Immune Metabolism of IL-4-Activated B Cells and Th2 Cells in the Context of Allergic Diseases

Over the last decades, the frequency of allergic disorders has steadily increased. Immunologically, allergies are caused by abnormal immune responses directed against otherwise harmless antigens derived from our environment. Two of the main cell types driving allergic sensitization and inflammation are IgE-producing plasma cells and Th2 cells. The acute activation of T and B cells, their differentiation into effector cells, as well as the formation of immunological memory are paralleled by distinct changes in cellular metabolism. Understanding the functional consequences of these metabolic changes is the focus of a new research field termed "immune metabolism". Currently, the contribution of metabolic changes in T and B cells to either the development or maintenance of allergies is not completely understood. Therefore, this mini review will introduce the fundamentals of energy metabolism, its connection to immune metabolism, and subsequently focus on the metabolic phenotypes of IL-4-activated B cells and Th2 cells.

Immune metabolism in allergies, does it matter?-A review of immune metabolic basics and adaptations associated with the activation of innate immune cells in allergy

Type I allergies are pathological, type 2 inflammatory immune responses against otherwise harmless environmental allergens that arise from complex interactions between different types of immune cells. Activated immune cells undergo extensive changes in phenotype and function to fulfill their effector functions. Hereby, activation, differentiation, proliferation, migration, and mounting of effector responses require metabolic reprogramming. While the metabolic changes associated with activation of dendritic cells, macrophages, and T cells are extensively studied, data about the metabolic phenotypes of the other cell types critically involved in allergic responses (epithelial cells, eosinophils, basophils, mast cells, and ILC2s) are rather limited. This review briefly covers the basics of cellular energy metabolism and its connection to immune cell function. In addition, it summarizes the current state of knowledge in terms of dendritic cell and macrophage metabolism and subsequently focuses on the metabolic changes associated with activation of epithelial cells, eosinophils, basophils, mast cells, as well as ILC2s in allergy. Interestingly, the innate key cell types in allergic inflammation were reported to change their metabolic phenotype during activation, shifting to either glycolysis (epithelial cells, M1 macrophages, DCs, eosinophils, basophils, acutely activated mast cells), oxidative phosphorylation (M2 macrophages, longer term activated mast cells), or fatty acid oxidation (ILC2s). Therefore, immune metabolism is of relevance in allergic diseases and its connection to immune cell effector function needs to be considered to better understand induction and maintenance of allergic responses. Further progress in this field will likely improve both our understanding of disease pathology and enable new treatment targets/strategies.

Mitofusin 2, a key coordinator between mitochondrial dynamics and innate immunity

Remodeling of mitochondrial dynamics and mitochondrial morphology plays a pivotal role in the maintenance of mitochondrial homeostasis in response to pathogenic attacks or stress stimuli. In addition to their role in metabolism and energy production, mitochondria participate in diverse biological functions, including innate immune responses driven by macrophages in response to infections or inflammatory stimuli. Mitofusin-2 (MFN2), a mitochondria-shaping protein regulating mitochondrial fusion and fission, plays a crucial role in linking mitochondrial function and innate immune responses. In this article, we review the role of MFN2 in the regulation of innate immune responses during viral and bacterial infections. We also summarize the current knowledge on the role of MFN2 in coordinating inflammatory, atherogenic, and fibrotic responses. MFN2-mediated crosstalk between mitochondrial dynamics and innate immune responses may determine the outcomes of pathogenic infections.

The Natural History of T Cell Metabolism

The cells of the immune system, particularly the T lymphocytes, have two main features that distinguish them from the cells of other tissues. They proliferate after activation and have the ability to move in tissues and organs. These characteristics compel them to develop metabolic plasticity in order to fulfil their immune function. This review focuses on the different known mechanisms that allow T cells to adapt their metabolism to the real-life circumstances they operate in, whether it is to exit quiescence, to differentiate into effector cells, or to participate in immune memory formation. Some of the metabolic adaptations to environmental variations that T cells are likely to undergo in their immune monitoring function are also discussed.

