Mechanisms of protective versus autoreactive B cells in systemic lupus erythematosus

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B lineage memory, mediated by antigen-specific memory B cells (mBC) and plasmablasts/plasma cells (PB/PC), plays a key role in both protective immune responses and autoimmune diseases, such as systemic lupus erythematosus (SLE). Both also form a defense strategy with providing stability and plasticity to allow adaptation against new antigens and immune effector (regulatory) functions. It has not been studied whether antigen-specific mBC and PB/PC in autoimmunity follow similar principles as those of protective immunity and if principles of a secondary immune response apply during flares.Previously, we characterized specific mBC and PB/PC arising from a primary and a secondary human immune response using Keyhole limpet hemocyanin (KLH) and tetanus toxoid (TT), respectively, as models. Primary KLH immunization resulted in recruitment of naive and cross-reactive mBC that generated low and highly Ig mutated KLH-specific PB, respectively, early after immunization. Circulating KLH-specific PB were mainly of IgA isotype, while IgG and IgM anti-KLH antibodies were also present. These features are distinct of a secondary immune response, that we model using TT vaccination, in which mBC and PB were mainly of the IgG isotype, appear with different kinetics, are clonally related and present increase levels of Ig mutations consistent with reactivation in GC reaction. The phenotypic and molecular data provide the basis to address whether autoreactive B lineage cells follow principles close to a primary or secondary challenge. Another goal of our application will be to delineate the role of checkpoint molecules on antigen-specific B cells. To address this, the study will use an established multiantigenic, polychromatic FACS (Ro52, TT, mutated citrullinated vimentin/MCV) of (auto)antigens to characterize patients with SLE a) during flares, b) after TT vaccination and c) upon anti-BAFF(belimumab) interventions. Ex vivo phenotypic analyses will identify the frequency of (auto)reactive incl. polyreactive cells under different conditions including the detection of checkpoint molecule expression and molecular studies of VDJ somatic hypermutations on the single cell level. In vitro T cell and BCR-independent differentiation experiments of mBC from patients versus controls will comparatively assess the frequency and capacity of mBC to differentiate into (auto)antigen-specific plasmablasts with their molecular and checkpoint molecule characteristics. Further studies will use BCR-dependent in vitro vaccination to identify the role of BCR stimulation in the differentiation of autoreactive and protective PC. These studies will provide fresh insights into mechanisms regulating autoreactive and protective B cells in SLE patients versus controls and hold promise of a new understanding of the role of (auto)reactive versus protective cells and may guide new treatment options.