Background/Purpose . B cell activating factor of tumor necrosis factor family (BAFF) is a positive regulator of B cell function and expressed in dendritic cells, monocytes/macrophages, and T cells. Clinical studies have demonstrated that BAFF is overexpressed in many patients with systemic lupus erythematosus (SLE). Furthermore, autocrine BAFF expression in B cells from SLE patients with high disease activity was reported. Although cyclophosphamide (CY) has remained the treatment of choice for major organ involvement in SLE, its precise therapeutic mechanism in SLE remains to be elucidated. At the 2010 ACR meeting, we reported CY predominantly and immediately suppresses activation and viability of plasmablasts in vivo and in vitro. In this study, we aimed to further clarify the precise effect of CY on human B cell subsets and BAFF expression on B cells in SLE.

Methods . We studied 10 consecutive patients with recent onset or flare of SLE in whom monthly intravenous CY and oral prednisolone were initiated, and 10 normal healthy controls (NHCs). Peripheral blood mononuclear cells (PBMCs) were isolated from heparinized whole blood of these subjects by density gradient centrifugation before and after CY initiation. Immunofluorescence staining of PBMCs for flow cytometric analyses was performed using anti–CD19, anti–CD27, anti–CD38, anti–IgD, anti–IgM, and anti–BAFF antibodies, which specifically recognizes membrane–bound BAFF and does not bind to shed soluble BAFF. Serum BAFF levels were measured by enzyme–linked immunosorbent assay (ELISA).

Results . The frequencies of plasmablasts (CD19⁺/CD27^high/CD38^high), transitional B cells (CD19⁺/CD27^–/CD38^high), and double negative memory B cells (CD19⁺/CD27^–/IgD^–) were significantly higher in SLE patients than in NHCs, whereas the frequencies of memory B cells (CD19⁺/CD27⁺) were significantly lower in NHCs (p < 0.01 in all comparisons). CD27⁺ memory B cells were further classified into CD27⁺IgD⁺ pre–switched memory B cells and CD27⁺IgD^– switched memory B cells. The frequencies of pre–switched and switched memory B cells were significantly lower in SLE patients than in NHCs (p = 0.0003 and 0.03, respectively). Two weeks after the initiation of CY, the frequencies of plasmablasts and transitional B cells in SLE patients significantly decreased (p < 0.01 in both comparisons), while the frequencies of pre–switched and switched memory B cells in SLE patients significantly increased (p = 0.02 and 0.002, respectively). The mean fluorescence intensity (MFI) of the membrane bound–BAFF on plasmablasts, memory B cells (CD19⁺/CD27⁺) and transitional B cells were significantly higher in SLE patients than in NHCs (p = 0.002, 0.02, and 0.007, respectively), and decreased significantly two weeks after the initiation of CY (p = 0.004, 0.01, and 0.01, respectively). Serum BAFF levels in SLE patients also significantly decreased with clinical amelioration following treatment (p = 0.002).

Conclusion . This study shows that administration of CY predominantly and immediately diminishes plasmablasts and transitional B cells, and the BAFF overexpression in these cells. These findings indicate that therapeutic effect of CY on SLE is at least partly exerted through this mechanism.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/cyclophosphamide-diminishes-plasmablasts-and-transitional-b-cells-and-suppresses-autocrine-production-of-b-cell-activating-factor-of-tumor-necrosis-factor-family-baff-in-these-cells-in-patients-with/