Background We have found that B cell activating factor (BAFF) robustly increased IL-6 production by peripheral monocytes from the patients with primary Sjögren's syndrome (pSS), and that the expression level of BAFF receptor (BR3) was significantly elevated in pSS monocytes compared to that of normal monocytes. These data collectively suggest that the elevated expression of BR3 is involved in increased IL-6 production by pSS monocytes. Since activation of IL-6 signaling pathways in B cells results in IgG production, our findings suggest that monocytes are involved in the development of hypergammaglobulinemia (HγG) which is often accompanied by pSS. However, the detailed molecular and cellular mechanism underlying HγG is not well clarified.
Objectives Our objectives is to show whether or not i) monocytes are actually involved in HγG, and ii) direct contact between monocytes and B cells is required for IgG overproduction.
Methods Peripheral pSS B cells were cultured in vitro with or without peripheral pSS monocytes and stimulated with soluble BAFF (sBAFF). The cells in a well were either allowed to contact to each other or separated with a transwell insert. An antibody against human soluble IL-6 receptor was added to the culture to investigate the effects of IL-6 on IgG production. The amounts of IL-6, IgG and soluble IL-6 receptor (sIL-6R) in the culture supernatants were measured by ELISA. FACS analysis of whole blood samples was employed to analyze the expression of BR3.
Results As reported previously, pSS monocytes produced a significantly higher amount of IL-6 than normal monocytes in vitro upon stimulation with sBAFF. In addition, production of sIL-6R by pSS monocytes was also elevated by stimulation with sBAFF. Remarkably, the proportion of BR3-positive monocytes to total monocytes (BR3+/CD14+ ratio) was positively and significantly correlated with BAFF-induced IL-6 production in pSS monocytes. Moreover, the BR3+/CD14+ ratio also has a significant and positive correlation with the serum IgG level of pSS patients. Taken together, these data suggest that sBAFF-triggered overproduction of IL-6 and sIL-6R by monocytes is involved in IgG overproduction by B cells in pSS patients. Concordantly, stimulation of co-culture of pSS B cells and pSS monocytes with sBAFF drastically enhanced IgG production in vitro, whereas the stimulation showed only marginal effects on IgG production when monocytes were eliminated from the co-culture. It should be noted that separation of these cells by a transwell insert in a well did not suppress the IgG production. Moreover, addition of an anti-human sIL-6R antibody to the co-culture significantly inhibited the IgG production. Since IL-6 triggers its signaling by binding with sIL-6R, these data strongly suggest that IL-6 and IL-6R produced by sBAFF-stimulated monocyte play a pivotal role in the IgG overproduction by pSS B cells.
Conclusions The data in the present study suggest that pSS monocytes are involved in IgG overproduction by pSS B cells. It is monocyte-derived humoral factors rather than direct contact between B cells and monocytes that are responsible for the overproduction. We presume that an IL-6 signaling pathway plays a pivotal role in the development of HγG in pSS patients.
Disclosure of Interest : None declared