Mucosal-associated invariant T cells repress group 2 innate lymphoid cells in Alternaria alternata-induced model of allergic airway inflammation

Emerging Concepts in Cytokine Regulation of Airway Remodeling in Asthma

Asthma, a chronic respiratory condition that has seen a dramatic rise in prevalence over the past few decades, now affects more than 300 million people globally and imposes a significant burden on healthcare systems. The key pathological features of asthma include inflammation, airway hyperresponsiveness, mucus cell metaplasia, smooth muscle hypertrophy, and subepithelial fibrosis. Cytokines released by lung epithelial cells, stromal cells, and immune cells during asthma are critical to pathological tissue remodeling in asthma. Over the past few decades, researchers have made great strides in understanding key cells involved in asthma and the cytokines that they produce. Epithelial cells as well as many adaptive and innate immune cells are activated by environmental signals to produce cytokines, namely, type 2 cytokines (IL-4, IL-5, IL-13), IFN-γ, IL-17, TGF-β, and multiple IL-6 family members. However, the precise mechanisms through which these cytokines contribute to airway remodeling remain elusive. Additionally, multiple cell types can produce the same cytokines, making it challenging to decipher how specific cell types and cytokines uniquely contribute to asthma pathogenesis. This review highlights recent advances and provides a comprehensive overview of the key cells involved in the production of cytokines and how these cytokines modulate airway remodeling in asthma.

Dual role of mucosal-associated invariant T cells (MAIT) in asthma

OBJECTIVE

Mucosal-associated invariant T cells (MAIT) are the predominant type of innate-like T cells in humans, and they represent a unique subset of microbiota-dependent invariant T cells. This Commentary reviews recent clinical studies and animal model research elucidating the multifaceted roles of MAIT cells in asthma.

METHOD

A literature search was performed using PubMed and Google Scholar, and covered the period from 1960 to 2024. The search yielded more than 50 articles, and only essential original research articles and selected review articles were evaluated.

RESULTS

Recent studies indicate that MAIT cell-derived effector molecules may play dual roles in asthma and allergic airway inflammation. While MAIT cells can produce the anti-inflammatory enzyme IL4I1 and the Th1 cytokine IFN-γ to repress allergic airway inflammation and airway hyperresponsiveness (AHR), they may also secrete IL-17. Which induces neutrophil infiltration and exacerbates AHR. In addition, some clinical studies from the literature search revealed a negative association between MAIT cell abundance and asthma. Regarding allergic airway inflammation, mouse model studies suggested that MAIT cells may play a protective role.

CONCLUSION

These findings raise critical questions as to whether MAIT cells are friend or foe in asthma, and whether distinct subsets of MAIT cells play different roles in allergic airway inflammation. Further studies are needed to better understand the implication of MAIT cells in asthma pathogenesis, particularly in patients with severe asthma.

The Role of Mucosal-Associated Invariant T Cells in Viral Infections and Their Function in Vaccine Development

Mucosal-Associated Invariant T (MAIT) cells, which bridge innate and adaptive immunity, have emerged as an important player in viral infections despite their inability to directly recognize viral antigens. This review provides a comprehensive analysis of MAIT cell responses across different viral infections, revealing consistent patterns in their behavior and function. We discuss the dynamics of MAIT cells during various viral infections, including changes in their frequency, activation status, and functional characteristics. Particular attention is given to emerging strategies for MAIT-cell-targeted vaccine development, including the use of MR1 ligands as mucosal adjuvants and the activation of MAIT cells through viral vectors and mRNA vaccines. Current knowledge of MAIT cell biology in viral infections provides promising approaches for harnessing their functions in vaccine development.

Mechanisms of microbe-mediated immune development in the context of antibiotics and asthma

The gut houses 70%-80% of the body's immune cells and represents the main point of contact between the immune system and the outside world. Immune maturation occurs largely after birth and is guided by the gut microbiota. In addition to the many human clinical studies that have identified relationships between gut microbiota composition and disease outcomes, experimental research has demonstrated a plethora of mechanisms by which specific microbes and microbial metabolites train the developing immune system. The healthy maturation of the gut microbiota has been well-characterized and discreet stages marked by changes in abundance of specific microbes have been identified. Building on Chapter 8, which discusses experimental models used to study the relationship between the gut microbiota and asthma, the present review aims to dive deeper into the specific microbes and metabolites that drive key processes in immune development. The implications of microbiota maturation patterns in the context of asthma and allergies, as well as the effects of antibiotics on microbe-immune crosstalk, will also be discussed.

