Inflammation switches the differentiation program of Ly6Chi monocytes from antiinflammatory macrophages to inflammatory dendritic cells in the colon

Single-Cell Protein and RNA Expression Analysis of Mononuclear Phagocytes in Intestinal Mucosa and Mesenteric Lymph Nodes of Ulcerative Colitis and Crohn's Disease Patients

Inflammatory bowel diseases (IBDs), which include Crohn's disease (CD) and ulcerative colitis (UC), are driven by an abnormal immune response to commensal microbiota in genetically susceptible hosts. In addition to epithelial and stromal cells, innate and adaptive immune systems are both involved in IBD immunopathogenesis. Given the advances driven by single-cell technologies, we here reviewed the immune landscape and function of mononuclear phagocytes in inflamed non-lymphoid and lymphoid tissues of CD and UC patients. Immune cell profiling of IBD tissues using scRNA sequencing combined with multi-color cytometry analysis identifies unique clusters of monocyte-like cells, macrophages, and dendritic cells. These clusters reflect either distinct cell lineages (nature), or distinct or intermediate cell types with identical ontogeny, adapting their phenotype and function to the surrounding milieu (nurture and tissue imprinting). These advanced technologies will provide an unprecedented view of immune cell networks in health and disease, and thus may offer a personalized medicine approach to patients with IBD.

Human Intestinal Mononuclear Phagocytes in Health and Inflammatory Bowel Disease

Inflammatory bowel disease (IBD), including Crohn's disease and ulcerative colitis, is a complex immune-mediated disease of the gastrointestinal tract that increases morbidity and negatively influences the quality of life. Intestinal mononuclear phagocytes (MNPs) have a crucial role in maintaining epithelial barrier integrity while controlling pathogen invasion by activating an appropriate immune response. However, in genetically predisposed individuals, uncontrolled immune activation to intestinal flora is thought to underlie the chronic mucosal inflammation that can ultimately result in IBD. Thus, MNPs are involved in fine-tuning mucosal immune system responsiveness and have a critical role in maintaining homeostasis or, potentially, the emergence of IBD. MNPs include monocytes, macrophages and dendritic cells, which are functionally diverse but highly complementary. Despite their crucial role in maintaining intestinal homeostasis, specific functions of human MNP subsets are poorly understood, especially during diseases such as IBD. Here we review the current understanding of MNP ontogeny, as well as the recently identified human intestinal MNP subsets, and discuss their role in health and IBD.

Identification of Novel Human Monocyte Subsets and Evidence for Phenotypic Groups Defined by Interindividual Variations of Expression of Adhesion Molecules

Monocytes contribute to immune responses as a source for subsets of dendritic cells and macrophages. Human blood monocytes are classified as classical, non-classical and intermediate cells. However, the particular functions of these subsets have been hard to define, with conflicting results and significant overlaps. One likely reason for these ambiguities is in the heterogeneity of these monocyte subsets regrouping cells with divergent functions. To better define monocyte populations, we have analysed expression of 17 markers by multicolour flow cytometry in samples obtained from 28 control donors. Data acquisition was tailored to detect populations present at low frequencies. Our results reveal the existence of novel monocyte subsets detected as larger CD14⁺ cells that were CD16⁺ or CD16^neg. These large monocytes differed from regular, smaller monocytes with respect to expression of various cell surface molecules, such as FcR, chemokine receptors, and adhesion molecules. Unsupervised multidimensional analysis confirmed the existence of large monocytes and revealed interindividual variations that were grouped according to unique patterns of expression of adhesion molecules CD62L, CD49d, and CD43. Distinct inflammatory responses to TLR agonists were found in small and large monocytes. Overall, refining the definition of monocyte subsets should lead to the identification of populations with specific functions.

Commensal microbiota drive the functional diversification of colon macrophages

Mononuclear phagocytes are a heterogeneous population of leukocytes essential for immune homeostasis that develop tissue-specific functions due to unique transcriptional programs driven by local microenvironmental cues. Single cell RNA sequencing (scRNA-seq) of colonic myeloid cells from specific pathogen free (SPF) and germ-free (GF) C57BL/6 mice revealed extensive heterogeneity of both colon macrophages (MPs) and dendritic cells (DCs). Modeling of developmental pathways combined with inference of gene regulatory networks indicate two major trajectories from common CCR2+ precursors resulting in colon MP populations with unique transcription factors and downstream target genes. Compared to SPF mice, GF mice had decreased numbers of total colon MPs, as well as selective proportional decreases of two major CD11c+CD206intCD121b+ and CD11c-CD206hiCD121b- colon MP populations, whereas DC numbers and proportions were not different. Importantly, these two major colon MP populations were clearly distinct from other colon MP populations regarding their gene expression profile, localization within the lamina propria (LP) and ability to phagocytose macromolecules from the blood. These data uncover the diversity of intestinal myeloid cell populations at the molecular level and highlight the importance of microbiota on the unique developmental as well as anatomical and functional fates of colon MPs.

Inflamed synovial fluid induces a homeostatic response in bone marrow mononuclear cells in vitro: Implications for joint therapy

Synovial inflammation is a central feature of osteoarthritis (OA), elicited when local regulatory macrophages (M2-like) become overwhelmed, activating an inflammatory response (M1-like). Bone marrow mononuclear cells (BMNC) are a source of naïve macrophages capable of reducing joint inflammation and producing molecules essential for cartilage metabolism. This study investigated the response of BMNC to normal (SF) and inflamed synovial fluid (ISF). Equine BMNC cultured in autologous SF or ISF (n = 8 horses) developed into macrophage-rich cultures with phenotypes similar to cells native to normal SF and became more confluent in ISF (~100%) than SF (~25%). BMNC cultured in SF or ISF were neither M1- nor M2-like, but exhibited aspects of both phenotypes and a regulatory immune response, characterized by increasing counts of IL-10⁺ macrophages, decreasing IL-1β concentrations and progressively increasing IL-10 and IGF-1 concentrations. Changes were more marked in ISF and suggest that homeostatic mechanisms were preserved over time and were potentially favored by progressive cell proliferation. Collectively, our data suggest that intra-articular BMNC could increase synovial macrophage counts, potentiating the macrophage- and IL-10-associated mechanisms of joint homeostasis lost during the progression of OA, preserving the production of cytokines involved in tissue repair (PGE₂ , IL-10) generally impaired by frequently used corticosteroids.

TLR2 Dimerization Blockade Allows Generation of Homeostatic Intestinal Macrophages under Acute Colitis Challenge

Recruited blood monocytes contribute to the establishment, perpetuation, and resolution of tissue inflammation. Specifically, in the inflamed intestine, monocyte ablation was shown to ameliorate colitis scores in preclinical animal models. However, the majority of intestinal macrophages that seed the healthy gut are also monocyte derived. Monocyte ablation aimed to curb inflammation would therefore likely interfere with intestinal homeostasis. In this study, we used a TLR2 trans-membrane peptide that blocks TLR2 dimerization that is critical for TLR2/1 and TLR2/6 heterodimer signaling to blunt inflammation in a murine colitis model. We show that although the TLR2 peptide treatment ameliorated colitis, it allowed recruited monocytes to give rise to macrophages that lack the detrimental proinflammatory gene signature and reduced potentially damaging neutrophil infiltrates. Finally, we demonstrate TLR blocking activity of the peptide on in vitro cultured human monocyte-derived macrophages. Collectively, we provide a significantly improved anti-inflammatory TLR2 peptide and critical insights in its mechanism of action toward future potential use in the clinic.

Exploring the complex role of chemokines and chemoattractants in vivo on leukocyte dynamics

Chemotaxis is fundamental for leukocyte migration in immunity and inflammation and contributes to the pathogenesis of many human diseases. Although chemokines and various other chemoattractants were initially appreciated as important mediators of acute inflammation, in the past years they have emerged as critical mediators of cell migration during immune surveillance, organ development, and cancer progression. Such advances in our knowledge in chemokine biology have paved the way for the development of specific pharmacological targets with great therapeutic potential. Chemoattractants may belong to different classes, including a complex chemokine system of approximately 50 endogenous molecules that bind to G protein-coupled receptors, which are expressed by a wide variety of cell types. Also, an unknown number of other chemoattractants may be generated by pathogens and damaged/dead cells. Therefore, blocking chemotaxis without causing side effects is an extremely challenging task. In this review, we focus on recent advances in understanding how the chemokine system orchestrates immune cell migration and positioning at the whole organ level in homeostasis, inflammation, and infection.

