CD39 and CD73 are key enzymes in the adenosine (ADO) pathway. ADO modulates pathophysiological responses of immune cells, including B cells. It has recently emerged that a subpopulation of ADO-producing CD39+CD73+ B cells has regulatory properties. Here, we define the CD39high subset of these cells as the major contributor to the regulatory network operated by human B lymphocytes. Peripheral blood B cells were sorted into CD39neg, CD39inter and CD39high subsets. The phenotype, proliferation and IL-10 secretion by these B cells were studied by flow cytometry. 5′-AMP and ADO levels were measured by mass spectrometry. Agonists or antagonists of A1R, A2AR and A3R were used to study ADO-receptor signaling in B cells. Inhibition of effector T-cell (Teff) activation/proliferation by B cells was assessed in co-cultures. Cytokine production was measured by Luminex. Upon in vitro activation and culture of B cells, the subset of CD39high B cells increased in frequency (p