Eosinophils downregulate lung alloimmunity by decreasing TCR signal transduction

Maintenance of graft tissue-resident Foxp3+ cells is necessary for lung transplant tolerance in mice

Mechanisms that mediate allograft tolerance differ between organs. We have previously shown that Foxp3+ T cell-enriched bronchus-associated lymphoid tissue (BALT) is induced in tolerant murine lung allografts and that these Foxp3+ cells suppress alloimmune responses locally and systemically. Here, we demonstrated that Foxp3+ cells that reside in tolerant lung allografts differed phenotypically and transcriptionally from those in the periphery and were clonally expanded. Using a mouse lung retransplant model, we showed that recipient Foxp3+ cells were continuously recruited to the BALT within tolerant allografts. We identified distinguishing features of graft-resident and newly recruited Foxp3+ cells and showed that graft-infiltrating Foxp3+ cells acquired transcriptional profiles resembling those of graft-resident Foxp3+ cells over time. Allografts underwent combined antibody-mediated rejection and acute cellular rejection when recruitment of recipient Foxp3+ cells was prevented. Finally, we showed that local administration of IL-33 could expand and activate allograft-resident Foxp3+ cells, providing a platform for the design of tolerogenic therapies for lung transplant recipients. Our findings establish graft-resident Foxp3+ cells as critical orchestrators of lung transplant tolerance and highlight the need to develop lung-specific immunosuppression.

Chronic Lung Allograft Dysfunction: Clinical Manifestations and Immunologic Mechanisms

The term "chronic lung allograft dysfunction" has emerged to describe the clinical syndrome of progressive, largely irreversible dysfunction of pulmonary allografts. This umbrella term comprises 2 major clinical phenotypes: bronchiolitis obliterans syndrome and restrictive allograft syndrome. Here, we discuss the clinical manifestations, diagnostic challenges, and potential therapeutic avenues to address this major barrier to improved long-term outcomes. In addition, we review the immunologic mechanisms thought to propagate each phenotype of chronic lung allograft dysfunction, discuss the various models used to study this process, describe potential therapeutic targets, and identify key unknowns that must be evaluated by future research strategies.

Spatial adaptation of eosinophils and their emerging roles in homeostasis, infection and disease

Eosinophils are bone marrow-derived granulocytes that are traditionally associated with type 2 immune responses, such as those that occur during parasite infections and allergy. Emerging evidence demonstrates the remarkable functional plasticity of this elusive cell type and its pleiotropic functions in diverse settings. Eosinophils broadly contribute to tissue homeostasis, host defence and immune regulation, predominantly at mucosal sites. The scope of their activities primarily reflects the breadth of their portfolio of secreted mediators, which range from cytotoxic cationic proteins and reactive oxygen species to multiple cytokines, chemokines and lipid mediators. Here, we comprehensively review basic eosinophil biology that is directly related to their activities in homeostasis, protective immunity, regeneration and cancer. We examine how dysregulation of these functions contributes to the physiopathology of a broad range of inflammatory diseases. Furthermore, we discuss recent findings regarding the tissue compartmentalization and adaptation of eosinophils, shedding light on the factors that likely drive their functional diversification within tissues.

Stress-induced eosinophil activation contributes to postoperative morbidity and mortality after lung resection

Respiratory failure occurs more frequently after thoracic surgery than abdominal surgery. Although the etiology for this complication is frequently attributed to underlying lung disease present in patients undergoing thoracic surgery, this notion is often unfounded because many patients with normal preoperative pulmonary function often require prolonged oxygen supplementation even after minimal resection of lung tissue. Using a murine model of pulmonary resection and peripheral blood samples from patients undergoing resection of the lung or abdominal organs, we demonstrated that lung surgery initiates a proinflammatory loop that results in damage to the remaining lung tissue, noncardiogenic pulmonary edema, hypoxia, and even death. Specifically, we demonstrated that resection of murine lung tissue increased concentrations of the homeostatic cytokine interleukin-7, which led to local and systemic activation of type 2 innate lymphoid cells. This process activated lung-resident eosinophils and facilitated stress-induced eosinophil maturation in the bone marrow in a granulocyte-macrophage colony-stimulating factor-dependent manner, resulting in systemic eosinophilia in both mice and humans. Up-regulation of inducible nitric oxide synthase in lung-resident eosinophils led to tissue nitrosylation, pulmonary edema, hypoxia, and, at times, death. Disrupting this activation cascade at any stage ameliorated deleterious outcomes and improved survival after lung resection in the mouse model. Our data suggest that repurposing US Food and Drug Administration-approved eosinophil-targeting strategies may potentially offer a therapeutic intervention to improve outcomes for patients who require lung resection for benign or malignant etiology.