Defining murine monocyte differentiation into colonic and ileal macrophages

Monocytes are circulating short-lived macrophage precursors that are recruited on demand from the blood to sites of inflammation and challenge. In steady state, classical monocytes give rise to vasculature-resident cells that patrol the luminal side of the endothelium. In addition, classical monocytes feed macrophage compartments of selected organs, including barrier tissues, such as the skin and intestine, as well as the heart. Monocyte differentiation under conditions of inflammation has been studied in considerable detail. In contrast, monocyte differentiation under non-inflammatory conditions remains less well understood. Here we took advantage of a combination of cell ablation and precursor engraftment to investigate the generation of gut macrophages from monocytes. Collectively, we identify factors associated with the gradual adaptation of monocytes to tissue residency. Moreover, comparison of monocyte differentiation into the colon and ileum-resident macrophages revealed the graduated acquisition of gut segment-specific gene expression signatures.

Deletion of IRF4 in Dendritic Cells Leads to Delayed Onset of T Cell-Dependent Colitis

Classical dendritic cells (cDC) can be classified into two major subsets: Irf8-dependent cDC1 and Irf4-expressing cDC2. Although these subsets play distinct roles in intestinal immune homeostasis, their functions in T cell-driven colitis remain unknown. To assess the role of IRF4 expression in cDC2 in T cell-driven colitis, CD11c-Cre.Irf4 ^fl/fl and Irf4 ^fl/fl mice were backcrossed onto a Rag-1 ^-/- background and used as recipients of CD45RB^hiCD4⁺ T cells. Colitis score and innate immune cell influx were reduced in Cre⁺ mice 4 wk posttransfer, and these changes were associated with reduced CD4⁺ T cell counts in both the mesenteric lymph nodes and colon. By 7 wk, colitis score and colon CD4⁺ T cell numbers were similar in Cre⁺ and Cre^- mice despite a selective reduction in Th17 cells in the colon of Cre⁺ mice and a continued reduction in CD4⁺ T cell numbers in mesenteric lymph nodes. Cotransfer of CD25⁺CD45RB^lo CD4⁺ T cells prevented CD45RB^hiCD4⁺ T cell-driven colitis in both Cre⁺ and Cre^- recipients, demonstrating that IRF4 expression by cDC is not required for CD4⁺ regulatory T cell-mediated control of colitis. Collectively these results suggest a role for IRF4 expression in cDC2 in the generation of colitogenic CD4⁺ T cells, which becomes redundant as colitis progresses.

Regulation of mononuclear phagocyte function by the microbiota at mucosal sites

Mucosal tissues contain distinct microbial communities that differ drastically depending on the barrier site, and as such, mucosal immune responses have evolved to be tailored specifically for their location. Whether protective or regulatory immune responses against invading pathogens or the commensal microbiota occur is controlled by local mononuclear phagocytes (MNPs). Comprising macrophages and dendritic cells (DCs), the functions of these cells are highly dependent on the local environment. For example, the intestine contains the greatest bacterial load of any site in the body, and hence, intestinal MNPs are hyporesponsive to bacterial stimulation. This is thought to be one of the major mechanisms by which harmful immune responses directed against the trillions of harmless bacteria that line the gut lumen are avoided. Regulation of MNP function by the microbiota has been characterized in the most depth in the intestine but there are several mucosal sites that also contain their own microbiota. In this review, we present an overview of how MNP function is regulated by the microbiota at mucosal sites, highlighting recent novel pathways by which this occurs in the intestine, and new studies elucidating these interactions at mucosal sites that have been characterized in less depth, including the urogenital tract.

The Role of Immune Cells and Cytokines in Intestinal Wound Healing

Intestinal wound healing is a complicated process that not only involves epithelial cells but also immune cells. In this brief review, we will focus on discussing the contribution and regulation of four major immune cell types (neutrophils, macrophages, regulatory T cells, and innate lymphoid cells) and four cytokines (interleukin-10, tumor necrosis factor alpha, interleukin-6, and interleukin-22) to the wound repair process in the gut. Better understanding of these immune factors will be important for developing novel targeted therapy.

Macrophages Are a Potent Source of Streptococcus-Induced IFN-β

IFN-β essentially modulates the host response against mucocutaneous colonizers and potential pathogens, such as group B Streptococcus (GBS). It has been reported that the dominant signaling cascade driving IFN-β in macrophages (MΦ) in streptococcal infection is the cGAS-STING pathway, whereas conventional dendritic cells (DC) exploit endosomal recognition by intracellular TLRs. In this study, we revisited this issue by precisely monitoring the phenotypic dynamics in mixed mouse MΦ/DC cultures with GM-CSF, which requires snapshot definition of cellular identities. We identified four mononuclear phagocyte populations, of which two were transcriptionally and morphologically distinct MΦ-DC-like subsets, and two were transitional types. Notably, GBS induced a TLR7-dependent IFN-β signal only in MΦ-like but not in DC-like cells. IFN-β induction did not require live bacteria (i.e., the formation of cytolytic toxins), which are essential for IFN-β induction via cGAS-STING. In contrast to IFN-β, GBS induced TNF-α independently of TLR7. Subsequent to the interaction with streptococci, MΦ changed their immunophenotype and gained some typical DC markers and DC-like morphology. In summary, we identify IFN-β formation as part of the antistreptococcal repertoire of GM-CSF differentiated MΦ in vitro and in vivo and delineate their plasticity.

The CSF-1-receptor inhibitor, JNJ-40346527 (PRV-6527), reduced inflammatory macrophage recruitment to the intestinal mucosa and suppressed murine T cell mediated colitis

BACKGROUND & AIMS

Originally believed to be primarily a disorder of T-cell signaling, evidence shows that macrophage-lineage cells also contribute to the pathogenesis of Crohn's disease (CD). Colony stimulating factor-1 (CSF-1) is a key regulator of the macrophage lineage, but its role in CD has not been well established. We examined transcriptional data from CD mucosa for evidence of CSF-1 pathway activation and tested JNJ-40346527 (PRV-6527), a small molecule inhibitor of CSF-1 receptor kinase (CSF-1R), for its ability to inhibit disease indices in murine colitis.

METHODS

A CSF-1 pathway gene set was created from microarray data of human whole blood cultured ex vivo with CSF-1 and compared to a TNFα-induced gene set generated from epithelial-lineage cells. Gene set variation analysis was performed using existing Crohn's mucosa microarray data comparing patients who either responded or failed to respond to anti-TNFα therapy. Commencing day 14 or day 21, mice with T-cell transfer colitis were treated with vehicle or JNJ-40346527 until study termination (day 42). Endpoints included colon weight/length ratios and histopathology scores, and macrophage and T cells were assessed by immunohistochemistry. Mucosal gene expression was investigated using RNAseq.

RESULTS

Both the CSF-1 and the TNFα gene sets were enriched in the colonic mucosal transcriptomes of Crohn's disease and in mouse colitis, and expression of both gene sets was highest in patients who did not respond to anti-TNFα therapy. In these patients neither set was reduced by therapy. In the mouse model, JNJ-40346527 inhibited the increase in colon weight/length ratio by ∼50%, reduced histological disease scores by ∼60%, and reduced F4/80+ mononuclear cell and CD3+ lymphocyte numbers. RNAseq analysis confirmed the CSF-1 gene set was sharply reduced in treated mice, as were gene sets enriched in "M1" inflammatory and "M0" resident macrophages and in activated T cells.

CONCLUSIONS

CSF-1 biology is activated in Crohn's disease and in murine T cell transfer colitis. Inhibition of CSF-1R by JNJ-40346527 was associated with attenuated clinical disease scores and reduced inflammatory gene expression in mice. These data provide rationale for testing JNJ-40346527 (PRV-6527) in human inflammatory bowel disease.

