Impaired regulatory T cell function in germ-free mice

Model systems to analyze the role of miRNAs and commensal microflora in bovine mucosal immune system development

Information is rapidly accumulating regarding the role of miRNAs as key regulators of immune system development and function. It is also increasingly evident that miRNAs play an important role in host-pathogen interactions through regulation of both innate and acquired immune responses. Little is known, however, about the specific role of miRNAs in regulating normal development of the mucosal immune system, especially during the neonatal period. Furthermore, there is limited knowledge regarding the possible role the commensal microbiome may play in regulating mucosal miRNAs expression, although evidence is emerging that a variety of enteric pathogens influence miRNA expression. The current review focuses on recent information that miRNAs play an important role in regulating early development of the bovine mucosal immune system. A possible role for the commensal microbiome in regulating mucosal development by altering miRNA expression is also discussed. Finally, we explore the potential advantages of using the newborn calf as a model to determine how interactions between developmental programming, maternal factors in colostrum, and colonization of the gastrointestinal tract by commensal bacteria may alter mucosal miRNA expression and immune development. Identifying the key factors that regulate mucosal miRNA expression is critical for understanding how the balance between protective immunity and inflammation is maintained to ensure optimal gastrointestinal tract function and health of the whole organism.

T-cell selection and intestinal homeostasis

Although intestinal bacteria live deep within the body, they are topographically on the exterior surface and thus outside the host. According to the classic notion that the immune system targets non-self rather than self, these intestinal bacteria should be considered foreign and therefore attacked and eliminated. While this appears to be true for some commensal bacterial species, recent data suggest that the immune system actively becomes tolerant to many bacterial organisms. The induction or activation of regulatory T (Treg) cells that inhibit, rather than promote, inflammatory responses to commensal bacteria appears to be a central component of mucosal tolerance. Loss of this mechanism can lead to inappropriate immune reactivity toward commensal organisms, perhaps contributing to mucosal inflammation characteristic of disorders such as inflammatory bowel disease.

Microbiome diversity and asthma and allergy risk

The prevalence of asthma and allergy has been constantly increasing in Westernized countries in the last decades. Asthma and allergies are complex diseases with a local tissue inflammation that are determined by genetic and environmental factors. Because the commensal microflora is crucial to maintain inflammatory homeostasis and to induce immune regulation, the microbiome may play an important role for the development of allergic conditions. New techniques such as next-generation sequencing methods give the opportunity to explore the microbial community structure of the human body comprehensively. In this review, we will discuss the available literature concerning the human microbiota and asthma and allergy development and occurrence. The focus is on studies of the local microbiome of the place of inflammation, the gastrointestinal microbiome, and the influence of intrinsic factors relating to the host and extrinsic factors relating to the external environment on the microbiome.

Blowing on embers: commensal microbiota and our immune system

Vertebrates have co-evolved with microorganisms resulting in a symbiotic relationship, which plays an important role in health and disease. Skin and mucosal surfaces are colonized with a diverse population of commensal microbiota, over 1000 species, outnumbering the host cells by 10-fold. In the past 40 years, studies have built on the idea that commensal microbiota is in constant contact with the host immune system and thus influence immune function. Recent studies, focusing on mutualism in the gut, have shown that commensal microbiota seems to play a critical role in the development and homeostasis of the host immune system. In particular, the gut microbiota appears to direct the organization and maturation of lymphoid tissues and acts both locally and systemically to regulate the recruitment, differentiation, and function of innate and adaptive immune cells. While the pace of research in the area of the mucosal–immune interface has certainly intensified over the last 10 years, we are still in the early days of this field. Illuminating the mechanisms of how gut microbes shape host immunity will enhance our understanding of the causes of immune-mediated pathologies and improve the design of next-generation vaccines. This review discusses the recent advances in this field, focusing on the close relationship between the adaptive immune system and commensal microbiota, a constant and abundant source of foreign antigens.

The microbiome and regulation of mucosal immunity

The gastrointestinal tract is a mucosal surface constantly exposed to foreign antigens and microbes, and is protected by a vast array of immunologically active structures and cells. Epithelial cells directly participate in immunological surveillance and direction of host responses in the gut and can express numerous pattern recognition receptors, including Toll-like receptor 5 (TLR5), TLR1, TLR2, TLR3, TLR9, and nucleotide oligomerization domain 2, as well as produce chemotactic factors for both myeloid and lymphoid cells following inflammatory stimulation. Within the epithelium and in the underlying lamina propria resides a population of innate lymphoid cells that, following stimulation, can become activated and produce effector cytokines and exert both protective and pathogenic roles during inflammation. Lamina propria dendritic cells play a large role in determining whether the response to a particular antigen will be inflammatory or anti-inflammatory. It is becoming clear that the composition and metabolic activity of the intestinal microbiome, as a whole community, exerts a profound influence on mucosal immune regulation. The microbiome produces short-chain fatty acids, polysaccharide A, α-galactosylceramide and tryptophan metabolites, which can induce interleukin-22, Reg3γ, IgA and interleukin-17 responses. However, much of what is known about microbiome-host immune interactions has come from the study of single bacterial members of the gastrointestinal microbiome and their impact on intestinal mucosal immunity. Additionally, evidence continues to accumulate that alterations of the intestinal microbiome can impact not only gastrointestinal immunity but also immune regulation at distal mucosal sites.

Regulatory T-cell differentiation and their function in immune regulation

Regulatory T-cells (Treg) represent a subset of CD4+ T-cells characterized by high suppressive capacity, which can be generated in the thymus or induced in the periphery. The deleterious phenotype of the Scurfy mouse, which develops an X-linked lymphoproliferative disease resulting from defective T-cell tolerance, clearly demonstrates the importance of Treg cells for the maintenance of immune homeostasis. Although significant progress has been achieved, much information regarding the development, characteristics and function of Treg cells remain lacking. This chapter highlights the most recent discoveries in the field of Treg biology, focusing on the development and role of this cell subset in the maintenance of immune balance.

The potential link between gut microbiota and IgE-mediated food allergy in early life

There has been a dramatic rise in the prevalence of IgE-mediated food allergy over recent decades, particularly among infants and young children. The cause of this increase is unknown but one putative factor is a change in the composition, richness and balance of the microbiota that colonize the human gut during early infancy. The coevolution of the human gastrointestinal tract and commensal microbiota has resulted in a symbiotic relationship in which gut microbiota play a vital role in early life immune development and function, as well as maintenance of gut wall epithelial integrity. Since IgE mediated food allergy is associated with immune dysregulation and impaired gut epithelial integrity there is substantial interest in the potential link between gut microbiota and food allergy. Although the exact link between gut microbiota and food allergy is yet to be established in humans, recent experimental evidence suggests that specific patterns of gut microbiota colonization may influence the risk and manifestations of food allergy. An understanding of the relationship between gut microbiota and food allergy has the potential to inform both the prevention and treatment of food allergy. In this paper we review the theory and evidence linking gut microbiota and IgE-mediated food allergy in early life. We then consider the implications and challenges for future research, including the techniques of measuring and analyzing gut microbiota, and the types of studies required to advance knowledge in the field.

