Microbial Components and Effector Molecules in T Helper Cell Differentiation and Function

The mammalian intestines harbor trillions of commensal microorganisms composed of thousands of species that are collectively called gut microbiota. Among the microbiota, bacteria are the predominant microorganism, with viruses, protozoa, and fungi (mycobiota) making up a relatively smaller population. The microbial communities play fundamental roles in the maturation and orchestration of the immune landscape in health and disease. Primarily, the gut microbiota modulates the immune system to maintain homeostasis and plays a crucial role in regulating the pathogenesis and pathophysiology of inflammatory, neuronal, and metabolic disorders. The microbiota modulates the host immune system through direct interactions with immune cells or indirect mechanisms such as producing short-chain acids and diverse metabolites. Numerous researchers have put extensive efforts into investigating the role of microbes in immune regulation, discovering novel immunomodulatory microbial species, identifying key effector molecules, and demonstrating how microbes and their key effector molecules mechanistically impact the host immune system. Consequently, recent studies suggest that several microbial species and their immunomodulatory molecules have therapeutic applicability in preclinical settings of multiple disorders. Nonetheless, it is still unclear why and how a handful of microorganisms and their key molecules affect the host immunity in diverse diseases. This review mainly discusses the role of microbes and their metabolites in T helper cell differentiation, immunomodulatory function, and their modes of action.

Signaling networks controlling ID and E protein activity in T cell differentiation and function

E and inhibitor of DNA binding (ID) proteins are involved in various cellular developmental processes and effector activities in T cells. Recent findings indicate that E and ID proteins are not only responsible for regulating thymic T cell development but also modulate the differentiation, function, and fate of peripheral T cells in multiple immune compartments. Based on the well-established E and ID protein axis (E-ID axis), it has been recognized that ID proteins interfere with the dimerization of E proteins, thus restricting their transcriptional activities. Given this close molecular relationship, the extent of expression or stability of these two protein families can dynamically affect the expression of specific target genes involved in multiple aspects of T cell biology. Therefore, it is essential to understand the endogenous proteins or extrinsic signaling pathways that can influence the dynamics of the E-ID axis in a cell-specific and context-dependent manner. Here, we provide an overview of E and ID proteins and the functional outcomes of the E-ID axis in the activation and function of multiple peripheral T cell subsets, including effector and memory T cell populations. Further, we review the mechanisms by which endogenous proteins and signaling pathways alter the E-ID axis in various T cell subsets influencing T cell function and fate at steady-state and in pathological settings. A comprehensive understanding of the functions of E and ID proteins in T cell biology can be instrumental in T cell-specific targeting of the E-ID axis to develop novel therapeutic modalities in the context of autoimmunity and cancer.

T Helper 2-Associated Immunity in the Pathogenesis of Systemic Lupus Erythematosus

Systemic Lupus Erythematosus (SLE) is a chronic autoimmune disease that mainly affects women in their reproductive years. A complex interaction of environmental and genetic factors leads to the disruption of immune tolerance towards self, causing overt immune activation and production of autoantibodies that attack multiple organs. Kidney damage, termed lupus nephritis, is the leading cause of SLE-related morbidity and mortality. Autoantibodies are central to propagating lupus nephritis through forming immune complexes and triggering complements. Immunoglobulin G (IgG) potently activates complement; therefore, autoantibodies were mainly considered to be of the IgG isotype. However, studies revealed that over 50% of patients produce autoantibodies of the IgE isotype. IgE autoantibodies actively participate in disease pathogenesis as omalizumab treatment, a humanized anti-IgE monoclonal antibody, improved disease severity in an SLE clinical trial. IgE is a hallmark of T helper 2-associated immunity. Thus, T helper 2-associated immunity seems to play a pathogenic role in a subset of SLE patients. This review summarizes human and animal studies that illustrate type 2 immune responses involved during the pathology of SLE.