Autologous bone marrow mononuclear cells modulate joint homeostasis in an equine in vivo model of synovitis

Osteoarthritis (OA) is characterized by macrophage-driven synovitis. Macrophages promote synovial health but become inflammatory when their regulatory functions are overwhelmed. Bone marrow mononuclear cells (BMNCs) are a rich source of macrophage progenitors used for treating chronic inflammation and produce essential molecules for cartilage metabolism. This study investigated the response to autologous BMNC injection in normal and inflamed joints. Synovitis was induced in both radiocarpal joints of 6 horses. After 8 h, 1 inflamed radiocarpal and 1 normal tarsocrural joint received BMNC injection. Contralateral joints were injected with saline. Synovial fluid was collected at 24, 96, and 144 h for cytology, cytokine quantification, and flow cytometry. At 144 h, horses were euthanatized, joints were evaluated, and synovium was harvested for histology and immunohistochemistry. Four days after BMNC treatment, inflamed joints had 24% higher macrophage counts with 10% more IL-10⁺ cells than saline-treated controls. BMNC-treated joints showed gross and analytical improvements in synovial fluid and synovial membrane, with increasing regulatory macrophages and synovial fluid IL-10 concentrations compared with saline-treated controls. BMNC-treated joints were comparable to healthy joints histologically, which remained abnormal in saline-treated controls. Autologous BMNCs are readily available, regulate synovitis through macrophage-associated effects, and can benefit thousands of patients with OA.-Menarim, B. C., Gillis, K. H., Oliver, A., Mason, C., Ngo, Y., Werre, S. R., Barrett, S. H., Luo, X., Byron, C. R., Dahlgren, L. A. Autologous bone marrow mononuclear cells modulate joint homeostasis in an equine in vivo model of synovitis.

Review article: the role of the autonomic nervous system in the pathogenesis and therapy of IBD

BACKGROUND

There is a growing body of evidence implicating a role for the brain-gut axis in the pathogenesis of inflammation in patients with IBD.

AIMS

To perform a narrative review of published literature regarding the association of the autonomic nervous system and intestinal inflammation and to describe the rationale for and emerging use of autonomic manipulation as a therapeutic agent METHODS: Current relevant literature was summarised and critically examined.

RESULTS

There is substantial pre-clinical and clinical evidence for a multifaceted anti-inflammatory effect of the vagus at both systemic and local intestinal levels. It acts via acetylcholine-mediated activation of α-7-acetylcholine receptors involving multiple cell types in innate and adaptive immunity and the enteric nervous system with subsequent protective influences on the intestinal barrier, inflammatory mechanisms and the microbiome. In patients with IBD, there is evidence for a sympatho-vagal imbalance, functional enteric neuronal depletion and hyporeactivity of the hypothalamic-pituitary-adrenal axis. Direct or transcutaneous vagal neuromodulation up-regulates the cholinergic anti-inflammatory pathway in pre-clinical and clinical models with down-regulation of systemic and local intestinal inflammation. This is supported by two small studies in Crohn's disease although remains to be investigated in ulcerative colitis.

CONCLUSIONS

Modulating the cholinergic anti-inflammatory pathway influences inflammation both systemically and at a local intestinal level. It represents a potentially underutilised anti-inflammatory therapeutic strategy. Given the likely pathogenic role of the autonomic nervous system in patients with IBD, vagal neuromodulation, an apparently safe and successful means of increasing vagal tone, warrants further clinical exploration.

IFNβ-producing CX3CR1+ macrophages promote T-regulatory cell expansion and tumor growth in the APCmin/+ / Bacteroides fragilis colon cancer model

Increased T-regulatory cell activity drives tumor progression in the compound APC^min/+/enterotoxic Bacteroides fragilis colon cancer model. At the same time, how microbially-induced inflammation promotes T-regulatory cell expansion in the dysplastic intestine remains poorly described. Analysis of post-infection immune cell kinetics in the colon lamina propria revealed that CD4+ Foxp3+ cell numbers increased by 25-fold between days 3–14. Importantly, T-regulatory cell expansion was preceded by a 12-fold spike in lamina propria CD11b⁺ cell numbers between days 0–4; suggesting a link between the myeloid compartment and the T-regulatory cells. Consistent with this notion, in vitro co-culture studies utilizing sorted myeloid cell subsets and CD4⁺ T-cells demonstrated that the CD11b⁺CX3CR1⁺ but not the CD11b⁺CX3CR1⁻ subset preferentially induced Foxp3 expression in CD4⁺ T-cells. Phenotypic analysis revealed that the CD11b⁺CX3CR1⁺ subset represented a homogenous CD64⁺CD24⁻CD103a⁻ macrophage population. Global CX3CR1 knockout or conditional depletion of CX3CR1⁺ myeloid cells resulted in diminished CD4⁺Foxp3⁺ cell expansion and a 3 to 6-fold reduction in tumor burden establishing CX3CR1⁺ macrophages as a major driver of the T-regulatory cell-tumor axis. Quantitative analysis of CD11b⁺ myeloid cell subsets for IFNβ mRNA revealed that the CX3CR1⁺ macrophages expressed 15-fold higher levels of IFNβ in comparison to the CX3CR1⁻ myeloid subset. Antibody mediated neutralization of IFNβ resulted in the suppression of CD4⁺Foxp3⁺ cell induction and tumor growth, demonstrating the central role of IFNβ in mediating CX3CR1⁺ macrophage-driven T-regulatory cell expansion. These studies shed new mechanistic light on the cellular ontogeny of pro-tumorigenic T-regulatory cells in the inflamed colon of the APC^min/+ mouse.

Comparison of anti-inflammatory mechanisms between doxofylline and theophylline in human monocytes

BACKGROUND

Methylxanthines are important pharmacological agents in the treatment of asthma and of chronic obstructive pulmonary diseases. The present study was designed to compare the ability of doxofylline and theophylline to modulate inflammatory pathways in human monocytes.

METHODS

Monocytes isolated from healthy anonymous human buffy coats were treated with doxofylline or theophylline in the presence of phorbol 12-myristate 13-acetate (PMA) or lipopolysaccharide (LPS), and their phenotype, the oxidative burst, cytokine expression and release, cAMP production, and protein kinase C (PKC) activity were evaluated.

RESULTS

Doxofylline and theophylline did not have overlapping effects on human monocytes. While sharing some common characteristics, they differed significantly in their selectivity. Theophylline affected LPS- above PMA-induced cellular responsivity, while doxofylline behaved in the opposite manner. Furthermore, when testing PKC activity, we found an inhibitory effect of doxofylline but not of theophylline, at equimolar doses.

CONCLUSIONS

In conclusion, our data support the growing hypothesis that doxofylline does not have a superimposable mechanism of action compared to theophylline, and this may both explain some differences in the risk/benefit ratio and may direct studies to tailor therapy for patients.

Microbial Colonization in Adulthood Shapes the Intestinal Macrophage Compartment

BACKGROUND AND AIMS

Contact with distinct microbiota early in life has been shown to educate the mucosal immune system, hence providing protection against immune-mediated diseases. However, the impact of early versus late colonization with regard to the development of the intestinal macrophage compartment has not been studied so far.

METHODS

Germ-free mice were colonized with specific-pathogen-free [SPF] microbiota at the age of 5 weeks. The ileal and colonic macrophage compartment were analysed by immunohistochemistry, flow cytometry, and RNA sequencing 1 and 5 weeks after colonization and in age-matched SPF mice, which had had contact with microbiota since birth. To evaluate the functional differences, dextran sulfate sodium [DSS]-induced colitis was induced, and barrier function analyses were undertaken.

RESULTS

Germ-free mice were characterized by an atrophied intestinal wall and a profoundly reduced number of ileal macrophages. Strikingly, morphological restoration of the intestine occurred within the first week after colonization. In contrast, ileal macrophages required 5 weeks for complete restoration, whereas colonic macrophages were numerically unaffected. However, following DSS exposure, the presence of microbiota was a prerequisite for colonic macrophage infiltration. One week after colonization, mild colonic inflammation was observed, paralleled by a reduced inflammatory response after DSS treatment, in comparison with SPF mice. This attenuated inflammation was paralleled by a lack of TNFα production of LPS-stimulated colonic macrophages from SPF and colonized mice, suggesting desensitization of colonized mice by the colonization itself.

