Objective
The accumulation of T-bethighCD21low B cells and homologous populations is associated with chronic immune activation as in chronic infection, autoimmune diseases or in the context of immune dysregulation in patients with common variable immunodeficiency (CVID). In autoimmune diseases these B cells have been associated with disease pathogenesis. The origin and role of this population is still incompletely understood. The transcription factor T-bet is strongly induced by IFNγ/anti-IgM stimulation and plays a dominant role in the transcriptional regulation of T-bethighCD21low B cells.
Our aim was to determine intrinsic and extrinsic factors involved in the differentiation of T-bethighCD21low B cell in vivo and in vitro.
 
Methods
B cells of healthy controls and patients with inborn errors of immunity (IEI) were analysed by FACS ex vivo and after in vitro cultivation. Sorted cells were analsed by RNAseq and ATACseq.
Results
In vitro cultivation of HC naïve B cells with anti-IgM, IFNγ and IL-21 with CpG or CD40L induced the differentiation of a T-bethighCD21low B-cell population. High expression of additional markers corroborated the phenotypic resemblance to T-bethighCD21low B cells ex vivo. IEI affecting IFNγR and canonical NF-κB signaling disturbed the differentiation of T-bethighCD21low-like B cells. While IL-21 was crucial for the differentiation in the context of CD40L stimulation, CpG-induced T-bethighCD21low-like B cells did not depend on IL-21. Reduced CD21low B cells in peripheral blood of patients with IEI affecting IFNγR-, canonical NF-κB- IL-21R- and CD40 signaling indicated a non-redundant role of these pathways for the differentiation of this B-cell phenotype in vivo.
Conclusion
The identification of IFNγ, IL21 and CD40 for the differentiation of T-bethighCD21low B cells in vivo provides pathways to target this potentially harmful B-cell population in the context of autoimmunity.