Control of T-cell responses can be achieved by several subsets of B cells with immunoregulatory functions, mostly acting by provision of the anti-inflammatory cytokine IL-10 or exhibiting killing properties through Fas ligand (Fas-L) or granzyme B-induced cell death. We herein describe the characterization as well as the cellular and molecular mechanisms mediating the suppressive properties of bone marrow immature innate pro-B cell progenitors that emerge upon transient activation of Toll-like receptor 9. They are licensed by activated T-cell-derived IFN-γ to become suppressive by up-regulating their Fas-L expression and inducing effector CD4(+) T-cell apoptosis. They also up-regulate their own IFN-γ production which dramatically reduces T-cell production of a major pathogenic cytokine, IL-21. A single adoptive transfer of as little as 60,000 of them efficiently prevents the onset of spontaneous type 1 diabetes in recipient nonobese diabetes (NOD) mice, highlighting the remarkable regulatory potency of these so-called CpG-proB cell progenitors compared to regulatory cells of diverse lineages so far described. The CpG-proB cell activity is prolonged in vivo by their differentiation after migration in the pancreas and the spleen into B-cell progeny with high Fas-L expression that can keep up inducing apoptosis of effector T cells in the long term.
Keywords: B-cell progenitors; Cell therapy; Fas-L; IFN-γ; IL-21; Killer B lymphocytes; Regulatory B cells; Tolerance; Toll-like receptors; Type 1 diabetes.