CONCLUSIONS

This study provides the first data indicating that after colonization of adult mice, the numeric, phenotypic, and functional restoration of the macrophage compartment requires the presence of intestinal microbiota and is time dependent.

Phenotypical and Functional Polymorphism of Liver Resident Macrophages

Liver diseases are one of the main causes of mortality. In this regard, the development of new ways of reparative processes stimulation is relevant. Macrophages play a leading role in the regulation of liver homeostasis in physiological conditions and in pathology. In this regard, the development of new liver treatment methods is impossible without taking into account this cell population. Resident macrophages of the liver, Kupffer cells, represent a unique cell population, first of all, due to their development. Most of the liver macrophages belong to the self-sustaining macrophage cell population, whose origin is not bone marrow. In addition, Kupffer cells are involved in such processes as regulation of hepatocyte proliferation and apoptosis, remodeling of the intercellular matrix, lipid metabolism, protective function, etc. Such a broad spectrum of liver macrophage functions indicates their high functional plasticity. The review summarizes recent data on the development, phenotypic and functional plasticity, and participation in the reparative processes of liver macrophages: resident macrophages (Kupffer cells) and bone marrow-derived macrophages.

Macrophages in intestinal inflammation and resolution: a potential therapeutic target in IBD

Macrophages are the gatekeepers of intestinal immune homeostasis as they discriminate between innocuous antigens and potential pathogens to maintain oral tolerance. However, in individuals with a genetic and environmental predisposition, regulation of intestinal immunity is impaired, leading to chronic relapsing immune activation and pathologies of the gastrointestinal tract, such as IBD. As evidence suggests a causal link between defects in the resolution of intestinal inflammation and altered monocyte-macrophage differentiation in patients with IBD, macrophages have been considered as a novel potential target to develop new treatment approaches. This Review discusses the molecular and cellular mechanisms involved in the differentiation and function of intestinal macrophages in homeostasis and inflammation, and their role in resolving the inflammatory process. Understanding the molecular pathways involved in the specification of intestinal macrophages might lead to a new class of targets that promote remission in patients with IBD.

Proliferation of Resident Macrophages Is Dispensable for Protection during Giardia duodenalis Infections

Infection with the intestinal parasite Giardia duodenalis is one of the most common causes of diarrheal disease in the world. Previous work has demonstrated that the cells and mechanisms of the adaptive immune system are critical for clearance of this parasite. However, the innate system has not been as well studied in the context of Giardia infection. We have previously demonstrated that Giardia infection leads to the accumulation of a population of CD11b⁺, F4/80⁺, ARG1⁺, and NOS2⁺ macrophages in the small intestinal lamina propria. In this report, we sought to identify the accumulation mechanism of duodenal macrophages during Giardia infection and to determine if these cells were essential to the induction of protective Giardia immunity. We show that F4/80⁺, CD11b⁺, CD11c^int, CX3CR1⁺, MHC class II⁺, Ly6C⁻, ARG1⁺, and NOS2⁺ macrophages accumulate in the small intestine during infections in mice. Consistent with this resident macrophage phenotype, macrophage accumulation does not require CCR2, and the macrophages incorporate EdU, indicating in situ proliferation rather than the recruitment of monocytes. Depletion of macrophages using anti-CSF1R did not impact parasite clearance nor development of regulatory T cell or Th17 cellular responses, suggesting that these macrophages are dispensable for protective Giardia immunity.

Low Dose of Cyanidin-3-O-Glucoside Alleviated Dextran Sulfate Sodium-Induced Colitis, Mediated by CD169+ Macrophage Pathway

BACKGROUND

Inflammatory bowel disease (IBD) is a chronic disease of the intestinal tract in which excessive activation of inflammatory response is correlated. Cyanidin-3-O-glucoside (C3G) is a powerful anti-inflammatory agent, widely existing in fruits and vegetables. However, the role of C3G has rarely been investigated in dextran sulfate sodium (DSS)-induced colitis.

METHODS

In an attempt to elucidate the possible mechanism of IBD and develop new efficient therapeutic methods for colitis, we evaluated the effects of C3G on DSS-induced colitis. DSS-induced colitic C57BL/6 mice were intraperitoneal injected with 1ug C3G or phosphate buffer every 2 days, a total of 3 times; the changes in macrophages and regular T cells were analyzed by flow cytometry and immunofluorescence. Cytokines and chemokines were measured by real-time quantitative polymerase chain reaction.

RESULTS

The results showed that C3G treatment did not cause changes in body weight and colon length as much as those of DSS-treated mice only. Cytokine expression levels such as interleukin (IL)- 6, IL-1β, IL-18, tumor necrosis factor α, interferon γ (IFN γ) in colons and mesenteric lymph nodes (mLNs) from C3G-treated mice were lower than those from colitic mice. Meanwhile, C3G injection inhibited the decrease in CCL22 levels and Tregs induction in colitic mice. Furthermore, the activation of macrophages by LPS and increase of CD169+ cells induced by type I IFN could be inhibited by C3G directly in vitro.

CONCLUSIONS

The study is the first to demonstrate strong effects of C3G to alleviate DSS-induced colonic damage in mice. The effect of C3G on DSS-induced colitis clearly showed a decrease of CD169+ macrophages in both the colon and mLNs. An increase of CD169+ cells induced by type I IFN could be inhibited by C3G. All these data suggest that the role of C3G in colitic inflammation was mediated at least partially by CD169+ cells and the type I IFN pathway.

Inflammation triggers immediate rather than progressive changes in monocyte differentiation in the small intestine

Bone marrow-derived circulating monocytes contribute to the replenishment and maintenance of the intestinal macrophage population. Intestinal monocytes undergo context-dependent phenotypic and functional adaptations to either maintain local immune balance or support intestinal inflammation. Here we use monocyte adoptive transfer to dissect the dynamics of monocyte-to-macrophage differentiation in normal and inflamed small intestine. We find that during homeostasis CCR2 and β7-integrin mediate constitutive homing of monocytes to the gut. By contrast, intestinal inflammation increases monocyte recruitment via CCR2, but not β7-integrin. In the non-inflamed intestine, monocytes gradually differentiate to express genes typically associated with tolerogenic macrophage functions. Conversely, immediately upon entry into the inflamed intestine, monocytes adapt a different expression pattern in a partly Trem-1-dependent manner. Our observations suggest that inflammation fundamentally changes the kinetics and modalities of monocyte differentiation in tissues.

Bone marrow-derived monocytes are recruited to the gut to replenish the local macrophage pool. Here the authors show that, while such replenishment constitutively occur under homeostasis, gut inflammation induces an immediate, Trem1-related transcription change to recruited monocyte to enable a context-dependent modulation of macrophage functions.

Frontline Science: Abnormalities in the gut mucosa of non-obese diabetic mice precede the onset of type 1 diabetes

Alterations in the composition of the intestinal microbiota have been associated with development of type 1 diabetes (T1D), but little is known about changes in intestinal homeostasis that contribute to disease pathogenesis. Here, we analyzed oral tolerance induction, components of the intestinal barrier, fecal microbiota, and immune cell phenotypes in non-obese diabetic (NOD) mice during disease progression compared to non-obese diabetes resistant (NOR) mice. NOD mice failed to develop oral tolerance and had defective protective/regulatory mechanisms in the intestinal mucosa, including decreased numbers of goblet cells, diminished mucus production, and lower levels of total and bacteria-bound secretory IgA, as well as an altered IEL profile. These disturbances correlated with bacteria translocation to the pancreatic lymph node possibly contributing to T1D onset. The composition of the fecal microbiota was altered in pre-diabetic NOD mice, and cross-fostering of NOD mice by NOR mothers corrected their defect in mucus production, indicating a role for NOD microbiota in gut barrier dysfunction. NOD mice had a reduction of CD103⁺ dendritic cells (DCs) in the MLNs, together with an increase of effector Th17 cells and ILC3, as well as a decrease of Th2 cells, ILC2, and Treg cells in the small intestine. Importantly, most of these gut alterations precede the onset of insulitis. Disorders in the intestinal mucosa of NOD mice can potentially interfere with the development of T1D due the close relationship between the gut and the pancreas. Understanding these early alterations is important for the design of novel therapeutic strategies for T1D prevention.

