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Type 3 innate lymphoid cells (ILC3s) are involved in maintenance of mucosal homeostasis; however, their role in immunoregulation has been unknown. Immature transitional regulatory B (itBreg) cells are innate-like B cells with immunosuppressive properties, and thein vivomechanisms by which they are induced have not been fully clarified.We aimed to investigate the ILC3–B-cell interaction that probably takes place in human tonsils.ILC3s were isolated from peripheral blood and palatine tonsils, expanded, and cocultured with naive B cells. Tonsillar ILC3s and regulatory B cells were visualized with immunofluorescence histology. ILC3 frequencies were measured in tonsil tissue of allergic and nonallergic patients and in peripheral blood of allergic asthmatic patients and healthy control subjects.A mutually beneficial relationship was revealed between ILC3s and B cells: ILC3s induced IL-15 production in B cells through B cell–activating factor receptor, whereas IL-15, a potent growth factor for ILC3s, induced CD40 ligand (CD40L) expression on circulating and tonsillar ILC3s. IL-15–activated CD40L+ ILC3s helped B-cell survival, proliferation, and differentiation of IL-10–secreting, PD-L1–expressing functional itBreg cells in a CD40L- and B cell–activating factor receptor–dependent manner. ILC3s and regulatory B cells were in close connection with each other in palatine tonsils. ILC3 frequency was reduced in tonsil tissue of allergic patients and in peripheral blood of allergic asthmatic patients.Human CD40L+ ILC3s provide innate B-cell help and are involved in an innate immunoregulatory mechanism through induction of itBreg cell differentiation, which takes place in palatine tonsilsin vivo. This mechanism, which can contribute to maintenance of immune tolerance, becomes insufficient in allergic diseases.