Food allergen selective thermal processing regimens may change oral tolerance in infancy

Food allergy can be considered a failure in the induction of oral tolerance. Recently, great interest has been focused on understanding the mechanisms and the contributing factors of oral tolerance development, hoping for new definitive interventions in the prevention and treatment of food allergy. Given that food processing may modify the properties and the nature of dietary proteins, several food processing methods could affect the allergenicity of these proteins and consequently may favour oral tolerance induction to food allergic children. Indeed, effective thermal food processing regimens of altering food proteins to reduce allergenicity have been recently reported in the literature. This article is mainly focused on the effect of selective thermal processing regimens on the main infant allergenic foods, with a potential clinical relevance on their allergenicity and therefore on oral tolerance induction. In the light of recent findings, the acquisition of tolerance in younger age and consequently the ability of young children to "outgrow" food allergy could be achieved through the application of selective thermal processing regimens on certain allergenic foods. Therefore, the ability of processed foods to circumvent clinical disease and at the same time to have an impact on the immune system and facilitate tolerance induction could be invaluable as a component of a successful therapeutic strategy. The opening in the new avenues of research in the use of processed foods in clinical practice for the amelioration of the impact on the quality of life of patients and possibly in food allergy prevention is warranted.

Lactobacilli regulate Staphylococcus aureus 161:2-induced pro-inflammatory T-cell responses in vitro

There seems to be a correlation between early gut microbiota composition and postnatal immune development. Alteration in the microbial composition early in life has been associated with immune mediated diseases, such as autoimmunity and allergy. We have previously observed associations between the presence of lactobacilli and Staphylococcus (S.) aureus in the early-life gut microbiota, cytokine responses and allergy development in children. Consistent with the objective to understand how bacteria modulate the cytokine response of intestinal epithelial cell (IEC) lines and immune cells, we exposed IEC lines (HT29, SW480) to UV-killed bacteria and/or culture supernatants (-sn) from seven Lactobacillus strains and three S. aureus strains, while peripheral blood mononuclear cells (PBMC) and cord blood mononuclear cells (CBMC) from healthy donors were stimulated by bacteria-sn or with bacteria conditioned IEC-sn. Although the overall IEC response to bacterial exposure was characterized by limited sets of cytokine and chemokine production, S. aureus 161:2-sn induced an inflammatory response in the IEC, characterized by CXCL1/GROα and CXCL8/IL-8 production, partly in a MyD88-dependent manner. UV-killed bacteria did not induce a response in the IEC line, and a combination of both UV-killed bacteria and the bacteria-sn had no additive effect to that of the supernatant alone. In PBMC, most of the Lactobacillus-sn and S. aureus-sn strains were able to induce a wide array of cytokines, but only S. aureus-sn induced the T-cell associated cytokines IL-2, IL-17 and IFN-γ, independently of IEC-produced factors, and induced up regulation of CTLA-4 expression and IL-10 production by T-regulatory cells. Notably, S. aureus-sn-induced T-cell production of IFN- γ and IL-17 was down regulated by the simultaneous presence of any of the different Lactobacillus strains, while the IEC CXCL8/IL-8 response was unaltered. Thus these studies present a possible role for lactobacilli in induction of immune cell regulation, although the mechanisms need to be further elucidated.

Regulatory T Cells Control Immune Responses through Their Non-Redundant Tissue Specific Features

Regulatory T cells (Treg) are needed in the control of immune responses and to maintain immune homeostasis. Of this subtype of regulatory lymphocytes, the most potent are Foxp3 expressing CD4+ T cells, which can be roughly divided into two main groups; natural Treg cells (nTreg), developing in the thymus, and induced or adaptive Treg cells (iTreg), developing in the periphery from naïve, conventional T cells. Both nTreg cells and iTreg cells have their own, non-redundant roles in the immune system, with nTreg cells mainly maintaining tolerance toward self-structures, and iTreg developing in response to externally delivered antigens or commensal microbes. In addition, Treg cells acquire tissue specific features and are adapted to function in the tissue they reside. This review will focus on some specific features of Treg cells in different compartments of the body.

Peripherally induced tregs - role in immune homeostasis and autoimmunity

Thymically derived Foxp3(+) regulatory T cells (tTregs) constitute a unique T cell lineage that is essential for maintaining immune tolerance to self and immune homeostasis. However, Foxp3 can also be turned on in conventional T cells as a consequence of antigen exposure in the periphery, under both non-inflammatory and inflammatory conditions. These so-called peripheral Tregs (pTregs) participate in the control of immunity at sites of inflammation, especially at the mucosal surfaces. Although numerous studies have assessed in vitro generated Tregs (termed induced or iTregs), these cells most often do not recapitulate the functional or phenotypic characteristics of in vivo generated pTregs. Thus, there are still many unanswered questions regarding the T cell receptor (TCR) repertoire and function of pTregs as well as conditions under which they are generated in vivo, and the degree to which these characteristics identify specialized features of pTregs versus features that are shared with tTregs. In this review, we summarize the current state of our understanding of pTregs and their relationship to the tTreg subset. We describe the recent discovery of unique cell surface markers and transcription factors (including Neuropilin-1 and Helios) that can be used to distinguish tTreg and pTreg subsets in vivo. Additionally, we discuss how the improved ability to distinguish these subsets provided new insights into the biology of tTregs versus pTregs and suggested differences in their function and TCR repertoire, consistent with a unique role of pTregs in certain inflammatory settings. Finally, these recent advances will be used to speculate on the role of individual Treg subsets in both tolerance and autoimmunity.

Thymic versus induced regulatory T cells - who regulates the regulators?

Physiological health must balance immunological responsiveness against foreign pathogens with tolerance toward self-components and commensals. Disruption of this balance causes autoimmune diseases/chronic inflammation, in case of excessive immune responses, and persistent infection/immunodeficiency if regulatory components are overactive. This homeostasis occurs at two different levels: at a resting state to prevent autoimmune disease, as autoreactive effector T-cells (Teffs) are only partially deleted in the thymus, and during inflammation to prevent excessive tissue injury, contract the immune response, and enable tissue repair. Adaptive immune cells with regulatory function (“regulatory T-cells”) are essential to control Teffs. Two sets of regulatory T cell are required to achieve the desired control: those emerging de novo from embryonic/neonatal thymus (“thymic” or tTregs), whose function is to control autoreactive Teffs to prevent autoimmune diseases, and those induced in the periphery (“peripheral” or pTregs) to acquire regulatory phenotype in response to pathogens/inflammation. The differentiation mechanisms of these cells determine their commitment to lineage and plasticity toward other phenotypes. tTregs, expressing high levels of IL-2 receptor alpha chain (CD25), and the transcription factor Foxp3, are the most important, since mutations or deletions in these genes cause fatal autoimmune diseases in both mice and men. In the periphery, instead, Foxp3⁺ pTregs can be induced from naïve precursors in response to environmental signals. Here, we discuss molecular signatures and induction processes, mechanisms and sites of action, lineage stability, and differentiating characteristics of both Foxp3⁺ and Foxp3⁻ populations of regulatory T cells, derived from the thymus or induced peripherally. We relate these predicates to programs of cell-based therapy for the treatment of autoimmune diseases and induction of tolerance to transplants.

