STAT6 controls the number of regulatory T cells in vivo, thereby regulating allergic lung inflammation

The regulation and differentiation of regulatory T cells and their dysfunction in autoimmune diseases

The discovery of FOXP3+ regulatory T (Treg) cells as a distinct cell lineage with a central role in regulating immune responses provided a deeper understanding of self-tolerance. The transcription factor FOXP3 serves a key role in Treg cell lineage determination and maintenance, but is not sufficient to enable the full potential of Treg cell suppression, indicating that other factors orchestrate the fine-tuning of Treg cell function. Moreover, FOXP3-independent mechanisms have recently been shown to contribute to Treg cell dysfunction. FOXP3 mutations in humans cause lethal fulminant systemic autoinflammation (IPEX syndrome). However, it remains unclear to what degree Treg cell dysfunction is contributing to the pathophysiology of common autoimmune diseases. In this Review, we discuss the origins of Treg cells in the periphery and the multilayered mechanisms by which Treg cells are induced, as well as the FOXP3-dependent and FOXP3-independent cellular programmes that maintain the suppressive function of Treg cells in humans and mice. Further, we examine evidence for Treg cell dysfunction in the context of common autoimmune diseases such as multiple sclerosis, inflammatory bowel disease, systemic lupus erythematosus and rheumatoid arthritis.

Regulatory T Cells, a Viable Target Against Airway Allergic Inflammatory Responses in Asthma

Asthma is a multifactorial disorder characterized by the airway chronic inflammation, hyper-responsiveness (AHR), remodeling, and reversible obstruction. Although asthma is known as a heterogeneous group of diseases with various clinical manifestations, recent studies suggest that more than half of the clinical cases are ‘‘T helper type 2 (Th2)-high’’ type, whose pathogenesis is driven by Th2 responses to an inhaled allergen from the environmental exposures. The intensity and duration of inflammatory responses to inhaled allergens largely depend on the balance between effector and regulatory cells, but many questions regarding the mechanisms by which the relative magnitudes of these opposing forces are remained unanswered. Regulatory T cells (Tregs), which comprise diverse subtypes with suppressive function, have long been attracted extensive attention owing to their capability to limit the development and progression of allergic diseases. In this review we seek to update the recent advances that support an essential role for Tregs in the induction of allergen tolerance and attenuation of asthma progression once allergic airway inflammation established. We also discuss the current concepts about Treg induction and Treg-expressed mediators relevant to controlling asthma, and the therapies designed based on these novel insights against asthma in clinical settings.

Identifying Function Determining Residues in Neuroimmune Semaphorin 4A

Semaphorin 4A (Sema4A) exerts a stabilizing effect on human Treg cells in PBMC and CD4+ T cell cultures by engaging Plexin B1. Sema4A deficient mice display enhanced allergic airway inflammation accompanied by fewer Treg cells, while Sema4D deficient mice displayed reduced inflammation and increased Treg cell numbers even though both Sema4 subfamily members engage Plexin B1. The main objectives of this study were: 1. To compare the in vitro effects of Sema4A and Sema4D proteins on human Treg cells; and 2. To identify function-determining residues in Sema4A critical for binding to Plexin B1 based on Sema4D homology modeling. We report here that Sema4A and Sema4D display opposite effects on human Treg cells in in vitro PBMC cultures; Sema4D inhibited the CD4+CD25+Foxp3+ cell numbers and CD25/Foxp3 expression. Sema4A and Sema4D competitively bind to Plexin B1 in vitro and hence may be doing so in vivo as well. Bayesian Partitioning with Pattern Selection (BPPS) partitioned 4505 Sema domains from diverse organisms into subgroups based on distinguishing sequence patterns that are likely responsible for functional differences. BPPS groups Sema3 and Sema4 into one family and further separates Sema4A and Sema4D into distinct subfamilies. Residues distinctive of the Sema3,4 family and of Sema4A (and by homology of Sema4D) tend to cluster around the Plexin B1 binding site. This suggests that the residues both common to and distinctive of Sema4A and Sema4D may mediate binding to Plexin B1, with subfamily residues mediating functional specificity. We mutated the Sema4A-specific residues M198 and F223 to alanine; notably, F223 in Sema4A corresponds to alanine in Sema4D. Mutant proteins were assayed for Plexin B1-binding and Treg stimulation activities. The F223A mutant was unable to stimulate Treg stability in in vitro PBMC cultures despite binding Plexin B1 with an affinity similar to the WT protein. This research is a first step in generating potent mutant Sema4A molecules with stimulatory function for Treg cells with a view to designing immunotherapeutics for asthma.