Of men in mice: the development and application of a humanized gnotobiotic mouse model for microbiome therapeutics

Considerable evidence points to the critical role of the gut microbiota in physiology and disease. The administration of live microbes as a therapeutic modality is increasingly being considered. However, key questions such as how to identify candidate microorganisms and which preclinical models are relevant to recapitulate human microbiota remain largely unanswered. The establishment of a humanized gnotobiotic mouse model through the fecal microbiota transplantation of human feces into germ-free mice provides an innovative and powerful tool to mimic the human microbial system. However, numerous considerations are required in designing such a model, as various elements, ranging from the factors pertaining to human donors to the mouse genetic background, affect how microbes colonize the gut. Thus, it is critical to match the murine context to that of human donors to provide a continuous and faithful progression of human flora in mice. This is of even greater importance when the need for accuracy and reproducibility across global research groups are taken into account. Here, we review the key factors that affect the formulation of a humanized mouse model representative of the human gut flora and propose several approaches as to how researchers can effectively design such models for clinical relevance.

Intestinal microbiota control acute kidney injury severity by immune modulation

Intestinal microbiota impacts the host immune system and influences the outcomes of chronic diseases. However, it remains uncertain whether acute kidney injury (AKI) impacts intestinal microbiota or vice versa. To determine this, we investigated the mechanistic link between AKI, microbiota, and immune response in ischemia/reperfusion injury. Microbiota alteration and its biological consequences after ischemia/reperfusion injury were examined and the effect of dysbiotic microbiota on the outcome of AKI was also assessed by colonizing germ-free mice with post-AKI microbiota. The role of Th17, Th1, Tregs cells and macrophage polarization in mediating the renoprotective effect of antibiotic induced microbiota depletion in ischemia/reperfusion injury was also determined. Increase of Enterobacteriacea, decrease of Lactobacilli, and Ruminococacceae were found to be the hallmarks of ischemia/reperfusion injury induced dysbiosis and were associated with a decreased levels of short-chain fatty acids, intestinal inflammation and leaky gut. Colonizing germ-free mice with post-AKI microbiota worsened ischemia/reperfusion injury severity with exaggerated inflammation in recipient mice compared to colonizing with microbiota from sham operated mice. Microbiota depletion by oral antibiotics protected against ischemia/reperfusion injury. This renoprotective effect was associated with reduced Th 17, Th 1 response along with expansion of regulatory T cells, and M2 macrophages. Our study demonstrated a unique bidirectional relationship between the kidney and the intestine during AKI. Intestinal dysbiosis, inflammation and leaky gut are consequences of AKI but they also represent an important modifier determining post-AKI severity. Thus, targeting the intestinal microbiota might provide a novel therapeutic strategy in AKI.

Lactobacillus pentosus Modulates Immune Response by Inducing IL-10 Producing Tr1 Cells

Several gut commensals have been shown to modulate host immune response. Recently, many food derived microbes have also been reported to affect the immune system. However, a mechanism to identify immunostimulatory and immunoregulatory microbes is needed. Here, we successfully established an in vitro screening system and identified an immunoregulatory bacterium, Lactobacillus pentosus KF340 (LP340), present in various fermented foods. LP340 induced a regulatory phenotype in mice Ag presenting cells which, in turn, induced IL-10 and IFN-γ producing Type 1 regulatory T cells (Tr1 cells) from naïve CD4+ T cells. Naïve CD4+ T cells co-cultured with LP340 treated dendritic cells highly expressed cytokine receptor IL-27R and were CD49b and lymphocyte-activation gene 3 double positive. Oral administration of LP340 in mice with atopic dermatitis reduced cellular infiltration in affected ear lobes and serum IgE levels, thus, ameliorating the disease symptoms. This suggests a systemic immunoregulatory effect of LP340. These findings demonstrate that LP340, a bacterium derived from food, prevents systemic inflammation through the induction of IL-10 producing Tr1 cells.

Cell surface polysaccharides of Bifidobacterium bifidum induce the generation of Foxp3+ regulatory T cells

Dysregulation of intestinal microflora is linked to inflammatory disorders associated with compromised immunosuppressive functions of Foxp3⁺ T regulatory (T_reg) cells. Although mucosa-associated commensal microbiota has been implicated in T_reg generation, molecular identities of the "effector" components controlling this process remain largely unknown. Here, we have defined Bifidobacterium bifidum as a potent inducer of Foxp3⁺ T_reg cells with diverse T cell receptor specificity to dietary antigens, commensal bacteria, and B. bifidum itself. Cell surface β-glucan/galactan (CSGG) polysaccharides of B. bifidum were identified as key components responsible for T_reg induction. CSGG efficiently recapitulated the activity of whole bacteria and acted via regulatory dendritic cells through a partially Toll-like receptor 2-mediated mechanism. T_reg cells induced by B. bifidum or purified CSGG display stable and robust suppressive capacity toward experimental colitis. By identifying CSGG as a functional component of T_reg-inducing bacteria, our studies highlight the immunomodulatory potential of CSGG and CSGG-producing microbes.

