesis will be examined in two aims, (i) to illuminate factors influencing anti-SARS-CoV-2 antibody durability, and (ii) to chart the functional evolution of anti-CoV memory B cell over time. For aim 1, human and mouse studies will be used to illuminate potential mechanistic insights and features connected to durable antibody responses. For aim 2, the durability and evolution of memory B cell repertoire antigen recognition capacity will be charted over time to assess the evolving relationship between secreted polyclonal and memory B cell repertoires. This work is expected to shed light on factors that influence longevity and evolution of antibody responses, which will be important for ongoing improvement of vaccine strategies.