Influence of intestinal microflora on murine bone marrow and spleen macrophage precursors

Monocyte regulation by gut microbial signals

Monocytes are innate immune cells that sense environmental changes and participate in the immunoregulation of autoimmune, neurologic, cardiovascular, and metabolic diseases as well as cancer. Recent studies have suggested that the gut microbiome shapes the biology of monocytes via microbial signals at extraintestinal sites. Interestingly, in chronic diseases, communication between microbial signals and monocytes can either promote or inhibit disease activity, suggesting that some of these pathways can be harnessed for clinical therapies. In this review, we discuss the newer concepts of regulation of monocyte homeostasis and function by gut microbial signals during steady state and inflammation. We also highlight the therapeutic potential of microbial signal-based approaches for modulation in the context of various diseases.

Inside out: Relations between the microbiome, nutrition, and eye health

Age-related macular degeneration (AMD) is a complex disease with increasing numbers of individuals being afflicted and treatment modalities limited. There are strong interactions between diet, age, the metabolome, and gut microbiota, and all of these have roles in the pathogenesis of AMD. Communication axes exist between the gut microbiota and the eye, therefore, knowing how the microbiota influences the host metabolism during aging could guide a better understanding of AMD pathogenesis. While considerable experimental evidence exists for a diet-gut-eye axis from murine models of human ocular diseases, human diet-microbiome-metabolome studies are needed to elucidate changes in the gut microbiome at the taxonomic and functional levels that are functionally related to ocular pathology. Such studies will reveal new ways to diminish risk for progression of- or incidence of- AMD. Current data suggest that consuming diets rich in dark fish, fruits, vegetables, and low in glycemic index are most retina-healthful during aging.

Gut dysbiosis is associated with acceleration of lupus nephritis

The gut microbiota (GM) exerts a strong influence over the host immune system and dysbiosis of this microbial community can affect the clinical phenotype in chronic inflammatory conditions. To explore the role of the GM in lupus nephritis, we colonized NZM2410 mice with Segmented Filamentous Bacteria (SFB). Gut colonization with SFB was associated with worsening glomerulonephritis, glomerular and tubular immune complex deposition and interstitial inflammation compared to NZM2410 mice free of SFB. With SFB colonization mice experienced an increase in small intestinal lamina propria Th17 cells and group 3 innate lymphoid cells (ILC3s). However, although serum IL-17A expression was elevated in these mice, Th17 cells and ILC3s were not detected in the inflammatory infiltrate in the kidney. In contrast, serum and kidney tissue expression of the macrophage chemoattractants MCP-1 and CXCL1 were significantly elevated in SFB colonized mice. Furthermore, kidney infiltrating F4/80+CD206+M2-like macrophages were significantly increased in these mice. Evidence of increased gut permeability or "leakiness" was also detected in SFB colonized mice. Finally, the intestinal microbiome of SFB colonized mice at 15 and 30 weeks of age exhibited dysbiosis when compared to uncolonized mice at the same time points. Both microbial relative abundance as well as biodiversity of colonized mice was found to be altered. Collectively, SFB gut colonization in the NZM2410 mouse exacerbates kidney disease, promotes kidney M2-like macrophage infiltration and overall intestinal microbiota dysbiosis.

Resistance Against Leishmania major Infection Depends on Microbiota-Guided Macrophage Activation

Innate immune cells present a dual role during leishmaniasis: they constitute the first line of host defense but are also the main host cells for the parasite. Response against the infection that results in the control of parasite growth and lesion healing depends on activation of macrophages into a classical activated phenotype. We report an essential role for the microbiota in driving macrophage and monocyte-derived macrophage activation towards a resistance phenotype against Leishmania major infection in mice. Both germ-free and dysbiotic mice showed a higher number of myeloid innate cells in lesions and increased number of infected cells, mainly dermal resident and inflammatory macrophages. Despite developing a Th1 immune response characterized by the same levels of IFN-γ production as the conventional mice, germ-free mice presented reduced numbers of iNOS+ macrophages at the peak of infection. Absence or disturbance of host microbiota impaired the capacity of bone marrow-derived macrophage to be activated for Leishmania killing in vitro, even when stimulated by Th1 cytokines. These cells presented reduced expression of inos mRNA, and diminished production of microbicidal molecules, such as ROS, while presenting a permissive activation status, characterized by increased expression of arginase I and il-10 mRNA and higher arginase activity. Colonization of germ-free mice with complete microbiota from conventional mice rescued their ability to control the infection. This study demonstrates the essential role of host microbiota on innate immune response against L. major infection, driving host macrophages to a resistance phenotype.

