Introduction Innate lymphoid cells ILCs are recently describ
Introduction Innate lymphoid beta amyloid (ILCs) are recently described immune cells of lymphoid origin and include cytotoxic natural killer (NK) cells and interleukin-7 receptor alpha (also known as CD127)+ subsets, which, similar to T helper (Th) lymphocytes, can be distinguished on the basis of signature transcription factors and effector cytokines: (1) ILC1s require the transcription factor T-BET and produce interferon-γ. (2) ILC2s express the transcription factor GATA3 and produce the type 2 cytokines interleukin 5 (IL-5) and IL-13. (3) ILC3s are dependent on the transcription factor RAR-related orphan receptor gamma t (RORγt) and have the ability to produce IL-17 and/or IL-22. ILC3s are enriched in the intestine, where they maintain healthy tissue function by orchestrating lymphoid-organ development, containment of commensal bacteria, tissue repair, host defense, and regulation of adaptive immunity (Artis and Spits, 2015, Diefenbach et al., 2014, Eberl et al., 2015, McKenzie et al., 2014, Serafini et al., 2015, Sonnenberg and Artis, 2015). ILC3s can be divided into two main populations with distinct ontogeny, transcriptional programs, and localization within the gut: (1) C-C motif chemokine receptor 6 (CCR6)− ILC3s co-expressing RORγt and T-BET are mainly found scattered throughout the lamina propria (Luci et al., 2009, Sanos et al., 2009, Satoh-Takayama et al., 2008). (2) CCR6+NKp46− fetal lymphoid tissue inducer (LTi) and adult LTi-like cells expressing c-KIT (also known as CD117) seed the gut during fetal development, develop along a pathway distinct from that of other ILCs, and promote lymphoid tissue development (Eberl and Littman, 2004, Eberl et al., 2004, Mebius et al., 1997, Sawa et al., 2010). Accordingly, LTi cells reside in intestinal lymphoid structures, called cryptopatches (CPs) (Kanamori et al., 1996) and isolated lymphoid follicles (ILFs) (Hamada et al., 2002), which are collectively referred to as solitary intestinal lymphoid tissues (SILTs) (Buettner and Lochner, 2016, Randall and Mebius, 2014). CPs are clusters of LTi-like ILC3s surrounded by dendritic cells (DCs) within a network of stromal cells, whereas ILFs additionally contain B cells. CPs and ILFs develop postnatally through the activity of adult LTi-like ILC3s that produce lymphotoxin (Eberl and Littman, 2004, Kruglov et al., 2013, Tsuji et al., 2008). Whereas lymphoid organogenesis in the small intestine has been well studied, the specific factors required for SILT development in the colon, beyond lymphotoxin, are unknown. Environmental cues, such as microbial, dietary, and neuronal signals, regulate the differentiation and function of ILC3s. However, the identities of any additional cues and the receptors that detect them remain unknown. An important class of proteins enabling cells to sense extracellular cues are G-protein-coupled receptors (GPCRs), which mediate cellular responses to diverse environmental signals. We therefore hypothesized that ILC3 function is regulated by GPCRs that recognize endogenous metabolites. In this regard, we focused on G-protein-coupled receptor 183 (GPR183, also known as EBI2), a GPCR that instructs antibody (Ab) responses in lymphoid organs through the positioning of B cells, T cells, and DCs (Gatto et al., 2009, Gatto et al., 2013, Li et al., 2016, Pereira et al., 2009, Yi and Cyster, 2013). GPR183 is a receptor for oxysterols (Hannedouche et al., 2011, Liu et al., 2011), hydroxylated metabolites of cholesterol, which have pleiotropic roles in lipid metabolism, immunity, and inflammation (Cyster et al., 2014). The most potent GPR183 ligand is 7α,25-hydroxycholesterol (7α,25-OHC). Synthesizing 7α,25-OHC from cholesterol requires the enzymes cholesterol 25-hydroxylase (CH25H) and cytochrome P450, family 7, subfamily b, polypeptide 1 (CYP7B1), whereas hydroxy-delta-5-steroid dehydrogenase, 3 beta- and steroid delta-isomerase 7 (HSD3B7) metabolizes 7α,25-OHC into bile acid precursors that lack GPR183 ligand activity. Given that GPR183 regulates immune cell migration, we reasoned that oxysterols could function as guidance cues for ILC3s.