Age-associated B cells (ABCs) develop from a CNS-localized progenitor pool into a pro-inflammatory phenotype after stroke.

Aging and age-related diseases like ischemic stroke induce chronic lymphocyte recruitment into the central nervous system (CNS). Conflicting effects on post-stroke functional recovery, however, are secondary to the differences in responding lymphocyte populations that shift immunophenotype with both ischemic injury and age. To better define CNS-localized B cell subsets, we used flow cytometry, single-cell RNA sequencing, and B cell receptor sequencing on B cells isolated from uninjured and post-stroke brains of aged male and female mice. We identified a novel B1b cell progenitor pool distinct from canonical pleural and peritoneal B1 niches. Trajectory analysis showed B1b progenitors transition into age-associated B cell (ABC) subsets, and clonal expansion of IgM+ ABCs (ABC/B1b) and plasma cells following ischemic stroke. We also confirmed analogous ABCs and developing B cell populations in post-mortem human parenchymal tissue isolated from aged brain donors. These studies reveal unique B cell populations that proliferate within the aging CNS and are associated with impaired post-stroke functional recovery in mice. Identification of inflammatory, CNS-resident ABC/B1b cells that are conserved across species is critical as they have the potential to be sequestered from peripheral immunotherapies and/or contribute to age-related neurodegenerative diseases.