The STAT6 inhibitor AS1517499 reduces the risk of asthma in mice with 2,4-dinitrochlorobenzene-induced atopic dermatitis by blocking the STAT6 signaling pathway

BACKGROUND

Epidemiological studies have revealed a link between atopic dermatitis (AD) and asthma. AS1517499, a selective signal transducer and activation of transcription 6 (STAT6) inhibitor, has been shown to effectively block this connection. In this study, we further explored the underlying mechanism by constructing an AD mouse model.

METHODS

Female BALB/c mice were randomly divided into four groups (n = 10/group). The AD mouse model was established by 2,4-dinitrochlorobenzene induction with repeated ovalbumin challenge. AS1517499 and corn oil were used as treatment interventions. The features of airway inflammation, remodeling, and hyperactivity were analyzed.

RESULTS

Active use of AS1517499 in AD mice effectively reduced Th2-related cytokine levels, alleviated airway eosinophil and lymphocyte infiltration, and regulated GATA3/Foxp3 levels and subepithelial collagen deposition. These changes might be due to specific blockade of the STAT6 signaling pathway.

CONCLUSION

AS1517499 could partially block the association between AD and asthma by specifically inhibiting the STAT6 signaling pathway.

Identification of Regulatory Factors and Prognostic Markers in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the progressive degeneration of motor neurons, leading to muscle atrophy, paralysis and even death. Immune disorder, redox imbalance, autophagy disorder, and iron homeostasis disorder have been shown to play critical roles in the pathogenesis of ALS. However, the exact pathogenic genes and the underlying mechanism of ALS remain unclear. The purpose of this study was to screen for pathogenic regulatory genes and prognostic markers in ALS using bioinformatics methods. We used Gene Ontology (GO) analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, gene set enrichment analysis (GSEA), and expression regulation network analysis to investigate the function of differentially expressed genes in the nerve tissue, lymphoid tissue, and whole blood of patients with ALS. Our results showed that the up-regulated genes were mainly involved in immune regulation and inflammation, and the down-regulated genes were mainly involved in energy metabolism and redox processes. Eleven up-regulated transcription factors (CEBPB, CEBPD, STAT5A, STAT6, RUNX1, REL, SMAD3, GABPB2, FOXO1, PAX6, and FOXJ1) and one down-regulated transcription factor (NOG) in the nerve tissue of patients with ALS likely play important regulatory roles in the pathogenesis of ALS. Based on construction and evaluation of the ALS biomarker screening model, cluster analysis of the identified characteristic genes, univariate Cox proportional hazards regression analysis, and the random survival forest algorithm, we found that MAEA, TPST1, IFNGR2, and ALAS2 may be prognostic markers regarding the survival of ALS patients. High expression of MAEA, TPST1, and IFNGR2 and low expression of ALAS2 in ALS patients may be closely related to short survival of ALS patients. Taken together, our results indicate that immune disorders, inflammation, energy metabolism, and redox imbalance may be the important pathogenic factors of ALS. CEBPB, CEBPD, STAT5A, STAT6, RUNX1, REL, SMAD3, GABPB2, FOXO1, PAX6, FOXJ1, and NOG may be important regulatory factors linked to the pathogenesis of ALS. MAEA, TPST1, IFNGR2, and ALAS2 are potential important ALS prognostic markers. Our findings provide evidence on the pathogenesis of ALS, potential targets for the development of new drugs for ALS, and important markers for predicting ALS prognosis.