The Transcription Factor Ets1 Suppresses T Follicular Helper Type 2 Cell Differentiation to Halt the Onset of Systemic Lupus Erythematosus

Single-nucleotide polymorphisms in ETS1 are associated with systemic lupus erythematosus (SLE). Ets1^-/- mice develop SLE-like symptoms, suggesting that dysregulation of this transcription factor is important to the onset or progression of SLE. We used conditional deletion approaches to examine the impact of Ets1 expression in different immune cell types. Ets1 deletion on CD4⁺ T cells, but not B cells or dendritic cells, resulted in the SLE autoimmunity, and this was associated with the spontaneous expansion of T follicular helper type 2 (Tfh2) cells. Ets1^-/- Tfh2 cells exhibited increased expression of GATA-3 and interleukin-4 (IL-4), which induced IgE isotype switching in B cells. Neutralization of IL-4 reduced Tfh2 cell frequencies and ameliorated disease parameters. Mechanistically, Ets1 suppressed signature Tfh and Th2 cell genes, including Cxcr5, Bcl6, and Il4ra, thus curbing the terminal Tfh2 cell differentiation process. Tfh2 cell frequencies in SLE patients correlated with disease parameters, providing evidence for the relevance of these findings to human disease.

Ets1 suppresses atopic dermatitis by suppressing pathogenic T cell responses

Atopic dermatitis (AD) is a complex inflammatory skin disease mediated by immune cells of both adaptive and innate types. Among them, CD4+ Th cells are one of major players of AD pathogenesis. Although the pathogenic role of Th2 cells has been well characterized, Th17/Th22 cells are also implicated in the pathogenesis of AD. However, the molecular mechanisms underlying pathogenic immune responses in AD remain unclear. We sought to investigate how the defect in the AD susceptibility gene, Ets1, is involved in AD pathogenesis in human and mice and its clinical relevance in disease severity by identifying Ets1 target genes and binding partners. Consistent with the decrease in ETS1 levels in severe AD patients and the experimental AD-like skin inflammation model, T cell-specific Ets1-deficient mice (Ets1ΔdLck) developed severe AD-like symptoms with increased pathogenic Th cell responses. A T cell-intrinsic increase of gp130 expression upon Ets1 deficiency promotes the gp130-mediated IL-6 signaling pathway, thereby leading to the development of severe AD-like symptoms. Functional blocking of gp130 by selective inhibitor SC144 ameliorated the disease pathogenesis by reducing pathogenic Th cell responses. Our results reveal a protective role of Ets1 in restricting pathogenic Th cell responses and suggest a potential therapeutic target for AD treatment.

Upregulation of Ets1 expression by NFATc2 and NFKB1/RELA promotes breast cancer cell invasiveness

Breast cancer is highly aggressive and is the leading cause of cancer-related mortality in women in developed countries. The ETS proto-oncogene 1 (Ets1) has versatile roles during the cellular processes of cancer development. It is often highly expressed in breast cancers and mediates migration and invasion of human breast cancer cells. However, underlying mechanisms of Ets1 gene expression is still ambiguous. Here, we identified a core-regulatory element (CRE) located in the Ets1 promoter region (−540/−80 bp from TSS) that contains elements responsible for associating with NFATs and NF-κBs. Compared with the less metastatic breast cancer cells, metastatic breast cancer cells (MDA-MB-231) show open chromatin configurations in the CRE, which facilitates direct binding of NFATc2 and/or NFKB1/RELA complex to trans-activate Ets1 transcription. Moreover, enhanced level of Nfatc2 and Nfkb1 positively correlated with Ets1 expression in the human breast cancer specimens. Deletion of the CRE region by CRISPR/Cas9 system resulted in significant reduction in Ets1 expression, which led to alterations of Ets1-mediated transcription programs including tumor invasiveness-related genes. Proper regulation of Ets1 gene expression by targeting the NFATc2 and NFKB1/RELA interaction could be a potential therapeutic target for Ets1-mediated metastatic breast cancer.