Identification of a novel antimicrobial peptide from human hepatitis B virus core protein arginine-rich domain (ARD)

The rise of multidrug-resistant (MDR) pathogens causes an increasing challenge to public health. Antimicrobial peptides are considered a possible solution to this problem. HBV core protein (HBc) contains an arginine-rich domain (ARD) at its C-terminus, which consists of 16 arginine residues separated into four clusters (ARD I to IV). In this study, we demonstrated that the peptide containing the full-length ARD I–IV (HBc147-183) has a broad-spectrum antimicrobial activity at micro-molar concentrations, including some MDR and colistin (polymyxin E)-resistant Acinetobacter baumannii. Furthermore, confocal fluorescence microscopy and SYTOX Green uptake assay indicated that this peptide killed Gram-negative and Gram-positive bacteria by membrane permeabilization or DNA binding. In addition, peptide ARD II–IV (HBc153-176) and ARD I–III (HBc147-167) were found to be necessary and sufficient for the activity against P. aeruginosa and K. peumoniae. The antimicrobial activity of HBc ARD peptides can be attenuated by the addition of LPS. HBc ARD peptide was shown to be capable of direct binding to the Lipid A of lipopolysaccharide (LPS) in several in vitro binding assays. Peptide ARD I–IV (HBc147-183) had no detectable cytotoxicity in various tissue culture systems and a mouse animal model. In the mouse model by intraperitoneal (i.p.) inoculation with Staphylococcus aureus, timely treatment by i.p. injection with ARD peptide resulted in 100-fold reduction of bacteria load in blood, liver and spleen, as well as 100% protection of inoculated animals from death. If peptide was injected when bacterial load in the blood reached its peak, the protection rate dropped to 40%. Similar results were observed in K. peumoniae using an IVIS imaging system. The finding of anti-microbial HBc ARD is discussed in the context of commensal gut microbiota, development of intrahepatic anti-viral immunity and establishment of chronic infection with HBV. Our current results suggested that HBc ARD could be a new promising antimicrobial peptide.

Antibiotics-resistant pathogens have been a major problem to our public health. Recently, in our studies of human hepatitis B virus (HBV), we accidentally discovered potent and broad spectrum antimicrobial peptides from HBV core protein (HBc) arginine-rich domain (ARD). The peptides are mainly composed of SPRRR repeats and are effective against both Gram-positive and Gram-negative bacteria, as well as fungi. We found different bactericidal mechanisms of the ARD peptides, which involved LPS binding, DNA binding and membrane permeabilization in various tested bacteria, such as P. aeruginosa, K. pneumoniae, E. coli and S. aureus. We also found that this ARD peptide was effective for colistin-resistant A. baumannii. The peptides exhibited no hemolysis activity to human red blood cells and no cytotoxicity to human hepatoma cells and kidney cells. Furthermore, the ARD peptide was shown to be safe and protective in the animal model. Recently, intestinal flora was found to influence the development of immunity. We discussed here the potential involvement of the antimicrobial activity of HBc ARD in the establishment of HBV chronic infection in the newborns. We proposed here that the HBc ARD peptides could serve as an alternative to the conventional antibiotics in clinical medicine.

Mucosal immunology and probiotics

The cross-talk between the mucosa-associated immune system and microbiota is critical in mucosal tissue homeostasis as well as in protection against infectious and inflammatory diseases occurring at mucosal sites. This recent evidence has paved the way to therapeutic approaches aimed at modulating the mucosa-associated immune system using probiotics. Different strains of probiotics possess the ability to finely regulate dendritic cell (DC) activation, polarizing the subsequent T cell activity toward Th1 (e.g. Lactobacillus (Lb) acidophilus), Th2 (Lb.reuteri and Bifidobacterium bifidum) or, as more recently demonstrated, Th17 responses induced by specific strains such as Lb.rhamnosus GG and Lac23a, the latter isolated in our laboratory. Here, we review some recent advances in our understanding of probiotics effects on mucosal immunology, particularly on cells of the innate immunity such as DCs. We also highlight our own experiences in modulating DC functions by commensal bacteria and discuss the relevance of probiotics administration in the treatment of human immunopathologies.

Commensal microbiota drive proliferation of conventional and Foxp3(+) regulatory CD4(+) T cells in mesenteric lymph nodes and Peyer's patches

Compelling evidence demonstrates that intestinal commensal microbiota modulate conventional and regulatory T cell (Treg) responses that are required for effective host defence against pathogens and avoidance of autoimmunity and other immunopathologic conditions. Here, we investigated the contribution of the commensal microbiota and Toll-like receptor (TLR) signaling to homeostasis of Foxp3(-) conventional CD4(+) T cells and Foxp3(+) Tregs. Upon long-term antibiotics treatment, we observed a significant reduction of conventional CD4(+) T cell proliferation in a systemic manner, whereas Foxp3(+) Treg proliferation was locally impaired in gut-draining mesenteric lymph nodes and Peyer's patches. The proliferative response to microbial components was not mediated by TLRs as MyD88- and various TLR-deficient mice displayed normal or even increased conventional T cell and Foxp3(+) Treg proliferation. Thus, commensal microbiota-derived stimuli support cycling of both conventional CD4(+) T cells and Foxp3(+) Tregs with TLR-mediated recognition of bacterial components not being the major mechanism controlling microbiota-driven T cell homeostasis.

The endogenous bacteria alter gut epithelial apoptosis and decrease mortality following Pseudomonas aeruginosa pneumonia

The endogenous bacteria have been hypothesized to play a significant role in the pathophysiology of critical illness, although their role in sepsis is poorly understood. The purpose of this study was to determine how commensal bacteria alter the host response to sepsis. Conventional and germ-free (GF) C57Bl/6 mice were subjected to Pseudomonas aeruginosa pneumonia. All GF mice died within 2 days, whereas 44% of conventional mice survived for 7 days (P = 0.001). Diluting the dose of bacteria 10-fold in GF mice led to similar survival in GF and conventional mice. When animals with similar mortality were assayed for intestinal integrity, GF mice had lower levels of intestinal epithelial apoptosis but similar levels of proliferation and intestinal permeability. Germ-free mice had significantly lower levels of tumor necrosis factor and interleukin 1β in bronchoalveolar lavage fluid compared with conventional mice without changes in systemic cytokine production. Under conventional conditions, sepsis unmasks lymphocyte control of intestinal epithelial apoptosis, because sepsis induces a greater increase in gut apoptosis in Rag-1 mice than in wild-type mice. However, in a separate set of experiments, gut apoptosis was similar between septic GF Rag-1 mice and septic GF wild-type mice. These data demonstrate that the endogenous bacteria play a protective role in mediating mortality from pneumonia-induced sepsis, potentially mediated through altered intestinal apoptosis and the local proinflammatory response. In addition, sepsis-induced lymphocyte-dependent increases in gut epithelial apoptosis appear to be mediated by the endogenous bacteria.

