Protection against bronchiolitis obliterans syndrome is associated with allograft CCR7+ CD45RA- T regulatory cells

Relationship between immune cells and the development of chronic lung allograft dysfunction

A major cause of death for lung transplant recipients (LTRs) is the advent of chronic lung allograft dysfunction (CLAD), which has long plagued the long-term post-transplant prognosis and quality of survival of transplant patients. The intricacy of its pathophysiology and the irreversibility of its illness process present major obstacles to the clinical availability of medications. Immunotherapeutic medications are available, but they only aim to slow down the course of CLAD rather than having any therapeutic impact on the disease's development. For this reason, understanding the pathophysiology of CLAD is essential for both disease prevention and proven treatment. The immunological response in particular, in relation to chronic lung allograft dysfunction, has received a great deal of interest recently. Innate immune cells like natural killer cells, eosinophils, neutrophils, and mononuclear macrophages, as well as adaptive immunity cells like T and B cells, play crucial roles in this process through the release of chemokines and cytokines. The present review delves into changes and processes within the immune microenvironment, with a particular focus on the quantity, subtype, and characteristics of effector immune cells in the peripheral and transplanted lungs after lung transplantation. We incorporate and solidify the documented role of immune cells in the occurrence and development of CLAD with the advancements in recent years.

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.

The CD8+ T cell content of transbronchial biopsies from patients with a first episode of clinically stable grade A1 cellular rejection is associated with future chronic lung allograft dysfunction

BACKGROUND

T cells drive acute cellular rejection (ACR) and its progression to chronic lung allograft dysfunction (CLAD) following lung transplantation. International Society for Heart and Lung Transplantation grade A1 ACR without associated allograft dysfunction is often untreated, yet some patients develop progressive graft dysfunction. T-cell composition of A1 ACR lesions may have prognostic value; therefore, protein-level and epigenetic techniques were applied to transbronchial biopsy tissue to determine whether differential T-cell infiltration in recipients experiencing a first episode of stable grade A1 ACR (StA1R) is associated with early CLAD.

METHODS

Sixty-two patients experiencing a first episode of StA1R were divided into those experiencing CLAD within 2 years (n = 13) and those remaining CLAD-free for 5 or more years (n = 49). Imaging mass cytometry (IMC) was used to profile the spectrum and distribution of intragraft T cell phenotypes on a subcohort (n = 16; 8 early-CLAD and 8 no early-CLAD). Immunofluorescence was used to quantify CD4+, CD8+, and FOXP3+ cells. Separately, CD3+ cells were fluorescently labeled, micro-dissected, and the degree of Treg-specific demethylated region methylation was determined.

RESULTS

PhenoGraph unsupervised clustering on IMC revealed 50 unique immune cell subpopulations. Methylation and immunofluorescence analyses demonstrated no significant differences in Tregs between early-CLAD and no early-CLAD groups. Immunofluorescence revealed that patients who developed CLAD within 2 years of lung transplantation showed greater CD8+ T cell infiltration compared to those who remained CLAD-free for 5 or more years.

CONCLUSIONS

In asymptomatic patients with a first episode of A1 rejection, greater CD8+ T cell content may be indicative of worse long-term outlook.

Immunological Landscapes in Lung Transplantation: Insights from T Cell Profiling in BAL and PBMC

Lung transplant recipients frequently encounter immune-related complications, including chronic lung allograft dysfunction (CLAD). Monitoring immune cells within the lung microenvironment is pivotal for optimizing post-transplant outcomes. This study examined the proportion of T cell subsets in paired bronchoalveolar lavage (BAL) and peripheral PBMC comparing healthy (n = 4) and lung transplantation patients (n = 6, no CLAD and n = 14 CLAD) using 14-color flow cytometry. CD4+ T cell proportions were reduced in CD3 cells in both PBMC and BAL, and positive correlations were discerned between T cell populations in peripheral PBMC and BAL, suggesting the prospect of employing less invasive PBMC sampling as a means of monitoring lung T cells. Furthermore, regulatory T cells (Tregs) were enriched in BAL when compared to peripheral PBMC for transplant recipients. A parallel positive correlation emerged between Treg proportions in BAL and peripheral PBMC, underscoring potential avenues for monitoring lung Tregs. Finally, the most promising biomarker was the Teff (CD8+Granzyme B+)-Treg ratio, which was higher in both the PBMC and BAL of transplant recipients compared to healthy individuals, and increased in the patients with CLAD compared to no CLAD and healthy patients. Conclusions: Distinct T cell profiles in BAL and peripheral PBMC underscore the significance of localized immune monitoring in lung transplantation. The Teff (CD8+granzyme B+)-Treg ratio, particularly within the context of CLAD, emerges as a promising blood and BAL biomarker reflective of inflammation and transplant-related complications. These findings emphasize the imperative need for personalized immune monitoring strategies that tailored to address the unique immunological milieu in post-transplant lungs.

