Background/Purpose:  Belimumab is a recombinant monoclonal antibody that inhibits soluble B lymphocyte stimulator (BLyS), also known as BAFF or TNFSF13B, and is approved for treating patients with systemic lupus erythematosus (SLE). In this observational prospective cohort study, we sought to identify B cell and T cell alterations in SLE patients during treatment with belimumab.

Methods:  Twenty-three SLE patients from the Karolinska University Hospital treated with belimumab were enrolled and followed longitudinally. Peripheral blood mononuclear cells were collected and cryopreserved at inclusion and at regular follow-up visits up to 3 years from treatment initiation. The sample series were assayed by mass cytometry (CyTOF) at the same time point, and analysed in the context of clinical parameters.

Results: Changes in B cell subsets were assessed by analysing the CyTOF data after adjustment for total lymphocyte counts from each visit. As expected, B cells decreased over time. More specifically, transitional and naïve B cells (CD19⁺CD20⁺IgD⁺IgM⁺CD27^–) showed a rapid reduction already at three months follow-up, and continued to decrease over time (p<0.0001). In contrast, the pre-switching (CD19⁺CD20⁺IgD⁺CD27⁺) and double negative memory B cells (CD19⁺CD20⁺IgD^–CD27^–) showed a more gradual decline (p=0.01 and p=0.0001, respectively). Moreover, we observed decreases (p<0.0001) in age-associated B cells (CD11c⁺CD21^–), a recently described cell subset implicated in humoral autoimmune responses. However, plasma cells (CD138⁺CD38⁺CD27⁺CD19⁺CD3e^–CD20^–) and switched memory B cells (CD19⁺CD20⁺CD27⁺IgD^–) remained stable (p=0.7 and p=0.3, respectively). In correlation analyses of individual markers with time from treatment initiation, average CD27 expression among B cells showed the greatest increase during the first 6 months of treatment (r=0.70) while expression of IgA (r=0.46), CD19 (r=0.39), and CD5 (r=0.34) also increased (<5% false discovery rate, FDR, for all). In contrast, IgD (r=-0.75), IgM (r=-0.67), CD22 (r=-0.64), Ki-67 (r=-0.62), CD20 (r=-0.58), CD38 (r=-0.49), and CD21 (r=-0.51) decreased (<5% FDR for all). During later follow-up times (from 6 months of treatment and during the remaining study period), CD5 (r=0.53) and CD38 (r=0.51) showed the greatest increases (<5% FDR for both), followed by CD20 (r=0.36) and Ki–67 (r=0.34) (p<0.05 for both), and CD22 continued to decrease (r=-0.32, p<0.05). In contrast to the clear B cell alterations following belimumab treatment, T cells and monocytes did not change significantly.

Conclusion: In this real-life clinical practice setting, belimumab resulted in an initial rapid reduction of naïve B cells and age-associated B cells, followed by a continuous gradual decrease, which also included pre-switching and double negative B cells. In contrast, memory B cells, plasma cells and T cells were preserved. Our observations of B cell alterations betiding in two distinct phases, a rapid early and a more gradual late phase, may have direct implications in the clinical use of belimumab, as early treatment evaluation and discontinuation might forfeit delayed clinical improvements reflecting the late B cell changes.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/observations-of-early-and-late-b-cell-alterations-during-belimumab-treatment-in-patients-with-systemic-lupus-erythematosus-using-mass-cytometry-cytof/