Background: B cells are central in the pathogenesis of anti-neutrophil cytoplasmic antibody (ANCA) associated vasculitides (AAV). The efficacy of remission induction and maintenance by B cell depleting therapy strongly underlines the importance of B cells in the disease and the use of rituximab (RTX) has successfully been implemented in the treatment of AAV. However, B cell depletion after application of RTX is markedly prolonged in AAV compared to other autoimmune diseases, suggesting an impairment of the B cell compartment.

Objectives: Our study aimed to understand the base of the defect in B cell reconstitution after RTX treatment in AAV, and to dissect B cell function and development in AAV.

Methods: We recruited 91 AAV patients and performed deep phenotyping of the peripheral B cell compartment by spectral flow cytometry. In a subgroup of patients, we analyzed the bone marrow using flow- as well as mass cytometry. In vitro modeling assays of B lymphopoiesis have been applied to study dynamic of development of AAV B cell precursors. BAFFR involvement into peripheral B cell survival and maturation have been studied by investigating serum BAFF concentration by ELISA, BAFFR expression of isolated B cells was determined by qPCR and Western Blot, and by in vitro survival assays.

Results: AAV patients show B-lymphocytopenia. Low transitional B cell numbers in treatment-naive patients indicate an impaired central B cell development. In RTX-treated AAV patients the median time of B cell depletion, defined as less than 5 cells/µL, was 17 months (IQR 9-36). We studied phenotype and development of bone marrow B cells and found a defect in early B cell development in both treatment-naive and RTX-treated AAV patients. Only in a subgroup of RTX-treated patients, transitional B cells were increased, indicative of beginning or ongoing B cell repopulation. The increased amount of transitional B cells did not lead to replenishment of the later stages of peripheral B cell maturation, suggesting a maturation stop in peripheral B cell reconstitution in these patients. We found low BAFFR expression in AAV peripheral B cells, caused by enhanced shedding of BAFFR, that resulted in a reduced B cell survival in response to BAFF.

Conclusion: Our data suggest that prolonged depletion of B cells in AAV patients after RTX therapy indicate a B cell defect that is unmasked by treatment. Impaired central B lymphopoiesis associated with disturbed peripheral B cell maturation because of enhanced BAFFR processing and shedding, resulting in reduced immature B cell survival, contribute to a delayed recovery of the peripheral B cell pool after RTX treatment in AAV. Our data point to a severe defect in B cell maturation and development in patients with AAV that may influence the therapeutic use of B cell depleting agents in this disease. In addition, our results emphasize the need for close immune monitoring after B cell depletion in AAV.

REFERENCES: NIL.

Acknowledgements: NIL.

Disclosure of Interests: None declared.