How defined microbes influence the skin immune system remains poorly understood. Here we demonstrate that
, dominant members of the skin microbiota, promote a dramatic increase in the number and ...activation of a defined subset of γδ T cells. This effect is long-lasting, occurs independently of other microbes, and is, in part, mediated by interleukin (IL)-23. Under steady-state conditions, the impact of
is discrete and noninflammatory. However, when applied to the skin of a host fed a high-fat diet,
alone promotes inflammation in an IL-23-dependent manner. Such effect is highly conserved among species of
and dependent on the expression of a dominant component of the cell envelope, mycolic acid. Our data uncover a mode of communication between the immune system and a dominant genus of the skin microbiota and reveal that the functional impact of canonical skin microbial determinants is contextually controlled by the inflammatory and metabolic state of the host.
The microbiota is known to influence the generation of hematopoietic progenitors, although the pathways underlying this process are still poorly understood. NOD1 and NOD2 are intracellular sensors ...for both Gram-positive and Gram-negative bacteria, but their role in steady-state hematopoiesis has never been characterized. We observed that stimulation with NOD1 or NOD2 ligand had no effect on the survival/proliferation of hematopoietic precursors. Nonetheless, NOD1, but not NOD2, ligand induced expression of multiple hematopoietic cytokines (interleukin-7 IL-7, Flt3L, stem cell factor SCF, ThPO, and IL-6) from bone marrow mesenchymal stromal cells (MSCs) in vitro. Moreover, in vivo administration of NOD1 ligand to germ-free mice restored the numbers of hematopoietic stem cells and precursors in bone marrow as well as serum concentrations of IL-7, Flt3L, SCF, and ThPO to the levels displayed by specific pathogen-free control animals. Based on these findings, we propose that NOD1 signaling in MSCs serves as an important pathway underlying the requirement for microbiota in the maintenance of steady-state hematopoiesis. This function is distinct from that triggered by lipopolysaccharide in both its broad effects on multiple progenitors and specific targeting of MSCs as cytokine producing intermediates.
•NOD1 ligand administration restores hematopoietic precursor pools in germ-free mice to the levels seen in specific pathogen-free animals.•NOD1 ligand–NOD1 signaling promotes steady-state hematopoiesis indirectly through the induction of cytokines by MSCs.
The central nervous system has historically been viewed as an immune-privileged site, but recent data have shown that the meninges-the membranes that surround the brain and spinal cord-contain a ...diverse population of immune cells
. So far, studies have focused on macrophages and T cells, but have not included a detailed analysis of meningeal humoral immunity. Here we show that, during homeostasis, the mouse and human meninges contain IgA-secreting plasma cells. These cells are positioned adjacent to dural venous sinuses: regions of slow blood flow with fenestrations that can potentially permit blood-borne pathogens to access the brain
. Peri-sinus IgA plasma cells increased with age and following a breach of the intestinal barrier. Conversely, they were scarce in germ-free mice, but their presence was restored by gut re-colonization. B cell receptor sequencing confirmed that meningeal IgA
cells originated in the intestine. Specific depletion of meningeal plasma cells or IgA deficiency resulted in reduced fungal entrapment in the peri-sinus region and increased spread into the brain following intravenous challenge, showing that meningeal IgA is essential for defending the central nervous system at this vulnerable venous barrier surface.
How early-life colonization and subsequent exposure to the microbiota affect long-term tissue immunity remains poorly understood. Here, we show that the development of mucosal-associated invariant T ...(MAIT) cells relies on a specific temporal window, after which MAIT cell development is permanently impaired. This imprinting depends on early-life exposure to defined microbes that synthesize riboflavin-derived antigens. In adults, cutaneous MAIT cells are a dominant population of interleukin-17A (IL-17A)-producing lymphocytes, which display a distinct transcriptional signature and can subsequently respond to skin commensals in an IL-1-, IL-18-, and antigen-dependent manner. Consequently, local activation of cutaneous MAIT cells promotes wound healing. Together, our work uncovers a privileged interaction between defined members of the microbiota and MAIT cells, which sequentially controls both tissue-imprinting and subsequent responses to injury.
Barrier tissues are primary targets of environmental stressors and are home to the largest number of antigen-experienced lymphocytes in the body, including commensal-specific T cells. We found that ...skin-resident commensal-specific T cells harbor a paradoxical program characterized by a type 17 program associated with a poised type 2 state. Thus, in the context of injury and exposure to inflammatory mediators such as interleukin-18, these cells rapidly release type 2 cytokines, thereby acquiring contextual functions. Such acquisition of a type 2 effector program promotes tissue repair. Aberrant type 2 responses can also be unleashed in the context of local defects in immunoregulation. Thus, commensal-specific T cells co-opt tissue residency and cell-intrinsic flexibility as a means to promote both local immunity and tissue adaptation to injury.