Potential Role of Gut Microbiota in Induction and Regulation of Innate Immune Memory

The gut microbiota significantly regulates the development and function of the innate and adaptive immune system. The attribute of immunological memory has long been linked only with adaptive immunity. Recent evidence indicates that memory is also present in the innate immune cells such as monocytes/macrophages and natural killer cells. These cells exhibit pattern recognition receptors (PRRs) that recognize microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs) expressed by the microbes. Interaction between PRRs and MAMPs is quite crucial since it triggers the sequence of signaling events and epigenetic rewiring that not only play a cardinal role in modulating the activation and function of the innate cells but also impart a sense of memory response. We discuss here how gut microbiota can influence the generation of innate memory and functional reprogramming of bone marrow progenitors that helps in protection against infections. This article will broaden our current perspective of association between the gut microbiome and innate memory. In the future, this knowledge may pave avenues for development and designing of novel immunotherapies and vaccination strategies.

Emerging Principles in Myelopoiesis at Homeostasis and during Infection and Inflammation

Myelopoiesis ensures the steady state of the myeloid cell compartment. Technological advances in fate mapping and genetic engineering, as well as the advent of single cell RNA-sequencing, have highlighted the heterogeneity of the hematopoietic system and revealed new concepts in myeloid cell ontogeny. These technologies are also shedding light on mechanisms of myelopoiesis at homeostasis and at different phases of infection and inflammation, illustrating important feedback loops between affected tissues and the bone marrow. We review these findings here and revisit principles in myelopoiesis in light of the evolving understanding of myeloid cell ontogeny and heterogeneity. We argue for the importance of system-wide evaluation of changes in myelopoiesis and discuss how even after the resolution of inflammation, long-lasting alterations in myelopoiesis may play a role in innate immune memory or trained immunity.

The influence of the commensal microbiota on distal tumor-promoting inflammation

Commensal microbes inhabit barrier surfaces, providing a first line of defense against invading pathogens, aiding in metabolic function of the host, and playing a vital role in immune development and function. Several recent studies have demonstrated that commensal microbes influence systemic immune function and homeostasis. For patients with extramucosal cancers, or cancers occurring distal to barrier surfaces, the role of commensal microbes in influencing tumor progression is beginning to be appreciated. Extrinsic factors such as chronic inflammation, antibiotics, and chemotherapy dysregulate commensal homeostasis and drive tumor-promoting systemic inflammation through a variety of mechanisms, including disruption of barrier function and bacterial translocation, release of soluble inflammatory mediators, and systemic changes in metabolic output. Conversely, it has also been demonstrated that certain immune therapies, immunogenic chemotherapies, and checkpoint inhibitors rely on the commensal microbiota to facilitate anti-tumor immune responses. Thus, it is evident that the mechanisms associated with commensal microbe facilitation of both pro- and anti-tumor immune responses are context dependent and rely upon a variety of factors present within the tumor microenvironment and systemic periphery. The goal of this review is to highlight the various contexts during which commensal microbes orchestrate systemic immune function with a focus on describing possible scenarios where the loss of microbial homeostasis enhances tumor progression.

Microbiota-myeloid cell crosstalk beyond the gut

The gut microbiota is a complex and dynamic microbial ecosystem that plays a fundamental role in host physiology. Locally, the gut commensal microbes/host symbiotic relationship is vital for barrier fortification, nutrient absorption, resistance against intestinal pathogens, and the development and maintenance of the mucosal immune system. It is now clear that the effects of the indigenous intestinal flora extend beyond the gut, ranging from shaping systemic immune responses to metabolic and behavioral functions. However, the underlying mechanisms of the gut microbiota/systemic immune system interactions remain largely unknown. Myeloid cells respond to microbial signals, including those derived from commensals, and initiate innate and adaptive immune responses. In this review, we focus on the impact of the gut microbiota on myeloid cells at extraintestinal sites. In particular, we discuss how commensal-derived signals affect steady-state myelopoiesis and cellular function and how that influences the response to infection and cancer therapy.

