Antigen affinity and site of immunization dictate B cell recall responses

Protective antibodies against HIV-1 require unusually high levels of somatic hypermutations (SHMs) introduced in germinal centers (GCs). To achieve this, a sequential vaccination approach was proposed. Using HIV-1 antibody knock-in mice with fate-mapping genes, we examined if antigen affinity affects the outcome of the B cell recall response. Compared to high affinity boost, low affinity boost resulted in decreased numbers of memory-derived B cells in secondary GCs, but with higher average SHM, indicating an affinity threshold for memory B cells to enter secondary GCs. Upon boosting local LNs, numbers of residual GC B cells increased independent on antigen affinity, while average SHM decreased. Our results demonstrate that antigen affinity and location of the boost affect the outcome of the B cell recall response. These results can help guide the design of vaccine immunogens aiming to selectively engage specific B cell clones for further SHM diversification.