Markers of rejection of a lung allograft: state of the art

Chronic lung allograft dysfunction (CLAD) affects approximately 50% of all lung transplant recipients by 5 post-operative years and is the leading cause of death in lung transplant recipients. Early CLAD diagnosis or ideally prediction of CLAD is essential to enable early intervention before significant lung injury occurs. New technologies have emerged to facilitate biomarker discovery, including epigenetic modification and single-cell RNA sequencing. This review examines new and existing technologies for biomarker discovery and the current state of research on biomarkers for identifying lung transplant rejection.

Effector immune cells in Chronic Lung Allograft Dysfunction: a Systematic Review

Chronic lung allograft dysfunction (CLAD) remains the major barrier to long‐term survival after lung transplantation and improved insight into its underlying immunological mechanisms is critical to better understand the disease and to identify treatment targets. We systematically searched the electronic databases of PubMed and EMBASE for original research publications, published between January 2000 and April 2021, to comprehensively assess current evidence on effector immune cells in lung tissue and bronchoalveolar lavage fluid from lung transplant recipients with CLAD. Literature search revealed 1351 articles, 76 of which met the criteria for inclusion in our analysis. Our results illustrate significant complexity in both innate and adaptive immune cell responses in CLAD, along with presence of numerous immune cell products, including cytokines, chemokines and proteases associated with tissue remodelling. A clear link between neutrophils and eosinophils and CLAD incidence has been seen, in which eosinophils more specifically predisposed to restrictive allograft syndrome. The presence of cytotoxic and T‐helper cells in CLAD pathogenesis is well‐documented, although it is challenging to draw conclusions about their role in tissue processes from predominantly bronchoalveolar lavage data. In restrictive allograft syndrome, a more prominent humoral immune involvement with increased B cells, immunoglobulins and complement deposition is seen. Our evaluation of published studies over the last 20 years summarizes the complex multifactorial immunopathology of CLAD onset and progression. It highlights the phenotype of several key effector immune cells involved in CLAD pathogenesis, as well as the paucity of single cell resolution spatial studies in lung tissue from patients with CLAD.

Chronic Lung Allograft Dysfunction: Evolving Concepts and Therapies

The primary factor that limits long-term survival after lung transplantation is chronic lung allograft dysfunction (CLAD). CLAD also impairs quality of life and increases the costs of medical care. Our understanding of CLAD continues to evolve. Consensus definitions of CLAD and the major CLAD phenotypes were recently updated and clarified, but it remains to be seen whether the current definitions will lead to advances in management or impact care. Understanding the potential differences in pathogenesis for each CLAD phenotype may lead to novel therapeutic strategies, including precision medicine. Recognition of CLAD risk factors may lead to earlier interventions to mitigate risk, or to avoid risk factors all together, to prevent the development of CLAD. Unfortunately, currently available therapies for CLAD are usually not effective. However, novel therapeutics aimed at both prevention and treatment are currently under investigation. We provide an overview of the updates to CLAD-related terminology, clinical phenotypes and their diagnosis, natural history, pathogenesis, and potential strategies to treat and prevent CLAD.