Gut Microbiota Promote Angiotensin II-Induced Arterial Hypertension and Vascular Dysfunction

Background

The gut microbiome is essential for physiological host responses and development of immune functions. The impact of gut microbiota on blood pressure and systemic vascular function, processes that are determined by immune cell function, is unknown.

Methods and Results

Unchallenged germ‐free mice (GF) had a dampened systemic T helper cell type 1 skewing compared to conventionally raised (CONV‐R) mice. Colonization of GF mice with regular gut microbiota induced lymphoid mRNA transcription of T‐box expression in T cells and resulted in mild endothelial dysfunction. Compared to CONV‐R mice, angiotensin II (AngII; 1 mg/kg per day for 7 days) infused GF mice showed reduced reactive oxygen species formation in the vasculature, attenuated vascular mRNA expression of monocyte chemoattractant protein 1 (MCP‐1), inducible nitric oxide synthase (iNOS) and NADPH oxidase subunit Nox2, as well as a reduced upregulation of retinoic‐acid receptor‐related orphan receptor gamma t (Rorγt), the signature transcription factor for interleukin (IL)‐17 synthesis. This resulted in an attenuated vascular leukocyte adhesion, less infiltration of Ly6G⁺ neutrophils and Ly6C⁺ monocytes into the aortic vessel wall, protection from kidney inflammation, as well as endothelial dysfunction and attenuation of blood pressure increase in response to AngII. Importantly, cardiac inflammation, fibrosis and systolic dysfunction were attenuated in GF mice, indicating systemic protection from cardiovascular inflammatory stress induced by AngII.

Conclusion

Gut microbiota facilitate AngII‐induced vascular dysfunction and hypertension, at least in part, by supporting an MCP‐1/IL‐17 driven vascular immune cell infiltration and inflammation.

Murine CD8+ T cells but not macrophages express the vitamin D 1α-hydroxylase

The active form of vitamin D, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] is synthesized by the 1α-hydroxylase, which is encoded by the Cyp27B1 gene. Using transgenic mice that have replaced the Cyp27B1 gene with the bacterial lacZ reporter gene (β-galactosidase), the inflammatory conditions that induce Cyp27B1 in the immune system were probed. A variety of stimuli including lipopolysaccharide, anti-CD3 or PMA/ionomycin were used to stimulate splenocytes and bone marrow derived macrophage in vitro. Only anti-CD3 stimulation resulted in a low induction of β-galactosidase activity in the spleen, indicating that T cells might be a source of Cyp27B1. In vivo, challenge with lipopolysaccharide, α-galactosylceramide, and Listeria monocytogenes failed to induce β-galactosidase activity outside of the kidneys. During more prolonged and severe inflammation there was staining in both the lungs and the gastrointestinal tract for β-galactosidase. Furthermore, wild-type reconstitution of the hematopoietic cell population in Cyp27B1 KO mice protected the mice from experimental colitis. T cell production of Cyp27B1 activity was shown to be from the CD8+ but not the CD4+ T cell population. CD8+ T cells expressed the reporter gene only after 48 h of stimulation. The data is consistent with a model where CD8+ T cells are activated to produce Cyp27B1 and 1,25(OH)2D3 that serves to turn off the local immune response.

The normal flora may contribute to the quantitative preponderance of myeloid cells under physiological conditions

