Poor Repair of Skeletal Muscle in Aging Mice Reflects a Defect in Local, Interleukin-33-Dependent Accumulation of Regulatory T Cells

Fibroblast and myofibroblast activation in normal tissue repair and fibrosis

The term 'fibroblast' often serves as a catch-all for a diverse array of mesenchymal cells, including perivascular cells, stromal progenitor cells and bona fide fibroblasts. Although phenotypically similar, these subpopulations are functionally distinct, maintaining tissue integrity and serving as local progenitor reservoirs. In response to tissue injury, these cells undergo a dynamic fibroblast-myofibroblast transition, marked by extracellular matrix secretion and contraction of actomyosin-based stress fibres. Importantly, whereas transient activation into myofibroblasts aids in tissue repair, persistent activation triggers pathological fibrosis. In this Review, we discuss the roles of mechanical cues, such as tissue stiffness and strain, alongside cell signalling pathways and extracellular matrix ligands in modulating myofibroblast activation and survival. We also highlight the role of epigenetic modifications and myofibroblast memory in physiological and pathological processes. Finally, we discuss potential strategies for therapeutically interfering with these factors and the associated signal transduction pathways to improve the outcome of dysregulated healing.

Immunomodulatory extracellular matrix hydrogel induces tissue regeneration in a model of partial glossectomy

While oropharyngeal cancer treatment regimens, including surgical resection, irradiation, and chemotherapy, are effective at removing tumors, they lead to muscle atrophy, denervation, and fibrosis, contributing to the pathogenesis of oropharyngeal dysphagia - difficulty swallowing. Current standard of care of rehabilitative tongue strengthening and swallowing exercises is ineffective. Here, we evaluate an alternative approach utilizing an acellular and injectable biomaterial to preserve muscle content and reduce fibrosis of the tongue after injury. Skeletal muscle extracellular matrix (SKM) hydrogel is fabricated from decellularized porcine skeletal muscle tissue. A partial glossectomy injury in the rat is used to induce tongue fibrosis, and SKM hydrogels along with saline controls are injected into the site of scarring two weeks after injury. Tissues are harvested at 3 and 7 days post-injection for gene expression and immunohistochemical analyses, and at 4 weeks post-injection to evaluate histomorphological properties. SKM hydrogel reduces scar formation and improves muscle regeneration at the site of injury compared to saline. SKM additionally modulates the immune response towards an anti-inflammatory phenotype. This study demonstrates the immunomodulatory and tissue-regenerative capacity of an acellular and minimally invasive ECM hydrogel in a rodent model of tongue injury.

IL-33 Facilitates Fibro-Adipogenic Progenitors to Establish the Pro-Regenerative Niche after Muscle Injury

During the process of muscle regeneration post-injury in adults, muscle stem cells (MuSCs) function is facilitated by neighboring cells within the pro-regenerative niche. However, the precise mechanism triggering the initiation of signaling in the pro-regenerative niche remains unknown. Using single-cell RNA sequencing, 14 different muscle cells are comprehensively mapped during the initial stage following injury. Among these, macrophages and fibro-adipogenic progenitor cells (FAPs) exhibit the most pronounced intercellular communication with other cells. In the FAP subclusters, the study identifies an activated FAP phenotype that secretes chemokines, such as CXCL1, CXCL5, CCL2, and CCL7, to recruit macrophages after injury. Il1rl1, encoding the protein of the interleukin-33 (IL-33) receptor, is identified as a highly expressed signature surface marker of the FAP phenotype. Following muscle injury, autocrine IL-33, an alarmin, has been observed to activate quiescent FAPs toward this inflammatory phenotype through the IL1RL1-MAPK/NF-κB signaling pathway. Il1rl1 deficiency results in decreased chemokine expression and recruitment of macrophages, accompanied by impaired muscle regeneration. These findings elucidate a novel mechanism involving the IL-33/IL1RL1 signaling pathway in promoting the activation of FAPs and facilitating muscle regeneration, which can aid the development of therapeutic strategies for muscle-related disorders and injuries.

Cross-talk between ILC2 and Gata3high Tregs locally constrains adaptive type 2 immunity

Regulatory T cells (Tregs) control adaptive immunity and restrain type 2 inflammation in allergic disease. Interleukin-33 promotes the expansion of tissue-resident Tregs and group 2 innate lymphoid cells (ILC2s); however, how Tregs locally coordinate their function within the inflammatory niche is not understood. Here, we show that ILC2s are critical orchestrators of Treg function. Using spatial, cellular, and molecular profiling of the type 2 inflamed niche, we found that ILC2s and Tregs engage in a direct (OX40L-OX40) and chemotaxis-dependent (CCL1-CCR8) cellular dialogue that enforces the local accumulation of Gata3high Tregs, which are transcriptionally and functionally adapted to the type 2 environment. Genetic interruption of ILC2-Treg communication resulted in uncontrolled type 2 lung inflammation after allergen exposure. Mechanistically, we found that Gata3high Tregs can modulate the local bioavailability of the costimulatory molecule OX40L, which subsequently controlled effector memory T helper 2 cell numbers. Hence, ILC2-Treg interactions represent a critical feedback mechanism to control adaptive type 2 immunity.

Biomaterial-Based Regenerative Strategies for Volumetric Muscle Loss: Challenges and Solutions

Significance: Volumetric muscle loss (VML) is caused by the loss of significant amounts of skeletal muscle tissue. VML cannot be repaired by intrinsic regenerative processes, resulting in permanent loss of muscle function and disability. Current rehabilitative-focused treatment strategies lack efficacy and do not restore muscle function, indicating the need for the development of effective regenerative strategies. Recent Advances: Recent developments implicate biomaterial-based approaches for promoting muscle repair and functional restoration post-VML. Specifically, bioscaffolds transplanted in the injury site have been utilized to mimic endogenous cues of the ablated tissue to promote myogenic pathways, increase neo-myofiber synthesis, and ultimately restore contractile function to the injured unit. Critical Issues: Despite the development and preclinical testing of various biomaterial-based regenerative strategies, effective therapies for patients are not available. The unique challenges posed for biomaterial-based treatments of VML injuries, including its scalability and clinical applicability beyond small-animal models, impede progress. Furthermore, production of tissue-engineered constructs is technically demanding, with reproducibility issues at scale and complexities in achieving vascularization and innervation of large constructs. Future Directions: Biomaterial-based regenerative strategies designed to comprehensively address the pathophysiology of VML are needed. Considerations for clinical translation, including scalability and regulatory compliance, should also be considered when developing such strategies. In addition, an integrated approach that combines regenerative and rehabilitative strategies is essential for ensuring functional improvement.

The tissue-resident regulatory T cell pool is shaped by transient multi-tissue migration and a conserved residency program

The tissues are the site of many important immunological reactions, yet how the immune system is controlled at these sites remains opaque. Recent studies have identified Foxp3+ regulatory T (Treg) cells in non-lymphoid tissues with unique characteristics compared with lymphoid Treg cells. However, tissue Treg cells have not been considered holistically across tissues. Here, we performed a systematic analysis of the Treg cell population residing in non-lymphoid organs throughout the body, revealing shared phenotypes, transient residency, and common molecular dependencies. Tissue Treg cells from different non-lymphoid organs shared T cell receptor (TCR) sequences, with functional capacity to drive multi-tissue Treg cell entry and were tissue-agnostic on tissue homing. Together, these results demonstrate that the tissue-resident Treg cell pool in most non-lymphoid organs, other than the gut, is largely constituted by broadly self-reactive Treg cells, characterized by transient multi-tissue migration. This work suggests common regulatory mechanisms may allow pan-tissue Treg cells to safeguard homeostasis across the body.