STAT6 Is Critical for the Induction of Regulatory T Cells In Vivo Controlling the Initial Steps of Colitis-Associated Cancer

Inflammation is the main driver of the tumor initiation and progression in colitis-associated colorectal cancer (CAC). Recent findings have indicated that the signal transducer and activator of transcription 6 (STAT6) plays a fundamental role in the early stages of CAC, and STAT6 knockout (STAT6^−/−) mice are highly resistant to CAC development. Regulatory T (Treg) cells play a major role in coordinating immunomodulation in cancer; however, the role of STAT6 in the induction and function of Treg cells is poorly understood. To clarify the contribution of STAT6 to CAC, STAT6^−/− and wild type (WT) mice were subjected to an AOM/DSS regimen, and the frequency of peripheral and local Treg cells was determined during the progression of CAC. When STAT6 was lacking, a remarkable reduction in tumor growth was observed, which was associated with decreased inflammation and an increased number of CD4+CD25+Foxp3+ cells in the colon, circulation, and spleen, including an over-expression of TGF-beta, IL-10, and Foxp3, compared to WT mice, during the early stages of CAC development. Conversely, WT mice showed an inverse frequency of Treg cells compared with STAT6^−/− mice, which was followed by intestinal tumor formation. Increased mucosal inflammation, histological damage, and tumorigenesis were restored to levels observed in WT mice when an early inhibition/depletion of Treg cells was performed in STAT6^−/− mice. Thus, with STAT6 deficiency, an increased number of Treg cells induce resistance against tumorigenesis, arresting tumor-promoting inflammation. We reported a direct role of STAT6 in the induction and function of Treg cells during CAC development and suggest that STAT6 is a potential target for the modulation of immune response in colitis and CAC.

Selective IL-13 inhibitors for the treatment of atopic dermatitis

Background

Atopic dermatitis (AD) is one of the most common chronic inflammatory skin diseases worldwide. AD pathogenesis is multifactorial, involving environmental and genetic factors. IL-13 stands out as one of the main cytokines in the pathophysiology of AD. Currently, dupilumab, which targets both IL-4 and IL-13 signalling, is the only biologic agent approved for the treatment of moderate-to-severe AD. New targeted biologic therapies are being developed, such as lebrikizumab and tralokinumab, two selective IL-13 inhibitors. This article reviews the role of IL-13 in AD and the most recent data on lebrikizumab and tralokinumab.

Methods

A narrative review of the literature was written after retrieving relevant articles in the PubMed database (up until December 2020) using the following keywords present in the title, abstract or body: atopic dermatitis; interleukin 13; IL-13; tralokinumab; lebrikizumab, biologic therapy.

Discussion

A phase IIb trial showed that all three dosing regimens evaluated (lebrikizumab 125 mg every 4 weeks (Q4W), 250 mg Q4W or 250 mg every 2 weeks) achieved rapid and dose-dependent efficacy concerning the signs and symptoms of AD, with a statistically significant improvement, at week 16. Tralokinumab was studied in three phase III clinical trials and reached its primary endpoints at week 16 (ECZTRA 1 and 2 in monotherapy and ECZTRA 3 with concomitant topical corticosteroids), with response maintained over time. Both lebrikizumab and tralokinumab exhibited good safety profiles in AD trials, with adverse effects usually being comparable between the control and treatment groups.

Conclusion

The evidence supports the hypothesis that selective antagonism of IL-13 is sufficient to control AD, providing an improvement in the patient’s quality of life. Therefore, the development of lebrikizumab and tralokinumab represents a new and exciting phase in the management of AD.