Locus-Specific Reversible DNA Methylation Regulates Transient IL-10 Expression in Th1 Cells

IL-10 is a pleiotropic cytokine with multifaceted functions in establishing immune homeostasis. Although expressed by Th1 and Th2 cells, conventional Th1 cells produce marginal levels of IL-10 compared with their Th2 counterparts. In this study, we investigated the epigenetic mechanisms of Il-10 gene expression in Th1 cells. Bioinformatics EMBOSS CpG plot analysis and bisulfite pyrosequencing revealed three CpG DNA methylation sites in the Il-10 gene locus. Progressive DNA methylation at all of the CpG regions of interest (ROIs) established a repressive program of Il-10 gene expression in Th1 cells. Interestingly, Th1 cells treated with IL-12 and IL-27 cytokines, thereby mimicking a chronic inflammatory condition in vivo, displayed a significant increase in IL-10 production that was accompanied by selective DNA demethylation at ROI 3 located in intron 3. IL-10-producing T cells isolated from lymphocytic choriomeningitis virus-infected mice also showed enhanced DNA demethylation at ROI 3. Binding of STAT1 and STAT3 to demethylated ROI 3 enhanced IL-10 expression in an IL-12/IL-27-dependent manner. Accordingly, CD4⁺ T cells isolated from STAT1- or STAT3-knockout mice were significantly defective in IL-10 production. Our data suggest that, although stably maintained DNA methylation at the promoter may repress IL-10 expression in Th1 cells, locus-specific reversible DNA demethylation may serve as a threshold platform to control transient Il-10 gene expression.

Probiotics as a Potential Immunomodulating Pharmabiotics in Allergic Diseases: Current Status and Future Prospects

The prevalence of allergic disorders has dramatically increased over the past decade, particularly in developed countries. Apart from gastrointestinal disorders, neoplasia, genital and dermatological diseases etc., dysregulation of gut microbiota (dysbiosis) has also been found to be associated with increased risk of allergies. Probiotics are increasingly being employed to correct dysbiosis and, in turn, to modulate allergic diseases. However, several factors like strain variations and effector metabolites or component of them in a bacterial species can affect the efficacy of those as probiotics. On the other hand, host variations like geographical locations, food habits etc. could also affect the expected results from probiotic usage. Thus, there is a glaring deficiency in our approach to establish probiotics as an irrefutable treatment avenue for suitable disorders. In this review, we explicate on the reported probiotics and their effects on certain allergic diseases like atopic dermatitis, food allergy and asthma to establish their utility. We propose possible measures like elucidation of effector molecules and functional mechanisms of probiotics towards establishing probiotics for therapeutic use. Certain probiotics studies have led to very alarming outcomes which could have been precluded, had effective guidelines been in place. Thus, we also propose ways to secure the safety of probiotics. Overall, our efforts tend to propose necessary discovery and quality assurance guidelines for developing probiotics as potential immunomodulatory ‘Pharmabiotics.’

Clinical Effect of IRT-5 Probiotics on Immune Modulation of Autoimmunity or Alloimmunity in the Eye

Background: Although the relation of the gut microbiota to a development of autoimmune and inflammatory diseases has been investigated in various animal models, there are limited studies that evaluate the effect of probiotics in the autoimmune eye disease. Therefore, we aimed to investigate the effect of IRT-5 probiotics consisting of Lactobacillus casei, Lactobacillus acidophilus, Lactobacillus reuteri, Bifidobacterium bifidum, and Streptococcus thermophilus on the autoimmunity of uveitis and dry eye and alloimmunity of corneal transplantation. Methods: Experimental autoimmune uveitis was induced by subcutaneous immunization with interphotoreceptor-binding protein and intraperitoneal injection of pertussis toxin in C57BL/6 (B6) mice. For an autoimmune dry eye model, 12-weeks-old NOD.B10.H2^b mice were used. Donor cornea of B6 mice was transplanted into BALB/C mice. IRT-5 probiotics or phosphate buffered saline (PBS) were administered for three weeks immediately after induction of uveitis or transplantation. The inflammation score of the retinal tissues, dry eye manifestations (corneal staining and tear secretion), and graft survival were measured in each model. The changes of T cells were evaluated in drainage lymph nodes using fluorescence-activated cell sorting. Results: Retinal histology score in IRT-5 group of uveitis was lower than that in PBS group (p = 0.045). Ocular staining score was lower (p < 0.0001) and tear secretion was higher (p < 0.0001) in the IRT-5 group of NOD.B10.H2^b mice than that in the PBS group. However, the graft survival in the IRT-5 group was not different from those of PBS group. The percentage of regulatory T cells was increased in the IRT-5-treated dry eye models (p = 0.032). The percentage of CD8⁺IL-17^hi (p = 0.027) and CD8⁺ interferon gamma (IFNγ)^hi cells (p = 0.022) were significantly decreased in the IRT-5-treated uveitis models and the percentage of CD8⁺IFNγ^hi cells was markedly reduced (p = 0.036) in IRT-5-treated dry eye model. Conclusion: Our results suggest that administration of IRT-5 probiotics may modulate clinical manifestations of autoimmunity in the eye, but not on alloimmunity of corneal transplantation.

