M cell maturation and cDC activation determine the onset of adaptive immune priming in the neonatal Peyer's patch.

Early-life immune development is critical to long-term host health. However, the mechanisms that determine the pace of postnatal immune maturation are not fully resolved. Here, we analyzed mononuclear phagocytes (MNPs) in small intestinal Peyer's patches (PPs), the primary inductive site of intestinal immunity. Conventional type 1 and 2 dendritic cells (cDC1 and cDC2) and RORgt+ antigen-presenting cells (RORgt+ APC) exhibited significant age-dependent changes in subset composition, tissue distribution, and reduced cell maturation, subsequently resulting in a lack in CD4+ T cell priming during the postnatal period. Microbial cues contributed but could not fully explain the discrepancies in MNP maturation. Type I interferon (IFN) accelerated MNP maturation but IFN signaling did not represent the physiological stimulus. Instead, follicle-associated epithelium (FAE) M cell differentiation was required and sufficient to drive postweaning PP MNP maturation. Together, our results highlight the role of FAE M cell differentiation and MNP maturation in postnatal immune development.

Some news from the unknown soldier, the Peyer's patch macrophage.

In mammals, macrophages (MF) are present in virtually all tissues where they serve many different functions linked primarily to the maintenance of homeostasis, innate defense against pathogens, tissue repair and metabolism. Although some of these functions appear common to all tissues, others are specific to the homing tissue. Thus, MF become adapted to perform particular functions in a given tissue. Accordingly, MF express common markers but also sets of tissue-specific markers linked to dedicated functions. One of the largest pool of MF in the body lines up the wall of the gut. Located in the small intestine, Peyer's patches (PP) are primary antigen sampling and mucosal immune response inductive sites. Surprisingly, although markers of intestinal MF, such as F4/80, have been identified more than 30 years ago, MF of PP escaped any kind of phenotypic description and remained "unknown" for decades. In absence of MF identification, the characterization of the PP mononuclear phagocyte system (MPS) functions has been impaired. However, taking into account that PP are privileged sites of entry for pathogens, it is important to understand how the latter are handled by and/or escape the PP MPS, especially MF, which role in killing invaders is well known. This review focuses on recent advances on the PP MPS, which have allowed, through new criteria of PP phagocyte subset identification, the characterization of PP MF origin, diversity, specificity, location and functions.

Peyer's patch dendritic cells sample antigens by extending dendrites through M cell-specific transcellular pores.

BACKGROUND & AIMS: Peyer's patches (PPs) of the small intestine are antigen sampling and inductive sites that help establish mucosal immunity. Luminal antigens are transported from the mucosal surface of PPs to the subepithelial dome (SED), through the specialized epithelial M cells of the follicle-associated epithelium. Among the SED resident dendritic cells (DCs), which are situated ideally for taking up these antigens, some express high levels of lysozyme (LysoDC) and have strong phagocytic activity. We investigated the mechanisms by which LysoDCs capture luminal antigens in vivo. METHODS: We performed 2-photon microscopy on explants of PPs from mice in which the enhanced green fluorescent protein gene was inserted into the lysozyme M locus (lys-EGFP mice), allowing fluorescence detection of LysoDC. RESULTS: LysoDC extended dendrites through M-cell-specific transcellular pores to the gut lumen. The M-cell adhesion molecules junctional adhesion molecule-A and epithelial cell adhesion molecule were recruited to sites of transcellular migration. Transcellular dendrites scanned the M-cell apical surface and the gut luminal content; they were able to take pathogenic bacteria and inert particles in the lumen before retracting back to the SED. CONCLUSIONS: We describe an antigen sampling mechanism that occurs in PPs and involves cooperation between M cells of the follicle-associated epithelium and DCs of the subepithelial dome. This process might be developed to target vaccines to the mucosa.

CD64 distinguishes macrophages from dendritic cells in the gut and reveals the Th1-inducing role of mesenteric lymph node macrophages during colitis.

