Interferon signatures fuel B cell hyperactivity and plasmablast expansion in systemic lupus erythematosus.

Systemic Lupus Erythematosus (SLE) is an autoimmune disease characterized by an array of autoantibodies, in particular anti-nuclear antibodies (ANA). The disease is also hallmarked by an expansion of plasmablasts (PB) and hypergammaglobulinemia. The mechanisms underlying this hyperactivity and its relation to autoantibody production is not clear. We aimed to characterize B cell hyperactivity in SLE to identify its underlying mechanisms. Using deep phenotyping with spectral flow cytometry and scRNAseq, we demonstrate that a high frequency of PB relative to memory B cells marks a subgroup of SLE patients, particularly those with higher disease activity and positive for Sm/RNP autoantibodies. We identified the origin of this phenotype in a prominent IFN signature in PB and increased activation in the switched CD27 memory B cell compartment. PB from this group of SLE patients displayed high levels of CD45RB and somatic hypermutation frequencies similar to memory B cells. Repertoire analysis revealed a highly polyclonal expansion of PB and skewing towards IgG1. B cell hyperactivity correlated with hypergammaglobulinemia, especially increased IgG serum levels. In summary, we show for the first time a direct relationship between IFN and PB expansion in a subgroup of SLE patients. Increased activation and differentiation of class-switched B cells driven by IFN may directly underlie PB expansion and hypergammaglobulinemia. These results provide insight into the pathways leading to B cell hyperactivity and autoantibody production which may guide the tailoring of B cell- and IFN-targeted therapies.