NFAT1 Regulates Systemic Autoimmunity through the Modulation of a Dendritic Cell Property

The transcription factor NFAT1 plays a pivotal role in the homeostasis of T lymphocytes. However, its functional importance in non-CD4⁺ T cells, especially in systemic immune disorders, is largely unknown. In this study, we report that NFAT1 regulates dendritic cell (DC) tolerance and suppresses systemic autoimmunity using the experimental autoimmune myasthenia gravis (EAMG) as a model. Myasthenia gravis and EAMG are T cell-dependent, Ab-mediated autoimmune disorders in which the acetylcholine receptor is the major autoantigen. NFAT1-knockout mice showed higher susceptibility to EAMG development with enhanced Th1/Th17 cell responses. NFAT1 deficiency led to a phenotypic alteration of DCs that show hyperactivation of NF-κB-mediated signaling pathways and enhanced binding of NF-κB (p50) to the promoters of IL-6 and IL-12. As a result, NFAT1-knockout DCs produced much higher levels of proinflammatory cytokines such as IL-1β, IL-6, IL-12, and TNF-α, which preferentially induce Th1/Th17 cell differentiation. Our data suggest that NFAT1 may limit the hyperactivation of the NF-κB-mediated proinflammatory response in DCs and suppress autoimmunity by serving as a key regulator of DC tolerance.

Modulation of gut microbiota and delayed immunosenescence as a result of syringaresinol consumption in middle-aged mice

Age-associated immunological dysfunction (immunosenescence) is closely linked to perturbation of the gut microbiota. Here, we investigated whether syringaresinol (SYR), a polyphenolic lignan, modulates immune aging and the gut microbiota associated with this effect in middle-aged mice. Compared with age-matched control mice, SYR treatment delayed immunosenescence by enhancing the numbers of total CD3⁺ T cells and naïve T cells. SYR treatment induced the expression of Bim as well as activation of FOXO3 in Foxp3⁺ regulatory T cells (Tregs). Furthermore, SYR treatment significantly enhanced the Firmicutes/Bacteroidetes ratio compared with that in age-matched controls by increasing beneficial bacteria, Lactobacillus and Bifidobacterium, while reducing the opportunistic pathogenic genus, Akkermansia. In addition, SYR treatment reduced the serum level of lipopolysaccharide-binding protein, an inflammatory marker, and enhanced humoral immunity against influenza vaccination to the level of young control mice. Taken together, these findings suggest that SYR may rejuvenate the immune system through modulation of gut integrity and microbiota diversity as well as composition in middle-aged mice, which may delay the immunosenescence associated with aging.

Flagellin suppresses experimental asthma by generating regulatory dendritic cells and T cells

BACKGROUND

Although the hygiene hypothesis suggests that microbial infections could subvert asthma and thus a microbial product might serve as a therapeutic adjuvant for asthma, the relationship between bacterial components and asthma is complex. Recently, low levels of flagellin, the Toll-like receptor (TLR) 5 ligand, have been reported to promote asthma.

OBJECTIVE

We show that a therapeutic dose of flagellin suppresses asthma and that the effect occurs through generating regulatory dendritic cells (rDCs) and regulatory T (Treg) cells.