Dendritic cells (DCs) and monocyte-derived macrophages (MΦs) are key components of intestinal immunity. However, the lack of surface markers differentiating MΦs from DCs has hampered understanding of their respective functions. Here, we demonstrate that, using CD64 expression, MΦs can be distinguished from DCs in the intestine of both mice and humans. On that basis, we revisit the phenotype of intestinal DCs in the absence of contaminating MΦs and we delineate a developmental pathway in the healthy intestine that leads from newly extravasated Ly-6C(hi) monocytes to intestinal MΦs. We determine how inflammation impacts this pathway and show that T cell-mediated colitis is associated with massive recruitment of monocytes to the intestine and the mesenteric lymph node (MLN). There, these monocytes differentiate into inflammatory MΦs endowed with phagocytic activity and the ability to produce inducible nitric oxide synthase. In the MLNs, inflammatory MΦs are located in the T-cell zone and trigger the induction of proinflammatory T cells. Finally, T cell-mediated colitis develops irrespective of intestinal DC migration, an unexpected finding supporting an important role for MLN-resident inflammatory MΦs in the etiology of T cell-mediated colitis.

Prolonged dysbiosis and altered immunity under nutritional intervention in a physiological mouse model of severe acute malnutrition.

Severe acute malnutrition (SAM) is a multifactorial disease affecting millions of children worldwide. It is associated with changes in intestinal physiology, microbiota, and mucosal immunity, emphasizing the need for multidisciplinary studies to unravel its full pathogenesis. We established an experimental model in which weanling mice fed a high-deficiency diet mimic key anthropometric and physiological features of SAM in children. This diet alters the intestinal microbiota (less segmented filamentous bacteria, spatial proximity to epithelium), metabolism (decreased butyrate), and immune cell populations (depletion of LysoDC in Peyer's patches and intestinal Th17 cells). A nutritional intervention leads to a fast zoometric and intestinal physiology recovery but to an incomplete restoration of the intestinal microbiota, metabolism, and immune system. Altogether, we provide a preclinical model of SAM and have identified key markers to target with future interventions during the education of the immune system to improve SAM whole defects.

Peyer's patch phagocytes acquire specific transcriptional programs that influence their maturation and activation profiles.

Peyer's patches (PPs) are secondary lymphoid organs in contact with the external environment via the intestinal lumen, thus combining antigen sampling and immune response initiation sites. Therefore, they provide a unique opportunity to study the entire process of phagocyte differentiation and activation in vivo. Here, we deciphered the transcriptional and spatial landscape of PP phagocyte populations from their emergence in the tissue to their final maturation state at homeostasis and under stimulation. Activation of monocyte-derived Lysozyme-expressing dendritic cells (LysoDCs) differs from that of macrophages by their upregulation of conventional DC (cDC) signature genes such as Ccr7 and downregulation of typical monocyte-derived cell genes such as Cx3cr1. We identified gene sets that distinguish PP cDCs from the villus ones and from LysoDCs. We also identified key immature, early, intermediate, and late maturation markers of PP phagocytes. Finally, exploiting the ability of the PP interfollicular region to host both villous and subepithelial dome emigrated cDCs, we showed that the type of stimulus, the subset, but also the initial location of cDCs shape their activation profile and thus direct the immune response. Our study highlights the importance of targeting the right phagocyte subset at the right place and time to manipulate the immune response.

Dendritic cell functions in the inductive and effector sites of intestinal immunity.

The intestine is constantly exposed to foreign antigens, which are mostly innocuous but can sometimes be harmful. Therefore, the intestinal immune system has the delicate task of maintaining immune tolerance to harmless food antigens while inducing tailored immune responses to pathogens and regulating but tolerating the microbiota. Intestinal dendritic cells (DCs) play a central role in these functions as sentinel cells able to prime and polarize the T cell responses. DCs are deployed throughout the intestinal mucosa but with local specializations along the gut length and between the diffuse effector sites of the gut lamina propria (LP) and the well-organized immune inductive sites comprising isolated lymphoid follicles (ILFs), Peyer's patches (PPs), and other species-specific gut-associated lymphoid tissues (GALTs). Understanding the specificities of each intestinal DC subset, how environmental factors influence DC functions, and how these can be modulated is key to harnessing the therapeutic potential of mucosal adaptive immune responses, whether by enhancing the efficacy of mucosal vaccines or by increasing tolerogenic responses in inflammatory disorders. In this review, we summarize recent findings related to intestinal DCs in steady state and upon inflammation, with a special focus on their functional specializations, highly dependent on their microenvironment.

