Influenza H1N1 Imprinting Uncouples Serological and B-Cell Responses in Mice

Antigenic drift reduces the protective potential of antibodies generated against earlier influenza variants. Unfortunately, a new infection or vaccination often preferentially recalls antibodies targeting previously encountered strains, thereby limiting responses to newly emerged variants. Although this phenomenon, known as immunological imprinting, has been well documented at the serological level, its underlying cellular mechanisms remain poorly understood. To resolve imprinting at both the serological and cellular level, we combined serum antibody profiling with targeted B-cell characterization in mice, including single-cell B-cell receptor (BCR) sequencing of germinal center (GC) B cells following secondary H1N1 challenge and bulk BCR sequencing of bone marrow plasma cells. We found that a primary infection with the 2009 pandemic H1N1 ("swine flu") strain largely prevented the generation of new serum antibodies following secondary exposure to the antigenically distinct H1N1 strain A/Puerto Rico/8/1934 (PR8), whereas the reciprocal infection sequence updated antibody specificities towards the last encountered virus. GC B cells did not mirror serological imprinting and instead generated updated BCR repertoires directed against the heterologous challenge strain. Likewise, BCR diversity in draining lymph node B cells failed to mirror the serological imprinting observed in the antibody response. Together, these findings reveal a striking uncoupling of cellular and serological immune responses during influenza imprinting, highlighting the need to study both compartments to better understand immune memory and guide the development of improved seasonal influenza vaccines across a diverse population.