Metabolic Interdependency of Th2 Cell-Mediated Type 2 Immunity and the Tumor Microenvironment

The function of T cells is critically dependent on their ability to generate metabolic building blocks to fulfil energy demands for proliferation and consecutive differentiation into various T helper (Th) cells. Th cells then have to adapt their metabolism to specific microenvironments within different organs during physiological and pathological immune responses. In this context, Th2 cells mediate immunity to parasites and are involved in the pathogenesis of allergic diseases including asthma, while CD8+ T cells and Th1 cells mediate immunity to viruses and tumors. Importantly, recent studies have investigated the metabolism of Th2 cells in more detail, while others have studied the influence of Th2 cell-mediated type 2 immunity on the tumor microenvironment (TME) and on tumor progression. We here review recent findings on the metabolism of Th2 cells and discuss how Th2 cells contribute to antitumor immunity. Combining the evidence from both types of studies, we provide here for the first time a perspective on how the energy metabolism of Th2 cells and the TME interact. Finally, we elaborate how a more detailed understanding of the unique metabolic interdependency between Th2 cells and the TME could reveal novel avenues for the development of immunotherapies in treating cancer.

Metabolic regulation by PPARγ is required for IL-33-mediated activation of ILC2s in lung and adipose tissue

Type 2 innate lymphoid cells (ILC2s) play a critical role early in the response to infection by helminths and in the development of allergic reactions. ILC2s are also involved in the physiologic regulation of adipose tissue and its metabolic response to cold shock. We find that the metabolic sensor peroxisome proliferator-activated receptor gamma (PPARγ) is highly expressed in ILC2s of the lung and adipose tissue and increases responsiveness to IL-33. In turn, activation of ILC2 by IL-33 leads to increased expression of PPARγ, a prerequisite for proliferation and expression of the effector cytokines IL-5 and IL-13. In contrast, pharmacological inhibition of PPARγ leads to decreased expression of CD36 and fatty acid uptake, a necessary source of energy for ILC2s and of potential ligands for PPARγ. As a consequence, treatment of mice with a PPARγ antagonist reduces the severity of an ILC2-dependent acute airway inflammation. Together, our results demonstrate the critical role of the metabolic sensor PPARγ for the functions of ILC2s.

A Praziquantel Treatment Study of Immune and Transcriptome Profiles in Schistosoma haematobium-Infected Gabonese Schoolchildren

Background

Although Schistosoma haematobium infection has been reported to be associated with alterations in immune function, in particular immune hyporesponsiveness, there have been only few studies that have used the approach of removing infection by drug treatment to establish this and to understand the underlying molecular mechanisms.

Methods

Schistosoma haematobium-infected schoolchildren were studied before and after praziquantel treatment and compared with uninfected controls. Cellular responses were characterized by cytokine production and flow cytometry, and in a subset of children RNA sequencing (RNA-Seq) transcriptome profiling was performed.

Results

Removal of S haematobium infection resulted in increased schistosome-specific cytokine responses that were negatively associated with CD4+CD25+FOXP3+ T-cells and accompanied by increased frequency of effector memory T-cells. Innate responses to Toll like receptor (TLR) ligation decreased with treatment and showed positive association with CD4⁺CD25⁺FOXP3⁺ T-cells. At the transcriptome level, schistosome infection was associated with enrichment in cell adhesion, whereas parasite removal was associated with a more quiescent profile. Further analysis indicated that alteration in cellular energy metabolism was associated with S haematobium infection and that the early growth response genes 2 and 3 (EGR 2 and EGR3), transcription factors that negatively regulate T-cell activation, may play a role in adaptive immune hyporesponsiveness.

Conclusions

Using a longitudinal study design, we found contrasting effects of schistosome infection on innate and adaptive immune responses. Whereas the innate immune system appears more activated, the adaptive immunity is in a hyporesponsive state reflected in alterations in CD4⁺CD25⁺FOXP3⁺ T-cells, cellular metabolism, and transcription factors involved in anergy.