PTPN2 Regulates Inflammasome Activation and Controls Onset of Intestinal Inflammation and Colon Cancer

Variants in the gene locus encoding protein tyrosine phosphatase non-receptor type 2 (PTPN2) are associated with inflammatory disorders, including inflammatory bowel diseases, rheumatoid arthritis, and type 1 diabetes. The anti-inflammatory role of PTPN2 is highlighted by the fact that PTPN2-deficient mice die a few weeks after birth because of systemic inflammation and severe colitis. However, the tissues, cells, and molecular mechanisms that contribute to this phenotype remain unclear. Here, we demonstrate that myeloid cell-specific deletion of PTPN2 in mice (PTPN2-LysMCre) promotes intestinal inflammation but protects from colitis-associated tumor formation in an IL-1β-dependent manner. Elevated levels of mature IL-1β production in PTPN2-LysMCre mice are a consequence of increased inflammasome assembly due to elevated phosphorylation of the inflammasome adaptor molecule ASC. Thus, we have identified a dual role for myeloid PTPN2 in directly regulating inflammasome activation and IL-1β production to suppress pro-inflammatory responses during colitis but promote intestinal tumor development.

Myeloid-Derived Suppressor Cells: Ductile Targets in Disease

Myeloid-derived suppressor cells (MDSCs) represent a heterogeneous population of immature myeloid cells with major regulatory functions and rise during pathological conditions, including cancer, infections and autoimmune conditions. MDSC expansion is generally linked to inflammatory processes that emerge in response to stable immunological stress, which alter both magnitude and quality of the myelopoietic output. Inability to reinstate physiological myelopoiesis would fall in an “emergency state” that perpetually reprograms myeloid cells toward suppressive functions. While differentiation and reprogramming of myeloid cells toward an immunosuppressive phenotype can be considered the result of a multistep process that originates in the bone marrow and culminates in the tumor microenvironment, the identification of its driving events may offer potential therapeutic approaches in different pathologies. Indeed, whereas expansion of MDSCs, in both murine and human tumor bearers, results in reduced immune surveillance and antitumor cytotoxicity, placing an obstacle to the effectiveness of anticancer therapies, adoptive transfer of MDSCs has shown therapeutic benefits in autoimmune disorders. Here, we describe relevant mechanisms of myeloid cell reprogramming leading to generation of suppressive MDSCs and discuss their therapeutic ductility in disease.

Nonclassical Monocytes in Health and Disease

Monocytes are innate blood cells that maintain vascular homeostasis and are early responders to pathogens in acute infections. There are three well-characterized classes of monocytes: classical (CD14+CD16−in humans and Ly6Chiin mice), intermediate (CD14+CD16+in humans and Ly6C+Treml4+in mice), and nonclassical (CD14−CD16+in humans and Ly6Cloin mice). Classical monocytes are critical for the initial inflammatory response. Classical monocytes can differentiate into macrophages in tissue and can contribute to chronic disease. Nonclassical monocytes have been widely viewed as anti-inflammatory, as they maintain vascular homeostasis. They are a first line of defense in recognition and clearance of pathogens. However, their roles in chronic disease are less clear. They have been shown to be protective as well as positively associated with disease burden. This review focuses on the state of the monocyte biology field and the functions of monocytes, particularly nonclassical monocytes, in health and disease.

Capturing the Fantastic Voyage of Monocytes Through Time and Space

Monocytes are a subset of cells that are categorized together with dendritic cells (DCs) and macrophages in the mononuclear phagocyte system (MPS). Despite sharing several phenotypic and functional characteristics with MPS cells, monocytes are unique cells with the ability to function as both precursor and effector cells in their own right. Before the development of hematopoietic stem cells (HSCs) in utero, monocytes are derived from erythro-myeloid precursors (EMPs) in the fetal liver that are important for populating the majority of tissue resident macrophages. After birth, monocytes arise from bone marrow (BM)-derived HSCs and are released into the circulation upon their maturation, where they survey peripheral tissues and maintain endothelial integrity. Upon sensing of microbial breaches or inflammatory stimuli, monocytes migrate into tissues where their plasticity allows them to differentiate into cells that resemble macrophages or DCs according to the environmental niche. Alternatively, they may also migrate into tissues in the absence of inflammation and remain in an undifferentiated state where they perform homeostatic roles. As monocytes are typically on the move, the availability of intravital imaging approaches has provided further insights into their trafficking patterns in distinct tissue compartments. In this review, we outline the importance of understanding their functional behavior in the context of tissue compartments, and how these studies may contribute towards improved vaccine and future therapeutic strategies.

The Resolution of Intestinal Inflammation: The Peace-Keeper's Perspective

The uncontrolled activation of the immune system toward antigens contained in the gut lumen in genetically predisposed subjects is believed to be the leading cause of inflammatory bowel disease (IBD). Two not mutually exclusive hypotheses can explain the pathogenic process leading to IBD. The first and mostly explored hypothesis states that the loss of tolerance toward gut microbiota antigens generates an aberrant inflammatory response that is perpetuated by continuous and unavoidable exposure to the triggering antigens. However, the discovery that the resolution of inflammation is not the mere consequence of clearing inflammatory triggers and diluting pro-inflammatory factors, but rather an active process in which molecular and cellular elements are involved, implies that a defect in the pro-resolving mechanisms might cause chronic inflammation in different immune-mediated diseases, including IBD. Here we review data on pro-resolving and counter-regulatory mechanisms involved in the resolution of inflammation, aiming to identify their possible involvement in the pathogenesis of IBD.

Interactions Between the Gut Microbiota and the Host Innate Immune Response Against Pathogens

The mammalian intestine is colonized by over a trillion microbes that comprise the "gut microbiota," a microbial community which has co-evolved with the host to form a mutually beneficial relationship. Accumulating evidence indicates that the gut microbiota participates in immune system maturation and also plays a central role in host defense against pathogens. Here we review some of the mechanisms employed by the gut microbiota to boost the innate immune response against pathogens present on epithelial mucosal surfaces. Antimicrobial peptide secretion, inflammasome activation and induction of host IL-22, IL-17, and IL-10 production are the most commonly observed strategies employed by the gut microbiota for host anti-pathogen defense. Taken together, the body of evidence suggests that the host gut microbiota can elicit innate immunity against pathogens.

Functions of Macrophages in the Maintenance of Intestinal Homeostasis

Intestinal macrophages constitute the largest pool of macrophages in the body and have emerged as crucial sentinels for pathogen recognition and elimination. The source and development of intestinal macrophages, as well as their distinct properties have been well documented. Intestinal macrophages exert their functions in the maintenance of intestinal homeostasis by shaping host-microbiota symbiosis, managing gut inflammation, crosstalking with T cells, and facilitating wound repair. Recently, nutritional regulation of intestinal macrophages has attracted substantial attention and is becoming a promising approach to disease prevention and control. Understanding the mechanisms employed by intestinal macrophages in mediating intestinal immune homeostasis and inflammation, as well as the mode of action of dietary nutrients in the modulating functions of intestinal macrophages, represents an opportunity to prevent and control inflammatory bowel diseases.

Intestinal macrophages and their interaction with the enteric nervous system in health and inflammatory bowel disease

Over the past decades, there has been an increasing understanding of cellular and molecular mechanisms that mediate modulation of the immune system by the autonomic nervous system. The discovery that vagal nerve stimulation (VNS) attenuates endotoxin-induced experimental sepsis paved the way for further studies investigating neuro-immune interaction. In particular, great attention is now given to intestinal macrophages: several studies report the existence of both intrinsic and extrinsic neural mechanisms by which intestinal immune homoeostasis can be regulated in different layers of the intestine, mainly by affecting macrophage activation through neurotransmitter release. Given the important role of inflammation in numerous disease processes, such as inflammatory bowel disease (IBD), cholinergic anti-inflammatory mechanisms are under intense investigation both from a basic and clinical science perspective in immune-mediated diseases such as IBD. This review discusses recent insights on the cross-talk between enteric neurons and the immune system, especially focusing on macrophages, and provides an overview of basic and translational aspects of the cholinergic anti-inflammatory response as therapeutic alternative to reinstall immune homoeostasis in intestinal chronic inflammation.

