• Autoimmune Diseases
• B cells
• Cytokines

B cells initiate and perpetuate autoimmune disease processes. Interleukin (IL)-4 and IL-21 produced by follicular helper T cells are required for B cell activation, germinal centre formation, immunoglobulin class switching and plasma cell differentiation.1 ,2 The JAK inhibitor tofacitinib is approved for treatment of rheumatoid arthritis. We recently reported that tofacitinib can suppress IL-17 and interferon-γ production by CD4+ T cells3 and inhibit the T cell stimulatory capacity of human monocyte-derived dendritic cells.4 However, whether this action involves B cell activation remains unclear.

Here we investigated the in vitro effects of tofacitinib on the gene regulatory network that controls B cell class switching and differentiation. Purified CD19+ B cells were stimulated with B cell antigen receptor (BCR), soluble CD40 ligand (sCD40L) and cytokines with/without tofacitinib. Culture medium was replenished on day 3. Cell viability tests revealed that, although B cell survival decreased considerably over time, the possibility of pharmacological toxicity by tofacitinib could be excluded (data not shown).

The expression of AICDA was slightly induced by cytokines or BCR/sCD40L alone, while costimulation with BCR, sCD40L and cytokines, especially IL-4, caused robust gene expression (figure 1A). BCL6 and XBP-1 exhibited similar expression patterns, at day 2 and day 5, respectively (figure 1A). BCR/sCD40L costimulated expression of AICDA and XBP-1was inhibited by tofacitinib in a dose-dependent manner (figure 1B, C). Tofacitinib did not inhibit BCL6 gene expression on day 2. In contrast with tofacitinib, inhibitors of the Src-family kinase (PP1, PP2) and a JAK2 inhibitor did not affect the expression of AICDA (figure 1B). Methotrexate exhibited a modest suppressive effect on AICDA expression compared with tofacitinib (figure 1D). In addition, AICDA gene expression levels after CpG stimulation were comparable in the presence/absence of tofacitinib (figure 1E). Finally, tofacitinib markedly and dose-dependently abrogated IgG production by B cells stimulated with BCR, sCD40L and IL-4 (figure 1F). These data describe tofacitinib as an effective inhibitor of B cell development.

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Figure 1

Effects of tofacitinib on the gene regulatory network involved in B cell class switching, differentiation and immunoglobulin (Ig) production. Peripheral blood CD19+ B cells from healthy donors were cultured with 1 μg/mL anti-B cell antigen receptor (BCR) monoclonal antibodies (mAbs), 2 μg/mL human soluble CD40 ligand (sCD40L), 2.5 μg/mL CpG oligodeoxynucleotides (CpG-ODN) 2006 and 100 ng/mL of interleukin (IL)-6, IL-21 and IL-4 for 2days or 5 days, with or without tofacitinib/PP1/PP2/JAK2 inhibitor/methotrexate (MTX), as indicated. (A–E) AICDA, BCL-6 and XBP-1 gene expression levels were detected by quantitative real-time PCR, with each sample 100 pg mRNA template/reaction. (F) IgG secretion was measured by ELISA. Data are mean±SEM of three independent experiments. *p<0.05, as determined by one-way analysis of variance with Games–Howell comparison test.

Next, the effect of tofacitinib on activated B cells was examined. Tofacitinib was added on day 2 of B cell stimulation, and its effects on gene expression and immunoglobulin G (IgG) production were assessed on day 5. Tofacitinib abrogated AICDA, BCL6, XBP-1 expression and IgG production to similar levels seen with BCR/sCD40L alone (figure 2A, B). BCL6 protein levels were also reduced by tofacitinib (data not shown). This suggested that tofacitinib may inhibit the cytokine-mediated maintenance of BCL6 at the later time point. Previous studies indicated that Bach2 impairs germinal centre formation and class switching,5 its downregulation enhances plasma cell differentiation from IgG1 memory B cells6 and that Bach2 also represses effector programmes that stabilise regulatory T cell function.7 Uniquely, Bach2 was induced by BCR and sCD40L, but expression was significantly inhibited by IL-4. Interestingly, the addition of tofacitinib resulted in the recovery of Bach2 expression in a dose-dependent manner (figure 2A).

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Figure 2

Effect of tofacitinib on highly activated B cells. Human PB CD19+ pan B cells from healthy subjects were cultured with 1 μg/mL anti- B cell antigen receptor (BCR) monoclonal antibodies (mAbs), 2 μg/mL human soluble CD40 ligand (sCD40L) and 100 ng/mL of interleukin (IL)-4 for 2 days, then treated with tofacitinib for another 3 days. (A) Gene expression levels were determined by quantitative real-time PCR, with each sample 100 pg mRNA template/reaction. (B) IgG secretion was measured by ELISA. (C) Cytokine production was measured by cytokine beads array. Data are mean±SEM of three independent experiments. *p<0.05, as determined by one-way analysis of variance with Games–Howell comparison test.

Finally, we investigated the effect of tofacitinib on the production of proinflammatory and anti-inflammatory cytokines by activated B cells. The strong induction of IL-6 gene expression and protein production by BCR, sCD40L and IL-4 stimulation was significantly abrogated by tofacitinib in a dose-dependent manner. Meanwhile, IL-10, which is produced by regulatory B cells,8 was not affected by tofacitinib even at the highest dose (figure 2C). These results suggest that tofacitinib suppresses B cell activation, differentiation and class switching, while maintaining B cell regulatory function.

In conclusion, our results provide new insights into the mechanisms of action of tofacitinib in the treatment of autoimmune diseases.