Gut microbiota and type 1 diabetes

The gut immune system has a key role in the development of autoimmune diabetes, and factors that control the gut immune system are also regulators of beta-cell autoimmunity. Gut microbiota modulate the function of the gut immune system by their effect on the innate immune system, such as the intestinal epithelial cells and dendritic cells, and on the adaptive immune system, in particular intestinal T cells. Due to the immunological link between gut and pancreas, e.g. the shared lymphocyte homing receptors, the immunological changes in the gut are reflected in the pancreas. According to animal studies, changes in gut microbiota alter the development of autoimmune diabetes. This has been demonstrated by antibiotics that induce changes in the gut microbiota. Furthermore, gut-colonizing microbes may modify the incidence of autoimmune diabetes in animal models. Deficient toll-like receptor (TLR) signaling, mediating microbial stimulus in immune cells, prevents autoimmune diabetes, which appears to be dependent on alterations in the intestinal microbiota. Although few studies have been conducted in humans, recent studies suggest that the abundance of Bacteroides and lack of butyrate-producing bacteria in fecal microbiota are associated with beta-cell autoimmunity and type 1 diabetes. It is possible that altered gut microbiota are associated with immunological aberrancies in type 1 diabetes. The changes in gut microbiota could lead to alterations in the gut immune system, such as increased gut permeability, small intestinal inflammation, and impaired tolerance to food antigens, all of which are observed in type 1 diabetes. Poor fitness of gut microbiota could explain why children who develop type 1 diabetes are prone to enterovirus infections, and do not develop tolerance to cow milk antigens. These candidate risk factors of type 1 diabetes may imply an increased risk of type 1 diabetes due to the presence of gut microbiota that do not support health. Despite the complex interaction of microbiota, host, environment, and disease mechanisms, gut microbiota are promising novel targets in the prevention of type 1 diabetes.

CD4+ CD25- FoxP3+ regulatory cells are the predominant responding regulatory T cells after human rotavirus infection or vaccination in gnotobiotic pigs

The distribution and dynamic changes of CD4(+) CD25(+) FoxP3(+) and CD4(+) CD25(-) FoxP3(+) regulatory T (Treg) cells induced by human rotavirus (HRV) infection and vaccination were examined in neonatal gnotobiotic pigs infected with virulent HRV (VirHRV) or vaccinated with attenuated HRV (AttHRV). Subsets of gnotobiotic pigs in the AttHRV and control groups were challenged with VirHRV at post-inoculation day (PID) 28. We demonstrated that VirHRV infection or AttHRV vaccination reduced frequencies and numbers of tissue-residing Treg cells, and decreased the frequencies of interleukin-10 (IL-10) and transforming growth factor-β (TGF-β) producing CD4(+) CD25(-) Treg cells in ileum, spleen and blood at PID 28. The frequencies of IL-10 and TGF-β producing CD4(+) CD25(-) Treg cells in all sites at PID 28 were significantly inversely correlated with the protection rate against VirHRV-caused diarrhoea (r = -1, P < 0.0001). Hence, higher frequencies of functional CD4(+) CD25(-) Treg cells can be an indicator for poorer protective immunity against rotavirus. Our results highlighted the importance of CD4(+) CD25(-) Treg cells over CD4(+) CD25(+) Treg cells in rotavirus infection and immunity. AttHRV vaccination (induction of immune effector responses) reduced the expansion of CD4(+) CD25(-) Treg cells in ileum seen in the challenged naive pigs during the acute phase of VirHRV infection and preserved normal levels of intestinal TGF-β producing Treg cells post-challenge. The reduced suppressive effect of Treg cells in AttHRV-vaccinated pigs would unleash effector/memory T-cell activation upon challenge. Preserving TGF-β producing CD4(+) CD25(-) Treg cells is important in maintaining homeostasis. Based on our findings, a model is proposed to depict the dynamic equilibrium course of Treg and effector T-cell responses after primary rotavirus infection/vaccination and challenge.

Immunoregulation by the gut microbiota

The human intestinal mucosa is constantly exposed to commensal microbiota. Since the gut microbiota is beneficial to the host, hosts have evolved intestine-specific immune systems to co-exist with the microbiota. On the other hand, the intestinal microbiota actively regulates the host's immune system, and recent studies have revealed that specific commensal bacterial species induce the accumulation of specific immune cell populations. For instance, segmented filamentous bacteria and Clostridium species belonging to clusters XIVa and IV induce the accumulation of Th17 cells in the small intestine and Foxp3(+) regulatory T cells in the large intestine, respectively. The immune cells induced by the gut microbiota likely contribute to intestinal homeostasis and influence systemic immunity in the host.

Delayed bacterial colonization of the gut alters the host immune response to oral sensitization against cow's milk proteins

SCOPE

Cow's milk allergy is the most prevalent food allergy in infants whose immune system development is critically stimulated during postnatal gut colonization by commensal bacteria. Allergenic potential of cow's milk β-lactoglobulin (BLG) and caseins (CAS) was investigated in germ-free (GF) BALB/c mice and in GF mice conventionalized (CVd) at 6 weeks of age.

METHODS AND RESULTS

Oral sensitization to cow's milk in the presence of cholera toxin led to higher BLG-specific IgE, IgG1, and IgG2a responses in GF mice than in conventional (CV) mice. No significant difference was observed for CAS-specific IgE responses although IgG1 responses to αS1- and κ-caseins were higher in GF mice than in CV mice. CVd mice, orally inoculated with fecal preparations from CV mice, also displayed biased antibody responses compared to CV mice. Secretion of Th2 cytokines by BLG- and CAS-reactivated splenocytes of CVd mice was similar to that of GF mice whereas cytokine production by reactivated cells from mesenteric lymph nodes of CVd mice was equivalent to that of CV mice.

CONCLUSION

Oral sensitization to BLG and CAS was differentially affected by the absence of gut microbiota and delayed bacterial colonization altered persistently the host immune response to oral sensitization against food antigens.

The microbiome of the lung

Investigation of the lung microbiome is a relatively new field. Although the lungs were classically believed to be sterile, recently published investigations have identified microbial communities in the lungs of healthy humans. At the present time, there are significant methodologic and technical hurdles that must be addressed in ongoing investigations, including distinguishing the microbiota of the upper and lower respiratory tracts. However, characterization of the lung microbiome is likely to provide important pathogenic insights into cystic fibrosis, respiratory disease of the newborn, chronic obstructive pulmonary disease, and asthma. In addition to characterization of the lung microbiome, the microbiota of the gastrointestinal tract have profound influence on the development and maintenance of lung immunity and inflammation. Further study of gastrointestinal-respiratory interactions is likely to yield important insights into the pathogenesis of pulmonary diseases, including asthma. As this field advances over the next several years, we anticipate that studies using larger cohorts, multicenter designs, and longitudinal sampling will add to our knowledge and understanding of the lung microbiome.

Differential cytokine expression in T-cell subsets of chicken caecal tonsils co-cultured with three species of Lactobacillus

Members of the intestinal microbiota play an important role in the development of T-cells. Little is known about responses of intestinal T-cell subsets of chickens to commensal bacteria. Therefore, we set out to characterise cytokine responses in T-cells after exposure to lactobacilli. Caecal tonsil mononuclear cells were isolated and co-cultured with Lactobacillus acidophilus, Lactobacillus reuteri and Lactobacillus salivarius for 12 hours. Subsequently the CD4+ and CD8+ cells were fractionated by flow cytometry and the expression of pro- and anti-inflammatory cytokines as well as Toll-like receptor 21 (TLR21) was determined. The results demonstrated that chicken CD4+ and CD8+ T-cells express TLR21 and that the various isolates of lactobacilli differentially induces the expression of interleukin 10, interferon-gamma and transforming growth factor beta. Our results demonstrate that different Lactobacillus species have the capacity to regulate intestinal T-cell responses and that these responses may be important to intestinal homeostasis.

