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Innate lymphoid cells. New players in tissue homeostasis and inflammatory responses.

In recent times, our understanding of the role of the immune system in different physiopathological situations has increased markedly. A new set of cells, generically known as innate lymphoid cells (ILC), has been discovered in the lymphoid compartment. Five ILC subsets can be recognized according to phenotypic and functional similarities with different subpopulations of T lymphocytes. Unlike T and B lymphocytes, ILC do not express antigen receptors nor undergo selection and clonal expansion upon activation. Instead, they respond rapidly to cytokines and danger signals in infected or inflamed tissues, producing cytokines that direct the immune response toward a type suitable for controlling the initial insult. In addition, ILC establish a crosstalk with other cells of the microenvironment that contributes to the maintenance and restoration of tissue homeostasis. Although many evidences on ILC were obtained from animal models, solid data confirm their existence in humans and their role in various inflammatory disorders. In this article, we address new knowledge on ILC, particularly on their role in the homeostasis of the immune system and in various inflammatory pathologies, in order to present new actors regulating immunity and immunopathology and affecting human health.