Antigen‐experienced CXCR5 ^‐ CD19 ^low B cells are plasmablast precursors expanded in SLE

Altered composition of the B cell compartment in the pathogenesis of systemic lupus erythematosus (SLE) is characterized by expanded plasmablast (PB) and IgD- CD27- double negative (DN) B cell populations. Previous studies showed that DN B cells represent a heterogeneous subset and further characterization is needed.Therefore, we analyzed two independent cohorts of healthy donors and SLE patients using a combined approach of flow (HD: n=16; SLE: n=28,) and mass cytometry (HD: n=18; SLE: n=24) and targeted RNA sequencing. To study B cell subsets formation in acute immune response versus autoimmunity we investigated HDs at various time points upon vaccination with BNT162b2 or during acute COVID-19 infection using flow cytometry.We have found that IgD- CD27+ switched and atypical IgD- CD27- memory B cells, which are increased in SLE, represent heterogeneous populations composed of three different subsets each. Populations of CXCR5+ CD19int , CXCR5- CD19high and CXCR5- CD19low are found in both compartments suggesting their relationship. We characterize a hitherto unknown and antigen-experienced CXCR5- CD19low subset enhanced in SLE carrying a PB phenotype with diminished B cell receptor responsiveness and expression of CD38, CD95, CD71, PRDM1, XBP-1, and IRF4. CXCR5- CD19low subsets are increased and correlate with PB frequencies in SLE and upon BNT162b2-vaccination of HD suggesting their interrelationship and contribution to plasmacytosis. The demonstration of CXCR5- CD19low B cells amongst both CD27+ and CD27- cells questions the role of CD27 as reliable marker for B cell differentiation.Our data suggest that CXCR5- CD19low B cells are precursors of plasmablasts, thus co-targeting this subset may have therapeutic value in SLE.