Regulatory T Cells Limit Pneumococcus-Induced Exacerbation of Lung Fibrosis in Mice

Patients with idiopathic pulmonary fibrosis (IPF) can experience life-threatening episodes of acute worsening of their disease, termed acute exacerbation of IPF, which may be caused by bacterial and/or viral infections. The potential for regulatory T cells (Tregs) to limit disease progression in bacterially triggered fibrosis exacerbation has not been explored so far. In the current study, we show that the number of Tregs was significantly increased in mice with established AdTGF-β1-induced lung fibrosis and further increased in mice with pneumococcal infection-induced lung fibrosis exacerbation. Diphtheria toxin-induced depletion of Tregs significantly worsened infection-induced fibrosis exacerbation as determined by increased lung collagen deposition, lung histology, and elevated pulmonary Th1/Th2 cytokine levels. Conversely, IL-2 complex-induced Treg expansion in wild-type mice with established lung fibrosis completely inhibited pneumococcal infection-induced fibrosis exacerbation as efficaciously as antibiotic treatment while preserving lung antibacterial immunity in mice. Collectively, these findings demonstrate the efficacy of Tregs as "silencers," suppressing infection-induced exacerbation of lung fibrosis in mice, and their expansion may offer a novel adjunctive treatment to limit acute exacerbations in patients with IPF.

Increased frequency of CD4+ CD25high CD127low T cells early after lung transplant is associated with improved graft survival - a retrospective study

In this retrospective study, we analyzed the presence of any association of three CD4⁺ CD25^high regulatory T-cell subpopulations at 3 weeks after lung transplantation with the later incidence of chronic lung allograft dysfunction and graft survival. Among lung-transplanted patients between January 2009 and April 2018, only patients with sufficient T-cell measurements at 3 weeks after transplantation were included into the study. Putative regulatory T cells were defined as CD4⁺ CD25^high T cells, detected in peripheral blood and further analyzed for CD127^low , FoxP3⁺ , and CD152⁺ using fluorescence-activated cell sorting (FACS) analysis. Associations of regulatory T cells with chronic lung allograft dysfunction (CLAD) and graft survival were evaluated using Cox analysis. During the study period, 724 (71%) patients were included into the study. Freedom from chronic lung allograft dysfunction (CLAD) and graft survival amounted to 66% and 68% at 5 years. At the multivariable analysis, increasing frequencies of CD127^low were associated with better freedom from CLAD (hazard ratio for each 1% increase of %CD127^low , HR = 0.989, 95% CI = 0.981-0.996, P = 0.003) and better graft survival (HR = 0.991, 95% CI = 0.984-0.999, P = 0.026). A higher frequency of CD127^low regulatory T cells in peripheral blood early after lung transplantation estimated a protective effect against chronic lung allograft dysfunction, mortality, and re-transplantation.

Autoantibodies in lung transplantation

Chronic lung allograft dysfunction (CLAD) comprises both bronchiolitis obliterans syndrome and restrictive allograft syndrome as subtypes. After lung transplantation, CLAD remains a major limitation for long-term survival, and lung transplant recipients therefore have poorer outcomes compared with recipients of other solid organ transplants. Although the number of lung transplants continues to increase globally, the field demands detailed understanding of immunoregulatory mechanisms and more effective individualized therapies to combat CLAD. Emerging evidence suggests that CLAD is multifactorial and involves a complex, delicate interplay of multiple factors, including perioperative donor characteristics, inflammation induced immediately following transplant, post-transplant infection and interplay between allo- and autoimmunity directed to donor antigens. Recently, identification of stress-induced exosome release from the transplanted organ has emerged as an underlying mechanism in the development of chronic rejection and promises to prompt novel strategies for future therapeutic interventions. In this review, we will discuss recent studies and ongoing research into the mechanisms for the development of CLAD, with emphasis on immune responses to lung-associated self-antigens-that is, autoimmunity.

Early Identification of Chronic Lung Allograft Dysfunction: The Need of Biomarkers

A growing number of patients with end-stage lung disease have benefited from lung transplantation (LT). Improvements in organ procurement, surgical techniques and intensive care management have greatly increased short-term graft survival. However, long-term outcomes remain limited, mainly due to the onset of chronic lung allograft dysfunction (CLAD), whose diagnosis is based on permanent loss of lung function after the development of irreversible lung lesions. CLAD is associated with high mortality and morbidity, and its exact physiopathology is still only partially understood. Many researchers and clinicians have searched for CLAD biomarkers to improve diagnosis, to refine the phenotypes associated with differential prognosis and to identify early biological processes that lead to CLAD to enable an early intervention that could modify the inevitable degradation of respiratory function. Donor-specific antibodies are currently the only biomarkers used in routine clinical practice, and their significance for accurately predicting CLAD is still debated. We describe here significant studies that have highlighted potential candidates for reliable and non-invasive biomarkers of CLAD in the fields of imaging and functional monitoring, humoral immunity, cell-mediated immunity, allograft injury, airway remodeling and gene expression. Such biomarkers would improve CLAD prediction and allow differential LT management regarding CLAD risk.

