As the tissue macrophages of the CNS, microglia are critically involved in diseases of the CNS. However, it remains unknown what controls their maturation and activation under homeostatic conditions. We observed substantial contributions of the host microbiota to microglia homeostasis, as germ-free (GF) mice displayed global defects in microglia with altered cell proportions and an immature phenotype, leading to impaired innate immune responses. Temporal eradication of host microbiota severely changed microglia properties. Limited microbiota complexity also resulted in defective microglia. In contrast, recolonization with a complex microbiota partially restored microglia features. We determined that short-chain fatty acids (SCFA), microbiota-derived bacterial fermentation products, regulated microglia homeostasis. Accordingly, mice deficient for the SCFA receptor FFAR2 mirrored microglia defects found under GF conditions. These findings suggest that host bacteria vitally regulate microglia maturation and function, whereas microglia impairment can be rectified to some extent by complex microbiota.

Type 2 innate lymphoid cells (ILC2s) both contribute to mucosal homeostasis and initiate pathologic inflammation in allergic asthma. However, the signals that direct ILC2s to promote homeostasis versus inflammation are unclear. To identify such molecular cues, we profiled mouse lung-resident ILCs using single-cell RNA sequencing at steady state and after in vivo stimulation with the alarmin cytokines IL-25 and IL-33. ILC2s were transcriptionally heterogeneous after activation, with subpopulations distinguished by expression of proliferative, homeostatic and effector genes. The neuropeptide receptor Nmur1 was preferentially expressed by ILC2s at steady state and after IL-25 stimulation. Neuromedin U (NMU), the ligand of NMUR1, activated ILC2s in vitro, and in vivo co-administration of NMU with IL-25 strongly amplified allergic inflammation. Loss of NMU–NMUR1 signalling reduced ILC2 frequency and effector function, and altered transcriptional programs following allergen challenge in vivo. Thus, NMUR1 signalling promotes inflammatory ILC2 responses, highlighting the importance of neuro-immune crosstalk in allergic inflammation at mucosal surfaces.

Type I and type III IFNs bind to different cell-surface receptors but induce identical signal transduction pathways, leading to the expression of antiviral host effector molecules. Despite the fact that type III IFN (IFN-λ) has been shown to predominantly act on mucosal organs, in vivo infection studies have failed to attribute a specific, nonredundant function. Instead, a predominant role of type I IFN was observed, which was explained by the ubiquitous expression of the type I IFN receptor. Here we comparatively analyzed the role of functional IFN-λ and type I IFN receptor signaling in the innate immune response to intestinal rotavirus infection in vivo, and determined viral replication and antiviral gene expression on the cellular level. We observed that both suckling and adult mice lacking functional receptors for IFN-λ were impaired in the control of oral rotavirus infection, whereas animals lacking functional receptors for type I IFN were similar to wild-type mice. Using Mx1 protein accumulation as marker for IFN responsiveness of individual cells, we demonstrate that intestinal epithelial cells, which are the prime target cells of rotavirus, strongly responded to IFN-λ but only marginally to type I IFN in vivo. Systemic treatment of suckling mice with IFN-λ repressed rotavirus replication in the gut, whereas treatment with type I IFN was not effective. These results are unique in identifying a critical role of IFN-λ in the epithelial antiviral host defense. IFNs play a critical role in the antimicrobial host defense. Whereas lymphocyte-derived type II IFN (also called IFN-γ) is associated with resistance against a broad range of intracellular microorganisms, type I and III IFN primarily mediate antiviral protection. IFN-α, IFN-β, and all other type I IFN family members use the same heterodimeric receptor complex (IFNAR) for signaling. Receptor engagement leads to activation of the Jak/STAT signaling pathway and expression of IFN-stimulated genes (ISG), which mediate the antiviral state (1). The type III IFN family consists of three members in humans, IFN-λ1, -λ2, and -λ3 that are also named IL29, IL28A, and IL28B, respectively, whereas mice only express IFN-λ2 and -λ3. Type III IFN are structurally different from type I IFN and bind to a distinct heterodimeric receptor (IL28R), consisting of the IL28Rα, also called IFN-λ receptor 1 (IFN-λR1), and the IL10Rβ chains (2-4). Type I and III IFN are both induced following stimulation of pattern recognition receptors of the innate immune system, such as Toll-like receptors and RIG-like helicases (5-7). IFN-λ-triggered signal transduction events and gene activation profiles are virtually indistinguishable from those of the type I IFN system (2,3,8,9). However, the type I and type III IFN systems differ strikingly with regard to the spectrum of responsive cell types. Whereas receptors for type I IFN seem to be present on most if not all nucleated cells, functional receptors for type III IFN are preferentially expressed on epithelial cells (10).Recent studies inves...

The type 2 cytokines interleukin (IL)-4, IL-5, IL-9 and IL-13 play critical roles in stimulating innate and adaptive immune responses required for resistance to helminth infection and promotion of allergic inflammation, metabolic homeostasis and tissue repair1–3. Group 2 innate lymphoid cells (ILC2s) are a potent source of type 2 cytokines and while significant advances have been made in understanding the cytokine milieu that promotes ILC2 responses4–9, there are fundamental gaps in knowledge regarding how ILC2 responses are regulated by other stimuli. In this report, we demonstrate that ILC2s in the gastrointestinal tract co-localize with cholinergic neurons that express the neuropeptide neuromedin U (NMU)10,11. In contrast to other hematopoietic cells, ILC2s selectively express the NMU receptor 1 (NMUR1). In vitro stimulation of ILC2s with NMU induced rapid cell activation, proliferation and secretion of type 2 cytokines IL-5, IL-9 and IL-13 that was dependent on cell-intrinsic expression of NMUR1 and Gαq protein. In vivo administration of NMU triggered potent type 2 cytokine responses characterized by ILC2 activation, proliferation and eosinophil recruitment that was associated with accelerated expulsion of the gastrointestinal nematode Nippostrongylus brasiliensis or induction of lung inflammation. Conversely, worm burden was higher in Nmur1−/− mice compared to control mice. Further, use of gene-deficient mice and adoptive cell transfer experiments revealed that ILC2s were necessary and sufficient to mount NMU-elicited type 2 cytokine responses. Together, these data indicate that the NMU-NMUR1 neuronal signaling circuit provides a selective and previously unrecognized mechanism through which the enteric nervous system and innate immune system integrate to promote rapid type 2 cytokine responses that can induce anti-microbial, inflammatory and tissue-protective type 2 responses at mucosal sites.

The epithelium is the major entry point for many viruses but the processes protecting barrier surfaces against viral infections are incompletely understood. We identify interleukin (IL)-22 produced by group 3 innate lymphoid cells (ILC3s) as an amplifier of interferon (IFN)-λ signaling, a synergism required to curtail replication of rotavirus, the leading cause of childhood gastroenteritis. Cooperation between IL-22 and IFN-λ receptors, both of which are preferentially expressed by intestinal epithelial cells, was required for optimal STAT1 transcription factor activation and expression of interferon-stimulated genes. This data suggests that epithelial cells are protected against virus replication by co-opting two evolutionarily related cytokine networks. These data may inform the design of novel immunotherapies of virus infections that are sensitive to IFNs.