Molecular and Cellular Dissection of the Nephrotic B-cell Response Reveals a Prominent Atypical B-cell Signature in Relapses of Idiopathic Nephrotic Syndrome

Abstract Text: The search for glucocorticoid (GC)-sparing treatments for idiopathic nephrotic syndrome (INS), the most common relapsing-remitting glomerular disorder in children, identified B-cell depletion as a successful approach in maintaining long-term remission from proteinuria. While this positioned B-cells as the central mediators of pathogenesis, the nature of the nephrotic B-cell response, including whether antibody-dependent or independent functions were involved, remained unknown. To dissect this, we carried out single cell RNA-sequencing (scRNA-seq) and high-parameter flow cytometry on peripheral blood B-cells from pediatric INS patients and healthy controls (HC). By scRNA-seq (N=4/group), we identified an expanded memory subset in INS characterized by a transcriptional profile consistent with atypical B-cells (atBCs) – a population associated with autoimmunity, viral infection, and aging. The atBCs in INS preferentially expressed genes associated with B-cell activation, antigen presentation, and immunoglobulin production. We confirmed the expansion of atBCs in INS using the flow-based unsupervised clustering algorithm FlowSOM (N>15/group). Moreover, we identified that this, alongside a marked expansion of plasmablasts, represented the predominant perturbation in peripheral B-cells in INS. While treatment with GC (N=15) diminished circulating atBC and plasmablast numbers, all memory B-cells were ablated in patients maintained in remission with the B-cell depleting biologic rituximab (RTX, N=14). As RTX-treated patients relapsed (N=4 longitudinally), scRNA-seq showed that the atBC subset partially rebounded and the pathogenic B-cell transcriptional signature was re-established. Our work proposes a central pathogenic role for atBCs in pediatric INS and thereby supports the understanding of INS as an autoimmune condition with relapses frequently taking place following viral infections.