Abnormal B-cell activation is implicated in the pathogenesis of autoimmune diseases, including systemic lupus erythematosus (SLE). The B-cell surface molecule CD22, which regulates activation through the B-cell receptor (BCR), is a potential target for inhibiting pathogenic B cells; however, the regulatory functions of CD22 remain poorly understood. In this study, we determined how targeting of CD22 with epratuzumab (Emab), a humanized anti-CD22 IgG1 monoclonal antibody, affects the activation of human B-cell subsets in response to Toll-like receptor 7 (TLR7) and BCR engagement.
B-cell subsets were isolated from human tonsils and stimulated with F(ab′) 2 anti-human IgM and/or the TLR7 agonist R848 in the presence of Emab or a human IgG1 isotype control. Changes in mRNA levels of genes associated with B-cell activation and differentiation were analyzed by quantitative PCR. Cytokine production was measured by ELISA. Cell proliferation, survival, and differentiation were assessed by flow cytometry.
Pretreatment of phenotypically naïve CD19 +CD10 –CD27 – cells with Emab led to a significant increase in IL-10 expression, and in some but not all patient samples to a reduction of IL-6 production in response to TLR7 stimulation alone or in combination with anti-IgM. Emab selectively inhibited the expression of PRDM1, the gene encoding B-lymphocyte-induced maturation protein 1 (Blimp-1) in activated CD10 –CD27 – B cells. CD10 –CD27 –IgD – cells were highly responsive to stimulation through TLR7 as evidenced by the appearance of blasting CD27 hiCD38 hi cells. Emab significantly inhibited the activation and differentiation of CD10 –CD27 –IgD – B cells into plasma cells.
Emab can both regulate cytokine expression and block Blimp1-dependent B-cell differentiation, although the effects of Emab may depend on the stage of B-cell development or activation. In addition, Emab inhibits the activation of CD27 –IgD – tonsillar cells, which correspond to so-called double-negative memory B cells, known to be increased in SLE patients with more active disease. These data may be relevant to the therapeutic effect of Emab in vivo via modulation of the production of pro-inflammatory and anti-inflammatory cytokines by B cells. Because Blimp-1 is required by B cells to mature into antibody-producing cells, inhibition of Blimp1 may reduce autoantibody production.