Immune disorders and its correlation with gut microbiome

Allergic disorders such as atopic dermatitis and asthma are common hyper-immune disorders in industrialized countries. Along with genetic association, environmental factors and gut microbiota have been suggested as major triggering factors for the development of atopic dermatitis. Numerous studies support the association of hygiene hypothesis in allergic immune disorders that a lack of early childhood exposure to diverse microorganism increases susceptibility to allergic diseases. Among the symbiotic microorganisms (e.g. gut flora or probiotics), probiotics confer health benefits through multiple action mechanisms including modification of immune response in gut associated lymphoid tissue (GALT). Although many human clinical trials and mouse studies demonstrated the beneficial effects of probiotics in diverse immune disorders, this effect is strain specific and needs to apply specific probiotics for specific allergic diseases. Herein, we briefly review the diverse functions and regulation mechanisms of probiotics in diverse disorders.

Pharyngeal microflora disruption by antibiotics promotes airway hyperresponsiveness after respiratory syncytial virus infection

BACKGROUND

Regulatory T cells (Treg cells), which are essential for regulation of immune response to respiratory syncytial virus (RSV) infection, are promoted by pharyngeal commensal pneumococcus. The effects of pharyngeal microflora disruption by antibiotics on airway responsiveness and relative immune responses after RSV infection have not been clarified.

METHODS

Female BALB/c mice (aged 3 weeks) were infected with RSV and then treated with either oral antibiotics or oral double distilled water (ddH(2)O) from 1 d post infection (pi). Changes in pharyngeal microflora were analyzed after antibiotic treatment for 7 d and 14 d. At 8 d pi and 15 d pi, the inflammatory cells in bronchoalveolar lavage fluid (BALF) were investigated in combination with tests of pulmonary histopathology, airway hyperresponsiveness (AHR), pulmonary and splenic Treg cells responses. Pulmonary Foxp3 mRNA expression, IL-10 and TGF-β1 in BALF and lung homogenate were investigated at 15 d pi. Ovalbumin (OVA) challenge was used to induce AHR after RSV infection.

RESULTS

The predominant pharyngeal commensal, Streptococcus, was cleared by antibiotic treatment for 7 d. Same change also existed after antibiotic treatment for 14 d. After RSV infection, AHR was promoted by antibiotic treatment at 15 d pi. Synchronous decreases of pulmonary Treg cells, Foxp3 mRNA and TGF-β1 were detected. Similar results were observed under OVA challenge.

CONCLUSIONS

After RSV infection, antibiotic treatment cleared pharyngeal commensal bacteria such as Streptococcus, which consequently, might induce AHR and decrease pulmonary Treg cells.

The induction of Treg cells by gut-indigenous Clostridium

Foxp3+ CD4+ cells are prominent immune regulatory T (Treg) cells that are most abundant in the intestine. Recent studies have suggested that intestinal Treg cells consist of thymically and extrathymically developed cells that have unique characteristics. A fraction of intestinal Treg cells express T cell receptors that recognize antigens that are derived from the gut microbiota. The presence of the gut microbiota, particularly the Clostridium species, affects the development and function of Treg cells. These intestinal bacteria-induced Treg cells are likely to play a role in the tolerance toward the gut microbiota. These recent advances provide new insight into how T cells are educated in the intestine to maintain homeostasis with the gut microbiota.

T cell tolerance and immunity to commensal bacteria

The commensal bacteria normally resident in the gastrointestinal tract represent an enormous pool of foreign antigen within the body. Although mechanical barriers limit entry of bacteria into the host, recent data suggest that T cells routinely interact with commensal bacteria using both antigen-specific and non-specific receptors. Depending on the bacterial species, either regulatory or effector T cell responses can be generated. For example, segmented filamentous bacteria (SFB) favor effector Th17 responses whereas Bacteroides fragilis and certain Clostridium species favor Foxp3+ regulatory T (Treg) cell responses. Thus, in contrast with the notion that only tolerogenic responses are required to self, gut homeostasis may require both tolerance and immunity to various constituents of the commensal microbiota.

Significance of the microbiome in obstructive lung disease

The composition of the lung microbiome contributes to both health and disease, including obstructive lung disease. Because it has been estimated that over 70% of the bacterial species on body surfaces cannot be cultured by currently available techniques, traditional culture techniques are no longer the gold standard for microbial investigation. Advanced techniques that identify bacterial sequences, including the 16S ribosomal RNA gene, have provided new insights into the depth and breadth of microbiota present both in the diseased and normal lung. In asthma, the composition of the microbiome of the lung and gut during early childhood development may play a key role in the development of asthma, while specific airway microbiota are associated with chronic asthma in adults. Early bacterial stimulation appears to reduce asthma susceptibility by helping the immune system develop lifelong tolerance to innocuous antigens. By contrast, perturbations in the microbiome from antibiotic use may increase the risk for asthma development. In chronic obstructive pulmonary disease, bacterial colonisation has been associated with a chronic bronchitic phenotype, increased risk of exacerbations, and accelerated loss of lung function. In cystic fibrosis, studies utilising culture-independent methods have identified associations between decreased bacterial community diversity and reduced lung function; colonisation with Pseudomonas aeruginosa has been associated with the presence of certain CFTR mutations. Genomic analysis of the lung microbiome is a young field, but has the potential to define the relationship between lung microbiome composition and disease course. Whether we can manipulate bacterial communities to improve clinical outcomes remains to be seen.

Regulation of T cells by gut commensal microbiota

PURPOSE OF REVIEW

Commensal bacteria in the gut shape the innate and adaptive immune systems of the host. An understanding of how these microbes direct the development of various immune cells will unravel mechanisms underlying host-microbial interaction at the cellular level. In this review, we describe the impact of microbial colonization on the modulation of individual T-cell subsets in health and disease.

RECENT FINDINGS

Compelling evidence demonstrates that the intestinal microbiota plays a pivotal role in the development of conventional and unconventional T cells both within and outside the intestine. Recent studies have documented an association of specific bacterial species with the development of certain T-cell subsets.

SUMMARY

It is increasingly clear that specific components of the microbiota selectively expand and activate different T-cell subsets under normal and/or pathological conditions. Modulation of the complex microbiota may provide opportunities for the treatment of T-cell-mediated diseases.

Induced CD4+Foxp3+ regulatory T cells in immune tolerance

Regulatory T lymphocytes are essential to maintain homeostasis of the immune system, limiting the magnitude of effector responses and allowing the establishment of immunological tolerance. Two main types of regulatory T cells have been identified--natural and induced (or adaptive)-and both play significant roles in tuning down effector immune responses. Adaptive CD4(+)Foxp3(+) regulatory T (iTreg) cells develop outside the thymus under a variety of conditions. These include not only antigen presentation under subimmunogenic or noninflammatory conditions, but also chronic inflammation and infections. We speculate that the different origin of iTreg cells (noninflammatory versus inflammatory) results in distinct properties, including their stability. iTreg cells are also generated during homeostasis of the gut and in cancer, although some cancers also favor expansion of natural regulatory T (nTreg) cells. Here we review how iTreg cells develop and how they participate in immunological tolerance, contrasting, when possible, iTreg cells with nTreg cells.