Diagnosing and managing bronchiolitis obliterans in children

INTRODUCTION

Bronchiolitis obliterans (BO) is a chronic and irreversible obstructive lung disease leading to the obstruction and/or obliteration of the small airways. Three main BO entities are distinguished: post-infectious BO (PIBO); BO post lung transplantation; and BO after bone marrow transplantation (BMT) or hematopoietic stem cell transplantation (HSCT). All three entities are separate, however, there are similarities in histopathological characteristics and possibly in aspects of the development pathway. Areas covered: We review current evidence of bronchiolitis obliterans diagnosis and management in children. The diagnosis of BO is usually based on a combination of history, clinical and radiological findings, although lung biopsy and histopathology remain the gold standard approaches to confirm BO. Expert opinion: At present, we do not have a clear understanding of the mechanisms of the development of BO and lack strong evidence for treatment. Although most BO in children is post-infectious, most of the current evidence for treatment originates from studies analyzing BO in adult lung transplant and HSCT patients. BO management requires multidisciplinary approach and care in specialized centers.

Transplant arteriosclerosis in humanized mice reflects chronic lung allograft dysfunction and is controlled by regulatory T cells

OBJECTIVE

Chronic lung allograft dysfunction (CLAD) is a severe complication of lung transplantation limiting long-term survival. We studied correlations between CLAD after clinical lung transplantation and leukocyte-mediated development of transplant arteriosclerosis (TA) in a humanized mouse model. The pericardiophrenic artery was procured from surplus tissue of donor lungs (n = 22) transplanted in our clinical program and was implanted into the abdominal aorta of immune-deficient mice.

METHODS

Allogeneic human peripheral blood mononuclear cells (PBMCs) had been procured 1 day after lung transplantation from the respective recipients with or without enriching for CD4⁺CD25^high T cells were used. TA was assessed in mice 28 days later by histology. The respective clinical lung recipients were later divided into 2 groups. Eight patients (36.3%) had developed CLAD 23 ± 5 months after lung transplantation, whereas the remaining 14 (63.6%) did not develop CLAD within 25 ± 5 months.

RESULTS

In the PBMC CLAD+ group of mouse experiments, TA was significantly more severe than in the PBMC CLAD- group (39.9% ± 13% vs 14.9% ± 4% intimal thickening; P = .0081). Then, intimal thickening was significantly inhibited in the PBMC+ regulatory T cells CLAD+ group compared with the PBMC CLAD+ group (0.4% ± 4% vs 39.9% ± 13%; P = .003). In the experiments using PBMCs from lung recipients without CLAD, enriching regulatory T cells also suppressed the development of TA (0.9% ± 3% PBMC CLAD- vs 14.9% ± 4% PBMC+ regulatory T cells CLAD-; P = .001).

CONCLUSIONS

Lung transplant recipients who later develop CLAD have peripheral leukocytes already at the time of transplant that transfer proinflammatory properties leading to TA in a humanized mouse model. TA remains sensitive to inhibition by autologous regulatory T cells, suggesting a cell therapy-based approach for the prevention of CLAD after lung transplantation.

High circulating CD4+CD25hiFOXP3+ T-cell sub-population early after lung transplantation is associated with development of bronchiolitis obliterans syndrome

BACKGROUND

Chronic bronchiolitis obliterans syndrome (BOS) remains a major limitation for long-term survival after lung transplantation. The immune mechanisms involved and predictive biomarkers have yet to be identified. The purpose of this study was to determine whether peripheral blood T-lymphocyte profile could predict BOS in lung transplant recipients.

METHODS

An in-depth profiling of CD4⁺ and CD8⁺ T cells was prospectively performed on blood cells from stable (STA) and BOS patients with a longitudinal follow-up. Samples were analyzed at 1 and 6 months after transplantation, at the time of BOS diagnosis, and at an intermediate time-point at 6 to 12 months before BOS diagnosis.