IL-33: Friend or foe in transplantation?

Several reports have highlighted the dichotomous nature of the Interleukin-33 (IL-33) system in cardiac and lung disease, where this cytokine can exert both protective effects and drive pro-inflammatory responses in a context-specific manner. This State-of-the-Art review focuses on preclinical mechanistic studies of the IL-33 system in development of allograft rejection in heart and lung transplantation. We address the scope of potential cellular sources of IL-33 and pathways for cellular release that may impact the study of this cytokine system in transplant models. We then highlight soluble IL-33 receptor as a biomarker in cardiac allograft rejection and detail preclinical models that collectively demonstrate a role for this cytokine in driving type-2 immune programs to protect cardiac allografts. We contrast this with investigation of IL-33 in lung transplantation, which has yielded mixed and somewhat conflicting results when comparing human studies with preclinical models, which have implicated the IL-33 system in both allograft tolerance and acceleration of chronic rejection. We summarize and interpret these results in aggregate and provide future directions for study of IL-33 in heart and lung transplantation.

The β-catenin C terminus links Wnt and sphingosine-1-phosphate signaling pathways to promote vascular remodeling and atherosclerosis

Canonical Wnt and sphingosine-1-phosphate (S1P) signaling pathways are highly conserved systems that contribute to normal vertebrate development, with key consequences for immune, nervous, and cardiovascular system function; despite these functional overlaps, little is known about Wnt/β-catenin-S1P cross-talk. In the vascular system, both Wnt/β-catenin and S1P signals affect vessel maturation, stability, and barrier function, but information regarding their potential coordination is scant. We report an instance of functional interaction between the two pathways, including evidence that S1P receptor 1 (S1PR1) is a transcriptional target of β-catenin. By studying vascular smooth muscle cells and arterial injury response, we find a specific requirement for the β-catenin carboxyl terminus, which acts to induce S1PR1, and show that this interaction is essential for vascular remodeling. We also report that pharmacological inhibition of the β-catenin carboxyl terminus reduces S1PR1 expression, neointima formation, and atherosclerosis. These findings provide mechanistic understanding of how Wnt/β-catenin and S1P systems collaborate during vascular remodeling and inform strategies for therapeutic manipulation.

BAL Fluid Eosinophilia Associates With Chronic Lung Allograft Dysfunction Risk: A Multicenter Study

BACKGROUND

Chronic lung allograft dysfunction (CLAD) is the leading cause of death among lung transplant recipients. Eosinophils, effector cells of type 2 immunity, are implicated in the pathobiology of many lung diseases, and prior studies suggest their presence associates with acute rejection or CLAD after lung transplantation.

RESEARCH QUESTION

Does histologic allograft injury or respiratory microbiology correlate with the presence of eosinophils in BAL fluid (BALF)? Does early posttransplant BALF eosinophilia associate with future CLAD development, including after adjustment for other known risk factors?

STUDY DESIGN AND METHODS

We analyzed BALF cell count, microbiology, and biopsy data from a multicenter cohort of 531 lung recipients with 2,592 bronchoscopies over the first posttransplant year. Generalized estimating equation models were used to examine the correlation of allograft histology or BALF microbiology with the presence of BALF eosinophils. Multivariable Cox regression was used to determine the association between ≥ 1% BALF eosinophils in the first posttransplant year and definite CLAD. Expression of eosinophil-relevant genes was quantified in CLAD and transplant control tissues.

RESULTS

The odds of BALF eosinophils being present was significantly higher at the time of acute rejection and nonrejection lung injury histologies and during pulmonary fungal detection. Early posttransplant ≥ 1% BALF eosinophils significantly and independently increased the risk for definite CLAD development (adjusted hazard ratio, 2.04; P = .009). Tissue expression of eotaxins, IL-13-related genes, and the epithelial-derived cytokines IL-33 and thymic stromal lymphoprotein were significantly increased in CLAD.