Characterizing the skeletal muscle immune microenvironment for sarcopenia: insights from transcriptome analysis and histological validation

Background

Sarcopenia is a condition characterized by the age-related loss of skeletal muscle mass and function. The pathogenesis of the disease is influenced by chronic low-grade inflammation. However, the specific changes in the immune landscape changes of sarcopenic muscle are not yet fully understood.

Methods

To gain insights into the immune cell composition and interactions, we combined single-nucleus RNA sequencing data, bulk RNA sequencing dataset, and comprehensive bioinformatic analyses on the skeletal muscle samples from young, aged, and sarcopenic individuals. Histological staining was then performed on skeletal muscles to validate the distribution of immune cells in clinical samples.

Results

We analyzed the transcriptomes of 101,862 single nuclei, revealing a total of 10 major cell types and 6 subclusters of immune cell types within the human skeletal muscle tissues. Notable variations were identified in the immune microenvironment between young and aged skeletal muscle. Among the immune cells from skeletal muscle microenvironment, macrophages constituted the largest fraction. A specific marker gene LYVE1 for skeletal muscle resident macrophages was further identified. Cellular subclasses included four distinct groups of resident macrophages, which play different roles in physiological or non-physiological conditions. Utilizing bulk RNA sequencing data, we observed a significant enrichment of macrophage-rich inflammation in sarcopenia.

Conclusions

Our findings demonstrate age-related changes in the composition and cross-talk of immune cells in human skeletal muscle microenvironment, which contribute to chronic inflammation in aged or sarcopenia muscle. Furthermore, macrophages emerge as a potential therapeutic target, thus advancing our understanding of the pathogenesis of sarcopenia.

Immunoengineering Biomaterials for Musculoskeletal Tissue Repair across Lifespan

Musculoskeletal diseases and injuries are among the leading causes of pain and morbidity worldwide. Broad efforts have focused on developing pro-regenerative biomaterials to treat musculoskeletal conditions; however, these approaches have yet to make a significant clinical impact. Recent studies have demonstrated that the immune system is central in orchestrating tissue repair and that targeting pro-regenerative immune responses can improve biomaterial therapeutic outcomes. However, aging is a critical factor negatively affecting musculoskeletal tissue repair and immune function. Hence, understanding how age affects the response to biomaterials is essential for improving musculoskeletal biomaterial therapies. This review focuses on the intersection of the immune system and aging in response to biomaterials for musculoskeletal tissue repair. The article introduces the general impacts of aging on tissue physiology, the immune system, and the response to biomaterials. Then, it explains how the adaptive immune system guides the response to injury and biomaterial implants in cartilage, muscle, and bone and discusses how aging impacts these processes in each tissue type. The review concludes by highlighting future directions for the development and translation of personalized immunomodulatory biomaterials for musculoskeletal tissue repair.

Immunological regulation of skeletal muscle adaptation to exercise

Exercise has long been acknowledged for its powerful disease-preventing, health-promoting effects. However, the cellular and molecular mechanisms responsible for the beneficial effects of exercise are not fully understood. Inflammation is a component of the stress response to exercise. Recent work has revealed that such inflammation is not merely a symptom of exertion; rather, it is a key regulator of exercise adaptations, particularly in skeletal muscle. The purpose of this piece is to provide a conceptual framework that we hope will integrate exercise immunology with exercise physiology, muscle biology, and cellular immunology. We start with an overview of early studies in the field of exercise immunology, followed by an exploration of the importance of stromal cells and immunocytes in the maintenance of muscle homeostasis based on studies of experimental muscle injury. Subsequently, we discuss recent advances in our understanding of the functions and physiological relevance of the immune system in exercised muscle. Finally, we highlight a potential immunological basis for the benefits of exercise in musculoskeletal diseases and aging.

Treg-tissue cell interactions in repair and regeneration

Regulatory T (Treg) cells are classically known for their critical immunosuppressive functions that support peripheral tolerance. More recent work has demonstrated that Treg cells produce pro-repair mediators independent of their immunosuppressive function, a process that is critical to repair and regeneration in response to numerous tissue insults. These factors act on resident parenchymal and structural cells to initiate repair in a tissue-specific context. This review examines interactions between Treg cells and tissue-resident non-immune cells-in the context of tissue repair, fibrosis, and cancer-and discusses areas for future exploration.

Inflammatory pathway communication with skeletal muscle-Does aging play a role? A topical review of the current evidence

Skeletal muscle plays an integral role in locomotion, but also as part of the integrative physiological system. Recent progress has identified crosstalk between skeletal muscle and various physiological systems, including the immune system. Both the musculoskeletal and immune systems are impacted by aging. Increased age is associated with decreased muscle mass and function, while the immune system undergoes "inflammaging" and immunosenescence. Exercise is identified as a preventative medicine that can mitigate loss of function for both systems. This review summarizes: (1) the inflammatory pathways active in skeletal muscle; and (2) the inflammatory and skeletal muscle response to unaccustomed exercise in younger and older adults. Compared to younger adults, it appears older individuals have a muted pro-inflammatory response and elevated anti-inflammatory response to exercise. This important difference could contribute to decreased regeneration and recovery following unaccustomed exercise in older adults, as well as in chronic disease. The current research provides specific information on the role inflammation plays in altering skeletal muscle form and function, and adaptation to exercise; however, the pursuit of more knowledge in this area will delineate specific interventions that may enhance skeletal muscle recovery and promote resiliency in this tissue particularly with aging.

Human skeletal muscle aging atlas

Skeletal muscle aging is a key contributor to age-related frailty and sarcopenia with substantial implications for global health. Here we profiled 90,902 single cells and 92,259 single nuclei from 17 donors to map the aging process in the adult human intercostal muscle, identifying cellular changes in each muscle compartment. We found that distinct subsets of muscle stem cells exhibit decreased ribosome biogenesis genes and increased CCL2 expression, causing different aging phenotypes. Our atlas also highlights an expansion of nuclei associated with the neuromuscular junction, which may reflect re-innervation, and outlines how the loss of fast-twitch myofibers is mitigated through regeneration and upregulation of fast-type markers in slow-twitch myofibers with age. Furthermore, we document the function of aging muscle microenvironment in immune cell attraction. Overall, we present a comprehensive human skeletal muscle aging resource ( https://www.muscleageingcellatlas.org/ ) together with an in-house mouse muscle atlas to study common features of muscle aging across species.

Increased EZH2 function in regulatory T cells promotes their capacity to suppress autoimmunity by driving effector differentiation prior to activation

The immunosuppressive function of regulatory T (Treg) cells is essential for maintaining immune homeostasis. Enhancer of zeste homolog 2 (EZH2), a histone H3 lysine 27 (H3K27) methyltransferase, plays a key role in maintaining Treg cell function upon CD28 co-stimulation, and Ezh2 deletion in Treg cells causes autoimmunity. Here we assessed whether increased EZH2 activity in Treg cells would improve Treg cell function. Using an Ezh2 gain-of-function mutation, Ezh2 Y641F , we found that Treg cells expressing Ezh2 Y641F displayed an increased effector Treg phenotype and were poised for improved homing to organ tissues. Expression of Ezh2 Y641F in Treg cells led to more rapid remission from autoimmunity. H3K27me3 profiling and transcriptomic analysis revealed a redistribution of H3K27me3, which prompted a gene expression profile in naïve Ezh2 Y641F Treg cells that recapitulated aspects of CD28-activated Ezh2 WT Treg cells. Altogether, increased EZH2 activity promotes the differentiation of effector Treg cells that can better suppress autoimmunity.