METHODS

Ovalbumin (OVA)-induced wild-type and TLR5 knockout asthmatic mice were treated intranasally with a mixture of OVA and 10 μg of a flagellin B (FlaB; of Vibrio vulnificus). OVA/FlaB-treated rDCs were adoptively transferred to mice with OVA-induced asthma. Anti-CD25 mAb was used to deplete Treg cells. A mixture of house dust mite (HDM) and FlaB was used to treat mice with HDM-induced asthma. Blood CD14(+) monocyte-derived dendritic cells from HDM-sensitive asthmatic patients were treated with FlaB and incubated with autologous CD4(+) T cells.

RESULTS

An OVA/FlaB mixture ameliorated OVA-induced asthma by inhibiting TH1/TH2/TH17 responses in a TLR5-dependent manner through generating rDCs and Treg cells. The adoptive transfer of OVA/FlaB-treated dendritic cells inhibited OVA-induced asthma, whereas the depletion of CD25(+) cells eliminated the inhibitory effect. A similar effect of FlaB was observed in mice with HDM-induced asthma. In patients with HDM-sensitive asthma, FlaB-treated rDCs inhibited HDM-stimulated TH1/TH2 responses while enhancing Treg cells in an IL-10-dependent manner.

CONCLUSION

These findings collectively suggest that flagellin could be used as a tolerogenic adjuvant to treat allergic asthma.

NFAT1 and JunB Cooperatively Regulate IL-31 Gene Expression in CD4+ T Cells in Health and Disease

IL-31 is a key mediator of itching in atopic dermatitis (AD) and is preferentially produced by activated CD4+ T cells and Th2 cells. Although pathophysiological functions of IL-31 have been suggested in diverse immune disorders, the molecular events underlying IL-31 gene regulation are still unclear. In this study we identified the transcription start site and functional promoter involved in IL-31 gene regulation in mouse CD4+ T cells. TCR stimulation–dependent IL-31 expression was found to be closely linked with in vivo binding of NFAT1 and JunB to the IL-31 promoter. Although NFAT1 alone enhanced IL-31 promoter activity, it was further enhanced in the presence of JunB. Conversely, knockdown of either NFAT1 or JunB resulted in reduced IL-31 expression. NFAT1-deficient CD4+ T cells showed a significant defect in IL-31 expression compared with wild-type CD4+ T cells. In agreement with these findings, mice subjected to atopic conditions showed much higher levels of IL-31, which were closely correlated with a significant increase in the number of infiltrated NFAT1+CD4+ T cells into the AD ears. Amelioration of AD progression by cyclosporin A treatment was well correlated with downregulation of IL-31 expressions in CD4+ T cells and total ear residual cells. In summary, our results suggest a functional cooperation between NFAT1 and JunB in mediating IL-31 gene expression in CD4+ T cells and indicate that interference with this interaction or their activity has the potential of reducing IL-31–mediated AD symptoms.

6-Methoxyflavone inhibits NFAT translocation into the nucleus and suppresses T cell activation

NFAT plays a crucial role in the immune system by regulating the transcription of inducible genes during immune responses. In T cells, NFAT proteins govern various cellular events related to T cell development, activation, tolerance induction, and differentiation. We previously reported the NFAT1-dependent enhancer activity of conserved noncoding sequence (CNS)-9, a distal cis-acting element, in the regulation of IL-10 transcription in T cells. In this study, we developed a T cell-based reporter system to identify compounds that modulate the regulatory activity of CNS-9. Among the identified candidates, 6-methoxyflavone (6-MF) significantly inhibited the enhancer activity of CNS-9, thereby reducing IL-10 expression in T cells without affecting cell viability. 6-MF also downregulated the transcription of NFAT1 target genes such as IL-4, IL-13, and IFN-γ. Treatment of 6-MF inhibited the translocation of NFAT1 into the nucleus, which consequently interrupted NFAT1 binding to the target loci, without affecting the expression or dephosphorylation of NFAT1. Treatment of 6-MF to CD4(+) T cells or B cells isolated from mice with atopic dermatitis significantly reduced disease-associated cytokine production, as well as the levels of IgE. In addition, oral administration of 6-MF to atopic dermatitis mice ameliorated disease symptoms by reducing serum IgE levels and infiltrating lymphocytes. Conclusively, our results suggest that 6-MF can be a potential candidate for the development of an effective immunomodulator via the suppression of NFAT-mediated T cell activation.