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

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⁺ CD27⁺ fraction in both controls and patients. IgM⁺ CD27⁺ B cells effectively suppressed interferon gamma (IFN-γ), tumor necrosis factor alpha (TNF-α), and IL-2 expression from CD4⁺ T cells, as well as IFN-γ and TNF-α expression from CD8⁺ T cells. The IgM⁺ CD27⁺ B cell-mediated suppression was restricted to the sample from the early healing time point in controls, as the IgM⁺ CD27⁺ B cells from normal healing patients later on or from DU/NU patients did not present significant regulatory function. In addition, culturing of CD4⁺ CD25⁺ Tregs with IgM⁺ CD27⁺ 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.