INTERPRETATION

BALF eosinophilia was an independent predictor of future CLAD risk across a multicenter lung recipient cohort. Additionally, type 2 inflammatory signals were induced in established CLAD. These data underscore the need for mechanistic and clinical studies to clarify the role of type 2 pathway-specific interventions in CLAD prevention or treatment.

Modeling tissue co-regulation estimates tissue-specific contributions to disease

Integrative analyses of genome-wide association studies and gene expression data have implicated many disease-critical tissues. However, co-regulation of genetic effects on gene expression across tissues impedes distinguishing biologically causal tissues from tagging tissues. In the present study, we introduce tissue co-regulation score regression (TCSC), which disentangles causal tissues from tagging tissues by regressing gene-disease association statistics (from transcriptome-wide association studies) on tissue co-regulation scores, reflecting correlations of predicted gene expression across genes and tissues. We applied TCSC to 78 diseases/traits (average n = 302,000) and gene expression prediction models for 48 GTEx tissues. TCSC identified 21 causal tissue-trait pairs at a 5% false discovery rate (FDR), including well-established findings, biologically plausible new findings (for example, aorta artery and glaucoma) and increased specificity of known tissue-trait associations (for example, subcutaneous adipose, but not visceral adipose, and high-density lipoprotein). TCSC also identified 17 causal tissue-trait covariance pairs at 5% FDR. In conclusion, TCSC is a precise method for distinguishing causal tissues from tagging tissues.

Protective and pathogenic functions of innate lymphoid cells in transplantation

Innate lymphoid cells (ILCs) are a family of lymphocytes with essential roles in tissue homeostasis and immunity. Along with other tissue-resident immune populations, distinct subsets of ILCs have important roles in either promoting or inhibiting immune tolerance in a variety of contexts, including cancer and autoimmunity. In solid organ and hematopoietic stem cell transplantation, both donor and recipient-derived ILCs could contribute to immune tolerance or rejection, yet understanding of protective or pathogenic functions are only beginning to emerge. In addition to roles in directing or regulating immune responses, ILCs interface with parenchymal cells to support tissue homeostasis and even regeneration. Whether specific ILCs are tissue-protective or enhance ischemia reperfusion injury or fibrosis is of particular interest to the field of transplantation, beyond any roles in limiting or promoting allograft rejection or graft-versus host disease. Within this review, we discuss the current understanding of ILCs functions in promoting immune tolerance and tissue repair at homeostasis and in the context of transplantation and highlight where targeting or harnessing ILCs could have applications in novel transplant therapies.

Adaptive Immunosuppression in Lung Transplant Recipients Applying Complementary Biomarkers: The Zurich Protocol

Achieving adequate immunosuppression for lung transplant recipients in the first year after lung transplantation is a key challenge. Prophylaxis of allograft rejection must be balanced with the adverse events associated with immunosuppressive drugs, for example infection, renal failure, and diabetes. A triple immunosuppressive combination is standard, including a steroid, a calcineurin inhibitor, and an antiproliferative compound beginning with the highest levels of immunosuppression and a subsequent tapering of the dose, usually guided by therapeutic drug monitoring and considering clinical results, bronchoscopy sampling results, and additional biomarkers such as serum viral replication or donor-specific antibodies. Balancing the net immunosuppression level required to prevent rejection without overly increasing the risk of infection and other complications during the tapering phase is not well standardized and requires repeated assessments for dose-adjustments. In our adaptive immunosuppression approach, we additionally consider results from the white blood cell counts, in particular lymphocytes and eosinophils, as biomarkers for monitoring the level of immunosuppression and additionally use them as therapeutic targets to fine-tune the immunosuppressive strategy over time. The concept and its rationale are outlined, and areas of future research mentioned.

Eosinophil-lymphocyte interactions in the tumor microenvironment and cancer immunotherapy

Eosinophils are important effector cells and therapeutic targets in allergic diseases. Emerging data indicate that eosinophils infiltrate a variety of solid tumor types and have pleiotropic activities by at least two non-mutually exclusive mechanisms: direct interactions with tumor cells, and intricate cross-talk with lymphocytes. In light of the immune checkpoint inhibition revolution in cancer therapy, we review eosinophil-lymphocyte interactions in the tumor microenvironment. We also analyze potential interactions between eosinophils and lymphocyte subsets, including T cells, natural killer cells and innate lymphoid cells. We provide perspectives on the consequences of these interactions and how eosinophils are accessory cells that can affect the response to various forms of T cell-mediated immunotherapies and might be therapeutically targeted to improve cancer immunotherapy.

