Background/Purpose:

Systemic lupus erythematosus (SLE) is characterized by B cell hyperactivity and production of autoantibodies. Treatment of patients with moderate-to-severe SLE with epratuzumab, a humanized monoclonal antibody that targets CD22, has resulted in clinically relevant, sustained improvements in disease activity. While epratuzumab is thought to modulate B cell function, its mechanism of action has not been completely elucidated. This study was undertaken to determine how epratuzumab affects the activation of different B cell subsets in response to Toll-like receptor (TLR) and/or B cell receptor (BCR) stimulation.

Methods:

CD20⁺ B cell subsets were isolated from human tonsils using magnetic bead cell separation and cell sorting based on their relative expression of CD10, CD27, IgD, CD38 and other surface markers. Cells were treated with anti-IgM and/or R848 (a TLR7 agonist) in the presence of epratuzumab or a human IgG control. Changes in mRNA levels of PRDM1, the gene encoding B lymphocyte-induced maturation protein 1 (Blimp-1), and a variety of other genes associated with B cell activation were analyzed by RT-PCR 12–24 hours after stimulation. Cell survival and plasma cell differentiation were assessed by flow cytometry after 3–5 days of in vitro cell culture.

Results:

Treatment of CD20⁺CD27^–CD10^– B cells with R848, anti-IgM, or a combination of R848 plus anti-IgM led to a significant (2–10-fold) increase in PRDM1 expression 12 hours after stimulation. Epratuzumab dramatically inhibited PRDM1 mRNA levels, but had no effect on the expression of a number of other genes, including AICDA, PAX5, BCL6, XBP1, and ETS1. In cell culture experiments R848, anti-IgM, or a combination of R848 plus anti-IgM stimulation did not have a significant effect on CD20⁺CD27⁺CD10^–IgD^– (switched memory cells), or CD20⁺CD27^–CD10^–IgD⁺ (naïve B cells), but significantly induced the activation of CD20⁺CD27^–CD10^–IgD^– cells, as evidenced by increased cell proliferation and by the appearance of blasting CD27^hiCD38^hi (pre-plasma) cells. Epratuzumab significantly inhibited CD38 expression and the generation of pre-plasma cells without significantly affecting cell survival.

Conclusion:

These results suggest that one therapeutic effect of epratuzumab may be via inhibiting the expression of PRDM1 (Blimp-1). Since Blimp-1 is required by B cells to mature into antibody-producing cells, epratuzumab-mediated inhibition of Blimp-1 may reduce autoantibody production in SLE patients. Epratuzumab inhibits the activation of CD20⁺CD27^–CD10^–IgD^– tonsillar B cells, which appear highly responsive to stimulation via TLR7. This population of cells might correspond to CD27^– memory B cells, found to be substantially increased in SLE patients. These data may have implications for understanding the effects of epratuzumab treatment on B cell function in SLE patients.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/inhibition-of-b-cell-activation-and-plasma-cell-differentiation-by-epratuzumab-a-humanized-monoclonal-antibody-targeting-cd22/