Spatial and temporal key steps in early-life intestinal immune system development and education.

Education of our intestinal immune system early in life strongly influences adult health. This education strongly relies on series of events that must occur in well-defined time windows. From initial colonization by maternal-derived microbiota during delivery to dietary changes from mother's milk to solid foods at weaning, these early-life events have indeed long-standing consequences on our immunity, facilitating tolerance to environmental exposures or, on the contrary, increasing the risk of developing noncommunicable diseases such as allergies, asthma, obesity, and inflammatory bowel diseases. In this review, we provide an outline of the recent advances in our understanding of these events and how they are mechanistically related to intestinal immunity development and education. First, we review the susceptibility of neonates to infections and inflammatory diseases, related to their immune system and microbiota changes. Then, we highlight the maternal factors involved in protection and education of the mucosal immune system of the offspring, the role of the microbiota, and the nature of neonatal immune system until weaning. We also present how the development of some immune responses is intertwined in temporal and spatial windows of opportunity. Finally, we discuss pending questions regarding the neonate particular immune status and the activation of the intestinal immune system at weaning.

Pathogenic bacteria and dead cells are internalized by a unique subset of Peyer's patch dendritic cells that express lysozyme.

BACKGROUND & AIMS: Lysozyme has an important role in preventing bacterial infection. In the gastrointestinal tract, lysozyme is thought to be mainly expressed by Paneth cells of the crypt epithelium. We investigated its expression in the Peyer's patch, a major intestinal site of antigen sampling and pathogen entry. METHODS: We performed immunostaining on normal and Salmonella Typhimurium-infected intestinal samples and analyzed them by confocal microscopy and flow cytometry. RESULTS: In Peyer's patch of mouse, rat, and human, lysozyme was strongly expressed in the germinal center of follicles by tingible body macrophages and in the subepithelial dome by a subset of myeloid dendritic cells (DC). Among DC subsets from mouse Peyer's patches, these lysozyme-expressing DC displayed the highest surface expression of class II major histocompatibility complex and costimulatory molecules; they were the most efficient at capturing microspheres in vitro. Moreover, they were the main DC subset involved in bacterial pathogen uptake and in dead cell clearance, including M cells. CONCLUSIONS: The subepithelial dome of Peyer's patches contains a unique population of intestinal DC that secretes high levels of lysozyme and internalizes bacteria and dead cells.

Contrasting roles of macrophages and dendritic cells in controlling initial pulmonary Brucella infection.

Control of pulmonary pathogens constitutes a challenging task as successful immune responses need to be mounted without damaging the lung parenchyma. Using immunofluorescence microscopy and flow cytometry, we analyzed in the mouse the initial innate immune response that follows intranasal inoculation of Brucella abortus. Bacteria were absent from parenchymal dendritic cells (DC) but present in alveolar macrophages in which they replicated. When the number of alveolar macrophages was reduced prior to Brucella infection, small numbers of pulmonary DC were infected and a massive recruitment of TNF-α- and iNOS-producing DC ensued. Coincidentally, Brucella disseminated to the lung-draining mediastinal lymph nodes (LN) where they replicated in both migratory DC and migratory alveolar macrophages. Together, these results demonstrate that alveolar macrophages are critical regulators of the initial innate immune response against Brucella within the lungs and show that pulmonary DC and alveolar macrophages play rather distinct roles in the control of microbial burden.

Natural killer cells and dendritic epidermal γδ T cells orchestrate type 1 conventional DC spatiotemporal repositioning toward CD8+ T cells.

Successful immune responses rely on a regulated delivery of the right signals to the right cells at the right time. Here we show that natural killer (NK) and dendritic epidermal γδ T cells (DETCs) use similar mechanisms to spatiotemporally orchestrate conventional type 1 dendritic cell (cDC1) functions in the spleen, skin, and its draining lymph nodes (dLNs). Upon MCMV infection in the spleen, cDC1 clusterize with activated NK cells in marginal zones. This XCR1-dependent repositioning of cDC1 toward NK cells allows contact delivery of IL-12 and IL-15/IL-15Rα by cDC1, which is critical for NK cell responses. NK cells deliver granulocyte-macrophage colony-stimulating factor (GM-CSF) to cDC1, guiding their CCR7-dependent relocalization into the T cell zone. In MCMV-infected skin, XCL1-secreting DETCs promote cDC1 migration from the skin to the dLNs. This XCR1-dependent licensing of cDC1 both in the spleen and skin accelerates antiviral CD8+ T cell responses, revealing an additional mechanism through which cDC1 bridge innate and adaptive immunity.