Solving the Conundrum of Eosinophils in Alloimmunity

Eosinophils are bone-marrow-derived granulocytes known for their ability to facilitate clearance of parasitic infections and their association with asthma and other inflammatory diseases. The purpose of this review is to discuss the currently available human observational and animal experimental data linking eosinophils to the immunologic response in solid organ transplantation. First, we present observational human studies that demonstrate a link between transplantation and eosinophils yet were unable to define the exact role of this cell population. Next, we describe published experimental models and demonstrate a defined mechanistic role of eosinophils in downregulating the alloimmune response to murine lung transplants. The overall summary of this data suggests that further studies are needed to define the role of eosinophils in multiple solid organ allografts and points to the possibility of manipulating this cell population to improve graft survival.

Ischemia reperfusion injury facilitates lung allograft acceptance through IL-33-mediated activation of donor-derived IL-5 producing group 2 innate lymphoid cells

Pathways regulating lung alloimmune responses differ from most other solid organs and remain poorly explored. Based on our recent work identifying the unique role of eosinophils in downregulating lung alloimmunity, we sought to define pathways contributing to eosinophil migration and homeostasis. Using a murine lung transplant model, we have uncovered that immunosuppression increases eosinophil infiltration into the allograft in an IL-5-dependent manner. IL-5 production depends on immunosuppression-mediated preservation of donor-derived group 2 innate lymphoid cells (ILC2). We further describe that ischemia reperfusion injury upregulates the expression of IL-33, which functions as the dominant and nonredundant mediator of IL-5 production by graft-resident ILC2. Our work thus identifies unique cellular mechanisms that contribute to lung allograft acceptance. Notably, ischemia reperfusion injury, widely considered to be solely deleterious to allograft survival, can also downregulate alloimmune responses by initiating unique pathways that promote IL-33/IL-5/eosinophil-mediated tolerance.

Tolerance, immunosuppression, and immune modulation: impacts on lung allograft survival

PURPOSE OF REVIEW

Immune responses following lung transplantation continue to result in high rates of allograft failure and rejection, and current immunosuppression does not address the unique immunologic properties of the lung. Here, we review recent studies on lung allograft tolerance and alloimmunity and discuss implications for immunosuppression.

RECENT FINDINGS

Processes governing tolerance and alloimmunity in lung allografts differ from other solid organs. Recent studies have suggested that allorecognition is regulated at the level of the lung graft. Furthermore, certain cell populations essential for lung allograft tolerance may facilitate rejection in other organs. Induction of lung allograft tolerance is associated with the formation of tertiary lymphoid organs, which are enriched in regulatory T cells and play an important role in preventing rejection.

SUMMARY

Recent discoveries regarding alloactivation and the regulation of tolerance following lung transplantation have introduced exciting potential avenues for the development of lung-specific immunosuppression.

Eosinophil Knockout Humans: Uncovering the Role of Eosinophils Through Eosinophil-Directed Biological Therapies

The enigmatic eosinophil has emerged as an exciting component of the immune system, involved in a plethora of homeostatic and inflammatory responses. Substantial progress has been achieved through experimental systems manipulating eosinophils in vivo, initially in mice and more recently in humans. Researchers using eosinophil knockout mice have identified a contributory role for eosinophils in basal and inflammatory processes and protective immunity. Primarily fueled by the purported proinflammatory role of eosinophils in eosinophil-associated diseases, a series of anti-eosinophil therapeutics have emerged as a new class of drugs. These agents, which dramatically deplete eosinophils, provide a valuable opportunity to characterize the consequences of eosinophil knockout humans. Herein, we comparatively describe mouse and human eosinophil knockouts. We put forth the view that human eosinophils negatively contribute to a variety of diseases and, unlike mouse eosinophils, do not yet have an identified role in physiological health; thus, clarifying all roles of eosinophils remains an ongoing pursuit.

