Innate lymphoid cell

Innate lymphoid cells (ILCs) are a group of innate immune cells that are derived from common lymphoid progenitor (CLP) and belong to the lymphoid lineage. These cells are defined by absence of antigen specific B or T cell receptor because of the lack of recombination activating gene (RAG). ILCs do not express myeloid or dendritic cell markers. Innate lymphoid cells (ILCs) are a group of innate immune cells that are derived from common lymphoid progenitor (CLP) and belong to the lymphoid lineage. These cells are defined by absence of antigen specific B or T cell receptor because of the lack of recombination activating gene (RAG). ILCs do not express myeloid or dendritic cell markers. This relatively newly described group of cells has varying physiological functions; some functions are analogous to helper T cells, while the group also includes cytotoxic NK cells. Accordingly, they have an important role in protective immunity and the regulation of homeostasis and inflammation, so their dysregulation can lead to immune pathology such as allergy, bronchial asthma and autoimmune disease. In addition, they can regulate adipose function and metabolic homeostasis, in part by eliciting beiging. ILCs can be divided based on the cytokines that they can produce, and the transcription factors that regulate their development and function. For each newly discovered branch of the ILC family, it will be important to determine whether a cell type represents a stable lineage or just a stage of differentiation or activation. The emerging body of data about the transcription factors and cytokine signals that differentiate ILCs contributes to the evolving classification system used to identify ILCs. In 2013 a nomenclature and classification system was proposed that divides the known ILCs into three groups. Group 1 ILCs constitutively express transcription factor T-bet and is able to produce Th1 cytokines (notably IFNγ and TNF) after stimulation with IL-12 or IL-18. ILC1 cells comprise NK cells, CD127low CD103+ intraepithelial ILC1s and CD127high ILC1s. Group 2 ILCs can produce type 2 cytokines (e.g. IL-4, IL-5, IL-9, IL-13). ILC2s (also termed natural helper cells, nuocytes, or innate helper 2 cells ) play the crucial role of secreting type 2 cytokines in response to helminth infection. They have also been implicated in the development of allergic lung inflammation. They express characteristic surface markers and receptors for chemokines, which are involved in distribution of lymphoid cells to specific organ sites. They require IL-7 for their development, which activates two transcription factors (both required by these cells)—RORα and GATA3. After stimulation with Th2 polarising cytokines (e.g. IL-25, IL-33, TSLP) ILC2s start to produce IL-5, IL-13, IL-9, IL-4. ILC2s are critical for primary responses to local Th2 antigens e.g. helminths and viruses and that is why ILC2s are abundant in tissues of skin, lungs, livers and gut. In mice, ILC2s can regulate adipose function and metabolic homeostasis in part via production of enkephalin peptides that elicit beiging.