Cellular and Molecular Therapeutic Targets in Inflammatory Bowel Disease-Focusing on Intestinal Barrier Function

The human gut relies on several cellular and molecular mechanisms to allow for an intact and dynamical intestinal barrier. Normally, only small amounts of luminal content pass the mucosa, however, if the control is broken it can lead to enhanced passage, which might damage the mucosa, leading to pathological conditions, such as inflammatory bowel disease (IBD). It is well established that genetic, environmental, and immunological factors all contribute in the pathogenesis of IBD, and a disturbed intestinal barrier function has become a hallmark of the disease. Genetical studies support the involvement of intestinal barrier as several susceptibility genes for IBD encode proteins with key functions in gut barrier and homeostasis. IBD patients are associated with loss in bacterial diversity and shifts in the microbiota, with a possible link to local inflammation. Furthermore, alterations of immune cells and several neuro-immune signaling pathways in the lamina propria have been demonstrated. An inappropriate immune activation might lead to mucosal inflammation, with elevated secretion of pro-inflammatory cytokines that can affect the epithelium and promote a leakier barrier. This review will focus on the main cells and molecular mechanisms in IBD and how these can be targeted in order to improve intestinal barrier function and reduce inflammation.

T lymphocytes in the intestinal mucosa: defense and tolerance

Although lymphocytes are known to circulate throughout lymphoid tissues and blood, they also establish residency in nonlymphoid organs, most prominently in barrier tissues, such as the intestines. The adaptation of T lymphocytes to intestinal environments requires constant discrimination between natural stimulation from commensal flora and food and pathogens that need to be cleared. Genetic variations that cause a defective defense or a break in tolerance along with environmental cues, such as infection or imbalances in the gut microbiota known as dysbiosis, can trigger several immune disorders via the activation of T lymphocytes in the intestines. Elucidation of the immune mechanisms that distinguish between commensal flora and pathogenic organisms may reveal therapeutic targets for the prevention or modulation of inflammatory diseases and boost the efficacy of cancer immunotherapy. In this review, we discuss the development and adaptation of T lymphocytes in the intestine, how these cells protect the host against pathogenic infections while tolerating food antigens and commensal microbiota, and the potential implications of targeting these cells for disease management and therapeutics.

T-Cell-Driven Inflammation as a Mediator of the Gut-Brain Axis Involved in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder affecting mainly the dopaminergic neurons of the nigrostriatal pathway, a neuronal circuit involved in the control of movements, thereby the main manifestations correspond to motor impairments. The major molecular hallmark of this disease corresponds to the presence of pathological protein inclusions called Lewy bodies in the midbrain of patients, which have been extensively associated with neurotoxic effects. Importantly, different research groups have demonstrated that CD4⁺ T-cells infiltrate into the substantia nigra of PD patients and animal models. Moreover, several studies have consistently demonstrated that T-cell deficiency results in a strong attenuation of dopaminergic neurodegeneration in animal models of PD, thus indicating a key role of adaptive immunity in the neurodegenerative process. Recent evidence has shown that CD4⁺ T-cell response involved in PD patients is directed to oxidised forms of α-synuclein, one of the main constituents of Lewy bodies. On the other hand, most PD patients present a number of non-motor manifestations. Among non-motor manifestations, gastrointestinal dysfunctions result especially important as potential early biomarkers of PD, since they are ubiquitously found among confirmed patients and occur much earlier than motor symptoms. These gastrointestinal dysfunctions include constipation and inflammation of the gut mucosa and the most distinctive pathologic features associated are the loss of neurons of the enteric nervous system and the generation of Lewy bodies in the gut. Moreover, emerging evidence has recently shown a pivotal role of gut microbiota in triggering the development of PD in genetically predisposed individuals. Of note, PD has been positively correlated with inflammatory bowel diseases, a group of disorders involving a T-cell driven inflammation of gut mucosa, which is strongly dependent in the composition of gut microbiota. Here we raised the hypothesis that T-cell driven inflammation, which mediates dopaminergic neurodegeneration in PD, is triggered in the gut mucosa. Accordingly, we discuss how structural components of commensal bacteria or how different mediators produced by gut-microbiota, including short-chain fatty acids and dopamine, may affect the behaviour of T-cells, triggering the development of T-cell responses against Lewy bodies, initially confined to the gut mucosa but later extended to the brain.

IL-10 produced by macrophages regulates epithelial integrity in the small intestine

Macrophages (Mϕs) are known to be major producers of the anti-inflammatory cytokine interleukin-10 (IL-10) in the intestine, thus playing an important role in maintaining gastrointestinal homeostasis. Mϕs that reside in the small intestine (SI) have been previously shown to be regulated by dietary antigens, while colonic Mϕs are regulated by the microbiota. However, the role which resident Mϕs play in SI homeostasis has not yet been fully elucidated. Here, we show that SI Mϕs regulate the integrity of the epithelial barrier via secretion of IL-10. We used an animal model of non-steroidal anti-inflammatory drug (NSAID)-induced SI epithelial injury to show that IL-10 is mainly produced by MHCII⁺ CD64⁺ Ly6C^low Mϕs early in injury and that it is involved in the restoration of the epithelial barrier. We found that a lack of IL-10, particularly its secretion by Mϕs, compromised the recovery of SI epithelial barrier. IL-10 production by MHCII⁺ CD64⁺ Ly6C^low Mϕs in the SI is not regulated by the gut microbiota, hence depletion of the microbiota did not influence epithelial regeneration in the SI. Collectively, these results highlight the critical role IL-10-producing Mϕs play in recovery from intestinal epithelial injury induced by NSAID.

Macrophages and Cardiovascular Health

Research during the last decade has generated numerous insights on the presence, phenotype, and function of myeloid cells in cardiovascular organs. Newer tools with improved detection sensitivities revealed sizable populations of tissue-resident macrophages in all major healthy tissues. The heart and blood vessels contain robust numbers of these cells; for instance, 8% of noncardiomyocytes in the heart are macrophages. This number and the cell's phenotype change dramatically in disease conditions. While steady-state macrophages are mostly monocyte independent, macrophages residing in the inflamed vascular wall and the diseased heart derive from hematopoietic organs. In this review, we will highlight signals that regulate macrophage supply and function, imaging applications that can detect changes in cell numbers and phenotype, and opportunities to modulate cardiovascular inflammation by targeting macrophage biology. We strive to provide a systems-wide picture, i.e., to focus not only on cardiovascular organs but also on tissues involved in regulating cell supply and phenotype, as well as comorbidities that promote cardiovascular disease. We will summarize current developments at the intersection of immunology, detection technology, and cardiovascular health.

Intestinal Dendritic Cells in Health and Gut Inflammation

Dendritic cells (DCs) mediate tolerance to food antigens, limit reactivity to the gut microbiota and are required for optimal response to intestinal pathogens. Intestinal DCs are heterogeneous but collectively generate both regulatory and effector T cell responses. The balance of outcomes is determined by the activity of functionally distinct DC subsets and their modulation by environmental cues. DCs constantly sample luminal content to monitor for pathogens; the significance of the various pathways by which this occurs is incompletely understood. Intestinal DC have distinctive properties shaped by local host, dietary and microbial signals. These properties include the ability to produce all-trans retinoic acid (RA) and imprint gut tropism on T cells they activate. In the steady-state, subsets of intestinal DC are potent generators of inducible Treg, aided by their ability to activate TGFβ and produce RA. However, responses induced by steady-state intestinal DCs are not exclusively regulatory in nature; effector T cells with specificity for commensal bacterial can be found in the healthy mucosa and these can be locally controlled to prevent inflammation. The ability of intestinal DCs to enhance effector responses in infection or sustain inflammation in disease is likely to involve both modulation of the local DC population and recruitment of additional populations. Immune pathways in the pathogenesis of inflammatory bowel disease can be mapped to DCs and in inflamed intestinal tissue, DCs show increased expression of microbial recognition machinery, activation, and production of key immunological mediators. Intestinal DCs may be targeted for disease therapy or to improve vaccine responses.