RESULTS

Although no significant difference was found for T-cell compartments at BOS diagnosis or several months beforehand, we identified an increase in the CD4⁺CD25^hiFoxP3⁺ T-cell sub-population in BOS patients at 1 and 6 months after transplantation (3.39 ± 0.40% vs 1.67 ± 0.22% in STA, p < 0.001). A CD4⁺CD25^hiFoxP3⁺ T-cell threshold of 2.4% discriminated BOS and stable patients at 1 month post-transplantation. This was validated on a second set of patients at 6 months post-transplantation. Patients with a proportion of CD4⁺CD25^hiFoxP3⁺ T cells up to 2.4% in the 6 months after transplantation had a 2-fold higher risk of developing BOS.

CONCLUSIONS

This study is the first to report an increased proportion of circulating CD4⁺CD25^hiFoxP3⁺ T cells early post-transplantation in lung recipients who proceed to develop BOS within 3 years, which supports its use as a BOS predictive biomarker.

Analysis of long term CD4+CD25highCD127- T-reg cells kinetics in peripheral blood of lung transplant recipients

Background

The role of CD4⁺CD25^highCD127⁻ T-reg cells in solid-organ Transplant (Tx) acceptance has been extensively studied. In previous studies on kidney and liver recipients, peripheral T-reg cell counts were associated to graft survival, while in lung Tx, there is limited evidence for similar findings. This study aims to analyze long term peripheral kinetics of T-reg-cells in a cohort of lung recipients and tests its association to several clinical variables.

Methods

From jan 2009 to dec 2014, 137 lung Tx recipients were submitted to an immunological follow up (median: 105.9 months (6.7–310.5)). Immunological follow up consisted of a complete blood peripheral immuno-phenotype, inclusive of CD4⁺CD25^highCD127⁻ T and FOXP3+ cells. We tested the association between T-reg and relevant variables by linear OR regression models for repeated measures, adjusting for time from Tx. Also, by ordered logistic models for panel data, the association between Chronic Lung Allograft Dysfuncton (CLAD) onset/progression and T-reg counts in the previous 3 months was tested.

Results

Among all variables analyzed at multivariate analysis: Bronchiolitis Obliterans Syndrome (OR −6.51, p < 0.001), Restrictive Allograft Syndrome (OR −5.19, p = 0.04) and Extracorporeal photopheresis (OR −5.65, p < 0.001) were significantly associated to T-reg cell. T-reg cell counts progressively decreased according to the severity of CLAD. Furthermore, patients with higher mean T-reg counts in a trimester had a significantly lower risk (OR 0.97, p = 0.012) of presenting CLAD or progressing in the graft dysfunction in the following trimester.

Conclusions

Our present data confirm animal observations on the possible role of T-reg in the evolution of CLAD.

Association of Higher CD4+ CD25high CD127low , FoxP3+ , and IL-2+ T Cell Frequencies Early After Lung Transplantation With Less Chronic Lung Allograft Dysfunction at Two Years

Regulatory T cells (Treg) can regulate alloantigens and may counteract chronic lung allograft dysfunction (CLAD) in lung transplantation. We analyzed Treg in peripheral blood prospectively and correlated percentages of subpopulations with the incidence of CLAD at 2 years. Among lung-transplanted patients between January 2009 and July 2011, only patients with sufficient Treg measurements were included into the study. Tregs were measured immediately before lung transplantation, at 3 weeks and 3, 6, 12, and 24 months after transplantation and were defined as CD4⁺ CD25^high T cells and further analyzed for CTLA4, CD127, FoxP3, and IL-2 expressions. Between January 2009 and July 2011, 264 patients were transplanted at our institution. Among the 138 (52%) patients included into the study, 31 (22%) developed CLAD within 2 years after transplantation. As soon as 3 weeks after lung transplantation, a statistically significant positive association was detected between Treg frequencies and later absence of CLAD. At the multivariate analysis, increasing frequencies of CD4⁺ CD25^high CD127^low , CD4⁺ CD25^high FoxP3⁺ and CD4⁺ CD25^high IL-2⁺ T cells at 3 weeks after lung transplantation emerged as protective factors against development of CLAD at 2 years. In conclusion, higher frequencies of specific Treg subpopulations early after lung transplantation are protective against CLAD development.

