The Role of Innate Lymphoid Cells in Homeostasis and in the Pathogenesis of Chronic Inflammatory Diseases

Innate lymphoid cells (ILCs) are lymphocytes that lack antigen-specific receptors. ILCs include NK cells, which were described in 1975, and lymphoid tissue inducer (LTi) cells, which were described in 1992, but it also includes the ILC1, ILC2 and ILC3 subsets, which were described in the late 2010s and produce cytokine profiles that are similar to those produced by conventional Th1, Th2 and Th17 cells. NK cells are cytotoxic lymphocytes that participate in the immune response to tumors and virus, and LTi cells are involved in the early development of secondary and tertiary lymphoid organs. In contrast, ILC1, ILC2 and ILC3 have been described as helper ILCs, because their main effector function is the production of cytokines that regulate other immune cells. ILC1 produce IFN-γ, which activates macrophages, Th1 cells and NK cells. ILC2 produce IL-4, IL-5, IL-9 and IL-13 and are thus involved in the pathogenesis of allergy and obesity. ILC3 produce IL-17 and IL-22, and are important for intestinal homeostasis. These three ILC subsets participate in the innate immune response to infections, and in the immunopathology of many chronic inflammatory diseases. Here we describe the role of ILCs during homeostasis, in the immune response to pathogens and in the pathogenesis of obesity, cancer, asthma, psoriasis and inflammatory bowel disease.