Brucella control of dendritic cell maturation is dependent on the TIR-containing protein Btp1.

Brucella is an intracellular pathogen able to persist for long periods of time within the host and establish a chronic disease. We show that soon after Brucella inoculation in intestinal loops, dendritic cells from ileal Peyer's patches become infected and constitute a cell target for this pathogen. In vitro, we found that Brucella replicates within dendritic cells and hinders their functional activation. In addition, we identified a new Brucella protein Btp1, which down-modulates maturation of infected dendritic cells by interfering with the TLR2 signaling pathway. These results show that intracellular Brucella is able to control dendritic cell function, which may have important consequences in the development of chronic brucellosis.

SAPHIR: a Shiny application to analyze tissue section images.

Study of cell populations in tissues using immunofluorescence is a powerful method for both basic and medical research. Image acquisitions performed by confocal microscopy notably allow excellent lateral resolution and more than 10 parameter measurements when using spectral or multiplex imaging. Analysis of such complex images can be very challenging and easily lead to bias and misinterpretation. Here, we have developed the Shiny Analytical Plot of Histological Image Results (SAPHIR), an R shiny application for histo-cytometry using scatterplot representation of data extracted by segmentation. It offers many features, such as filtering of spurious data points, selection of cell subsets on scatterplot, visualization of scatterplot selections back into the image, statistics of selected data and data annotation. Our application allows to characterize labeled cells, from their phenotype to their number and location in the tissue, as well as their interaction with other cells. SAPHIR is available from: https://github.com/elodiegermani/SAPHIR.

From Species to Regional and Local Specialization of Intestinal Macrophages.

Initially intended for nutrient uptake, phagocytosis represents a central mechanism of debris removal and host defense against invading pathogens through the entire animal kingdom. In vertebrates and also many invertebrates, macrophages (MFs) and MF-like cells (e.g., coelomocytes and hemocytes) are professional phagocytic cells that seed tissues to maintain homeostasis through pathogen killing, efferocytosis and tissue shaping, repair, and remodeling. Some MF functions are common to all species and tissues, whereas others are specific to their homing tissue. Indeed, shaped by their microenvironment, MFs become adapted to perform particular functions, highlighting their great plasticity and giving rise to high population diversity. Interestingly, the gut displays several anatomic and functional compartments with large pools of strikingly diversified MF populations. This review focuses on recent advances on intestinal MFs in several species, which have allowed to infer their specificity and functions.

Differentiation Paths of Peyer's Patch LysoDCs Are Linked to Sampling Site Positioning, Migration, and T Cell Priming.

The monocyte-derived phagocytes termed LysoDCs are hallmarks of Peyer's patches, where their main function is to sample intestinal microorganisms. Here, we study their differentiation pathways in relation with their sampling, migratory, and T cell-priming abilities. Among four identified LysoDC differentiation stages displaying similar phagocytic activity, one is located in follicles, and the others reside in subepithelial domes (SED), where they proliferate and mature as they get closer to the epithelium. Mature LysoDCs but not macrophages express a gene set in common with conventional dendritic cells and prime naive helper T cells in vitro. At steady state, they do not migrate into naive T cell-enriched interfollicular regions (IFRs), but upon stimulation, they express the chemokine receptor CCR7 and migrate from SED to the IFR periphery, where they strongly interact with proliferative immune cells. Finally, we show that LysoDCs populate human Peyer's patches, strengthening their interest as targets for modulating intestinal immunity.

Dendritic cell aggresome-like induced structures are dedicated areas for ubiquitination and storage of newly synthesized defective proteins.