Highlights

EZH2 function promotes effector differentiation of Treg cells.EZH2 function promotes Treg cell migration to organ tissues.EZH2 function in Treg cells improves remission from autoimmunity.EZH2 function poises naïve Treg cells to adopt a CD28-activated phenotype.

MST1/2 regulates fibro/adipogenic progenitor fate decisions in skeletal muscle regeneration

Defective skeletal muscle regeneration is often accompanied by fibrosis. Fibroblast/adipose progenitors (FAPs) are important in these processes, however, the regulation of FAP fate decisions is unclear. Here, using inducible conditional knockout mice, we show that blocking mammalian Ste20-like kinases 1/2 (MST1/2) of FAPs prevented apoptosis and reduced interleukin-6 secretion in vivo and in vitro, which impaired myoblast proliferation and differentiation, as well as impaired muscle regeneration. Deletion of Mst1/2 increased co-localization of Yes-associated protein (YAP) with Smad2/3 in nuclei and promoted differentiation of FAPs toward myofibroblasts, resulting in excessive collagen deposition and skeletal muscle fibrosis. Meanwhile, inhibition of MST1/2 increased YAP/Transcriptional co-activator with PDZ-binding motif activation, which promoted activation of the WNT/β-catenin pathway and impaired the differentiation of FAPs toward adipocytes. These results reveal a new mechanism for MST1/2 action in disrupted skeletal muscle regeneration and fibrosis via regulation of FAP apoptosis and differentiation. MST1/2 is a potential therapeutic target for the treatment of some myopathies.

Effects of the immune system on muscle regeneration

Muscle regeneration is a complex process orchestrated by multiple steps. Recent findings indicate that inflammatory responses could play central roles in bridging initial muscle injury responses and timely muscle injury reparation. The various types of immune cells and cytokines have crucial roles in muscle regeneration process. In this review, we provide an overview of the functions of acute inflammation in muscle regeneration.

Regulatory T cells limit age-associated retinal inflammation and neurodegeneration

BACKGROUND

Ageing is the principal risk factor for retinal degenerative diseases, which are the commonest cause of blindness in the developed countries. These conditions include age-related macular degeneration or diabetic retinopathy. Regulatory T cells play a vital role in immunoregulation of the nervous system by limiting inflammation and tissue damage in health and disease. Because the retina was long-considered an immunoprivileged site, the precise contribution of regulatory T cells in retinal homeostasis and in age-related retinal diseases remains unknown.

METHODS

Regulatory T cells were selectively depleted in both young (2-4 months) and aged (18-23 months) FoxP3-DTR mice. We evaluated neuroretinal degeneration, gliosis, subretinal space phagocyte infiltration, and retinal pigmented epithelium morphology through immunofluorescence analysis. Subsequently, aged Treg depleted animals underwent adoptive transfer of both young and aged regulatory T cells from wild-type mice, and the resulting impact on neurodegeneration was assessed. Statistical analyses employed included the U-Mann Whitney test, and for comparisons involving more than two groups, 1-way ANOVA analysis followed by Bonferroni's post hoc test.

RESULTS

Our study shows that regulatory T cell elimination leads to retinal pigment epithelium cell dysmorphology and accumulation of phagocytes in the subretinal space of young and aged mice. However, only aged mice experience retinal neurodegeneration and gliosis. Surprisingly, adoptive transfer of young but not aged regulatory T cells reverse these changes.

CONCLUSION

Our findings demonstrate an essential role for regulatory T cells in maintaining age retinal homeostasis and preventing age-related neurodegeneration. This previously undescribed role of regulatory T cells in limiting retinal inflammation, RPE/choroid epithelium damage and subsequently photoreceptor loss with age, opens novel avenues to explore regulatory T cell neuroprotective and anti-inflammatory properties as potential therapeutic approaches for age-related retinal diseases.

Cellular and Molecular Mechanisms of Heterotopic Ossification in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is a debilitating genetic disorder characterized by recurrent episodes of heterotopic ossification (HO) formation in muscles, tendons, and ligaments. FOP is caused by a missense mutation in the ACVR1 gene (activin A receptor type I), an important signaling receptor involved in endochondral ossification. The ACVR1R206H mutation induces increased downstream canonical SMAD-signaling and drives tissue-resident progenitor cells with osteogenic potential to participate in endochondral HO formation. In this article, we review aberrant ACVR1R206H signaling and the cells that give rise to HO in FOP. FOP mouse models and lineage tracing analyses have been used to provide strong evidence for tissue-resident mesenchymal cells as cellular contributors to HO. We assess how the underlying mutation in FOP disrupts muscle-specific dynamics during homeostasis and repair, with a focus on muscle-resident mesenchymal cells known as fibro-adipogenic progenitors (FAPs). Accumulating research points to FAPs as a prominent HO progenitor population, with ACVR1R206H FAPs not only aberrantly differentiating into chondro-osteogenic lineages but creating a permissive environment for bone formation at the expense of muscle regeneration. We will further discuss the emerging role of ACVR1R206H FAPs in muscle regeneration and therapeutic targeting of these cells to reduce HO formation in FOP.

IL-33-ST2 signaling in fibro-adipogenic progenitors alleviates immobilization-induced muscle atrophy in mice

BACKGROUND

The regenerative and adaptive capacity of skeletal muscles reduces with age, leading to severe disability and frailty in the elderly. Therefore, development of effective therapeutic interventions for muscle wasting is important both medically and socioeconomically. In the present study, we aimed to elucidate the potential contribution of fibro-adipogenic progenitors (FAPs), which are mesenchymal stem cells in skeletal muscles, to immobilization-induced muscle atrophy.

METHODS

Young (2-3 months), adult (12-14 months), and aged (20-22 months) mice were used for analysis. Muscle atrophy was induced by immobilizing the hind limbs with a steel wire. FAPs were isolated from the hind limbs on days 0, 3, and 14 after immobilization for transcriptome analysis. The expression of ST2 and IL-33 in FAPs was evaluated by flow cytometry and immunostaining, respectively. To examine the role of IL-33-ST2 signaling in vivo, we intraperitoneally administered recombinant IL-33 or soluble ST2 (sST2) twice a week throughout the 2-week immobilization period. After 2-week immobilization, the tibialis anterior muscles were harvested and the cross-sectional area of muscle fibers was evaluated.

RESULTS

The number of FAPs increased with the progression of muscle atrophy after immobilization in all age-groups. Transcriptome analysis of FAPs collected before and after immobilization revealed that Il33 and Il1rl1 transcripts, which encode the IL-33 receptor ST2, were transiently induced in young mice and, to a lesser extent, in aged mice. The number of FAPs positive for ST2 increased after immobilization in young mice. The number of ST2-positive FAPs also increased after immobilization in aged mice, but the difference from the baseline was not statistically significant. Immunostaining for IL-33 in the muscle sections revealed a significant increase in the number of FAPs expressing IL-33 after immobilization. Administration of recombinant IL-33 suppressed immobilization-induced muscle atrophy in aged mice but not in young mice.

CONCLUSIONS

Our data reveal a previously unknown protective role of IL-33-ST2 signaling against immobilization-induced muscle atrophy in FAPs and suggest that IL-33-ST2 signaling is a potential new therapeutic target for alleviating disuse muscle atrophy, particularly in older adults.

