Regulatory B cell is critical in bone union process through suppressing proinflammatory cytokines and stimulating Foxp3 in Treg cells

Summary Bone fractures may result in delayed union ( DU ) or non‐union ( NU ) in some patients. Evidence suggests that the skewing of the immune system toward the proinflammatory type is a contributing factor. Because B cells were previously found to infiltrate the fracture healing site at abundant levels, we examined the regulatory B cells (Bregs) in DU / NU patients. In bone fracture patients with normal healing, the frequency of interleukin ( IL )‐10‐expressing B cells was significantly upregulated in the early healing process (6 weeks post‐surgery) and was downregulated later on (18 weeks post‐surgery), whereas in DU / NU patients, the early upregulation of IL ‐10‐expressing B cells was missing. The majority of IL ‐10‐expressing B cells were concentrated in the IgM + CD 27 + fraction in both controls and patients. IgM + CD 27 + B cells effectively suppressed interferon gamma ( IFN ‐γ), tumor necrosis factor alpha ( TNF ‐α), and IL ‐2 expression from CD 4 + T cells, as well as IFN ‐γ and TNF ‐α expression from CD 8 + T cells. The IgM + CD 27 + B cell‐mediated suppression was restricted to the sample from the early healing time point in controls, as the IgM + CD 27 + B cells from normal healing patients later on or from DU / NU patients did not present significant regulatory function. In addition, culturing of CD 4 + CD 25 + Tregs with IgM + CD 27 + B cells from controls at early healing time point resulted in higher Foxp3 expression, a function absent in controls at later time point, or in DU / NU patients. In conclusion, our results support a role of B cell‐mediated regulation early during the bone healing process.