Spatial distribution of LTi-like cells in intestinal mucosa regulates type 3 innate immunity

Lymphoid tissue inducer (LTi)-like cells are tissue resident innate lymphocytes that rapidly secrete cytokines that promote gut epithelial integrity and protect against extracellular bacterial infections.Here, we report that the retention of LTi-like cells in conventional solitary intestinal lymphoid tissue (SILT) is essential for controlling LTi-like cell function and is maintained by expression of the chemokine receptor CXCR5. Deletion of Cxcr5 functionally unleashed LTi-like cells in a cell intrinsic manner, leading to uncontrolled IL-17 and IL-22 production. The elevated production of IL-22 in Cxcr5-deficient mice improved gut barrier integrity and protected mice during infection with the opportunistic pathogen Clostridium difficile. Interestingly, Cxcr5^−/− mice developed LTi-like cell aggregates that were displaced from their typical niche at the intestinal crypt, and LTi-like cell hyperresponsiveness was associated with the local formation of this unconventional SILT. Thus, LTi-like cell positioning within mucosa controls their activity via niche-specific signals that temper cytokine production during homeostasis.

Lymphoid tissue inducer (LTi)-like cells belong to a family of tissue resident innate lymphocytes that lack rearranged antigen-specific receptors and act as a first line of defense at barrier tissues. LTi-like cells, along with other group 3 innate lymphoid cells (ILC3), maintain intestinal homeostasis by producing the cytokines IL-22 and IL-17A, which promote gut epithelial cell proliferation, anti-microbial peptide production, and tight junction protein abundance (1, 2). The conditioning of epithelial cells by these cytokines contributes to balanced interactions between the host and commensal microbiota under steady-state conditions, and LTi-like cell-derived IL-22 promotes barrier integrity and protective immunity during infection with the enteric pathogenic bacteria (3).In addition to providing effector functions, LTi-like cells and their fetal LTi counterparts are required for early steps in lymphoid tissue development. Fetal LTi induce lymph node and Peyer’s patch development during gestation by activating lymphoid tissue organizer cells at primordial lymphoid organs with lymphotoxin (LT)-α1β2 (4–6). Similarly, LTi-like cells are required for the postnatal development of cryptopatches, small lymphoid aggregates in the intestine that have the potential to mature into isolated lymphoid follicles (ILF) in response to signals from microbes (7, 8). In line with their roles in lymphoid tissue organogenesis and maturation, LTi-like cells in adult mouse intestines preferentially localize in solitary intestinal lymphoid tissue (SILT). The microenvironments of these highly specialized niches are expected to support and regulate LTi-like cells; however, their impact on LTi-like cell behavior has not been fully explored.LTi-like cells express multiple G protein–coupled receptors that facilitate their migration in tissue (9–12). Among these, CXCR5 has a predominant role in the migration of LTi to developing lymphoid structures, with Cxcr5^−/− mice exhibiting defects in lymph node and Peyer’s patch development (13). Mice deficient in CXCR5 or its ligand CXCL13 also have delayed cryptopatch development and fail to convert cryptopatches to mature ILF because of impaired recruitment of B cells to these structures (14–16). Dendritic cells (DCs) have been shown to be a local source of CXCL13 in SILT (16) and thus likely retain B cells and LTi-like cells at these structures under homeostatic conditions via the CXCL13–CXCR5 signaling axis. The retention of LTi-like cells in SILT is expected to bring these cells in close proximity to activating and inhibitory signals provided by specialized myeloid cells, neurons that express the vasoactive intestinal peptide (VIP), and lymphocyte populations localized at these sites (17–20). However, the impact of CXCR5 on functions of LTi-like cells beyond those associated with lymphoid tissue maintenance and development remains unknown.In the current study, we show that CXCR5 expression regulates LTi-like cell function. Deletion of Cxcr5 led to increased numbers of LTi-like cells in the small intestine (SI) and enhanced their ability to produce IL-17A and IL-22. Cxcr5 regulated LTi-like cells via a cell-intrinsic mechanism that did not involve direct suppression by CXCL13. Heightened LTi-like cell activity in Cxcr5-deficient mice was associated with the development of abnormal LTi-like cell aggregates in the SI that were localized in villus lamina propria instead of at the intestinal crypt base. Importantly, augmented production of IL-22 in Cxcr5^−/− mice was protective during acute infection with the opportunistic pathogen Clostridium difficile. These data reveal that CXCR5-dependent migration can control innate type 3 immunity by altering the niche of LTi-like cells in intestinal lamina propria.