Mammalian barrier surfaces are constitutively colonized by numerous microorganisms. We explored how the microbiota was sensed by the immune system and the defining properties of such responses. Here, ...we show that a skin commensal can induce T cell responses in a manner that is restricted to non-classical MHC class I molecules. These responses are uncoupled from inflammation and highly distinct from pathogen-induced cells. Commensal-specific T cells express a defined gene signature that is characterized by expression of effector genes together with immunoregulatory and tissue-repair signatures. As such, non-classical MHCI-restricted commensal-specific immune responses not only promoted protection to pathogens, but also accelerated skin wound closure. Thus, the microbiota can induce a highly physiological and pleiotropic form of adaptive immunity that couples antimicrobial function with tissue repair. Our work also reveals that non-classical MHC class I molecules, an evolutionarily ancient arm of the immune system, can promote homeostatic immunity to the microbiota.
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•Non-classical MHC class I molecules promote homeostatic immunity to the microbiota•Commensal-specific T cells express immunoregulatory and tissue repair signatures•Commensal-specific T cells accelerate wound closure
Microbiota induce a form of adaptive immunity that couples antimicrobial function with tissue repair.
The gut microbiota influences both local and systemic inflammation. Inflammation contributes to development, progression, and treatment of cancer, but it remains unclear whether commensal bacteria ...affect inflammation in the sterile tumor microenvironment. Here, we show that disruption of the microbiota impairs the response of subcutaneous tumors to CpG-oligonucleotide immunotherapy and platinum chemotherapy. In antibiotics-treated or germ-free mice, tumor-infiltrating myeloid-derived cells responded poorly to therapy, resulting in lower cytokine production and tumor necrosis after CpG-oligonucleotide treatment and deficient production of reactive oxygen species and cytotoxicity after chemotherapy. Thus, optimal responses to cancer therapy require an intact commensal microbiota that mediates its effects by modulating myeloid-derived cell functions in the tumor microenvironment. These findings underscore the importance of the microbiota in the outcome of disease treatment.
Intestinal commensal bacteria induce protective and regulatory responses that maintain host-microbial mutualism. However, the contribution of tissue-resident commensals to immunity and inflammation ...at other barrier sites has not been addressed. We found that in mice, the skin microbiota have an autonomous role in controlling the local inflammatory milieu and tuning resident T lymphocyte function. Protective immunity to a cutaneous pathogen was found to be critically dependent on the skin microbiota but not the gut microbiota. Furthermore, skin commensals tuned the function of local T cells in a manner dependent on signaling downstream of the interleukin-1 receptor. These findings underscore the importance of the microbiota as a distinctive feature of tissue compartmentalization, and provide insight into mechanisms of immune system regulation by resident commensal niches in health and disease.
Immuno-surveillance networks operating at barrier sites are tuned by local tissue cues to ensure effective immunity. Site-specific commensal bacteria provide key signals ensuring host defense in the ...skin and gut. However, how the oral microbiome and tissue-specific signals balance immunity and regulation at the gingiva, a key oral barrier, remains minimally explored. In contrast to the skin and gut, we demonstrate that gingiva-resident T helper 17 (Th17) cells developed via a commensal colonization-independent mechanism. Accumulation of Th17 cells at the gingiva was driven in response to the physiological barrier damage that occurs during mastication. Physiological mechanical damage, via induction of interleukin 6 (IL-6) from epithelial cells, tailored effector T cell function, promoting increases in gingival Th17 cell numbers. These data highlight that diverse tissue-specific mechanisms govern education of Th17 cell responses and demonstrate that mechanical damage helps define the immune tone of this important oral barrier.
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•Distinct signals shape the Th17 cell network at the oral barrier•Oral barrier Th17 cells develop independently of commensal microbe colonization•Physiologic damage through mastication promotes the generation of oral Th17 cells•Barrier damage triggers oral Th17-cell-mediated protective immunity and inflammation
The signals regulating immunity at the gingiva, a key oral barrier, remain unclear. Dutzan et al. show that oral barrier Th17 cells are induced in response to mastication rather than commensal colonization, identifying physiologic mechanical damage as a unique tissue-specific cue conditioning local immunity and inflammation at the oral barrier.
Using a model of lethal oral infection with
Toxoplasma gondii, we examined the fate of both induced and natural regulatory T (Treg) cells in the face of strong inflammatory responses occurring in a ...tolerogenic-prone environment. We found that during highly T helper 1 (Th1) cell-polarized mucosal immune responses, Treg cell numbers collapsed via multiple pathways, including blockade of Treg cell induction and disruption of endogenous Treg cell homeostasis. In particular, shutdown of interleukin 2 (IL-2) in the highly Th1 cell-polarized environment triggered by infection directly contributes to Treg cell incapacity to suppress effector responses and eventually leads to immunopathogenesis. Furthermore, we found that environmental cues provided by both local dendritic cells and effector T cells can induce the expression of T-bet transcription factor and IFN-γ by Treg cells. These data reveal a mechanism for Th1 cell pathogenicity that extends beyond their proinflammatory program to limit Treg cell survival.
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