Control of T-Cell Activation and Signaling by Amino-Acid Catabolizing Enzymes

Amino acids are essential for protein synthesis, epigenetic modification through the methylation of histones, and the maintenance of a controlled balance of oxidoreduction via the production of glutathione and are precursors of certain neurotransmitters. T lymphocytes are particularly sensitive to fluctuations in amino acid levels. During evolution, the production of amino-acid catabolizing enzymes by mainly antigen-presenting cells has become a physiological mechanism to control T-cell activation and polarization. The action of these enzymes interferes with TCR and co-stimulation signaling, allowing tuning of the T-cell response. These capacities can be altered in certain pathological conditions, with relevant consequences for the development of disease.

Eosinophils in transbronchial biopsies: a predictor of chronic lung allograft dysfunction and reduced survival after lung transplantation - a retrospective single-center cohort study

Long-term outcomes after lung transplantation remain inferior to those of other solid organ groups. The significance of eosinophils detected on transbronchial biopsies (TBBx) after lung transplantation and their relationship to long-term outcomes remain unknown. A retrospective single-center cohort study was performed of patients transplanted between January 01, 2001, and July 31, 2018, who had at least 1 TBBx with evaluable parenchymal tissue. Multivariable Cox proportional hazard models were used to assess the associations between eosinophil detection and: all-cause mortality and Chronic Lung Allograft Dysfunction (CLAD). 8887 TBBx reports from 1440 patients were reviewed for the mention of eosinophils in the pathology report. 112 (7.8%) patients were identified with eosinophils on at least one TBBx. The median (95% CI) survival time for all patients was 8.28 (7.32-9.31) years. Multivariable analysis, adjusted for clinical variables known to affect post-transplant outcomes, showed that the detection of eosinophils was independently associated with an increased risk of death (HR 1.51, 95% CI 1.24-1.85, p < 0.01) and CLAD (HR 1.35, 95% CI 1.07-1.70, P = 0.01). Eosinophils detected in TBBx are associated with an increased risk of CLAD and death. There may be benefit in specifically reporting the presence of eosinophils in TBBx reports and incorporating their presence in clinical decision-making.

Screening and identification of key regulatory connections and immune cell infiltration characteristics for lung transplant rejection using mucosal biopsies

This study aimed to explore key regulatory connections underlying lung transplant rejection. The differentially expressed genes (DEGs) between rejection and stable lung transplantation (LTx) samples were screened using R package limma, followed by functional enrichment analysis and protein-protein interaction network construction. Subsequently, a global triple network, including miRNAs, mRNAs, and transcription factors (TFs), was constructed. Furthermore, immune cell infiltration characteristics were analyzed to investigate the molecular immunology of lung transplant rejection. Finally, potential drug-target interactions were generated. In brief, 739 DEGs were found between rejection and stable LTx samples. PTPRC, IL-6, ITGAM, CD86, TLR8, TYROBP, CXCL10, ITGB2, and CCR5 were defined as hub genes. Eight TFs, including STAT1, SPIB, NFKB1, SPI1, STAT5A, RUNX1, VENTX, and BATF, and five miRNAs, including miR-335-5p, miR-26b-5p, miR-124-3p, miR-1-3p, and miR-155-5p, were involved in regulating hub genes. The immune cell infiltration analysis revealed higher proportions of activated memory CD4 T cells, follicular helper T cells, γδ T cells, monocytes, M1 and M2 macrophages, and eosinophils in rejection samples, besides lower proportions of resting memory CD4 T cells, regulatory T cells, activated NK cells, M0 macrophages, and resting mast cells. This study provided a comprehensive perspective of the molecular co-regulatory network underlying lung transplant rejection.

Frontline Science: Superior mouse eosinophil depletion in vivo targeting transgenic Siglec-8 instead of endogenous Siglec-F: Mechanisms and pitfalls

