Single‐Cell Profiling of Bone Marrow B Cells and Early B Cell Developmental Disorders Associated With Systemic Lupus Erythematosus

Objective The peripheral B cell compartment is heavily disturbed in systemic lupus erythematosus (SLE), but whether B cells develop aberrantly in the bone marrow (BM) is largely unknown. Methods We performed single‐cell RNA/B cell receptor (BCR) sequencing and immune profiling of BM B cells and classified patients with SLE into two groups: early B cell (Pro‐B and Pre‐B) normal (EB nor ) and EB defective/low (EB lo ) groups. Results The SLE‐EB lo group exhibited more severe disease activity and proinflammatory status, overaction of type I interferon signaling and metabolic pathways within the B cell compartment, and aberrant BCR repertoires compared with the SLE‐EB nor group. Moreover, in one patient with SLE who was initially classified in the SLE‐EB lo group, early B cell deficiency and associated abnormalities were largely rectified in a second BM sample at the remission phase. Conclusion In summary, this study suggests that early B cell loss in BM defines a unique pathological state in a subset of patients with SLE that may play an active role in the dysregulated autoimmune responses.