Systemic lupus erythematosus (SLE) is an incurable autoimmune B cell disorder that is in part due to ineffective clearance of nuclear antigens and activation of the TLR pathway. The extrafollicular (EF) pathway of antibody secreting cells (ASC) is recognized to play a prominent role in generating pathogenic antibodies in both mice and humans. For example, a novel population of CD21^loCD11c⁺ B cells identified in both mice (age- associated B cells) and human (DN2 cells) are thought to be a major source of autoreactive EF ASCs. However, the developmental kinetics and cellular origin of EF ASCs remain unexplored.

To track early events of B cell break of tolerance and EF ASC generation, an adoptive transfer system was established, where WT B cells are transferred into the autoreactive BCR transgenic 564Igi host enriched in nuclear antigens. By introducing competing populations of WT and TLR7 deficient B cells, we demonstrated that the differentiation of EF ASCs requires TLR7.

To examine the kinetics of autoreactive B cell proliferation and EF ASC differentiation, a Cell Trace labeling approach was used. We found that donor B cells require at least 7 divisions to differentiate to EF ASCs and that TLR7 deficient B cells are gradually outcompeted by WT at each division. Interestingly, CD21^loCD23^- B cells were highly proliferative, expressed CD11c, and were sensitive to TLR7 defect. Our interpretation is that they are likely the immediate developmental precursors of EF ASCs. The loss of CD21 was reversible by receptor blockade and directly links to donor cell proliferation.

These findings advance our current understanding of the fundamental biological circuitry behind EF-derived autoantibody producing cells and potentially point to future avenues for therapeutic development.