T cell immunoglobulin and mucin domain protein 3 inhibits glycolysis in RAW 264.7 macrophages through Hexokinase 2

T cell immunoglobulin and mucin domain-3 (Tim-3), an immune checkpoint molecule, plays critical roles in maintaining innate immune homeostasis; however, the mechanisms underlying these roles remain to be determined. Here, we determined that Tim-3 controls glycolysis in macrophages and thus contributes to phenotype shifting. Tim-3 signal blockade significantly increases lactate production by macrophages, but does not influence cell proliferation or apoptosis. Tim-3 attenuates glucose uptake by inhibiting hexokinase 2 (HK2) expression in macrophages. Tim-3-mediated inhibition of macrophage glycolysis and the expression of proinflammatory cytokines, tumour necrosis factor (TNF)-α and interleukin (IL)-1β are reversed by HK2 silencing. Finally, we demonstrated that Tim-3 inhibits HK2 expression via the STAT1 pathway. We have thus discovered a new way by which Tim-3 modulates macrophage function.

Mitochondrial dysfunction in lung ageing and disease

Mitochondrial biology has seen a surge in popularity in the past 5 years, with the emergence of numerous new avenues of exciting mitochondria-related research including immunometabolism, mitochondrial transplantation and mitochondria-microbe biology. Since the early 1960s mitochondrial dysfunction has been observed in cells of the lung in individuals and in experimental models of chronic and acute respiratory diseases. However, it is only in the past decade with the emergence of more sophisticated tools and methodologies that we are beginning to understand how this enigmatic organelle regulates cellular homeostasis and contributes to disease processes in the lung. In this review, we highlight the diverse role of mitochondria in individual lung cell populations and what happens when these essential organelles become dysfunctional with ageing and in acute and chronic lung disease. Although much remains to be uncovered, we also discuss potential targeted therapeutics for mitochondrial dysfunction in the ageing and diseased lung.

Evolving concepts in how viruses impact asthma: A Work Group Report of the Microbes in Allergy Committee of the American Academy of Allergy, Asthma & Immunology

Over the past decade, there have been substantial advances in our understanding about how viral infections regulate asthma. Important lessons have been learned from birth cohort studies examining viral infections and subsequent asthma and from understanding the relationships between host genetics and viral infections, the contributions of respiratory viral infections to patterns of immune development, the impact of environmental exposure on the severity of viral infections, and how the viral genome influences host immune responses to viral infections. Further, there has been major progress in our knowledge about how bacteria regulate host immune responses in asthma pathogenesis. In this article, we also examine the dynamics of bacterial colonization of the respiratory tract during viral upper respiratory tract infection, in addition to the relationship of the gut and respiratory microbiomes with respiratory viral infections. Finally, we focus on potential interventions that could decrease virus-induced wheezing and asthma. There are emerging therapeutic options to decrease the severity of wheezing exacerbations caused by respiratory viral infections. Primary prevention is a major goal, and a strategy toward this end is considered.

Lipid-Droplet Formation Drives Pathogenic Group 2 Innate Lymphoid Cells in Airway Inflammation

Innate lymphoid cells (ILCs) play an important role in the control and maintenance of barrier immunity. However, chronic activation of ILCs results in immune-mediated pathology. Here, we show that tissue-resident type 2 ILCs (ILC2s) display a distinct metabolic signature upon chronic activation. In the context of allergen-driven airway inflammation, ILC2s increase their uptake of both external lipids and glucose. Externally acquired fatty acids are transiently stored in lipid droplets and converted into phospholipids to promote the proliferation of ILC2s. This metabolic program is imprinted by interleukin-33 (IL-33) and regulated by the genes Pparg and Dgat1, which are both controlled by glucose availability and mTOR signaling. Restricting dietary glucose by feeding mice a ketogenic diet largely ablated ILC2-mediated airway inflammation by impairing fatty acid metabolism and the formation of lipid droplets. Together, these results reveal that pathogenic ILC2 responses require lipid metabolism and identify ketogenic diet as a potent intervention strategy to treat airway inflammation.