Intestinal microbiota: shaping local and systemic immune responses

Recent studies have highlighted the fundamental role of commensal microbes in the maintenance of host homeostasis. For instance, commensals can play a major role in the control of host defense, metabolism and tissue development. Over the past few years, abundant experimental data also support their central role in the induction and control of both innate and adaptive responses. It is now clearly established that commensals are not equal in their capacity to trigger control regulatory or effector responses, however, the molecular basis of these differences has only recently begun to be explored. This review will discuss recent findings evaluating how commensals shape both effector and regulatory responses at steady state and during infections and the consequence of this effect on local and systemic protective and inflammatory responses.

Induction of Treg cells in the mouse colonic mucosa: a central mechanism to maintain host-microbiota homeostasis

CD4+ regulatory T (Treg) cells expressing the transcription factor forkhead box P3 (Foxp3) play a critical role in maintaining immunological homeostasis. Treg cells are highly abundant in the mouse intestinal lamina propria, particularly in the colon. Recent studies using germ-free and gnotobiotic mice have revealed that specific components of the intestinal microbiota influence the number and function of Treg cells. Substantial changes in the composition of microbiota have been associated with inflammatory bowel disease. In this review, we will discuss recent findings that associate intestinal microbiota in mice with Treg responses and with the maintenance of intestinal immune homeostasis.

Molecular and functional heterogeneity of T regulatory cells

Naturally occurring CD4+ T regulatory (Treg) cells are produced during maturation in the thymus and have a mandatory role in maintaining homeostasis and immune quiescence. Development and function of Treg cells depends on the transcription factor forkhead box P3 (Foxp3), which is necessary and sufficient for Treg cell function. Currently emerging evidence indicates Treg cells display molecular and functional heterogeneity and can be categorized into naïve and effector- or memory-like cells, which can produce effector cytokines supporting the idea that Treg cells retain plasticity. The role of Treg cells that acquire these properties remains unclear and is currently under intense investigation. In this review, we summarize recent advances on the differentiation of effector- or memory-like Treg cells, the impact of the cytokine milieu on the molecular and functional heterogeneity of Treg cells, and the clinical implications of the heterogeneity and specialization of Treg cells.

Commensal gut flora reduces susceptibility to experimentally induced colitis via T-cell-derived interleukin-10

BACKGROUND

Regulatory cytokines are well known to modify experimental colitis in mice. The aim of this study was to elucidate the effect of interleukin (IL)-10 derived from different cellular sources and the effect of commensal gut flora in dextran sulfate sodium (DSS)-induced colitis in mice.

METHODS

Wildtype (WT) and IL-10 deficient (IL-10(-/-) ) mice either harboring a characterized specific pathogen-free (SPF) gut flora or germfree were exposed to 2% DSS. Moreover, cell type-specific IL-10, IL-4, and IL-12 knockout mice and animals combining the T-cell-specific IL-10 knockout with a deficiency in IL-12 or IL-4 were exposed to DSS.

RESULTS

SPF IL-10(-/-) mice showed an increased susceptibility to DSS-induced colitis compared to WT mice determined by histopathology and proinflammatory cytokine and chemokine responses. Under germfree conditions, both WT and IL-10(-/-) mice were highly susceptible to DSS. IL-10 mRNA was increased upon DSS exposure in WT SPF but not in germfree mice. Mice carrying a specific deletion of IL-10 in T-cells exhibited a tendency towards an enhanced susceptibility to DSS. The lack of T-cell-derived IL-10 in combination with the lack of IL-4 increased the susceptibility to DSS colitis, as did the lack of IL-12 alone.

CONCLUSIONS

IL-10 is a crucial factor inhibiting the innate proinflammatory immune response induced by DSS. Intestinal bacteria are necessary for the induction of protective IL-10, which is mainly T-cell-derived. T-cell-derived IL-10 can only mediate its protective effect in a Th1-dominated milieu. If the balance is shifted towards a Th2 response, IL-10 is not protective.

Bacterial heat shock protein 60, GroEL, can induce the conversion of naïve T cells into a CD4 CD25(+) Foxp3-expressing phenotype

Recent publications report that heat shock proteins (HSPs) can endow regulatory responses to the systemic immune system when administered via the mucosal route, leading to an amelioration of atherosclerosis and allergy. However, it remains poorly understood if HSP antigens exist in the luminal contents of the gastrointestinal tract and which types of HSP induce regulatory responses. Here we addressed these problems, considering that numerous gut microflora and foods are natural sources of HSPs. SDS-PAGE and immunoblotting with the anti-HSP60 antibody demonstrated the intact and degraded forms of HSP60 mainly in appendix and large intestine of the gastrointestinal tract. No reactivity with this antibody was observed for any of the luminal contents derived from germ-free animals, suggesting gut microflora to be a source of the intestinal HSPs because of lack of HSPs in animal chow diet. GroEL, a typical member of bacterial HSP60, showed a tendency to stimulate splenocytes in germ-free mice, compared to that in conventional mice, suggesting that resident commensal bacterial GroEL may stimulate HSP-reactive T cells as regulatory cells in conventional animals. Importantly, GroEL, but not mouse-derived HSP60, caused naïve T cells to differentiate into CD4(+) CD25(+) Foxp3(+) T cells, indicating that the production of regulatory T cells depends on the type of HSP. Thus, HSPs derived from commensal microbes can be utilized to stimulate immunoregulatory pathways for the maintenance of intestinal homeostasis.

Allergenic and immunogenic potential of cow's milk β-lactoglobulin and caseins evidenced without adjuvant in germ-free mice

SCOPE

In most animal models of allergy, the development of an IgE response requires the use of an adjuvant. Germ-free (GF) mice exhibit Th2-polarized antibody responses combined with defective immunosuppressive mechanisms. The sensitizing potential of milk proteins was investigated in GF mice in the absence of adjuvant.

METHODS AND RESULTS

β-lactoglobulin (BLG) and whole casein (CAS) allergenicity was evaluated by means of intraperitoneal injections without adjuvant. Injections of BLG induced significant IgE and IgG1 responses in GF mice, while CAS injections provoked the production of IgG1 toward κ- and αS1-caseins. No significant antibody response was evidenced in conventional (CV) mice. After in vitro BLG-reactivation, IL-4, IL-5, IL-13 and IFN-γ productions by splenocytes were higher in GF mice than in CV mice. Heat-treatment decreased BLG allergenicity as indicated by the absence of IgE production in GF mice. However, heat-treatment increased protein immunogenicity and led to the production of anti-BLG and anti-κ-casein IgG1 in both GF and CV mice. This correlated with enhanced productions of IL-4, IL-5 and IL-13 in BLG-reactivated splenocytes from CV mice.

CONCLUSION

Gut colonization by commensal bacteria appeared then to significantly reduce the susceptibility of mice toward the intrinsic allergenic and immunogenic potential of milk proteins.

Effects of a germ-free environment on gut immune regulation and diabetes progression in non-obese diabetic (NOD) mice

AIMS/HYPOTHESIS

Microbial factors influence the development of diabetes in NOD mice. Studies in germ-free animals have revealed important roles of microbiota in the regulation of Th17 and forkhead box P3 (FOXP3)(+) T regulatory (Treg) activation in the intestine. However, the effects of intestinal microbiota in immune regulation and diabetes development in NOD mice are still poorly understood.