Restoring Mitochondrial Function and Muscle Satellite Cell Signaling: Remedies against Age-Related Sarcopenia

Sarcopenia has a complex pathophysiology that encompasses metabolic dysregulation and muscle ultrastructural changes. Among the drivers of intracellular and ultrastructural changes of muscle fibers in sarcopenia, mitochondria and their quality control pathways play relevant roles. Mononucleated muscle stem cells/satellite cells (MSCs) have been attributed a critical role in muscle repair after an injury. The involvement of mitochondria in supporting MSC-directed muscle repair is unclear. There is evidence that a reduction in mitochondrial biogenesis blunts muscle repair, thus indicating that the delivery of functional mitochondria to injured muscles can be harnessed to limit muscle fibrosis and enhance restoration of muscle function. Injection of autologous respiration-competent mitochondria from uninjured sites to damaged tissue has been shown to reduce infarct size and enhance cell survival in preclinical models of ischemia-reperfusion. Furthermore, the incorporation of donor mitochondria into MSCs enhances lung and cardiac tissue repair. This strategy has also been tested for regeneration purposes in traumatic muscle injuries. Indeed, the systemic delivery of mitochondria promotes muscle regeneration and restores muscle mass and function while reducing fibrosis during recovery after an injury. In this review, we discuss the contribution of altered MSC function to sarcopenia and illustrate the prospect of harnessing mitochondrial delivery and restoration of MSCs as a therapeutic strategy against age-related sarcopenia.

Muscle stem cell niche dynamics during muscle homeostasis and regeneration

The process of skeletal muscle regeneration involves a coordinated interplay of specific cellular and molecular interactions within the injury site. This review provides an overview of the cellular and molecular components in regenerating skeletal muscle, focusing on how these cells or molecules in the niche regulate muscle stem cell functions. Dysfunctions of muscle stem cell-to-niche cell communications during aging and disease will also be discussed. A better understanding of how niche cells coordinate with muscle stem cells for muscle repair will greatly aid the development of therapeutic strategies for treating muscle-related disorders.

Origins and functions of eosinophils in two non-mucosal tissues

Eosinophils are a type of granulocyte named after the presence of their eosin-stained granules. Traditionally, eosinophils have been best known to play prominent roles in anti-parasitic responses and mediating allergic reactions. Knowledge of their behaviour has expanded with time, and they are now recognized to play integral parts in the homeostasis of gastrointestinal, respiratory, skeletal muscle, adipose, and connective tissue systems. As such, they are implicated in a myriad of pathologies, and have been the target of several medical therapies. This review focuses on the lifespan of eosinophils, from their origins in the bone marrow, to their tissue-resident role. In particular, we wish to highlight the functions of eosinophils in non-mucosal tissues with skeletal muscle and the adipose tissues as examples, and to discuss the current understanding of their participation in diseased states in these tissues.

Ageing impairs the regenerative capacity of regulatory T cells in mouse central nervous system remyelination

Myelin regeneration (remyelination) is essential to prevent neurodegeneration in demyelinating diseases such as Multiple Sclerosis, however, its efficiency declines with age. Regulatory T cells (Treg) recently emerged as critical players in tissue regeneration, including remyelination. However, the effect of ageing on Treg-mediated regenerative processes is poorly understood. Here, we show that expansion of aged Treg does not rescue age-associated remyelination impairment due to an intrinsically diminished capacity of aged Treg to promote oligodendrocyte differentiation and myelination in male and female mice. This decline in regenerative Treg functions can be rescued by a young environment. We identified Melanoma Cell Adhesion Molecule 1 (MCAM1) and Integrin alpha 2 (ITGA2) as candidates of Treg-mediated oligodendrocyte differentiation that decrease with age. Our findings demonstrate that ageing limits the neuroregenerative capacity of Treg, likely limiting their remyelinating therapeutic potential in aged patients, and describe two mechanisms implicated in Treg-driven remyelination that may be targetable to overcome this limitation.

Intersection of Immunity, Metabolism, and Muscle Regeneration in an Autoimmune-Prone MRL Mouse Model

Due to the limited capacity of mammals to regenerate complex tissues, researchers have worked to understand the mechanisms of tissue regeneration in organisms that maintain that capacity. One example is the MRL/MpJ mouse strain with unique regenerative capacity in ear pinnae that is absent from other strains, such as the common C57BL/6 strain. The MRL/MpJ mouse has also been associated with an autoimmune phenotype even in the absence of the mutant Fas gene described in its parent strain MRL/lpr. Due to these findings, the differences between the responses of MRL/MpJ versus C57BL/6 strain are evaluated in volumetric muscle injury and subsequent material implantation. One salient feature of the MRL/MpJ response to injury is robust adipogenesis within the muscle. This is associated with a decrease in M2-like polarization in response to biologically derived extracellular matrix scaffolds. In pro-fibrotic materials, such as polyethylene, there are fewer foreign body giant cells in the MRL/MpJ mice. As there are reports of both positive and negative influences of adipose tissue and adipogenesis on wound healing, this model can provide an important lens to investigate the interplay between stem cells, adipose tissue, and immune responses in trauma and material implantation.

Sporadic Inclusion Body Myositis at the Crossroads between Muscle Degeneration, Inflammation, and Aging

Sporadic inclusion body myositis (sIBM) is the most common muscle disease of older people and is clinically characterized by slowly progressive asymmetrical muscle weakness, predominantly affecting the quadriceps, deep finger flexors, and foot extensors. At present, there are no enduring treatments for this relentless disease that eventually leads to severe disability and wheelchair dependency. Although sIBM is considered a rare muscle disorder, its prevalence is certainly higher as the disease is often undiagnosed or misdiagnosed. The histopathological phenotype of sIBM muscle biopsy includes muscle fiber degeneration and endomysial lymphocytic infiltrates that mainly consist of cytotoxic CD8+ T cells surrounding nonnecrotic muscle fibers expressing MHCI. Muscle fiber degeneration is characterized by vacuolization and the accumulation of congophilic misfolded multi-protein aggregates, mainly in their non-vacuolated cytoplasm. Many players have been identified in sIBM pathogenesis, including environmental factors, autoimmunity, abnormalities of protein transcription and processing, the accumulation of several toxic proteins, the impairment of autophagy and the ubiquitin-proteasome system, oxidative and nitrative stress, endoplasmic reticulum stress, myonuclear degeneration, and mitochondrial dysfunction. Aging has also been proposed as a contributor to the disease. However, the interplay between these processes and the primary event that leads to the coexistence of autoimmune and degenerative changes is still under debate. Here, we outline our current understanding of disease pathogenesis, focusing on degenerative mechanisms, and discuss the possible involvement of aging.

Muscle stem cells as immunomodulator during regeneration

The skeletal muscle is well known for its remarkable ability to regenerate after injuries. The regeneration is a complex and dynamic process that involves muscle stem cells (also called muscle satellite cells, MuSCs), fibro-adipogenic progenitors (FAPs), immune cells, and other muscle-resident cell populations. The MuSCs are the myogenic cell populaiton that contribute nuclei directly to the regenerated myofibers, while the other cell types collaboratively establish a microenvironment that facilitates myogenesis of MuSCs. The myogenic process includes activation, proliferation and differentiationof MuSCs, and subsequent fusion their descendent mononuclear myocytes into multinuclear myotubes. While the contributions of FAPs and immune cells to this microenvironment have been well studied, the influence of MuSCs on other cell types remains poorly understood. This review explores recent evidence supporting the potential role of MuSCs as immunomodulators during muscle regeneration, either through cytokine production or ligand-receptor interactions.