Under physiological conditions, the innate immune cells derived from myeloid lineage absolutely outnumber the lymphoid cells. At present, two theories are attributed to the maintenance of haemopoiesis: the asymmetric cell division and the bone marrow hematopoietic microenvironment or "niche". However, the former only explains the self-renewal of haemopoietic stem cell (HSC) and the start of haemopoietic differentiation but fails to address the inducers of cell fate decisions; the latter has to admit that the hematopoietic cytokines, despite their significance in the maintenance of haemopoiesis, have no specific effect on lineage commitment. Given these flaws, the advantageous mechanism of myeloid haemopoiesis has not yet been uncovered in the current theories. The discoveries that bacterial components (lipopolysaccharide, LPS) and intestinal decontamination affect the mobilization of HSC trigger the interest in normal flora, which together with their components may have an effect on haemopoiesis. In the experiments in dogs and mice, researchers documented that the generation of myeloid cells has undergone changes in the bone marrow and periphery when antibiotics are used to regulate the normal intestinal flora and the concentration of its components. However, the same changes are not involved in lymphoid cells. Therefore, we hypothesize that in human body normal flora and its components are a driving force to maintain myeloid haemopoiesis under physiological conditions. To account for the selectiveness in haemopoiesis, these facts should be taken into consideration, such as HSC and mesenchymal stem cells (MSC) functionally expressed pattern recognition receptors (PRR), and both of them can self-migrate or be recruited by normal flora or its components into periphery. Dynamically monitoring the myeloid haemopoiesis may provide an important complementary program that precludes the abuse of antibiotics, which prevents diseases triggered by the imbalance of normal flora. Meanwhile, the regulation of normal flora and the use of purified microecological modulator may serve as valuable auxiliary treatments to mobilize HSC prior to the HSC transplantation as well as to promote hematopoietic recovery after transplantation or chemotherapy in the blood diseases.

Craniofacial ciliopathies: A new classification for craniofacial disorders

Craniofacial anomalies are some of the most variable and common defects affecting the population. Herein, we examine a group of craniofacial disorders that are the result of defects in primary cilia; ubiquitous, microtubule-based organelles that transduce molecular signals and facilitate the interactions between the cell and its environment. Based on the frequent appearance of craniofacial phenotypes in diseases born from defective primary cilia (ciliopathies) we propose a new class of craniofacial disorders referred to as craniofacial ciliopathies. We explore the most frequent phenotypes associated with ciliopathic conditions and the ciliary gene mutations responsible for craniofacial defects. Finally, we propose that some non-classified disorders may now be classified as craniofacial ciliopathies.

Eradication of the commensal intestinal microflora by oral antimicrobials interferes with the host response to lipopolysaccharide

The host components and commensal microorganisms of the intestinal microenvironment play roles in the development and maintenance of the host defence. Recent observations have suggested that toll-like receptors (TLRs) are involved in the recognition of innate immunity against intestinal microbes. However, little is known regarding the role of TLR in the maintenance of systemic host defence by intestinal microorganisms. We studied the expression and function of TLR4 and TLR2 on alveolar and peritoneal macrophages in mice after 3 weeks of oral administration of streptomycin and cefotaxime. After active treatment, the intestinal microorganisms were nearly completely eradicated, and the surface expression of TLR4 and TLR2 on the peritoneal macrophages was prominently downregulated. When the actively treated mice were challenged with lipopolysaccharide (LPS), a TLR4 ligand, the host response was markedly impaired. Our results suggest that the oral administration of antimicrobials downregulates the expression of surface TLR on the peritoneal macrophages and modulates the host immune responses against LPS by modifying the intestinal environment.

The effects of whole mushrooms during inflammation

BACKGROUND

Consumption of edible mushrooms has been suggested to improve health. A number of isolated mushroom constituents have been shown to modulate immunity. Five commonly consumed edible mushrooms were tested to determine whether whole mushrooms stimulate the immune system in vitro and in vivo.

RESULTS

The white button (WB) extracts readily stimulated macrophage production of TNF-alpha. The crimini, maitake, oyster and shiitake extracts also stimulated TNF-alpha production in macrophage but the levels were lower than from WB stimulation. Primary cultures of murine macrophage and ovalbumin (OVA) specific T cells showed that whole mushroom extracts alone had no effect on cytokine production but co-stimulation with either lipopolysaccharide or OVA (respectively) induced TNF-alpha, IFN-gamma, and IL-1beta while decreasing IL-10. Feeding mice diets that contained 2% WB mushrooms for 4 weeks had no effect on the ex vivo immune responsiveness or associated toxicity (changes in weight or pathology of liver, kidney and gastrointestinal tract). Dextran sodium sulfate (DSS) stimulation of mice that were fed 1% WB mushrooms were protected from DSS induced weight loss. In addition, 2% WB feeding protected the mice from transient DSS induced colonic injury. The TNF-alpha response in the colon and serum of the DSS challenged and 2% WB fed mice was higher than controls.