STAT6 Pathway Is Critical for the Induction and Function of Regulatory T Cells Induced by Mucosal B Cells

B cells could convert naïve T cells into regulatory T cells (so-called Treg-of-B cells) which have the ability to treat animal models of inflammatory diseases, including allergic asthma, collagen-induced arthritis and colitis; however, the mechanisms of Treg-of-B cell generation remain unclear. In this study, we investigated the role of STAT6 in the generation of Treg-of-B (P) cells, which Treg cells were generated by Peyer’s patch B cells (P stands for Peyer’s patch). CD4+CD25- T cells from wild type, STAT6 knockout and IL-4 knockout mice were cocultured with wild type Peyer’s patch B cells for Treg-of-B (P) cell generation. A murine asthmatic model was used to analyze the in vivo regulatory function of Treg-of-B (P) cells. The data demonstrated that STAT6 played a critical role in the generation of Treg-of-B (P) cells, which confirmed with STAT6-deficient T cells and the STAT6 inhibitor AS1517499. When STAT6 was lacking, Treg-of-B (P) cells exerted impaired suppressive ability with decreased LAG3 expression. Furthermore, Peyer’s patch B cells played an essential role in regulatory T cell generation. In the absence of Peyer’s patch B cells, T cells expressed decreased phosphorylated STAT6, which was followed by decreased LAG3 expression and impaired suppressive ability, suggesting that Peyer’s patch B cells provided the critical signal to activate STAT6 phosphorylation in T cells. Moreover, STAT6 deficient Treg-of-B (P) cells could not alleviate inflammation in an animal model of asthma in vivo. IL-4 was downstream of phosphorylated STAT6 and maintained Treg-of-B (P) cell survival with increased expression of Bcl-2 and Bcl_XL. We reported a novel finding that the STAT6-LAG3 signaling axis is important for the induction and function of Treg-of-B (P) cells.

Helminth-induced regulation of T-cell transfer colitis requires intact and regulated T cell Stat6 signaling in mice

Infection with parasitic worms (helminths) alters host immune responses and can inhibit pathogenic inflammation. Helminth infection promotes a strong Th2 and T regulatory response while suppressing Th1 and Th17 function. Th2 responses are largely dependent on transcriptional programs directed by Stat6-signaling. We examined the importance of intact T cell Stat6 signaling on helminth-induced suppression of murine colitis that results from T cell transfer into immune-deficient mice. Colonization with the intestinal nematode Heligmosomoides polygyrus bakeri resolves WT T cell transfer colitis. However, if the transferred T cells lack intact Stat6 then helminth exposure failed to attenuate colitis or suppress MLN T cell IFN-γ or IL17 production. Loss of Stat6 signaling resulted in decreased IL10 and increased IFN-γ co-expression by IL-17⁺ T cells. We also transferred T cells from mice with constitutive T cell expression of activated Stat6 (Stat6VT). These mice developed a severe eosinophilic colitis that also was not attenuated by helminth infection. These results show that T cell expression of intact but regulated Stat6 signaling is required for helminth infection-associated regulation of pathogenic intestinal inflammation.

IL-4Rα signaling in CD4+CD25+FoxP3+ T regulatory cells restrains airway inflammation via limiting local tissue IL-33

Impaired tolerance to innocuous particles during allergic asthma has been linked to increased plasticity of FoxP3+ regulatory T cells (Tregs) reprogramming into pathogenic effector cells, thus exacerbating airway disease. However, failure of tolerance mechanisms is driven by Th2 inflammatory signals. Therefore, the in vivo role of canonical IL-4 receptor α (IL-4Rα) signaling, an essential driver of Th2-type airway responses to allergens, on the regulatory function of FoxP3+ Tregs in allergic asthma was explored. Here, we used transgenic Foxp3cre IL-4Rα-/lox and littermate control mice to investigate the role of IL-4 and IL-13 signaling via Tregs in house dust mite-induced (HDM-induced) allergic airway disease. We sensitized mice intratracheally on day 0, challenged them on days 6-10, and analyzed airway hyperresponsiveness (AHR), airway inflammation, mucus production, and cellular profile on day 14. In the absence of IL-4Rα responsiveness on FoxP3+ Tregs, exacerbated AHR and airway inflammation were shown in HDM-sensitized mice. Interestingly, reduced induction of FoxP3+ Tregs accompanied increased IL-33 alarmin production and type 2 innate lymphoid cell activation in the lung, exacerbating airway hyperreactivity and lung eosinophilia. Taken together, our findings indicate that IL-4Rα-unresponsive FoxP3+ Tregs result in exaggerated innate Th2-type, IL-33-dependent airway inflammation and a break in tolerance during allergic asthma.

