g. IL-5 and IL-13), and play a critical role in immune responses to parasitic worm infection [75-77]. These type 2 ILCs have not been shown to produce IL-17; RORγt± ILCs that include fetal lymphoid tissue inducer (LTi) cells and adult LTi-like cells. Fetal LTi cells are essential for initiating development of lymph nodes and Peyer’s patches [71, 78-80]. Adult LTi-like cells are present after birth and initiate

development of cryptopatches and lymphoid follicles in the small and large intestine. LTi-like cells are also present at a lower frequency in the spleen Selleckchem Kinase Inhibitor Library and lymph nodes [5]. It is thought that these cells help to maintain and repair secondary lymphoid tissues

in response to infection and inflammation [81]. Since the identification of RORγt as a critical transcription factor essential for IL-17 production by Th17 cells, numerous reports have shown that RORγt+ ILCs also produce IL-17 [3, 82, 83]. Type 1 and type 2 ILCs do not express RORγt; however, RORγt plays an important role in the differentiation and maintenance of the third type of ILCs, which includes LTi and LTi-like cells, as these cells constitutively express RORγt [84-86]. RORγt+ ILCs can be further divided into at least three different subsets: (i) classical Sorafenib chemical structure LTi-like ILCs, (ii) Sca-1+ ILCs and, (iii) NKR-LTi cells. Classical LTi-like ILCs are defined as lineage negative (CD3−CD19−NK1.1−NKp46−Gr.1−CD11c−) CD45+c-kit+IL-7R+ and around 50% of these cells in mice express CD4 [87]. Both mouse and human LTi cells constitutively Tryptophan synthase express IL-17 in the intestine in the developing fetus [82, 88] and studies in

mice have shown that when microbial colonization occurs after birth secretion of IL-17 by LTi-like cells begins to decrease and is not detectable by 8 weeks of age. Sca-1+ ILCs have been identified in mice and are nonclassical intestinal LTi-like ILCs that are lineage negative RORγt+IL-7R+CCR6+, but unlike LTi cells, they are Sca-1+c-kit−CD4− [3]. These Sca-1+ ILCs have been shown to secrete both IL-17 and IFN-γ upon stimulation with IL-23 [3]. NKR-LTi cells are characterized by their expression of NK cytotoxicity receptors: NKp46 in mice and NKp44 in humans. These NKR-LTi cells have been identified in the intestine and tonsils in humans [82], and in mice these cells exist in the small intestine, large intestine, and Peyer’s patches, and at lower frequencies in the mesenteric lymph nodes [5]. NKR-LTi cells constitutively secrete IL-22, but have also been shown to produce IL-17 in humans. IL-22 production is further enhanced by stimulation with IL-23 alone or with IL-1β [5, 89-92].