Roles of Myeloid-Derived Suppressor Cell Subpopulations in Autoimmune Arthritis

Emerging evidence suggests the promise of the use of myeloid-derived suppressor cells (MDSCs) in inflammatory disorders based on their unique immune-intervention properties. However, the roles of MDSCs in autoimmune arthritis are not completely understood. Indeed, their immunosuppressive functions in arthritic conditions remain controversial, with heterogeneity among MDSCs and differential effects among subpopulations receiving much attention. As a result, it is necessary to determine the roles of MDSC subpopulations in autoimmune arthritis to clarify their diagnostic and therapeutic potential. Interestingly, in the inflammation niche of autoimmune arthritis, each MDSC subpopulation can exhibit both alternatives of a given characteristic. Moreover, polymorphonuclear MDSCs (PMN-MDSCs) are likely to be more suppressive and stable compared with monocytic MDSCs (MO-MDSCs). Although various important cytokines associated with the differentiation of MDSCs or MDSC subpopulations from immature myeloid precursors, such as granulocyte colony-stimulating factor (G-CSF), have been largely applied in external inductive systems, their roles are not entirely clear. Moreover, MDSC-based clinical treatments in rheumatoid arthritis (RA) continue to represent a significant challenge, as also reported for other autoimmune diseases. In this review, we describe the effects and actions of MDSC subpopulations on the development of autoimmune arthritis and analyze several types of MDSC-based therapeutic strategies to provide comprehensive information regarding immune networks and a foundation for more effective protocols for autoimmune arthritis.

Origin, Differentiation, and Function of Intestinal Macrophages

Macrophages are increasingly recognized as essential players in the maintenance of intestinal homeostasis and as key sentinels of the intestinal immune system. However, somewhat paradoxically, they are also implicated in chronic pathologies of the gastrointestinal tract, such as inflammatory bowel disease (IBD) and are therefore considered potential targets for novel therapies. In this review, we will discuss recent advances in our understanding of intestinal macrophage heterogeneity, their ontogeny and the potential factors that regulate their origin. We will describe how the local environment of the intestine imprints the phenotypic and functional identity of the macrophage compartment, and how this changes during intestinal inflammation and infection. Finally, we highlight key outstanding questions that should be the focus of future research.

Dynamics of Colon Monocyte and Macrophage Activation During Colitis

Background: Macrophages are pivotal in coordinating a range of important processes in the intestines, including controlling intracellular infections and limiting damaging inflammation against the microbiota. However, it is not clear how gut macrophages, relative to recruited blood monocytes and other myeloid cells, contribute to the intestinal inflammatory milieu, nor how macrophages and their monocyte precursors mediate recruitment of other immune cells to the inflamed intestine. Methods: Myeloid cell populations isolated from colonic inflammatory bowel disease (IBD) or murine dextran sulphate sodium (DSS) induced colitis were assessed using flow cytometry and compared to healthy controls. In addition, mRNA expression profiles in human and murine colon samples, and in macrophages and monocytes from healthy and inflamed murine colons, were analysed by quantitative PCR (qPCR) and mRNA microarray. Results: We show that the monocyte:macrophage balance is disrupted in colon inflammation to favour recruitment of CD14+HLA-DRInt cells in humans, and Ly6CHi monocytes in mice. In addition, we identify that murine blood monocytes receive systemic signals enabling increased release of IL-1β prior to egress from the blood into the colon. Further, once within the colon and relative to other myeloid cells, monocytes represent the dominant local source of both IL-1β and TNF. Finally, our data reveal that, independent of inflammation, murine colon macrophages act as a major source of Ccl7 and Ccl8 chemokines that trigger further recruitment of their pro-inflammatory monocyte precursors. Conclusions: Our work suggests that strategies targeting macrophage-mediated monocyte recruitment may represent a promising approach for limiting the chronic inflammation that characterises IBD.

Impaired COMMD10-Mediated Regulation of Ly6Chi Monocyte-Driven Inflammation Disrupts Gut Barrier Function

Ly6C^hi monocyte tissue infiltrates play important roles in mediating local inflammation, bacterial elimination and resolution during sepsis and inflammatory bowel disease (IBD). Yet, the immunoregulatory pathways dictating their activity remain poorly understood. COMMD family proteins are emerging as key regulators of signaling and protein trafficking events during inflammation, but the specific role of COMMD10 in governing Ly6C^hi monocyte-driven inflammation is unknown. Here we report that COMMD10 curbs canonical and non-canonical inflammasome activity in Ly6C^hi monocytes in a model of LPS-induced systemic inflammation. Accordingly, its deficiency in myeloid cells, but not in tissue resident macrophages, resulted in increased Ly6C^hi monocyte liver and colonic infiltrates, elevated systemic cytokine storm, increased activation of caspase-1 and-11 in the liver and colon, and augmented IL-1β production systemically and specifically in LPS-challenged circulating Ly6C^hi monocytes. These inflammatory manifestations were accompanied by impaired intestinal barrier function with ensuing bacterial dissemination to the mesenteric lymph nodes and liver leading to increased mortality. The increased inflammasome activity and intestinal barrier leakage were ameliorated by the inducible ablation of COMMD10-deficient Ly6C^hi monocytes. In consistence with these results, COMMD10-deficiency in Ly6C^hi monocytes, but not in intestinal-resident lamina propria macrophages, led to increased IL-1β production and aggravated colonic inflammation in a model of DSS-induced colitis. Finally, COMMD10 expression was reduced in Ly6C^hi monocytes and their corresponding human CD14^hi monocytes sorted from mice subjected to DSS-induced colitis or from IBD patients, respectively. Collectively, these results highlight COMMD10 as a negative regulator of Ly6C^hi monocyte inflammasome activity during systemic inflammation and IBD.

Monocytes accumulate in the airways of children with fatal asthma

BACKGROUND

Activated T helper type 2 (Th2) cells are believed to play a pivotal role in allergic airway inflammation, but which cells attract and activate Th2 cells locally have not been fully determined. Recently, it was shown in an experimental human model of allergic rhinitis (AR) that activated monocytes rapidly accumulate in the nasal mucosa after local allergen challenge, where they promote recruitment of Th2 cells and eosinophils.

OBJECTIVE

To investigate whether monocytes are recruited to the lungs in paediatric asthma.

METHODS

Tissue samples obtained from children and adolescents with fatal asthma attack (n = 12), age-matched non-atopic controls (n = 9) and allergen-challenged AR patients (n = 8) were subjected to in situ immunostaining.

RESULTS

Monocytes, identified as CD68+S100A8/A9+ cells, were significantly increased in the lower airway mucosa and in the alveoli of fatal asthma patients compared with control individuals. Interestingly, cellular aggregates containing CD68+S100A8/A9+ monocytes obstructing the lumen of bronchioles were found in asthmatics (8 out of 12) but not in controls. Analysing tissue specimens from challenged AR patients, we confirmed that co-staining with CD68 and S100A8/A9 was a valid method to identify recently recruited monocytes. We also showed that the vast majority of accumulating monocytes both in the lungs and in the nasal mucosa expressed matrix metalloproteinase 10, suggesting that this protein may be involved in their migration within the tissue.

CONCLUSIONS AND CLINICAL RELEVANCE

Monocytes accumulated in the lungs of children and adolescents with fatal asthma attack. This finding strongly suggests that monocytes are directly involved in the immunopathology of asthma and that these pro-inflammatory cells are potential targets for therapy.

Aquaporin 9 Contributes to the Maturation Process and Inflammatory Cytokine Secretion of Murine Dendritic Cells

Dendritic cells (DCs) are the most potent antigen-presenting cells able to trigger the adaptive immune response to specific antigens. When non-self-antigens are captured, DCs switch from an “immature” to a “mature” state to fulfill their function. Among the several surface proteins involved in DCs maturation, the role of aquaporins (AQPs) is still poorly understood. Here we investigated the expression profile of Aqps in murine bone marrow derived dendritic cells (BMDCs). Among the Aqps analyzed, Aqp9 was the most expressed by DCs. Its expression level was significantly upregulated 6 h following LPS exposure. Chemical inhibition of Aqp9 led to a decreased inflammatory cytokines secretion. BMDCs from AQP9-KO mice release lower amount of inflammatory cytokines and chemokines and increased release of IL-10. Despite the reduced release of inflammatory cytokines, Aqp9-KO mice were not protected from DSS induced colitis. All together, our data indicate that AQP9 blockade can be an efficient strategy to reduce DCs inflammatory response but it is not sufficient to protect from acute inflammatory insults such as DSS induced colitis.