Therapeutic Potential of Lebrikizumab in the Treatment of Atopic Dermatitis

Background

Atopic dermatitis (AD) is a chronic, relapsing skin condition with a wide disease spectrum. Moderate-to-severe cases often need systemic treatment. Conventional immunosuppressants have extensive side effect profiles and require close monitoring. In recent decades, there has been increasing interest in developing targeted systemic immunomodulators for AD, as they have been shown to have efficacy for AD as well as favorable safety profiles. Herein, we review the recent data on lebrikizumab, an interleukin (IL)-13 inhibitor, and its potential role in the treatment of AD.

Objective

Review the mechanism of action, and available data on the efficacy and safety of lebrikizumab for the treatment of AD.

Methods

PubMed, Google Scholar, and clinicaltrials.gov searches were performed with the following terms: “atopic dermatitis,” “dermatitis,” “eczema,” “lebrikizumab,” “IL-4,” and “IL-13.”

Results

Two Phase II randomized controlled clinical trials have been conducted to evaluate the use of lebrikizumab in a total of 289 patients with moderate-severe AD and inadequate response to topical corticosteroids. Patients treated with lebrikizumab experienced significantly more improvement in their AD compared to placebo, as measured by Eczema Area and Severity Index (EASI)-50 and EASI-75 scores, pruritus scores, and reduction in body surface area (BSA). Its clinical efficacy appears to be dose-dependent, and it has a favorable side effect profile and is generally well tolerated.

Conclusion

Lebrikizumab appears to be a promising emerging targeted biologic for the treatment of moderate-to-severe AD. Further Phase III studies investigating optimal dosing regimens and safety profile are needed.

Interleukin-13: A promising therapeutic target for autoimmune disease

Interleukin-13 (IL-13) was previously thought to be a redundant presence of IL-4, but in recent years its role in immunity, inflammation, fibrosis, and allergic diseases has become increasingly prominent. IL-13 can regulate several subtypes of T helper (Th) cells and affect their transformation, including Th1, Th2, T17, etc., thus it may play an important role in immune system. Previous studies have revealed that IL-13 is implicated in the pathogenesis of autoimmune diseases, such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), systemic sclerosis (SSc), ulcerative colitis (UC), type 1 diabetes (T1D), sjogren's syndrome (SS), etc. In this review, we will briefly discuss the biological features of IL-13 and summarize recent advances in the role of IL-13 in the development and pathogenesis of autoimmune diseases. This information may provide new perspectives and suggestions for the selection of therapeutic targets for autoimmune diseases.

A Computational Method for Classifying Different Human Tissues with Quantitatively Tissue-Specific Expressed Genes

Tissue-specific gene expression has long been recognized as a crucial key for understanding tissue development and function. Efforts have been made in the past decade to identify tissue-specific expression profiles, such as the Human Proteome Atlas and FANTOM5. However, these studies mainly focused on "qualitatively tissue-specific expressed genes" which are highly enriched in one or a group of tissues but paid less attention to "quantitatively tissue-specific expressed genes", which are expressed in all or most tissues but with differential expression levels. In this study, we applied machine learning algorithms to build a computational method for identifying "quantitatively tissue-specific expressed genes" capable of distinguishing 25 human tissues from their expression patterns. Our results uncovered the expression of 432 genes as optimal features for tissue classification, which were obtained with a Matthews Correlation Coefficient (MCC) of more than 0.99 yielded by a support vector machine (SVM). This constructed model was superior to the SVM model using tissue enriched genes and yielded MCC of 0.985 on an independent test dataset, indicating its good generalization ability. These 432 genes were proven to be widely expressed in multiple tissues and a literature review of the top 23 genes found that most of them support their discriminating powers. As a complement to previous studies, our discovery of these quantitatively tissue-specific genes provides insights into the detailed understanding of tissue development and function.