CONCLUSION

The data support a model whereby edible mushrooms regulate immunity in vitro. The in vivo effects of edible mushrooms required a challenge with DSS to detect small changes in TNF-alpha and transient protection from colonic injury. There are modest effects of in vivo consumption of edible mushrooms on induced inflammatory responses. The result is not surprising since it would certainly be harmful to strongly induce or suppress immune function following ingestion of a commonly consumed food.

Bacteroides thetaiotaomicron in the gut: molecular aspects of their interaction

The gut microflora can be considered a metabolically active organ composed of a vast and complex community of microorganisms that has an important role in the stability and functional activity of the intestinal ecosystem. Recently, thanks to microarray technology, a global screening of the microflora's regulated genes has allowed the analysis of the complex bacteria-host interplay. In particular, most of our knowledge comes from studies on Bacteroides thetaiotaomicron, a prominent member of the intestinal microflora of mice and humans. The results of published studies have revealed that Bacteroides thetaiotaomicron modulate the expression of a large quantity of genes implicated in different aspect of host physiology. This review aims to illustrate the specific contributions of this intestinal microorganism in three important aspects of host physiology: mucosal barrier reinforcement, immune system modulation and nutrients metabolism. In particular, we focus on recent insights about the molecular mechanisms by which Bacteroides thetaiotaomicron help the host in these important functions.

Role of the innate immune system and host-commensal mutualism

Host organisms live in intimate contact with indigenous microflora. The interactions between the host and commensal microbiota are highly complex and heterogeneous. A growing body of evidence indicates that commensal symbionts provide many benefits to the host physiology, particularly in the gastrointestinal system. The molecular mechanisms of the mutualistic interactions between the host and commensals are largely unknown but can be due either to bioactivity of the commensals or to the reaction of the host immune system to the commensal-derived products. Recent advances in our understanding of the innate immune system allow re-evaluation of some of the older findings regarding the mechanisms of benefits conferred by microflora. Here we review the examples of the benefits of host-commensal interactions that are due to recognition of commensal microbial products by the host innate immune system.

Treatment and prevention of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is the most common serious, acquired gastrointestinal disorder in the newborn infant. Although many variables are associated with development of NEC, only prematurity has been consistently identified in case-controlled studies. Traditionally, the diving seal reflex has been invoked as the mechanism responsible for ischaemic injury and necrosis. Intestinal ischaemia is likely to be the final common pathway in NEC; however, it is due to the release of vasoconstricting substances, such as platelet activating factor, rather than perinatal asphyxia. Bacteria and/or bacterial toxins are likely to have a key role in the pathogenesis of NEC by fostering production of inflammatory mediators. The role of feeding practices in the pathogenesis of NEC remains controversial. Treatment of infants with NEC generally includes a regimen of bowel rest, gastric decompression, systemic antibiotics and parenteral nutrition. Infants with perforation are generally operated upon; however, there has been recent interest in primary peritoneal drainage as an alternative. Prevention of NEC still remains elusive. Avoidance of preterm birth, use of antenatal steroids and breast-milk feeding are practices that offer the greatest potential benefits. Use of any other strategy should await further trials.

Effect of hemorrhagic shock on gut barrier function and expression of stress-related genes in normal and gnotobiotic mice

We sought to determine whether gut-derived microbial factors influence the hepatic or intestinal inflammatory response to hemorrhagic shock and resuscitation (HS/R). Conventional and gnotobiotic mice contaminated with a defined microbiota without gram-negative bacteria were subjected to either a sham procedure or HS/R. Tissue samples were obtained 4 h later for assessing ileal mucosal permeability to FITC dextran and hepatic and ileal mucosal steady-state IL-6, inducible nitric oxide synthase (iNOS), cyclooxygenase (COX)-2, and TNF mRNA levels. Whereas HS/R significantly increased ileal mucosal permeability in conventional mice, this effect was not apparent in gnotobiotic animals. HS/R markedly increased hepatic mRNA levels for several proinflammatory genes in both conventional and gnotobiotic mice. HS/R increased ileal mucosal IL-6 and COX-2 mRNA expression in conventional but not gnotobiotic mice. If gnotobiotic mice were contaminated with Escherichia coli C25, HS/R increased ileal mucosal permeability and upregulated expression of IL-6 and COX-2. These data support the view that the hepatic inflammatory response to HS/R is largely independent of the presence of potentially pathogenic gram-negative bacteria colonizing the gut, whereas the local mucosal response to HS/R is profoundly influenced by the microbial ecology within the lumen during and shortly after the period of hemorrhage.