Niche-specific control of tissue function by regulatory T cells-Current challenges and perspectives for targeting metabolic disease

Tissue regulatory T cells (Tregs) exert pivotal functions in both immune and metabolic regulation, maintaining local tissue homeostasis, integrity, and function. Accordingly, Tregs play a crucial role in controlling obesity-induced inflammation and supporting efficient muscle function and repair. Depending on the tissue context, Tregs are characterized by unique transcriptomes, growth, and survival factors and T cell receptor (TCR) repertoires. This functional specialization offers the potential to selectively target context-specific Treg populations, tailoring therapeutic strategies to specific niches, thereby minimizing potential side effects. Here, we discuss challenges and perspectives for niche-specific Treg targeting, which holds promise for highly efficient and precise medical interventions to combat metabolic disease.

A type 1 immunity-restricted promoter of the IL-33 receptor gene directs antiviral T-cell responses

The pleiotropic alarmin interleukin-33 (IL-33) drives type 1, type 2 and regulatory T-cell responses via its receptor ST2. Subset-specific differences in ST2 expression intensity and dynamics suggest that transcriptional regulation is key in orchestrating the context-dependent activity of IL-33-ST2 signaling in T-cell immunity. Here, we identify a previously unrecognized alternative promoter in mice and humans that is located far upstream of the curated ST2-coding gene and drives ST2 expression in type 1 immunity. Mice lacking this promoter exhibit a selective loss of ST2 expression in type 1- but not type 2-biased T cells, resulting in impaired expansion of cytotoxic T cells (CTLs) and T-helper 1 cells upon viral infection. T-cell-intrinsic IL-33 signaling via type 1 promoter-driven ST2 is critical to generate a clonally diverse population of antiviral short-lived effector CTLs. Thus, lineage-specific alternative promoter usage directs alarmin responsiveness in T-cell subsets and offers opportunities for immune cell-specific targeting of the IL-33-ST2 axis in infections and inflammatory diseases.

MuSCs and IPCs: roles in skeletal muscle homeostasis, aging and injury

Skeletal muscle is a highly specialized tissue composed of myofibres that performs crucial functions in movement and metabolism. In response to external stimuli and injuries, a range of stem/progenitor cells, with muscle stem cells or satellite cells (MuSCs) being the predominant cell type, are rapidly activated to repair and regenerate skeletal muscle within weeks. Under normal conditions, MuSCs remain in a quiescent state, but become proliferative and differentiate into new myofibres in response to injury. In addition to MuSCs, some interstitial progenitor cells (IPCs) such as fibro-adipogenic progenitors (FAPs), pericytes, interstitial stem cells expressing PW1 and negative for Pax7 (PICs), muscle side population cells (SPCs), CD133-positive cells and Twist2-positive cells have been identified as playing direct or indirect roles in regenerating muscle tissue. Here, we highlight the heterogeneity, molecular markers, and functional properties of these interstitial progenitor cells, and explore the role of muscle stem/progenitor cells in skeletal muscle homeostasis, aging, and muscle-related diseases. This review provides critical insights for future stem cell therapies aimed at treating muscle-related diseases.

Matrix-bound nanovesicle-associated IL-33 supports functional recovery after skeletal muscle injury by initiating a pro-regenerative macrophage phenotypic transition

Injuries to skeletal muscle are among the most common injuries in civilian and military populations, accounting for nearly 60% of extremity injuries. The standard of care for severe extremity injury has been focused upon limb salvage procedures and the utilization of tissue grafts or orthotics in conjunction with rehabilitation to avoid amputation. Nonetheless, many patients have persistent strength and functional deficits that permanently impact their quality of life. Preclinical and clinical studies have shown that partial restoration of functional skeletal muscle tissue following injury can be achieved by the implantation of a biologic scaffold composed of extracellular matrix (ECM). These favorable outcomes are mediated, at least in part, through local immunomodulation. The mechanisms underlying this immunomodulatory effect, however, are poorly understood. The present study investigates a potential mechanistic driver of the immunomodulatory effects; specifically, the effect of selected ECM components upon inflammation resolution and repair. Results show that the host response to skeletal muscle injury is profoundly altered and functional recovery decreased in il33-/- mice compared to age- and sex-matched wildtype counterparts by 14 days post-injury. Results also show that IL-33, contained within matrix-bound nanovesicles (MBV), supports skeletal muscle regeneration by regulating local macrophage activation toward a pro-remodeling phenotype via canonical and non-canonical pathways to improve functional recovery from injury compared to untreated il33-/- counterparts. Taken together, these data suggest that MBV and their associated IL-33 cargo represent a novel homeostatic signaling mechanism that contributes to skeletal muscle repair.

Beyond FOXP3: a 20-year journey unravelling human regulatory T-cell heterogeneity

The initial idea of a distinct group of T-cells responsible for suppressing immune responses was first postulated half a century ago. However, it is only in the last three decades that we have identified what we now term regulatory T-cells (Tregs), and subsequently elucidated and crystallized our understanding of them. Human Tregs have emerged as essential to immune tolerance and the prevention of autoimmune diseases and are typically contemporaneously characterized by their CD3+CD4+CD25high CD127lowFOXP3+ phenotype. It is important to note that FOXP3+ Tregs exhibit substantial diversity in their origin, phenotypic characteristics, and function. Identifying reliable markers is crucial to the accurate identification, quantification, and assessment of Tregs in health and disease, as well as the enrichment and expansion of viable cells for adoptive cell therapy. In our comprehensive review, we address the contributions of various markers identified in the last two decades since the master transcriptional factor FOXP3 was identified in establishing and enriching purity, lineage stability, tissue homing and suppressive proficiency in CD4+ Tregs. Additionally, our review delves into recent breakthroughs in innovative Treg-based therapies, underscoring the significance of distinct markers in their therapeutic utilization. Understanding Treg subsets holds the key to effectively harnessing human Tregs for immunotherapeutic approaches.

Immunomodulatory leptin receptor+ sympathetic perineurial barrier cells protect against obesity by facilitating brown adipose tissue thermogenesis

Adipose tissues (ATs) are innervated by sympathetic nerves, which drive reduction of fat mass via lipolysis and thermogenesis. Here, we report a population of immunomodulatory leptin receptor-positive (LepR+) sympathetic perineurial barrier cells (SPCs) present in mice and humans, which uniquely co-express Lepr and interleukin-33 (Il33) and ensheath AT sympathetic axon bundles. Brown ATs (BATs) of mice lacking IL-33 in SPCs (SPCΔIl33) had fewer regulatory T (Treg) cells and eosinophils, resulting in increased BAT inflammation. SPCΔIl33 mice were more susceptible to diet-induced obesity, independently of food intake. Furthermore, SPCΔIl33 mice had impaired adaptive thermogenesis and were unresponsive to leptin-induced rescue of metabolic adaptation. We therefore identify LepR+ SPCs as a source of IL-33, which orchestrate an anti-inflammatory BAT environment, preserving sympathetic-mediated thermogenesis and body weight homeostasis. LepR+IL-33+ SPCs provide a cellular link between leptin and immune regulation of body weight, unifying neuroendocrinology and immunometabolism as previously disconnected fields of obesity research.