Interleukin-4 Supports the Suppressive Immune Responses Elicited by Regulatory T Cells

Interleukin-4 (IL-4) has been considered as one of the tolerogenic cytokines in many autoimmune animal models and clinical settings. Despite its role in antagonizing pathogenic Th1 responses, little is known about whether IL-4 possesses functions that affect regulatory T cells (Tregs). Tregs are specialized cells responsible for the maintenance of peripheral tolerance through their immune modulatory capabilities. Interestingly, it has been suggested that IL-4 supplement at a high concentration protects responder T cells (Tresps) from Treg-mediated immune suppression. In addition, such supplement also impedes TGF-β-induced Treg differentiation in vitro. However, these phenomena may contradict the tolerogenic role of IL-4, and the effects of IL-4 on Tregs are therefore needed to be further elucidated. In this study, we utilized IL-4 knockout (KO) mice to validate the role of IL-4 on Treg-mediated immune suppression. Although IL-4 KO and control animals harbor similar frequencies of Tregs, Tregs from IL-4 KO mice weakly suppressed autologous Tresp activation. In addition, IL-4 deprivation impaired the ability of Tregs to modulate immune response, whereas IL-4 supplementation reinforced IL-4 KO Tregs in their function in suppressing Tresps. Finally, the presence of IL-4 was associated with increased cell survival and granzyme expression of Tregs. These results suggest the essential role of IL-4 in supporting Treg-mediated immune suppression, which may benefit the development of therapeutic strategies for autoimmune diseases.

Forkhead box protein-3 (Foxp3)-producing dendritic cells suppress allergic response

BACKGROUND

The generation of the tolerogenic dendritic cells (DC) is not fully understood yet. Forkhead box protein-3 (Foxp3) is an important molecule in the immune tolerance. This study tests a hypothesis that DCs express Foxp3, which can be upregulated by Staphylococcal enterotoxin B (SEB).

METHODS

The expression of Foxp3 by DCs was evaluated by real-time RT-PCR, Western blotting, flow cytometry, and chromatin immunoprecipitation assay.

RESULTS

We observed that mice treated with SEB at 0.25-0.5 μg/mouse showed high frequencies of transforming growth factor (TGF)-β-producing CD4⁺ T cells and TGF-β-producing DCs in the intestine, while the IL-4⁺ CD4⁺ T cells and TIM4⁺ DCs were dominated in the intestine in mice treated with SEB at 1-10 μg/mouse. Treating DCs with SEB in the culture induced high levels of Foxp3 at the TGF-β promoter locus. The function of Foxp3 was blocked by STAT6 (signal transducer and activator transcription-6); the latter was induced by exposing DCs to SEB in the culture at doses of 100-400 ng/ml. Treating allergic mice with specific immunotherapy (SIT) together with SEB significantly promoted the therapeutic effects on the allergic responses than treating with SIT alone.

CONCLUSION

Dendritic cells have the capacity to express Foxp3, which can be upregulated by exposure to SEB.

Immune Responses to Tissue-Restricted Nonmajor Histocompatibility Complex Antigens in Allograft Rejection

Chronic diseases that result in end-stage organ damage cause inflammation, which can reveal sequestered self-antigens (SAgs) in that organ and trigger autoimmunity. The thymus gland deletes self-reactive T-cells against ubiquitously expressed SAgs, while regulatory mechanisms in the periphery control immune responses to tissue-restricted SAgs. It is now established that T-cells reactive to SAgs present in certain organs (e.g., lungs, pancreas, and intestine) are incompletely eliminated, and the dysregulation of peripheral immuneregulation can generate immune responses to SAgs. Therefore, chronic diseases can activate self-reactive lymphocytes, inducing tissue-restricted autoimmunity. During organ transplantation, donor lymphocytes are tested against recipient serum (i.e., cross-matching) to detect antibodies (Abs) against donor human leukocyte antigens, which has been shown to reduce Ab-mediated hyperacute rejection. However, primary allograft dysfunction and rejection still occur frequently. Because donor lymphocytes do not express tissue-restricted SAgs, preexisting Abs against SAgs are undetectable during conventional cross-matching. Preexisting and de novo immune responses to tissue-restricted SAgs (i.e., autoimmunity) play a major role in rejection. In this review, we discuss the evidence that supports autoimmunity as a contributor to rejection. Testing for preexisting and de novo immune responses to tissue-restricted SAgs and treatment based on immune responses after organ transplantation may improve short- and long-term outcomes after transplantation.

