MO246: Belimumab Disrupts Memory B Cell Trafficking in Patients with Systemic Lupus Erythematosus

Abstract BACKGROUND AND AIMS Belimumab (BEL), a recombinant human monoclonal antibody directed against B-cell activating factor (BAFF), is the first approved biological agent for patients with systemic lupus erythematosus (SLE) and a high level of disease activity or lupus nephritis (LN). BEL inhibits primary humoral immune responses by depleting naive B-cells that are dependent on BAFF for their survival while secondary humoral immune responses by memory B cells (MBCs) remain intact. Indeed, some studies reported an increase of circulating MBCs following neutralization of BAFF [1–3]. So far these effects of BEL on the MBC compartment in SLE patients have not been investigated. This study aimed to establish the dynamics of circulating MBCs in patients with SLE treated with BEL and to perform an in-depth analysis of the impact of BEL on the MBC compartment. METHOD First, extensive B cell subset phenotyping was performed prospectively by employing high-sensitivity flow cytometry (HSFC) based on EuroFlow protocols [4] in severe SLE/LN patients treated with BEL [5]. Additionally, in-depth characterisation of surging MBCs in circulation was performed by single-cell RNA sequencing (scRNA-seq). RESULTS HSFC established that the increase in MBCs was non-specific and observed in a broad range of MBC immunoglobulin subclasses peaking as early as 2 weeks after BEL initiation. Subsequent scRNA-seq analysis of the emerging MBCs revealed a non-proliferating phenotype with a prominent decrease in activation status. In these circulating MBCs, a large amount of migration and adhesion genes were downregulated suggesting that the accumulation of MBCs by BEL initiation was related to their impaired cell–cell adhesion disrupting cell-trafficking and preventing extravasation. CONCLUSION After initiation of BEL treatment, a substantial increase of circulating MBCs was firmly established in patients with SLE/LN. The surge of circulating MBCs appeared to be associated with disrupted lymphocyte trafficking of MBCs, thereby suggesting a new potential therapeutic mechanism of BEL on MBCs in SLE. These findings have important implications to our understanding and consequent improvement of B-cell targeted treatment strategies in patients with active SLE and LN as MBC accumulation in circulation might allow for more efficient targeting of the B-cell compartment.