Background In our previous study, we reported that soluble BAFF (sBAFF) robustly enhanced IL-6 production by peripheral monocytes of patients with primary Sjogren's syndrome (pSS) and that the expression level of a BAFF receptor (BR3) was significantly elevated in pSS monocytes. We also found that the proportion of BR3-positive monocytes to total monocytes was positively and significantly correlated with the serum IgG level of pSS patients. Investigation of the interaction of monocytes and B cells showed that IgG production by B cells was enhanced by sBAFF-stimulated monocytes. These data collectively suggest that the elevated expression of BR3 on monocytes is involved in IgG overproduction by B cells which is often observed in pSS, and that BAFF signaling via BR3 is a possible therapeutic target to treat pSS. A high throughput screening of a low molecular weight compound library successfully discovered two pyrrolopryimidine derivatives, BIK12 and BIK13, which inhibit sBAFF binding to BR3. We found that these compounds inhibited not only IL-6 production by BAFF-stimulated monocytes, but also IgG production by B cells co-cultured with the monocytes.

Objectives To elucidate the mechanism of inhibitory activities of these compounds on BAFF signaling pathways, we measured production of IL-10 as well as IL-6 by monocytes in vitro, both of which work for B cell activation. In addition, we analyzed in vivo effects of the compounds on production of autoantibody and cytokines in autoimmune disease model mice.

Methods Peripheral monocytes and B cells were prepared from healthy individuals. The monocytes were stimulated with sBAFF and cultured in vitro with or without peripheral B cells in the presence of BIK-12 or BIK-13. The amounts of IL-6, IL-10 and IgG were measured by ELISA. BIK-13 was administered intraperitoneally to MRL/lpr mice, an animal model of autoimmune diseases, three times a week for 6 months. Serum levels of an anti-ds DNA antibody, IL-6 and IL-10 were measured by ELISA.

Results sBAFF enhanced the production of not only IL-6, but also IL-10 by peripheral monocytes in vitro. As expected, BIK12 and BIK-13 significantly suppressed production of IL-6 and IL-10 by BAFF-stimulated monocytes in vitro in a dose dependent manner. Notably, IgG production by B cells co-cultured in vitro with sBAFF-stimulated monocytes was significantly suppressed by these compounds. The compounds did not exhibit toxicities to the cells in the dose range. These data suggest that BAFF signaling via BR3 lead to production of IL-6 and IL-10 in monocytes, and the cytokines in turn mediates the signal to B cells resulting in IgG overproduction. Interestingly, serum levels of an anti-dsDNA antibody, IL-6 and IL-10 in MRL/lpr mice received BIK13 simultaneously declined as compared to control mice. These data collectively suggest that the compound was also efficacious in vivo.

Conclusions BIK-13, a pyrrolopyrimidine derivative, suppressed IgG production by B cells through affecting monocytes. IL-6 and/or IL-10 may intermediate the effect. Our findings strongly suggest that BAFF signaling via BR3 is a possible therapeutic target for drug discovery to treat pSS or other intractable autoimmune diseases which accompany hypergammaglobulinemia. Moreover, these compounds may provide novel tools to explore the pathological mechanism of the development of these autoimmune diseases.

Disclosure of Interest K. Yoshimoto: None declared, K. Suzuki: None declared, K. Sugahara Employee of: Mitsubishi Tanabe Pharma Corporation, T. Takeuchi Grant/research support from: Mitsubishi Tanabe Pharma Corporation