CD200R/Foxp3-mediated signalling regulates microglial activation

The heterogeneity of microglial functions have either beneficial or detrimental roles in specific physiological or pathological environments. However, the details of what transcriptional mechanisms induce microglia to take beneficial phenotypes remain unknown. Here, we report that Foxp3 is essential for beneficial outcome of the microglial response and depends upon signalling by the immunoglobulin CD200 through its receptor (CD200R). Foxp3 expression was up-regulated in microglia activated by excitotoxicity-induced hippocampal neuroinflammation. Suppression of CD200R prevented anti-inflammatory phenotype of microglia, but over-expression of Foxp3 enhanced it. Phosphorylation of STAT6, a downstream effector of CD200R, modulated transcription of Foxp3. Finally, CD200R/Foxp3-mediated signalling enhanced hippocampal neuronal viability and conferred a degree of neuroprotection, presumably by counteracting inducible nitric oxide synthase. We conclude that enhancement of Foxp3 through CD200R could be neuroprotective by targeting the microglia.

Expansion of brain T cells in homeostatic conditions in lymphopenic Rag2(-/-) mice

The concept of the brain as an immune privileged organ is rapidly evolving in light of new findings outlining the sophisticated relationship between the central nervous and the immune systems. The role of T cells in brain development and function, as well as modulation of behavior has been demonstrated by an increasing number of studies. Moreover, recent studies have redefined the existence of a brain lymphatic system and the presence of T cells in specific brain structures, such as the meninges and choroid plexus. Nevertheless, much information is needed to further the understanding of brain T cells and their relationship with the central nervous system under non-inflammatory conditions. In the present study we employed the Rag2(-/-) mouse model of lymphocyte deficiency and reconstitution by adoptive transfer to study the temporal and anatomical expansion of T cells in the brain under homeostatic conditions. Lymphopenic Rag2(-/-) mice were reconstituted with 10 million lymphoid cells and studied at one, two and four weeks after transfer. Moreover, lymphoid cells and purified CD4(+) and CD8(+) T cells from transgenic GFP expressing mice were used to define the neuroanatomical localization of transferred cells. T cell numbers were very low in the brain of reconstituted mice up to one week after transfer and significantly increased by 2weeks, reaching wild type values at 4weeks after transfer. CD4(+) T cells were the most abundant lymphocyte subtype found in the brain followed by CD8(+) T cells and lastly B cells. Furthermore, proliferation studies showed that CD4(+) T cells expand more rapidly than CD8(+) T cells. Lymphoid cells localize abundantly in meningeal structures, choroid plexus, and circumventricular organs. Lymphocytes were also found in vascular and perivascular spaces and in the brain parenchyma across several regions of the brain, in particular in structures rich in white matter content. These results provide proof of concept that the brain meningeal system, as well as vascular and perivascular spaces, are homing sites of lymphocytes and suggest the possibility of a brain specific T cell subtype.

Strategies targeting the IL-4/IL-13 axes in disease

IL-4 and IL-13 are pleiotropic Th2 cytokines produced by a wide variety of different cell types and responsible for a broad range of biology and functions. Physiologically, Th2 cytokines are known to mediate host defense against parasites but they can also trigger disease if their activities are dysregulated. In this review we discuss the rationale for targeting the IL-4/IL-13 axes in asthma, atopic dermatitis, allergic rhinitis, COPD, cancer, inflammatory bowel disease, autoimmune disease and fibrotic disease as well as evaluating the associated clinical data derived from blocking IL-4, IL-13 or IL-4 and IL-13 together.