A peptide derived from bovine beta-casein modulates functional properties of bone marrow-derived macrophages from germfree and human flora-associated mice

The purpose of this study was to evaluate the immunomodulatory activity of a peptide derived from bovine beta-casein (beta-CN), the beta-CN (193-209) peptide, on mouse macrophages that were obtained either from germfree (GF) or from human flora-associated (HF) mice. Macrophages were derived from bone marrow (BMDM) in the presence of recombinant macrophage colony-stimulating factor and exposed to the peptide or lipopolysaccharide (LPS). Membrane marker expression [F4/80, Mac-1, major histocompatibility complex (MHC) class II antigens] and phagocytic activity were assessed by flow cytometry. Production of tumor necrosis factor-alpha and interleukin (IL)-6 was measured by bioassays and production of IL-1alpha, IL-1beta and IL-12 by ELISA. The expression of cytokine mRNA was determined using semi-quantitative reverse transcription-polymerase chain reaction. The beta-CN (193-209) peptide up-regulated MHC class II antigen expression and phagocytic activity of BMDM from GF and HF mice. Its enhancing effect on phagocytosis was greater than that after LPS stimulation (P < 0.01). The peptide induced notable levels of cytokine mRNA in BMDM from GF and HF mice, but it was a significantly weaker inducer of cytokine secretion than LPS. Nevertheless, although flora implantation had no stimulatory influence on basal MHC class II and basal cytokine levels, cells from HF mice were more susceptible than those from GF mice to the peptide effects on these variables. These results indicate that the beta-CN (193-209) peptide could enhance antimicrobial activity of macrophages without proinflammatory effects.

The gastrointestinal ecosystem: a precarious alliance among epithelium, immunity and microbiota

The gastrointestinal (GI) tract is a complex ecosystem generated by the alliance of GI epithelium, immune cells and resident microbiota. The three components of the GI ecosystem have co-evolved such that each relies on the presence of the other two components to achieve its normal function and activity. Experimental systems such as cell culture, germ-free animal models and intestinal isografts have demonstrated that each member of the GI ecosystem can follow a predetermined developmental pathway, even if isolated from the other components of the ecosystem. However, the presence of all three components is required for full physiological function. Genetic or functional alterations of any one component of this ecosystem can result in a broken alliance and subsequent GI pathology. A more detailed understanding of the interactions among microbiota, GI epithelium and the immune system should provide insight into multiple human disease states.

Priming by microbial antigens from the intestinal flora determines the ability of CD4+ T cells to rapidly secrete IL-4 in BALB/c mice infected with Leishmania major

Infection of BALB/c mice with Leishmania major results in the rapid accumulation of IL-4 transcripts within CD4(+) T cells that react to the parasite Leishmania homologue of mammalian RACK1 (LACK) Ag. Because memory/effector cells secrete IL-4 more rapidly than naive cells, we sought to analyze the phenotype of these lymphocytes before infection. Indeed, a fraction of LACK-specific CD4(+) T cells expressed a typical CD62 ligand(low)CD44(high)CD45RB(low) phenotype in uninfected mice. LACK-specific T cells were primed in gut-associated lymphoid tissues by cross-reactive microbial Ags as demonstrated by their reactivity with bacterial extracts and by the ability of APCs from the mesenteric LN of BALB/c mice to induce their proliferation. Also, mice in which the digestive tract has been decontaminated exhibited a reduced proportion of LACK-specific T cells expressing a memory/effector phenotype and did not exhibit the early accumulation of IL-4 transcripts induced by L. major. Thus, LACK-specific T cells represent a subset of CD4(+) T cells which have acquired the ability to rapidly secrete IL-4 as the result of their priming by cross-reactive microbial Ags. Tracking the fate of these cells may provide information about the regulation of cell-mediated immune responses to gut Ags in physiological and pathological situations.