METHODS

A colony of germ-free NOD mice was established to evaluate the effects of intestinal microbiota on regulatory immunity in the gut, and on the development of insulitis and diabetes in NOD mice.

RESULTS

Diabetes developed in roughly equal numbers in germ-free and specific pathogen-free NOD mice. Insulitis was accentuated in germ-free NOD mice; yet insulin preservation was unaltered. Germ-free NOD mice showed increased levels of Il17 (also known as Il17a) mRNA in the colon, and of Th17 and Th1 cells in the mesenteric and pancreatic lymph nodes, while Foxp3 mRNA and FOXP3(+) Tregs were reduced. In the islet infiltrates, FOXP3(+)CD4(+) T cells were slightly increased in germ-free mice. B cells appeared less activated in the peritoneum and were less abundant in islet infiltrates.

CONCLUSIONS/INTERPRETATION

These results indicate that lack of intestinal microbiota promotes an imbalance between Th1, Th17 and Treg differentiation in the intestine. This imbalance is associated with accelerated insulitis, but intact recruitment of FOXP3(+) Tregs into islets, suggesting: (1) a microbial dependence of local induction of Treg in the gut and draining lymph nodes; but (2) a potentially compensatory function of naturally occurring Tregs in the islets, which may help control diabetogenic T cells.

Role of natural killer and dendritic cell crosstalk in immunomodulation by commensal bacteria probiotics

A cooperative dialogue between natural killer (NK) cells and dendritic cells (DCs) has been elucidated in the last years. They help each other to acquire their complete functions, both in the periphery and in the secondary lymphoid organs. Thus, NK cells' activation by dendritic cells allows the killing of transformed or infected cells in the periphery but may also be important for the generation of adaptive immunity. Indeed, it has been shown that NK cells may play a key role in polarizing a Th1 response upon interaction with DCs exposed to microbial products. This regulatory role of DC/NK cross-talk is of particular importance at mucosal surfaces such as the intestine, where the immune system exists in intimate association with commensal bacteria such as lactic acid bacteria (LAB). We here review NK/DC interactions in the presence of gut-derived commensal bacteria and their role in bacterial strain-dependent immunomodulatory effects. We particularly aim to highlight the ability of distinct species of commensal bacterial probiotics to differently affect the outcome of DC/NK cross-talk and consequently to differently influence the polarization of the adaptive immune response.

Dysregulation of allergic airway inflammation in the absence of microbial colonization

RATIONALE

The incidence of allergic disorders is increasing in developed countries and has been associated with reduced exposure to microbes and alterations in the commensal bacterial flora.

OBJECTIVES

To ascertain the relevance of commensal bacteria on the development of an allergic response, we used a model of allergic airway inflammation in germ-free (GF) mice that lack any exposure to pathogenic or nonpathogenic microorganisms.

METHODS

Allergic airway inflammation was induced in GF, specific pathogen-free (SPF), or recolonized mice by sensitization and challenge with ovalbumin. The resulting cellular infiltrate and cytokine production were measured.

MEASUREMENTS AND MAIN RESULTS

Our results show that the total number of infiltrating lymphocytes and eosinophils were elevated in the airways of allergic GF mice compared with control SPF mice, and that this increase could be reversed by recolonization of GF mice with the complex commensal flora of SPF mice. Exaggerated airway eosinophilia correlated with increased local production of Th2-associated cytokines, elevated IgE production, and an altered number and phenotype of conventional dendritic cells. Regulatory T-cell populations and regulatory cytokine levels were unaltered, but GF mice exhibited an increased number of basophils and decreased numbers of alveolar macrophages and plasmacytoid dendritic cells.

CONCLUSIONS

These data demonstrate that the presence of commensal bacteria is critical for ensuring normal cellular maturation, recruitment, and control of allergic airway inflammation.

Influence of gastrointestinal commensal bacteria on the immune responses that mediate allergy and asthma

The human intestine contains more than 100 trillion microorganisms that maintain a symbiotic relationship with the host. Under normal conditions, these bacteria are not pathogenic and in fact confer health benefits to the host. The microbiota interacts with the innate and adaptive arms of the host's intestinal mucosal immune system and through these mechanisms drives regulatory cell differentiation in the gut that is critically involved in maintaining immune tolerance. Specifically, the microbiota can activate distinct tolerogenic dendritic cells in the gut and through this interaction can drive regulatory T-cell differentiation. In addition, the microbiota is important in driving T(H)1 cell differentiation, which corrects the T(H)2 immune skewing that is thought to occur at birth. If appropriate immune tolerance is not established in early life and maintained throughout life, this represents a risk factor for the development of inflammatory, autoimmune, and allergic diseases. Early-life events are instrumental in establishing the microbiota, the composition of which throughout life is influenced by various environmental and lifestyle pressures. Significant efforts are now being made to establish interventional approaches that can create a healthy microbiota that confers maximum tolerogenic immunomodulatory effects in the gut and that will protect against systemic inflammatory disease pathologies.

Staphylococcal enterotoxin A: a candidate for the amplification of physiological immunoregulatory responses in the gut

Staphylococcal enterotoxin A (SEA) is one of the bacterial products tested for modulation of unwanted immune responses. Of all the staphylococcal enterotoxins, SEA is the most potent stimulator of T cells. When administered orally, SEA acts as a superantigen (SA), producing unspecific stimulation of intra-epithelial lymphocytes (IELs) in the intestinal mucosa. This stimulation results in amplification of the normal local immunologic responses, which are mainly regulatory. This amplification is based on increased local production of IFN-γ by IELs, which acts on the nearby enterocytes. As a result, the enterocytes produce large amounts of tolerosomes, cellular corpuscles which detach themselves from the basal poles of the enterocytes and contain antigenic peptides that are conditioned to be interpreted as tolerogenic by the gut immune system. Tolerosomes are physiologically produced as a response to dietary peptides; it is already known that enterocytes posses the molecular mechanisms for processing peptides in a similar manner to lymphocytes. The fate of tolerosomes is not precisely known, but it seems that they merge with intestinal dendritic cells, conveying to them the information that orally administered peptides must be interpreted as tolerogens. SEA can stimulate this mechanism, thus favoring the development of tolerance to peptides/proteins administered subsequently via the oral route. This characteristic of SEA might be useful in therapy for regulating immune responses. The present paper reviews the current status of research regarding the impact of SEA on the enteric immune system and its potential use in the treatment of allergic and autoimmune diseases.

Environmental influences on T regulatory cells in inflammatory bowel disease

Inflammatory bowel disease (IBD) is characterized by chronic, idiopathic inflammation of the intestine. The disease is thought to result from a combination of genetic and environmental factors which ultimately leads to a mucosal immune system that overreacts to normal constituents of the mucosal microbiota. The inflammation in IBD is primarily mediated by inappropriate production of proinflammatory cytokines by CD4(+) T effector cells, effects that are suppressed by CD4(+) T regulatory cells. Defects in both the function of T regulatory cells, and the ability of T effector cells to be suppressed, have been implicated in IBD. In this review we will discuss environmental factors, including cytokines, vitamins A and D, and commensal bacteria, which influence the phenotype and function of regulatory T cells and thereby alter the course of IBD. We will also discuss how these environmental signals can be manipulated therapeutically in order to improve the function of regulatory T cells and ultimately restore mucosal homeostasis in patients with IBD.

