A high-resolution analysis of cells in inflamed tissue offers the possibility of uncovering previously unappreciated pathogenic mechanisms and of identifying new therapeutic targets. In this project, we will characterize age/autoimmunity associated B cells (ABCs) in kidney and synovium of patients with systemic lupus and rheumatoid arthritis, respectively. We will explore their heterogeneity, derivation, localization, and antigenic targets as well as their functional interactions with T cell subsets, most specifically T follicular and T peripheral helper cells (Tfh/Tph). These efforts will provide a broad assessment of the features and potential functions of this highly expanded B cell population in human autoimmunity.