Such cells were not observed in the FAE of 4 serial sectioned SM-ILF (data not shown), although cells with M-cell like morphology could be detected in the FAE of SM-ILF by scanning electron microscopy (Figure 1N). mucosal isolated lymphoid follicles (SM-ILF and M-ILF), as well as in GALT-free intestinal lamina propria (LP). SM-ILF and M-ILF showed unique patterns of distribution along the length of the intestine, were linked to the systemic blood circulation through MAdCAM1+ high endothelial venules and efferent lymphatics, and experienced immune profiles consistent with immune inductive sites. IgA sequencing analysis indicated that human ILF are sites where intestinal adaptive immune responses are initiated in an anatomically restricted smanner. Our findings position ILF as important inductive hubs for regional immunity in the human intestine and the methods presented will allow future assessment of these compartments in health and disease. eTOC Our understanding of the function of lymphoid follicles in the human intestine remains limited. Here, Fenton et al. describe a method for identifying and isolating lymphoid follicles along the length of the human intestine and use it to show a role for these structures in regionalized adaptive immune responses. Introduction The intestine consists of several anatomically and functionally specialized segments with unique environmental pressures that include variations in bacterial weight and diversity, nutrient concentrations and microbiota-derived metabolites. Given these challenges it is perhaps unsurprising that this intestine contains the best number and diversity of immune cells in the body. Crosstalk between these cells and their local environment is essential for the development and function of the immune system and alterations in this communication can rewire immune cell behavior and contribute to the development and chronicity of a wide range of disorders (Durack and Lynch, 2019). The intestine contains numerous immune niches including the gut associated lymphoid tissues (GALT), which are thought to serve as sites of adaptive immune cell priming and differentiation. Conversely, the intestinal epithelium and lamina propria (LP) are sites where antigen-experienced lymphocytes accumulate and can persist long-term as committed effector or regulatory cells. Murine GALT include the macroscopically visible Peyers patches (PP) of the ileum and colonic patches, together with the far more numerous solitary isolated lymphoid tissues (SILT) that are distributed throughout the small and large intestine. SILT encompass a Fangchinoline spectrum of maturation says from immature cryptopatches (CP) that contain lymphoid tissue inducer cells (LTi) and dendritic cells, to mature, B cell rich, isolated lymphoid follicles (ILF) (Hamada et al., 2002; Kanamori et al., 1996; Knoop et al., 2011; McDonald et al., 2010; Pabst et al., 2006). While murine SILT contain few T cells and may contribute to T cell-independent IgA responses (Tsuji et al., 2008; Wang et al., 2006), murine PP and caecal patches are thought to represent major sites of T-dependent IgA responses (Craig and Cebra, 1971). Murine PP were recently implicated in the generation of IgA+ plasmablasts destined for the small intestine (SI), whereas caecal patches were shown to generate IgA+ plasmablasts destined for both the SI and colon (Masahata et al., 2014). Whether such regionalized immune responses occur in the human intestine and how unique compartments of the human Rabbit polyclonal to EPHA4 GALT contribute to intestine-wide or region-specific immunity remains unclear. Current understanding Fangchinoline of human GALT is derived primarily from immunohistochemistry and electron microscopy studies. Human PP develop (Hoorweg and Cupedo, 2008) and consist of unique B follicles interspersed with T cell-rich inter-follicular regions (Farstad et al., 2000), a sub-epithelial Fangchinoline dome (SED) region (Brandtzaeg and Bjerke, 1990) and a microfold (M) cell-containing follicle-associated epithelium (FAE) (Owen and Jones, 1974). Human PP may consist of several hundred follicles (Cornes, 1965a), although Fangchinoline their size varies, peaking during adolescence and declining continuously thereafter (Cornes, 1965b). They constitutively contain germinal centers (GC) and the B cells present in PP share clonal overlap with SI plasma cells (PC) (Dunn-Walters et al., 1997), indicating that they likely represent inductive sites for the generation of intestinal PC. Structures resembling ILF have also been observed throughout the human intestine (Buettner and Lochner, 2016; Dukes and Bussey, 1926), although their frequency may vary with age and between intestinal segments (Moghaddami et al., 1998; OLeary and Sweeney, 1986; Senda et al., 2019; Spencer et al., 2019). ILF in both SI and colon express mRNA (Barone et al., 2009), which encodes activation induced cytidine deaminase (AID) required for class switch recombination. Some human ILF also contain GC (Meier et al., 2014; Moghaddami et al., 1998;.