Germ-free status and altered caecal subdominant microbiota are associated with a high susceptibility to cow's milk allergy in mice

Studies suggesting that the development of atopy is linked to gut microbiota composition are inconclusive on whether dysbiosis precedes or arises from allergic symptoms. Using a mouse model of cow's milk allergy, we aimed at investigating the link between the intestinal microbiota, allergic sensitization, and the severity of symptoms. Germ-free and conventional mice were orally sensitized with whey proteins and cholera toxin, and then orally challenged with β-lactoglobulin (BLG). Allergic responses were monitored with clinical symptoms, plasma markers of sensitization, and the T-helper Th1/Th2/regulatory-T-cell balance. Microbiota compositions were analysed using denaturing gradient gel electrophoresis and culture methods. Germ-free mice were found to be more responsive than conventional mice to sensitization, displaying a greater reduction of rectal temperature upon challenge, higher levels of blood mouse mast cell protease-1 (mMCP-1) and BLG-specific immunoglobulin G1 (IgG1), and a systemic Th2-skewed response. This may be explained by a high susceptibility to release mMCP-1 even in the presence of low levels of IgE. Sensitization did not alter the microbiota composition. However, the absence of or low Staphylococcus colonization in the caecum was associated with high allergic manifestations. This work demonstrates that intestinal colonization protects against oral sensitization and allergic response. This is the first study to show a relationship between alterations within the subdominant microbiota and severity of food allergy.

Regulatory T cells in infection

Infectious agents have intimately co-evolved with the host immune system, acquiring a portfolio of highly sophisticated mechanisms to modulate immunity. Among the common strategies developed by viruses, bacteria, protozoa, helminths, and fungi is the manipulation of the regulatory T cell network in order to favor pathogen survival and transmission. Treg activity also benefits the host in many circumstances by controlling immunopathogenic reactions to infection. Interestingly, some pathogens are able to directly induce the conversion of naive T cells into suppressive Foxp3-expressing Tregs, while others activate pre-existing natural Tregs, in both cases repressing pathogen-specific effector responses. However, Tregs can also act to promote immunity in certain settings, such as in initial stages of infection when effector cells must access the site of infection, and subsequently in ensuring generation of effector memory. Notably, there is little current information on whether infections selectively drive pathogen-specific Tregs, and if so whether these cells are also reactive to self-antigens. Further analysis of specificity, together with a clearer picture of the relative dynamics of Treg subsets over the course of disease, should lead to rational strategies for immune intervention to optimize immunity and eliminate infection.

Has the microbiota played a critical role in the evolution of the adaptive immune system?

Although microbes have been classically viewed as pathogens, it is now well established that the majority of host-bacterial interactions are symbiotic. During development and into adulthood, gut bacteria shape the tissues, cells, and molecular profile of our gastrointestinal immune system. This partnership, forged over many millennia of coevolution, is based on a molecular exchange involving bacterial signals that are recognized by host receptors to mediate beneficial outcomes for both microbes and humans. We explore how specific aspects of the adaptive immune system are influenced by intestinal commensal bacteria. Understanding the molecular mechanisms that mediate symbiosis between commensal bacteria and humans may redefine how we view the evolution of adaptive immunity and consequently how we approach the treatment of numerous immunologic disorders.

Neonatal tolerance under breastfeeding influence

Diseases due to defect in tolerance induction such as allergy, celiac disease, or Type 1 Diabetes develop mostly in childhood indicating the necessity of early intervention for primary prevention. Epidemiological studies report that breastfeeding could protect from these diseases. However, data are controversial and the mechanisms unclear. Experimental data suggest that breastfeeding-induced protection might rely on tolerance induction as long as some criteria are fulfilled. Thus, the tolerogenic potential of breast milk would depend on maternal exposure to common environmental and dietary antigens and the efficiency of antigen transfer across mammary epithelium. Induction of tolerance upon breast milk-mediated antigen transfer will also depend on the presence of immunomodulatory factors in breast milk and of its impact on neonatal gut and immune system maturation. The better understanding of maternal influence on tolerance induction through breastfeeding should allow the development of new strategies to prevent immune-mediated diseases.

Regulatory T-cell stability and plasticity in mucosal and systemic immune systems

Regulatory T cells (Treg) express the forkhead box p3 (Foxp3) transcription factor and suppress pathological immune responses against self and foreign antigens, including commensal microorganisms. Foxp3 has been proposed as a master key regulator for Treg, required for their differentiation, maintenance, and suppressive functions. Two types of Treg have been defined. Natural Treg (nTreg) are usually considered to be a separate sublineage arising during thymus differentiation. Induced Treg (iTreg) originate upon T cell receptor (TCR) stimulation in the presence of tumor growth factor beta. Although under homeostatic conditions most Treg in the periphery are nTreg, special immune challenges in the intestine promote more frequently the generation of iTreg. Furthermore, recent observations have challenged the notion that Treg are a stable sublineage, and they suggest that, particularly under lymphopenic and/or inflammatory conditions, Treg may lose Foxp3 and/or acquire diverse effector functions, especially in the intestine, which may contribute to uncontrolled inflammation.

Microbiote et probiotiques : impact en santé humaine

All accessible mucous membranes of the human body are colonized by an abundant and diversified microbial flora called microbiota. Recent studies have shown that these microorganisms, long regarded as purely commensal, have essential beneficial effects on human health. Thus, numerous human ailments are linked to dysbiosis; that is, imbalances in the microflora composition. The administration of probiotic microorganisms could, in some situations, provide substantial relief from such disorders. These live microorganisms, which, according to the definition, confer a health benefit to the host when administered in adequate amounts, are often derived from human flora and belong mostly to lactic acid bacteria, in particular to the genus Lactobacillus . The constant improvement of knowledge of the role of human microbiota and the growing popularity of probiotics are now opening the door to new prophylactic and therapeutic strategies in human health.

Guidance for substantiating the evidence for beneficial effects of probiotics: prevention and management of allergic diseases by probiotics

Allergy is a hypersensitivity reaction mediated by specific antibody-mediated or cell-mediated immunologic mechanisms and clinically manifested as atopic eczema, allergic rhinoconjunctivitis, or asthma. During the recent decades there has been an increase in allergy prevalence, which is attributed to changes in environmental factors. The so-called "hygiene hypothesis" suggests that a lack of exposure to microbial stimulus early in childhood is a major factor involved in this trend. This provides a rationale for using probiotics to modify the gut microbiota and thereby shaping the immune response of the host, especially in infancy. Most success has been obtained in primary prevention of atopic eczema. A limited number of studies also provided evidence for a beneficial effect of different probiotics in the management of allergic diseases (atopic eczema, allergic rhinitis). However, choice of probiotic strains as well as timing of the intervention are important variables. The exact in vivo mechanism of probiotics in shaping the immune response still needs to be determined. Future studies should use uniform criteria for diagnosis and symptom scoring of atopic diseases and may identify the genes predisposing to allergic disease. There is encouraging evidence that specific probiotics can become valuable tools in the prevention and management of allergic diseases.