Bone Morphogenetic Proteins Inhibit CD40L- and IL-21-Induced Ig Production In Human B Cells: Differential Effects of BMP-6 and BMP-7

TGF-β has been shown to play an important role in directing class switch recombination in human B cells. Bone morphogenetic proteins (BMPs) are members of the TGF-β family, and we have previously shown that BMP-6 inhibits proliferation of B cell progenitors as well as mature B cells in humans. However, how various BMP members affect Ig production and B-cell maturation to plasma cells have not been investigated. Here we studied the effects of various BMPs in CD27^- naive and CD27⁺ memory B cells from peripheral blood of healthy human donors.

We used flow immunomagnetic beads and flow cytometry cell sorting to purify CD19⁺CD27^- naive and CD19⁺CD27⁺ memory B cells from peripheral blood of healthy donors, and investigated BMP induced effects on in vitro proliferation, cell death and Ig production as measured by thymidine incorporation assay, propidium iodide staining and ELISA, respectively. To separate direct inhibition of plasma cell differentiation from indirect effects via suppression of proliferation and induction of apoptosis, CFSE tracking of cell division was combined with immunophenotyping of cultured cells to identify CD27⁺CD38⁺ plasma cells. Expression of transcription factors and AICDA were measured by quantitative real-time PCR.

BMP-2, -4, -6 and -7 specifically inhibited CD40L- and IL-21-induced production of IgM, IgG and IgA. BMP-6 was the most potent inhibitor, reducing the Ig production by 70% in memory B cells and more than 50% in naive B cells. By investigating the mechanisms for reduced Ig production, we found a striking difference between the structurally similar BMP-6 and BMP-7. BMP-6 directly inhibited differentiation to CD27⁺CD38⁺ plasma cells, whereas BMP-7 only had minor effects on differentiation. Instead, BMP-7 mainly affected Ig production indirectly by inducing apoptosis. Furthermore, we explored BMP-6-induced signaling and gene regulation in more detail in memory B cells. BMP-6 up-regulated Id1, Id2 and Id3 gene expression in CD40L- and IL-21-stimulated cells (6.1-fold, 1.7-fold and 4.0-fold induction after 48 hours culture with BMP-6, respectively). In contrast, BMP-6 potently inhibited CD40L- and IL-21-induced upregulation of the transcription factor XBP-1, necessary for the late stages of plasmacytic differentiation. Expression of transcription factors regulating earlier stages (IRF4, PRDM1) was not affected, indicating that BMP-6 only modulates late events of plasma cell development.

These results show that BMPs potently suppress Ig production in mature human B cells by inhibiting differentiation or by inducing cell death.

No relevant conflicts of interest to declare.