The Influence of Dietary Fatty Acids on Immune Responses

Diet-derived fatty acids (FAs) are essential sources of energy and fundamental structural components of cells. They also play important roles in the modulation of immune responses in health and disease. Saturated and unsaturated FAs influence the effector and regulatory functions of innate and adaptive immune cells by changing membrane composition and fluidity and by acting through specific receptors. Impaired balance of saturated/unsaturated FAs, as well as n-6/n-3 polyunsaturated FAs has significant consequences on immune system homeostasis, contributing to the development of many allergic, autoimmune, and metabolic diseases. In this paper, we discuss up-to-date knowledge and the clinical relevance of the influence of dietary FAs on the biology, homeostasis, and functions of epithelial cells, macrophages, dendritic cells, neutrophils, innate lymphoid cells, T cells and B cells. Additionally, we review the effects of dietary FAs on the pathogenesis of many diseases, including asthma, allergic rhinitis, food allergy, atopic dermatitis, rheumatoid arthritis, multiple sclerosis as well as type 1 and 2 diabetes.

Extrinsic and Intrinsic Immunometabolism Converge: Perspectives on Future Research and Therapeutic Development for Obesity

PURPOSE OF REVIEW

Research over the past decade has shown that immunologic and metabolic pathways are intricately linked. This burgeoning field of immunometabolism includes intrinsic and extrinsic pathways and is known to be associated with obesity-accelerated metabolic disease. Intrinsic immunometabolism includes the study of fuel utilization and bioenergetic pathways that influence immune cell function. Extrinsic immunometabolism includes the study of immune cells and products that influence systemic metabolism.

RECENT FINDINGS

Th2 immunity, macrophage iron handling, adaptive immune memory, and epigenetic regulation of immunity, which all require intrinsic metabolic changes, play a role in systemic metabolism and metabolic function, linking the two arms of immunometabolism. Together, this suggests that targeting intrinsic immunometabolism can directly affect immune function and ultimately systemic metabolism. We highlight important questions for future basic research that will help improve translational research and provide therapeutic targets to help establish new treatments for obesity and associated metabolic disorders.

Shaping the diversity of Th2 cell responses in epithelial tissues and its potential for allergy treatment

Th2 cells have evolved to protect from large helminth infections and to exert tissue protective functions in response to nonmicrobial noxious stimuli. The initiation, maintenance, and execution of these functions depend on the integration of diverse polarizing cues by cellular sensors and molecular programs as well as the collaboration with cells that are coopted for signal exchange. The complexity of input signals and cellular collaboration generates tissue specific Th2 cell heterogeneity and specialization. In this review, we aim to discuss the advances and recent breakthroughs in our understanding of Th2 cell responses and highlight developmental and functional differences among T cells within the diversifying field of type 2 immunity. We will focus on factors provided by the tissue microenvironment and highlight factors with potential implications for the pathogenesis of allergic skin and lung diseases. Especially new insights into the role of immunometabolism, the microbiota and ionic signals enhance the complexity of Th2 cell regulation and warrant a critical evaluation. Finally, we will discuss how this ensemble of established knowledge and recent breakthroughs about Th2 immunobiology advance our understanding of the pathogenesis of allergic diseases and how this could be exploited for future immunotherapies.

The lung environment controls alveolar macrophage metabolism and responsiveness in type 2 inflammation

Fine control of macrophage activation is needed to prevent inflammatory disease, particularly at barrier sites such as the lungs. However, the dominant mechanisms that regulate the activation of pulmonary macrophages during inflammation are poorly understood. We found that alveolar macrophages (AlvMs) were much less able to respond to the canonical type 2 cytokine IL-4, which underpins allergic disease and parasitic worm infections, than macrophages from lung tissue or the peritoneal cavity. We found that the hyporesponsiveness of AlvMs to IL-4 depended upon the lung environment but was independent of the host microbiota or the lung extracellular matrix components surfactant protein D (SP-D) and mucin 5b (Muc5b). AlvMs showed severely dysregulated metabolism relative to that of cavity macrophages. After removal from the lungs, AlvMs regained responsiveness to IL-4 in a glycolysis-dependent manner. Thus, impaired glycolysis in the pulmonary niche regulates AlvM responsiveness during type 2 inflammation.