Lung Injury Combined with Loss of Regulatory T Cells Leads to De Novo Lung-Restricted Autoimmunity

More than one third of patients with chronic lung disease undergoing lung transplantation have pre-existing Abs against lung-restricted self-Ags, collagen type V (ColV), and k-α1 tubulin (KAT). These Abs can also develop de novo after lung transplantation and mediate allograft rejection. However, the mechanisms leading to lung-restricted autoimmunity remain unknown. Because these self-Ags are normally sequestered, tissue injury is required to expose them to the immune system. We previously showed that respiratory viruses can induce apoptosis in CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs), the key mediators of self-tolerance. Therefore, we hypothesized that lung-tissue injury can lead to lung-restricted immunity if it occurs in a setting when Tregs are impaired. We found that human lung recipients who suffer respiratory viral infections experienced a decrease in peripheral Tregs. Pre-existing lung allograft injury from donor-directed Abs or gastroesophageal reflux led to new ColV and KAT Abs post respiratory viral infection. Similarly, murine parainfluenza (Sendai) respiratory viral infection caused a decrease in Tregs. Intratracheal instillation of anti-MHC class I Abs, but not isotype control, followed by murine Sendai virus infection led to development of Abs against ColV and KAT, but not collagen type II (ColII), a cartilaginous protein. This was associated with expansion of IFN-γ-producing CD4(+) T cells specific to ColV and KAT, but not ColII. Intratracheal anti-MHC class I Abs or hydrochloric acid in Foxp3-DTR mice induced ColV and KAT, but not ColII, immunity, only if Tregs were depleted using diphtheria toxin. We conclude that tissue injury combined with loss of Tregs can lead to lung-tissue-restricted immunity.

Molecular Mechanisms of Airway Hyperresponsiveness in a Murine Model of Steroid-Resistant Airway Inflammation

IL-13 and IL-17A, produced mainly by Th2 and Th17 cells, respectively, have an influential role in asthma pathogenesis. We examined the role of IL-13 and IL-17A in mediating airway hyperresponsiveness (AHR), lung inflammation, and mucus metaplasia in a dual Th2/Th17 model of asthma. IL-13 and/or IL-17A were neutralized using mAbs. Th2/Th17 adoptive transfer induced a mixed asthma phenotype characterized by elevated eosinophilia and neutrophilia, tissue inflammation, mucus metaplasia, and AHR that were partially reversible with steroid treatment. Pulmonary inflammation and quasi-static lung compliance were largely unaffected by neutralization of IL-13 and/or IL-17A. However, neutralization of IL-13 alone or in combination with IL-17A significantly attenuated AHR and mucus metaplasia. Further, STAT6 activation was attenuated following IL-13 and IL-13/IL-17A Ab treatment. We next assessed the role of STAT6 in Th2/Th17-mediated allergic airway disease using STAT6(-/-) mice. STAT6(-/-) mice adoptively transferred with Th2/Th17 cells had decreased AHR compared with controls. These data suggest that IL-13 drives AHR and mucus metaplasia in a STAT6-dependent manner, without directly contributing to airway or tissue inflammation. IL-17A independently contributes to AHR, but it only partially mediates inflammation and mucus metaplasia in a mixed Th2/Th17 model of steroid-resistant asthma.

Regulatory tone and mucosal immunity in asthma

The lung is constantly exposed to a variety of inhaled foreign antigens, many of which are harmless to the body. Therefore, the mucosal immune system must not only have the capacity to distinguish self from non-self, but also harmless versus dangerous non-self. To address this, mucosal immune cells establish an anti-inflammatory steady state in the lung that must be overcome by inflammatory signals in order to mount an effector immune response. In the case of inhaled allergens, the false detection of dangerous non-self results in inappropriate immune activation and eventual allergic asthma. Both basic and clinical studies suggest that the balance between tolerogenic and inflammatory immune responses is a key feature in the outcome of health or disease. This review is focused on what we term 'regulatory tone': the immunosuppressive environment in the lung that must be overcome to induce inflammatory responses. We will summarize the current literature on this topic, with a particular focus on the role of regulatory T cells in preventing allergic disease of the lung. We propose that inter-individual differences in regulatory tone have the potential to not only establish the threshold for immune activation in the lung, but also shape the quality of resulting effector responses following tolerance breakdown.