CD21 primes extrafollicular differentiation of autoreactive B cells in a TLR7-driven lupus model

The extrafollicular (EF) B cell differentiation pathway has emerged as a prominent source of autoantibody-secreting cells (ASCs) in systemic lupus erythematosus (SLE). CD21 lo CD11c + B cells are associated with aging, infection, and autoimmunity. They are key contributors to EF ASCs, yet their developmental trajectory and receptor programming are unclear. To study EF mechanics of autoreactive B cells, we adoptively transferred naïve B cell populations into 564Igi mice, which act as an autoreactive host enriched for autoantigens and T cell help. Time-resolved analyses revealed a Toll-like receptor 7 (TLR7)–dependent early escape of peripheral tolerance and a pre–ASC division program. We identified naïve-derived CD21 lo cells as precursors of EF ASCs exhibiting elevated reliance on TLR7. Repertoire analysis delineated protoautoreactive B cell selection and receptor evolution toward self-reactivity. Continuous complement receptor 2 (CR2/CD21)–complement C3d and CD21–complement iC3b engagement triggered receptor down-regulation before proliferation. We reveal CD21 as an initiator of TLR7-dependent autoimmune EF proliferation and target for suppressing autoreactivity.