Regulatory T cells in dominant immunologic tolerance

Regulatory T cells expressing the transcription factor forkhead box protein 3 mediate peripheral immune tolerance both to self-antigens and to the commensal flora. Their defective function due to inborn errors of immunity or acquired insults is associated with a broad range of autoimmune and immune dysregulatory diseases. Although their function in suppressing autoimmunity and enforcing commensalism is established, a broader role for regulatory T cells in tissue repair and metabolic regulation has emerged, enabled by unique programs of tissue adaptability and specialization. In this review, we focus on the myriad roles played by regulatory T cells in immunologic tolerance and host homeostasis and the potential to harness these cells in novel therapeutic approaches to human diseases.

Dysfunction of the aging female mouse urethra is associated with striated muscle loss and increased fibrosis: an initial report

The decline of urethral function with advancing age plays a major role in urinary incontinence in women, impairing quality of life and economically burdening the health care system. However, none of the current urinary incontinence treatments address the declining urethral function with aging, and the mechanisms by which aging impacts urethra physiology remain little known or explored. Here, we have compared functional, morphometric, and global gene expression of urethral tissues between young and old female mice. Bladder leak point pressure (LPP) measurement showed that the aged female mice had 26.55% lower LPP compared to younger mice. Vectorized Scale-Invariant Pattern Recognition (VIPR) analysis of the relative abundance of different tissue components revealed that the mid-urethra of old female mice contains less striated muscle, more extracellular matrix/fibrosis, and diminished elastin fibers ratio compared to young mice. Gene expression profiling analysis (bulk RNA-seq of the whole urethra) showed more down-regulated genes in aged than young mice. Immune response and muscle-related (striated and smooth) pathways were predominantly enriched. In contrast, keratinization, skin development, and cell differentiation pathways were significantly downregulated in aged urethral tissues compared to those from young female mice. These results suggest that molecular pathways (i.e., ACVR1/FST signaling and CTGF/TGF-β signaling) leading to a decreased striated muscle mass and an increase in fibrous extracellular matrix in the process of aging deserve further investigation for their roles in the declined urethral function.

Emerging role of immune cells as drivers of pulmonary fibrosis

The pathogenesis of pulmonary fibrosis, including idiopathic pulmonary fibrosis (IPF) and other forms of interstitial lung disease, involves a complex interplay of various factors including host genetics, environmental pollutants, infection, aberrant repair and dysregulated immune responses. Highly variable clinical outcomes of some ILDs, in particular IPF, have made it difficult to identify the precise mechanisms involved in disease pathogenesis and thus the development of a specific cure or treatment to halt and reverse the decline in patient health. With the advent of in-depth molecular diagnostics, it is becoming evident that the pathogenesis of IPF is unlikely to be the same for all patients and therefore will likely require different treatment approaches. Chronic inflammation is a cardinal feature of IPF and is driven by both innate and adaptive immune responses. Inflammatory cells and activated fibroblasts secrete various pro-inflammatory cytokines and chemokines that perpetuate the inflammatory response and contribute to the recruitment and activation of more immune cells and fibroblasts. The balance between pro-inflammatory and regulatory immune cell subsets, as well as the interactions between immune cell types and resident cells within the lung microenvironment, ultimately determines the extent of fibrosis and the potential for resolution. This review examines the role of the innate and adaptive immune responses in pulmonary fibrosis, with an emphasis on IPF. The role of different immune cell types is discussed as well as novel anti-inflammatory and immunotherapy approaches currently in clinical trial or in preclinical development.

Regulatory T cells shield muscle mitochondria from interferon-γ-mediated damage to promote the beneficial effects of exercise

Exercise enhances physical performance and reduces the risk of many disorders such as cardiovascular disease, type 2 diabetes, dementia, and cancer. Exercise characteristically incites an inflammatory response, notably in skeletal muscles. Although some effector mechanisms have been identified, regulatory elements activated in response to exercise remain obscure. Here, we have addressed the roles of Foxp3+CD4+ regulatory T cells (Tregs) in the healthful activities of exercise via immunologic, transcriptomic, histologic, metabolic, and biochemical analyses of acute and chronic exercise models in mice. Exercise rapidly induced expansion of the muscle Treg compartment, thereby guarding against overexuberant production of interferon-γ and consequent metabolic disruptions, particularly mitochondrial aberrancies. The performance-enhancing effects of exercise training were dampened in the absence of Tregs. Thus, exercise is a natural Treg booster with therapeutic potential in disease and aging contexts.

[What if the origin of FAPs was contributing to their heterogeneity in muscle?]

Fibro-adipogenic progenitors (FAPs) are resident mesenchymal stromal cells (MSCs) of skeletal muscle. They play a crucial role in muscle homeostasis and regeneration through their paracrine activity. Recent technological advances in single-cell RNA sequencing have allowed the characterization of the heterogeneity within this cell population. In this article, we will present the different subpopulations of FAPs under basal, injury, or degenerative conditions, as well as their associated functions in mice and humans. We will then discuss the potential extramuscular origin of a post-injury FAP population. Indeed, our recent work demonstrates that MSCs from adipose tissue, infiltrating the muscle, could contribute to FAP heterogeneity.

Immunomodulation of Myocardial Fibrosis

Immunotherapy is a potential cornerstone in the treatment of myocardial fibrosis. During a myocardial insult or heart failure, danger signals stimulate innate immune cells to produce chemokines and profibrotic cytokines, which initiate self-escalating inflammatory processes by attracting and stimulating adaptive immune cells. Stimulation of fibroblasts by inflammatory processes and the need to replace damaged cardiomyocytes fosters reshaping of the cardiac fibroblast landscape. In this review, we discuss new immunomodulatory strategies that manipulate and direct cardiac fibroblast activation and differentiation. In particular, we highlight immunomodulatory strategies that target fibroblasts such as chimeric antigen receptor T cells, interleukin-11, and invariant natural killer T-cells. Moreover, we discuss the potential of manipulating both innate and adaptive immune system components for the translation into clinical validation. Clearly, multiple pathways should be considered to develop innovative approaches to ameliorate myocardial fibrosis and hence to reduce the risk of heart failure.

The amphiregulin/EGFR axis has limited contribution in controlling autoimmune diabetes

Conventional immunosuppressive functions of CD4+Foxp3+ regulatory T cells (Tregs) in type 1 diabetes (T1D) pathogenesis have been well described, but whether Tregs have additional non-immunological functions supporting tissue homeostasis in pancreatic islets is unknown. Within the last decade novel tissue repair functions have been ascribed to Tregs. One function is production of the epidermal growth factor receptor (EGFR) ligand, amphiregulin, which promotes tissue repair in response to inflammatory or mechanical tissue injury. However, whether such pathways are engaged during autoimmune diabetes and promote tissue repair is undetermined. Previously, we observed that upregulation of amphiregulin at the transcriptional level was associated with functional Treg populations in the non-obese diabetic (NOD) mouse model of T1D. From this we postulated that amphiregulin promoted islet tissue repair and slowed the progression of diabetes in NOD mice. Here, we report that islet-infiltrating Tregs have increased capacity to produce amphiregulin, and that both Tregs and beta cells express EGFR. Moreover, we show that amphiregulin can directly modulate mediators of endoplasmic reticulum stress in beta cells. Despite this, NOD amphiregulin deficient mice showed no acceleration of spontaneous autoimmune diabetes. Taken together, the data suggest that the ability for amphiregulin to affect the progression of autoimmune diabetes is limited.