Eosinophils are important multifunctional granulocytes. When studying eosinophil function and its contribution to diseases, mouse models are often used. Mouse eosinophils selectively express sialic acid-binding immunoglobulin-like lectin (Siglec)-F. Its closest functional paralog on human eosinophils is Siglec-8. These Siglecs are being used to target eosinophils when exploring their mechanistic roles in disease and for potential therapeutic benefit. In order to facilitate preclinical studies of human Siglec-8, we developed transgenic mouse strains expressing human Siglec-8 only on the surface of eosinophils with or without endogenous Siglec-F and have begun characterizing various cellular functions in vitro and in vivo. Eosinophils from Siglec-8+ mice, with or without Siglec-F, responded to Siglec-8 antibody engagement in vitro by up-regulating surface CD11b, whereas Siglec-F antibody had no such effect. Engagement of Siglec-F or Siglec-8 with respective antibodies in vitro resulted in only modest increases in cell death. Administration of rat Siglec-F antibodies to mice led to a significant decrease in Siglec-F surface expression on eosinophils due to internalization, and thus appeared to decrease eosinophil numbers based on Siglec-F+ cells, but with proper gaiting strategies did not in fact result in significant eosinophil depletion. In marked contrast, administration of mouse Siglec-8 antibodies rapidly and effectively depleted eosinophils from blood and spleens of mice, but an F(ab')2 version did not, indicating an Fc-mediated mechanism for eosinophil depletion in vivo. Siglec-8 expressing mice with or without endogenous Siglec-F will be useful to study Siglec-8-based therapeutics, and may be a preferred approach when acute or chronic eosinophil depletion is needed.

Role of FcγRI in Antigen-Dependent Eosinophil Activation in Patients With Allergic Rhinitis

Background

The eosinophil (Eo) activation is a crucial factor evoking allergic rhinitis (AR) attacks; factors; the mechanism of triggering Eo activation remains to be further investigated. The interaction of antigen (Ag) and antibody plays a critical role in evoking allergy attacks. This study aims to elucidate the role of FcγRI, the high affinity receptor of IgG, in the Ag-mediated Eo activation.

Methods

Nasal lavage fluids (NLF) were collected from AR patients and healthy control (HC) subjects. Eos were isolated by flow cytometry cell sorting and analyzed by pertinent immunological approaches.

Results

Eos composed more than 60% of the cellular components in AR NLF. Exposure to specific Ags (sAgs) in the culture triggered Eos to release inflammatory mediators. High levels of FcγRI were detected on the surface of AR NLF Eos. Exposure to lipopolysaccharide markedly increased the FcγRI expression in naive Eos, which could be bound by Ag-specific IgG (sIgG) to form complexes on the surface of Eos; this made Eos at the sensitized status. Eos bore with the sIgG/FcγRI complexes could be activated upon exposure to sIgG in the culture; these Eos can be designated as Ag-specific Eos. Passive transfer of Ag-specific Eos resulted in profound AR response in mice upon sAg challenge. Depletion of FcγRI on Eos efficiently abolished AR response in mice.

Conclusions

AR Eos express high levels FcγRI, that can be bound by sIgG to make Eos sensitized. Re-exposure to specific Ags can activate the sensitized Eos.

Deciphering the role of eosinophils in solid organ transplantation

Eosinophils are rare granulocytes that belong to the innate arm of the immune system. This cell population is traditionally defined as a destructive and cytotoxic mediator in asthma and helminth infection. Limited data in transplantation have suggested that eosinophils play a similar role in potentiating deleterious organ inflammation and immunologic rejection. Contrary to this long-held notion, recent data have uncovered the possibility that eosinophils play an alternative role in immune homeostasis, defense against a wide range of pathogens, as well as downregulation of deleterious inflammation. Specifically, translational data from small animal models of lung transplantation have demonstrated a critical role for eosinophils in the downregulation of alloimmunity. These findings shed new light on the unique immunologic features of the lung allograft and demonstrate that environmental polarization may alter the phenotype and function of leukocyte populations previously thought to be static in nature. In this review, we provide an update on eosinophils in the homeostasis of the lung as well as other solid organs.

The emerging role of regulatory T cells following lung transplantation

Regulatory T cells (Treg) have proven to be a powerful immunologic force in nearly every organ system and hold therapeutic potential for a wide range of diseases. Insights gained from non-transplant pathologies, such as infection, cancer, and autoimmunity, are now being translated to the field of solid organ transplantation, particularly for livers and kidneys. Recent insights from animal models of lung transplantation have established that Tregs play a vital role in suppressing rejection and facilitating tolerance of lung allografts, and such discoveries are being validated in human studies and preclinical trials. Given that long-term outcomes following lung transplantation remain profoundly limited by chronic rejection, Treg therapy holds the potential to significantly improve patient outcomes and should be aggressively investigated.