In response to inflammatory stimulation, dendritic cells (DCs) have a remarkable pattern of differentiation (maturation) that exhibits specific mechanisms to control antigen processing and presentation. One of these mechanisms is the sorting of polyubiquitinated proteins in large cytosolic aggregates called dendritic cell aggresome-like induced structures (DALIS). DALIS formation and maintenance are tightly linked to protein synthesis. Here, we took advantage of an antibody recognizing the antibiotic puromycin to follow the fate of improperly translated proteins, also called defective ribosomal products (DRiPs). We demonstrate that DRiPs are rapidly stored and protected from degradation in DALIS. In addition, we show that DALIS contain the ubiquitin-activating enzyme E1, the ubiquitin-conjugating enzyme E225K, and the COOH terminus of Hsp70-interacting protein ubiquitin ligase. The accumulation of these enzymes in the central area of DALIS defines specific functional sites where initial DRiP incorporation and ubiquitination occur. Therefore, DCs are able to regulate DRiP degradation in response to pathogen-associated motifs, a capacity likely to be important for their immune functions.

The Peyer's Patch Mononuclear Phagocyte System at Steady State and during Infection.

The gut represents a potential entry site for a wide range of pathogens including protozoa, bacteria, viruses, or fungi. Consequently, it is protected by one of the largest and most diversified population of immune cells of the body. Its surveillance requires the constant sampling of its encounters by dedicated sentinels composed of follicles and their associated epithelium located in specialized area. In the small intestine, Peyer's patches (PPs) are the most important of these mucosal immune response inductive sites. Through several mechanisms including transcytosis by specialized epithelial cells called M-cells, access to the gut lumen is facilitated in PPs. Although antigen sampling is critical to the initiation of the mucosal immune response, pathogens have evolved strategies to take advantage of this permissive gateway to enter the host and disseminate. It is, therefore, critical to decipher the mechanisms that underlie both host defense and pathogen subversive strategies in order to develop new mucosal-based therapeutic approaches. Whereas penetration of pathogens through M cells has been well described, their fate once they have reached the subepithelial dome (SED) remains less well understood. Nevertheless, it is clear that the mononuclear phagocyte system (MPS) plays a critical role in handling these pathogens. MPS members, including both dendritic cells and macrophages, are indeed strongly enriched in the SED, interact with M cells, and are necessary for antigen presentation to immune effector cells. This review focuses on recent advances, which have allowed distinguishing the different PP mononuclear phagocyte subsets. It gives an overview of their diversity, specificity, location, and functions. Interaction of PP phagocytes with the microbiota and the follicle-associated epithelium as well as PP infection studies are described in the light of these new criteria of PP phagocyte identification. Finally, known alterations affecting the different phagocyte subsets during PP stimulation or infection are discussed.

Gene expression profiling of the Peyer's patch mononuclear phagocyte system.

Peyer's patches (PPs) are primary inductive sites of mucosal immunity. The PP mononuclear phagocyte system, which encompasses both dendritic cells (DCs) and macrophages, is essential for the initiation of the mucosal immune response. We recently developed a method to isolate each mononuclear phagocyte subset of PP (Bonnardel et al., 2015). We performed a transcriptional analysis of three of these subsets: the CD11b(+) conventional DC, the lysozyme-expressing monocyte-derived DC termed LysoDC and the CD11c(hi) lysozyme-expressing macrophages. Here, we provide details of the gating strategy we used to isolate each phagocyte subset and show the quality controls and analysis associated with our gene array data deposited into Gene Expression Omnibus (GEO) under GSE65514.

Innate and adaptive immune functions of peyer's patch monocyte-derived cells.

Peyer's patches (PPs) are primary inductive sites of mucosal immunity. Defining PP mononuclear phagocyte system (MPS) is thus crucial to understand the initiation of mucosal immune response. We provide a comprehensive analysis of the phenotype, distribution, ontogeny, lifespan, function, and transcriptional profile of PP MPS. We show that monocytes give rise to macrophages and to lysozyme-expressing dendritic cells (LysoDCs), which are both involved in particulate antigen uptake, display strong innate antiviral and antibacterial gene signatures, and, upon TLR7 stimulation, secrete IL-6 and TNF, but neither IL-10 nor IFNγ. However, unlike macrophages, LysoDCs display a rapid renewal rate, strongly express genes of the MHCII presentation pathway, and prime naive helper T cells for IFNγ production. Our results show that monocytes differentiate locally into LysoDCs and macrophages, which display distinct features from their adjacent villus counterparts.