Mucosal associated invariant T cells: Powerhouses of the lung

The lungs face constant environmental challenges from harmless molecules, airborne pathogens and harmful agents that can damage the tissue. The lungs' immune system includes numerous tissue-resident lymphocytes that contribute to maintain tissue homeostasis and to the early initiation of immune responses. Amongst tissue-resident lymphocytes, Mucosal Associated Invariant T (MAIT) cells are present in human and murine lungs and emerging evidence supports their contribution to immune responses during infections, chronic inflammatory disorders and cancer. This review explores the mechanisms underpinning MAIT cell functions in the airways, their impact on lung immunity and the potential for targeting pulmonary MAIT cells in a therapeutic context.

Potential of MAIT cells to modulate asthma

Despite recent advances in asthma treatments, the search for novel therapies remains necessary because there are still patients with recurrent asthma exacerbations and poor responses to the existing treatments. Since group 2 innate lymphoid cells (ILC2) play a pivotal role in asthma by triggering and exacerbating type 2 inflammation, controlling ILC2s function is key to combating severe asthma. Mucosal-associated invariant T (MAIT) cells are innate-like T cells abundant in humans and are activated both in a T cell receptor-dependent and -independent manner. MAIT cells are composed of MAIT1 and MAIT17 based on the expression of transcription factors T-bet and RORγt, respectively. MAIT cells play pivotal roles in host defense against pathogens and in tissue repair and are essential for the maintenance of immunity and hemostasis. Our recent studies revealed that MAIT cells inhibit both ILC2 proliferation and functions in a mouse model of airway inflammation. MAIT cells may alleviate airway inflammation in two ways, by promoting airway epithelial cell barrier repair and by repressing ILC2s. Therefore, reagents that promote MAIT cell-mediated suppression of ILC2 proliferation and function, or designer MAIT cells (genetically engineered to suppress ILC2s or promote repair of airway damage), may be effective therapeutic agents for severe asthma.

Current Perspectives and Challenges of MAIT Cell-Directed Therapy for Tuberculosis Infection

Mucosal-associated invariant T (MAIT) cells are a distinct population of non-conventional T cells that have been preserved through evolution and possess properties of both innate and adaptive immune cells. They are activated through the recognition of antigens presented by non-polymorphic MR1 proteins or, alternately, can be stimulated by specific cytokines. These cells are multifaceted and exert robust antimicrobial activity against bacterial and viral infections, direct the immune response through the modulation of other immune cells, and exhibit a specialized tissue homeostasis and repair function. These distinct characteristics have instigated interest in MAIT cell biology for immunotherapy and vaccine development. This review describes the current understanding of MAIT cell activation, their role in infections and diseases with an emphasis on tuberculosis (TB) infection, and perspectives on the future use of MAIT cells in immune-mediated therapy.

A flow-cytometry-based assay to assess the cytolytic activity against tumor cells by combination of mouse MAIT cells and natural killer cells

Mucosal-associated invariant T (MAIT) cells are innate-like T cells responsible for mucosal immunity in the respiratory and intestinal tracts. Here we present a flow-cytometry-based assay to measure the cytolytic activity of murine MAIT cells and natural killer (NK) cells. We describe steps for differentiating MAIT-like cells from the induced pluripotent stem cells prepared from MAIT cells (reMAIT cells), NK cell isolation, co-culture with target tumor cells, and staining to distinguish dead cells from live cells. For complete details on the use and execution of this protocol, please refer to Sugimoto et al. (2022).1.

Mitochondrial Damage-Associated Molecular Patterns and Metabolism in the Regulation of Innate Immunity

The innate immune system, as the host's first line of defense against intruders, plays a critical role in recognizing, identifying, and reacting to a wide range of microbial intruders. There is increasing evidence that mitochondrial stress is a major initiator of innate immune responses. When mitochondria's integrity is disrupted or dysfunction occurs, the mitochondria's contents are released into the cytosol. These contents, like reactive oxygen species, mitochondrial DNA, and double-stranded RNA, among others, act as damage-related molecular patterns (DAMPs) that can bind to multiple innate immune sensors, particularly pattern recognition receptors, thereby leading to inflammation. To avoid the production of DAMPs, in addition to safeguarding organelles integrity and functionality, mitochondria may activate mitophagy or apoptosis. Moreover, mitochondrial components and specific metabolic regulations modify properties of innate immune cells. These include macrophages, dendritic cells, innate lymphoid cells, and so on, in steady state or in stimulation that are involved in processes ranging from the tricarboxylic acid cycle to oxidative phosphorylation and fatty acid metabolism. Here we provide a brief summary of mitochondrial DAMPs' initiated and potentiated inflammatory response in the innate immune system. We also provide insights into how the state of activation, differentiation, and functional polarization of innate immune cells can be influenced by alteration to the metabolic pathways in mitochondria.