Dendritic cells are what they eat: how their metabolism shapes T helper cell polarization

Dendritic cells (DCs) are professional antigen-presenting cells that play a crucial role in the priming and differentiation of CD4⁺ T cells into several distinct subsets including effector T helper (Th) 1, Th17 and Th2 cells, as well as regulatory T cells (Tregs). It is becoming increasingly clear that cellular metabolism shapes the functional properties of DCs. Specifically, the ability of DCs to drive polarization of different Th cell subsets may be orchestrated by the engagement of distinct metabolic pathways. In this review, we will discuss the recent advances in the DC metabolism field, by focusing on how cellular metabolism of DCs shapes their priming and polarization of distinct Th cell responses.

Early-life undernutrition reprograms CD4+ T-cell glycolysis and epigenetics to facilitate asthma

BACKGROUND

Exposure to early-life undernutrition is closely related to higher risks of adverse immunologic outcomes in adulthood. Although it has been suggested that asthma has its origins in early life, its underlying mechanisms remain largely unknown.

OBJECTIVE

We characterized the effects of early-life undernutrition on T lymphocytes, which play a pivotal role in immune diseases, and we investigated whether this contributes to susceptibility to asthma in adulthood.

METHODS

Pregnant mice were fed a protein restriction diet (PRD) to establish an early-life undernutrition model. Naive CD4⁺ T cells (CD4⁺CD62L^hiCD44^-) from offspring were used throughout the study. T_H2 differentiation was examined by using fluorescence-activated cell sorting and ELISA under T_H2-polarized conditions in vitro and through ovalbumin-induced experimental asthma in vivo. T-cell metabolism was measured with a Seahorse XF96 Analyzer. DNA methylation levels were measured by using bisulfite sequencing.

RESULTS

PRD CD4⁺ T cells displayed increased activation and proliferation and were prone to differentiate into T_H2 cells both in vitro and in vivo, leading to susceptibility to experimental asthma. Mechanistically, early-life undernutrition upregulated mechanistic target of rapamycin 1-dependent glycolysis and induced conserved noncoding DNA sequence 1 DNA hypomethylation in the T_H2 cytokine locus of CD4⁺ T cells. Glycolysis blockades undermined increased T_H2 skewing and alleviated experimental asthma in PRD mice.

CONCLUSION

Early-life undernutrition induced mechanistic target of rapamycin 1-dependent glycolysis upregulation and T_H2 cytokine locus hypomethylation in CD4⁺ T cells, resulting in increased T-cell activation, proliferation, and T_H2 skewing and further susceptibility to experimental asthma.

Modulating T Cell Responses via Autophagy: The Intrinsic Influence Controlling the Function of Both Antigen-Presenting Cells and T Cells

Autophagy is a homeostatic and inducible process affecting multiple aspects of the immune system. This intrinsic cellular process is involved in MHC-antigen (Ag) presentation, inflammatory signaling, cytokine regulation, and cellular metabolism. In the context of T cell responses, autophagy has an influential hand in dictating responses to self and non-self by controlling extrinsic factors (e.g., MHC-Ag, cytokine production) in antigen-presenting cells (APC) and intrinsic factors (e.g., cell signaling, survival, cytokine production, and metabolism) in T cells. These attributes make autophagy an attractive therapeutic target to modulate T cell responses. In this review, we examine the impact autophagy has on T cell responses by modulating multiple aspects of APC function; the importance of autophagy in the activation, differentiation and homeostasis of T cells; and discuss how the modulation of autophagy could influence T cell responses.