Ectopic adipogenesis in response to injury and material implantation in an autoimmune mouse model

Due to the limited capacity of mammals to regenerate complex tissues, researchers have worked to understand the mechanisms of tissue regeneration in organisms that maintain that capacity. One example is the MRL/MpJ mouse strain with unique regenerative capacity in ear pinnae that is absent from other strains, such as the common C57BL/6 strain. The MRL/MpJ mouse has also been associated with an autoimmune phenotype even in the absence of the mutant Fas gene described in its parent strain MRL/lpr. Due to these findings, we evaluated the differences between the responses of MRL/MpJ versus C57BL/6 strain in traumatic muscle injury and subsequent material implantation. One salient feature of the MRL/MpJ response to injury was a robust adipogenesis within the muscle. This was associated with a decrease in M2-like polarization in response to biologically derived extracellular matrix scaffolds. In pro-fibrotic materials, such as polyethylene, there were fewer foreign body giant cells in the MRL/MpJ mice. As there are reports of both positive and negative influences of adipose tissue and adipogenesis on wound healing, this model could provide an important lens to investigate the interplay between stem cells, adipose tissue, and immune responses in trauma and materials implantation.

Regulatory T cells require IL6 receptor alpha signaling to control skeletal muscle function and regeneration

Muscle-residing regulatory T cells (Tregs) control local tissue integrity and function. However, the molecular interface connecting Treg-based regulation with muscle function and regeneration remains largely unexplored. Here, we show that exercise fosters a stable induction of highly functional muscle-residing Tregs with increased expression of amphiregulin (Areg), EGFR, and ST2. Mechanistically, we find that mice lacking IL6Rα on T cells (TKO) harbor significant reductions in muscle Treg functionality and satellite and fibro-adipogenic progenitor cells, which are required for muscle regeneration. Using exercise and sarcopenia models, IL6Rα TKO mice demonstrate deficits in Tregs, their functional maturation, and a more pronounced decline in muscle mass. Muscle injury models indicate that IL6Rα TKO mice have significant disabilities in muscle regeneration. Treg gain of function restores impaired muscle repair in IL6Rα TKO mice. Of note, pharmacological IL6R blockade in WT mice phenocopies deficits in muscle function identified in IL6Rα TKO mice, thereby highlighting the clinical implications of the findings.

Laboratory mice with a wild microbiota generate strong allergic immune responses

Allergic disorders are caused by a combination of hereditary and environmental factors. The hygiene hypothesis postulates that early-life microbial exposures impede the development of subsequent allergic disease. Recently developed "wildling" mice are genetically identical to standard laboratory specific pathogen-free (SPF) mice but are housed under seminatural conditions and have rich microbial exposures from birth. Thus, by comparing conventional SPF mice with wildlings, we can uncouple the impact of lifelong microbial exposures from genetic factors on the allergic immune response. We found that wildlings developed larger populations of antigen-experienced T cells than conventional SPF mice, which included interleukin-10-producing CD4 T cells specific for commensal Lactobacilli strains and allergy-promoting T helper 2 (TH2) cells. In models of airway exposure to house dust mite (HDM), recombinant interleukin-33, or Alternaria alternata, wildlings developed strong allergic inflammation, characterized by eosinophil recruitment, goblet cell metaplasia, and antigen-specific immunoglobulin G1 (IgG1) and IgE responses. Wildlings developed robust de novo TH2 cell responses to incoming allergens, whereas preexisting TH2 cells could also be recruited into the allergic immune response in a cytokine-driven and TCR-independent fashion. Thus, wildling mice, which experience diverse and lifelong microbial exposures, were not protected from developing pathological allergic immune responses. Instead, wildlings mounted robust allergic responses to incoming allergens, shedding new light on the hygiene hypothesis.

Stromal vascular fraction in the treatment of myositis

Muscle regeneration is a physiological process that converts satellite cells into mature myotubes under the influence of an inflammatory environment progressively replaced by an anti-inflammatory environment, with precise crosstalk between immune and muscular cells. If the succession of these phases is disturbed, the immune system can sometimes become auto-reactive, leading to chronic muscular inflammatory diseases, such as myositis. The triggers of these autoimmune myopathies remain mostly unknown, but the main mechanisms of pathogenesis are partially understood. They involve chronic inflammation, which could be associated with an auto-reactive immune response, and gradually with a decrease in the regenerative capacities of the muscle, leading to its degeneration, fibrosis and vascular architecture deterioration. Immunosuppressive treatments can block the first part of the process, but sometimes muscle remains weakened, or even still deteriorates, due to the exhaustion of its capacities. For patients refractory to immunosuppressive therapies, mesenchymal stem cells have shown interesting effects but their use is limited by their availability. Stromal vascular fraction, which can easily be extracted from adipose tissue, has shown good tolerance and possible therapeutic benefits in several degenerative and autoimmune diseases. However, despite the increasing use of stromal vascular fraction, the therapeutically active components within this heterogeneous cellular product are ill-defined and the mechanisms by which this therapy might be active remain insufficiently understood. We review herein the current knowledge on the mechanisms of action of stromal vascular fraction and hypothesise on how it could potentially respond to some of the unmet treatment needs of refractory myositis.

Regulatory T cells: a new therapeutic link for Sjögren syndrome?

Great advancements have been made in understanding the pathogenesis of SS, but there remain unmet needs for effective and targeted treatments. Glandular and extraglandular dysfunction in SS is associated with autoimmune lymphocytic infiltration that invades the epithelial structures of affected organs. Regulatory T (Treg) cells are a subset of CD4+ T lymphocytes that maintain self-tolerance during physiological conditions. Besides inhibiting excessive inflammation and autoimmune response by targeting various immune cell subsets and tissues, Treg cells have also been shown to promote tissue repair and regeneration in pathogenic milieus. The changes of quantity and function of Treg cells in various autoimmune and chronic inflammatory disorders have been reported, owing to their effects on immune regulation. Here we summarize the recent findings from murine models and clinical data about the dysfunction of Treg cells in SS pathogenesis and discuss the therapeutic strategies of direct or indirect targeting of Treg cells in SS. Understanding the current knowledge of Treg cells in the development of SS will be important to elucidate disease pathogenesis and may guide research for successful therapeutic intervention in this disease.

The immune function of dermal fibroblasts in skin defence against pathogens

Dermal fibroblasts are the main resident cells of the dermis. They have several significant functions related to wound healing, extracellular matrix production and hair cycling. Dermal fibroblasts can also act as sentinels in defence against infection. They express pattern recognition receptors such as toll-like receptors to sense pathogen components, followed by the synthesis of pro-inflammatory cytokines (including IL-6, IFN-β and TNF-α), chemokines (such as IL-8 and CXCL1) and antimicrobial peptides. Dermal fibroblasts also secrete other molecules-like growth factors and matrix metalloproteinases to benefit tissue repair from infection. Crosstalk between dermal fibroblasts and immune cells may amplify the immune response against infection. Moreover, the transition of a certain adipogenic fibroblasts to adipocytes protects skin from bacterial infection. Together, we discuss the role of dermal fibroblasts in the war against pathogens in this review. Dermal fibroblasts have important immune functions in anti-infection immunity, which should not be overlooked.