Interferons as negative regulators of ILC2s in allergic lung inflammation and respiratory viral infections

Group 2 innate lymphoid cells (ILC2s), characterized by a lack of antigen receptors, have been regarded as an important component of type 2 pulmonary immunity. Analogous to Th2 cells, ILC2s are capable of releasing type 2 cytokines and amphiregulin, thus playing an essential role in a variety of diseases, such as allergic diseases and virus-induced respiratory diseases. Interferons (IFNs), an important family of cytokines with potent antiviral effects, can be triggered by microbial products, microbial exposure, and pathogen infections. Interestingly, the past few years have witnessed encouraging progress in revealing the important role of IFNs and IFN-producing cells in modulating ILC2 responses in allergic lung inflammation and respiratory viral infections. This review underscores recent progress in understanding the role of IFNs and IFN-producing cells in shaping ILC2 responses and discusses disease phenotypes, mechanisms, and therapeutic targets in the context of allergic lung inflammation and infections with viruses, including influenza virus, rhinovirus (RV), respiratory syncytial virus (RSV), and severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2).

Classic costimulatory interactions in MAIT cell responses: from gene expression to immune regulation

Mucosa-associated invariant T (MAIT) cells are evolutionarily conserved, innate-like T lymphocytes with enormous immunomodulatory potentials. Due to their strategic localization, their invariant T cell receptor (iTCR) specificity for major histocompatibility complex-related protein 1 (MR1) ligands of commensal and pathogenic bacterial origin, and their sensitivity to infection-elicited cytokines, MAIT cells are best known for their antimicrobial characteristics. However, they are thought to also play important parts in the contexts of cancer, autoimmunity, vaccine-induced immunity, and tissue repair. While cognate MR1 ligands and cytokine cues govern MAIT cell maturation, polarization, and peripheral activation, other signal transduction pathways, including those mediated by costimulatory interactions, regulate MAIT cell responses. Activated MAIT cells exhibit cytolytic activities and secrete potent inflammatory cytokines of their own, thus transregulating the biological behaviors of several other cell types, including dendritic cells, macrophages, natural killer cells, conventional T cells, and B cells, with significant implications in health and disease. Therefore, an in-depth understanding of how costimulatory pathways control MAIT cell responses may introduce new targets for optimized MR1/MAIT cell-based interventions. Herein, we compare and contrast MAIT cells and mainstream T cells for their expression of classic costimulatory molecules belonging to the immunoglobulin superfamily and the tumor necrosis factor (TNF)/TNF receptor superfamily, based not only on the available literature but also on our transcriptomic analyses. We discuss how these molecules participate in MAIT cells' development and activities. Finally, we introduce several pressing questions vis-à-vis MAIT cell costimulation and offer new directions for future research in this area.

Increased Number of Mucosal-Associated Invariant T Cells Is Associated with the Inhibition of Nonalcoholic Fatty Liver Disease in High Fat Diet-Fed Mice

Nonalcoholic fatty liver disease (NAFLD) is an emerging worldwide health concern. The disease may involve immune cells including T cells, but little is known about the role(s) of the innate-like T cells in the liver. Furthermore, the most abundant innate-like T cells in the human liver are mucosal-associated invariant T (MAIT) cells, but the involvement of MAIT cells in NAFLD remains largely unexplored because of their paucity in mice. In this study, we used a novel mouse line, Vα19, in which the number of MAIT cells is equivalent to or greater than that in humans. Compared with the control mice, Vα19 mice fed a high-fat diet (HFD) exhibited a reduction in lipid accumulation, NAFLD activity score, and transcripts relevant to lipogenesis. In addition, serum triglyceride and non-esterified fatty acids were lower in Vα19 mice fed normal chow or HFD. In contrast, the Vα19 mice showed little or no change in glucose tolerance, insulin sensitivity, inflammation in adipose tissues, or intestinal permeability compared with the controls, irrespective of diet. These results suggest that the presence of MAIT cells is associated with reduced lipogenesis and lipid accumulation in the liver; however, further studies are needed to clarify the role of MAIT cells in hepatic lipid metabolism.