Insights into endotoxin-mediated lung inflammation and future treatment strategies

INTRODUCTION

Airway inflammatory disorders are prevalent diseases in need of better management and new therapeutics. Immunotherapies offer a solution to the problem of corticosteroid resistance. Areas covered: The current review focuses on lipopolysaccharide (Gram-negative bacterial endotoxin)-mediated inflammation in the lung and the animal models used to study related diseases. Endotoxin-induced lung pathology is usually initiated by antigen presenting cells (APC). We will discuss different subsets of APC including lung dendritic cells and macrophages, and their role in responding to endotoxin and environmental challenges. Expert commentary: The pharmacotherapeutic considerations to combat airway inflammation should cost-effectively improve quality of life with sustainable and safe strategies. Selectively targeting APCs in the lung offer the potential for a promising new strategy for the better management and treatment of inflammatory lung disease.

Neuroimmune Communication in Health and Disease

The immune and nervous systems are tightly integrated, with each system capable of influencing the other to respond to infectious or inflammatory perturbations of homeostasis. Recent studies demonstrating the ability of neural stimulation to significantly reduce the severity of immunopathology and consequently reduce mortality have led to a resurgence in the field of neuroimmunology. Highlighting the tight integration of the nervous and immune systems, afferent neurons can be activated by a diverse range of substances from bacterial-derived products to cytokines released by host cells. While activation of vagal afferents by these substances dominates the literature, additional sensory neurons are responsive as well. It is becoming increasingly clear that although the cholinergic anti-inflammatory pathway has become the predominant model, a multitude of functional circuits exist through which neuronal messengers can influence immunological outcomes. These include pathways whereby efferent signaling occurs independent of the vagus nerve through sympathetic neurons. To receive input from the nervous system, immune cells including B and T cells, macrophages, and professional antigen presenting cells express specific neurotransmitter receptors that affect immune cell function. Specialized immune cell populations not only express neurotransmitter receptors, but express the enzymatic machinery required to produce neurotransmitters, such as acetylcholine, allowing them to act as signaling intermediaries. Although elegant experiments have begun to decipher some of these interactions, integration of these molecules, cells, and anatomy into defined neuroimmune circuits in health and disease is in its infancy. This review describes these circuits and highlights continued challenges and opportunities for the field.

Inhaled IL-10 Suppresses Lung Tumorigenesis via Abrogation of Inflammatory Macrophage-Th17 Cell Axis

Intratracheal administration of a novel IL-10 formulation suppressed IL-17-driven, CD4⁺ T cell-dependent tumorigenesis in the LSL-K-ras^G12D murine lung cancer model. Analysis of lung lymphocyte populations demonstrated that antitumor activity of IL-10 was associated with a 5-fold decline in Th17 cell prevalence and a concurrent suppression of inflammatory M1-like macrophage activity. Further phenotypic characterization revealed that macrophages and dendritic cells, but not Th17 cells, expressed IL-10RA on the cell surface with the CD11b⁺F4/80⁺CX3CR1⁺ interstitial macrophages representing the dominant IL-10RA⁺ subset. Consistent with these observations, in vitro stimulation of sorted CD4⁺ T cells with IL-10 did not affect their ability to produce IL-17, whereas similar treatment of purified interstitial macrophages resulted in a dramatic M1 to M2 phenotypic switch. Importantly, preconditioning of macrophages (but not of CD4⁺ T cells) with IL-10 led to potent suppression of CD4⁺ T cell IL-17 production in an in vitro coculture assay, suggesting that IL-10 suppressed Th17 cell activity primarily via its upstream effects on macrophages. In support of this notion, in vivo macrophage depletion resulted in a 5-fold decline in Th17 cell numbers and a concurrent 6-fold reduction in tumor burden. Collectively, these data demonstrate that in the LSL-K-ras^G12D murine lung cancer model, inflammatory macrophage-Th17 cell axis is critical to tumorigenesis and that IL-10 blocks this process primarily via a direct effect on the former. Inhaled IL-10 formulations may be of use in prophylaxis against lung cancer in high-risk patients.

Impaired wound healing in type 1 diabetes is dependent on 5-lipoxygenase products

Type 1 diabetes is associated with systemic low grade inflammation (LGI). We have previously shown that LGI in diabetic mice depends on systemic circulation of leukotriene (LTB₄) which potentiates the toll-like/IL1β receptors response in macrophages. Impaired wound healing is an important co-morbidity in diabetes, and macrophages play a key role in this process. Here, we investigated the role of leukotrienes on monocytes and macrophages phenotype and in the impaired wound healing in diabetic mice. Type 1 diabetes was induced with streptozotocin in 129SvE wild-type (WT) and leukotrienes-deficient 5LO^−/− (5-lipoxygenase knockout) mice. In diabetics, the systemic levels of LTB4, TNF-α, IL-6, IL-10, IL-12 and IFNγ were increased as well as the frequency of pro-inflammatory monocytes (CD11b⁺Ly6C^highLy6G⁻) compared to healthy mice. In diabetic 5LO^−/− mice, these parameters were similar to those in healthy mice. Resident peritoneal macrophages from diabetic WT mice showed a classically activated M1-like phenotype (high Nos2, Stat and Il12 expression, and nitrite levels). Macrophages from diabetic 5LO^−/− mice presented alternatively activated M2-macrophages markers (high Arg1 and Chi3l3 expression and arginase activity) and when stimulated with IL4, enhanced phosphorylated-STAT6. Cutaneous wound healing in diabetic WT mice was impaired, which correlated with the decreased frequency of M2-macrophages (CD45⁺F4/80⁺CD206⁺) in the lesions. In diabetic 5LO^−/− mice, the frequency of M2-macrophages in the wound was similar to that in healthy mice, suggesting that the impaired healing of diabetic mice depends on 5LO products. The inhibition of leukotrienes or antagonism of its receptors could be a therapeutic alternative for diabetic patients with impaired healing.

The Gut-Brain Axis in Alzheimer's Disease and Omega-3. A Critical Overview of Clinical Trials

Despite intensive study, neurodegenerative diseases remain insufficiently understood, precluding rational design of therapeutic interventions that can reverse or even arrest the progressive loss of neurological function. In the last decade, several theories investigating the causes of neurodegenerative diseases have been formulated and a condition or risk factor that can contribute is described by the gut-brain axis hypothesis: stress, unbalanced diet, and drugs impact altering microbiota composition which contributes to dysbiosis. An altered gut microbiota may lead to a dysbiotic condition and to a subsequent increase in intestinal permeability, causing the so-called leaky-gut syndrome. Herein, in this review we report recent findings in clinical trials on the risk factor of the gut-brain axis in Alzheimer's disease and on the effect of omega-3 supplementation, in shifting gut microbiota balance towards an eubiosis status. Despite this promising effect, evidences reported in selected randomized clinical trials on the effect of omega-3 fatty acid on cognitive decline in Alzheimer's disease are few. Only Mild Cognitive Impairment, a prodromal state that could precede the progress to Alzheimer's disease could be affected by omega-3 FA supplementation. We report some of the critical issues which emerged from these studies. Randomized controlled trials in well-selected AD patients considering the critical points underlined in this review are warranted.

Transcriptional profiling reveals monocyte-related macrophages phenotypically resembling DC in human intestine

The tissue dendritic cell (DC) compartment is heterogeneous, and the ontogeny and functional specialization of human tissue conventional DC (cDC) subsets and their relationship with monocytes is unresolved. Here we identify monocyte-related CSF1R⁺Flt3^- antigen presenting cells (APCs) that constitute about half of the cells classically defined as SIRPα⁺ DCs in the steady-state human small intestine. CSF1R⁺Flt3^- APCs express calprotectin and very low levels of CD14, are transcriptionally related to monocyte-derived cells, and accumulate during inflammation. CSF1R⁺Flt3^- APCs show typical macrophage characteristics functionally distinct from their Flt3⁺ cDC counterparts: under steady-state conditions they excel at antigen uptake, have a lower migratory potential, and are inefficient activators of naïve T cells. These results have important implications for the understanding of the ontogenetic and functional heterogeneity within human tissue DCs and their relation to the monocyte lineage.