Amphiregulin couples IL1RL1+ regulatory T cells and cancer-associated fibroblasts to impede antitumor immunity

Regulatory T (Treg) cells and cancer-associated fibroblasts (CAFs) jointly promote tumor immune tolerance and tumorigenesis. The molecular apparatus that drives Treg cell and CAF coordination in the tumor microenvironment (TME) remains elusive. Interleukin 33 (IL-33) has been shown to enhance fibrosis and IL1RL1+ Treg cell accumulation during tumorigenesis and tissue repair. We demonstrated that IL1RL1 signaling in Treg cells greatly dampened the antitumor activity of both IL-33 and PD-1 blockade. Whole tumor single-cell RNA sequencing (scRNA-seq) analysis and blockade experiments revealed that the amphiregulin (AREG)-epidermal growth factor receptor (EGFR) axis mediated cross-talk between IL1RL1+ Treg cells and CAFs. We further demonstrated that the AREG/EGFR axis enables Treg cells to promote a profibrotic and immunosuppressive functional state of CAFs. Moreover, AREG mAbs and IL-33 concertedly inhibited tumor growth. Our study reveals a previously unidentified AREG/EGFR-mediated Treg/CAF coupling that controls the bifurcation of fibroblast functional states and is a critical barrier for cancer immunotherapy.

A link between mitochondrial damage and the immune microenvironment of delayed onset muscle soreness

BACKGROUND

Delayed onset muscle soreness (DOMS) is a self-healing muscle pain disorder. Inflammatory pain is the main feature of DOMS. More and more researchers have realized that changes in mitochondrial morphology are related to pain. However, the role of mitochondria in the pathogenesis of DOMS and the abnormal immune microenvironment is still unknown.

METHODS

Mitochondria-related genes and gene expression data were obtained from MitoCarta3.0 and NCBI GEO databases. The network of mitochondrial function and the immune microenvironment of DOMS was constructed by computer algorithm. Subsequently, the skeletal muscle of DOMS rats was subjected to qPCR to verify the bioinformatics results. DOMS and non-DOMS histological samples were further studied by staining and transmission electron microscopy.

RESULTS

Bioinformatics results showed that expression of mitochondria-related genes was changed in DOMS. The results of qPCR showed that four hub genes (AMPK, PGC1-α, SLC25A25, and ARMCX1) were differentially expressed in DOMS. These hub genes are related to the degree of skeletal muscle immune cell infiltration, mitochondrial respiratory chain complex, DAMPs, the TCA cycle, and mitochondrial metabolism. Bayesian network inference showed that IL-6 and PGC1-α may be the main regulatory genes of mitochondrial damage in DOMS. Transmission electron microscopy revealed swelling of skeletal muscle mitochondria and disorganization of myofilaments.

CONCLUSIONS

Our study found that skeletal muscle mitochondrial damage is one of the causes of inflammatory factor accumulation in DOMS. According to the screened-out hub genes, this study provides a reference for follow-up clinical application.

Single-cell transcriptomic analysis of the identity and function of fibro/adipogenic progenitors in healthy and dystrophic muscle

Fibro/adipogenic progenitors (FAPs) are skeletal muscle stromal cells that support regeneration of injured myofibers and their maintenance in healthy muscles. FAPs are related to mesenchymal stem cells (MSCs/MeSCs) found in other adult tissues, but there is poor understanding of the extent of similarity between these cells. Using single-cell RNA sequencing (scRNA-seq) datasets from multiple mouse tissues, we have performed comparative transcriptomic analysis. This identified remarkable transcriptional similarity between FAPs and MeSCs, confirmed the suitability of PDGFRα as a reporter for FAPs, and identified extracellular proteolysis as a new FAP function. Using PDGFRα as a cell surface marker, we isolated FAPs from healthy and dysferlinopathic mouse muscles and performed scRNA-seq analysis. This revealed decreased FAP-mediated Wnt signaling as a potential driver of FAP dysfunction in dysferlinopathic muscles. Analysis of FAPs in dysferlin- and dystrophin-deficient muscles identified a relationship between the nature of muscle pathology and alteration in FAP gene expression.

The Amphiregulin/EGFR axis has limited contribution in controlling autoimmune diabetes

Conventional immunosuppressive functions of CD4+Foxp3+ regulatory T cells (Tregs) in type 1 diabetes (T1D) pathogenesis have been well described, but whether Tregs have additional non-immunological functions supporting tissue homeostasis in pancreatic islets is unknown. Within the last decade novel tissue repair functions have been ascribed to Tregs. One function is production of the epidermal growth factor receptor (EGFR) ligand, amphiregulin, which promotes tissue repair in response to inflammatory or mechanical tissue injury. Whether such pathways are engaged during autoimmune diabetes and promote tissue repair is undetermined. Previously, we observed upregulation of amphiregulin at the transcriptional level was associated with functional Treg populations in the non-obese diabetic (NOD) mouse model of T1D. We postulated that amphiregulin promoted islet tissue repair and slowed the progression of diabetes in NOD mice. Here, we report that islet-infiltrating Tregs have increased capacity to produce amphiregulin and both Tregs and beta cells express EGFR. Moreover, we show that amphiregulin can directly modulate mediators of endoplasmic reticulum (ER) stress in beta cells. Despite this, NOD amphiregulin deficient mice showed no acceleration of spontaneous autoimmune diabetes. Taken together, the data suggest that the ability for amphiregulin to affect the progression of autoimmune diabetes is limited.

Differentiation, regulation and function of regulatory T cells in non-lymphoid tissues and tumors

Regulatory T cells (Tregs) play a substantial role in inhibiting excessive immune response. A large number of studies have focused on the tissue homeostasis maintenance and remodeling characteristics of Tregs in non-lymphoid tissues, such as the skin, colon, lung, brain, muscle, and adipose tissues. Herein, we overview the kinetics of Treg migration to non-lymphoid tissues and adaptation to the specific tissue microenvironment through the development of tissue-specific chemokine receptors, transcription factors, and phenotypes. Additionally, tumor-infiltrating Tregs (Ti-Tregs) play an important role in tumor generation and immunotherapy resistance. The phenotypes of Ti-Tregs are related to the histological location of the tumor and there is a large overlap between the transcripts of Ti-Tregs and those of tissue-specific Tregs. We recapitulate the molecular underpinnings of tissue-specific Tregs, which might shed new light on Treg-based therapeutic targets and biomarkers for inflammatory diseases and cancer.

Isoalantolactone relieves depression-like behaviors in mice after chronic social defeat stress via the gut-brain axis

RATIONALE

The management of depression continues to be challenging despite the variety of available antidepressants. Herbal medicines are used in many cultures but lack stringent testing to understand their efficacy and mechanism of action. Isoalantolactone (LAT) from Elecampane (Inula helenium) improved the chronic social defeat stress (CSDS)-induced anhedonia-like phenotype in mice comparable to fluoxetine, a selective serotonin reuptake inhibitor (SSRI).

OBJECTIVES

Compare the effects of LAT and fluoxetine on depression-like behaviors in mice exposed to CSDS.

RESULT

The CSDS-induced decrease in protein expression of postsynaptic density (PSD95), brain derived neurotrophic factor (BDNF), and glutamate receptor subunit-1 (GluA1) in the prefrontal cortex was restored by LAT. LAT showed robust anti-inflammatory activity and can lessen the increase in IL-6 and TNF-α caused by CSDS. CSDS altered the gut microbiota at the taxonomic level, resulting in significant changes in α- and β-diversity. LAT treatment reestablished the bacterial abundance and diversity and increased the production of butyric acid in the gut that was inhibited by CSDS. The levels of butyric acid were negatively correlated with the abundance of Bacteroidetes, and positively correlated with those of Proteobacteria and Firmicutes across all treatment groups.

CONCLUSIONS

The current data suggest that, similar to fluoxetine, LAT show antidepressant-like effects in mice exposed to CSDS through the modulation of the gut-brain axis.