In Vivo Development of Transplant Arteriosclerosis in Humanized Mice Reflects Alloantigen Recognition and Peripheral Treg Phenotype of Lung Transplant Recipients

Experimentally, regulatory T cells inhibit rejection. In clinical transplantations, however, it is not known whether T cell regulation is the cause for, or an epiphenomenon of, long-term allograft survival. Here, we study naïve and alloantigen-primed T cell responses of clinical lung transplant recipients in humanized mice. The pericardiophrenic artery procured from human lung grafts was implanted into the aorta of NODrag^-/- /IL-2rγc^-/- mice reconstituted with peripheral blood mononuclear cells (PBMCs) from the respective lung recipient. Naïve or primed allogeneic PBMCs procured 21 days post-lung transplantation with or without enriching for CD4⁺ CD25^high T cells were used. Transplant arteriosclerosis was assessed 28 days later by histology. Mice reconstituted with alloantigen-primed PBMCs showed significantly more severe transplant arteriosclerosis than did mice with naïve PBMCs (p = 0.005). Transplant arteriosclerosis was equally suppressed by enriching for autologous naïve (p = 0.012) or alloantigen-primed regulatory T cells (Tregs) (p = 0.009). Alloantigen priming in clinical lung recipients can be adoptively transferred into a humanized mouse model. Transplant arteriosclerosis elicited by naïve or alloantigen-primed PBMCs can be similarly controlled by potent autologous Tregs. Cellular therapy with expanded autologous Tregs in lung transplantation might be a promising future strategy.

Altered Exosomal RNA Profiles in Bronchoalveolar Lavage from Lung Transplants with Acute Rejection

RATIONALE

The mechanism by which acute allograft rejection leads to chronic rejection remains poorly understood despite its common occurrence. Exosomes, membrane vesicles released from cells within the lung allograft, contain a diverse array of biomolecules that closely reflect the biologic state of the cell and tissue from which they are released. Exosome transcriptomes may provide a better understanding of the rejection process. Furthermore, biomarkers originating from this transcriptome could provide timely and sensitive detection of acute cellular rejection (AR), reducing the incidence of severe AR and chronic lung allograft dysfunction and improving outcomes.

OBJECTIVES

To provide an in-depth analysis of the bronchoalveolar lavage fluid exosomal shuttle RNA population after lung transplantation and evaluate for differential expression between acute AR and quiescence.

METHODS

Serial bronchoalveolar lavage specimens were ultracentrifuged to obtain the exosomal pellet for RNA extraction, on which RNA-Seq was performed.

MEASUREMENTS AND MAIN RESULTS

AR demonstrates an intense inflammatory environment, skewed toward both innate and adaptive immune responses. Novel, potential upstream regulators identified offer potential therapeutic targets.

CONCLUSIONS

Our findings validate bronchoalveolar lavage fluid exosomal shuttle RNA as a source for understanding the pathophysiology of AR and for biomarker discovery in lung transplantation.

Altered Exosomal RNA Profiles in Bronchoalveolar Lavage from Lung Transplants with Acute Rejection

RATIONALE

The mechanism by which acute allograft rejection leads to chronic rejection remains poorly understood despite its common occurrence. Exosomes, membrane vesicles released from cells within the lung allograft, contain a diverse array of biomolecules that closely reflect the biologic state of the cell and tissue from which they are released. Exosome transcriptomes may provide a better understanding of the rejection process. Furthermore, biomarkers originating from this transcriptome could provide timely and sensitive detection of acute cellular rejection (AR), reducing the incidence of severe AR and chronic lung allograft dysfunction and improving outcomes.

OBJECTIVES

To provide an in-depth analysis of the bronchoalveolar lavage fluid exosomal shuttle RNA population after lung transplantation and evaluate for differential expression between acute AR and quiescence.

METHODS

Serial bronchoalveolar lavage specimens were ultracentrifuged to obtain the exosomal pellet for RNA extraction, on which RNA-Seq was performed.

MEASUREMENTS AND MAIN RESULTS

AR demonstrates an intense inflammatory environment, skewed toward both innate and adaptive immune responses. Novel, potential upstream regulators identified offer potential therapeutic targets.

CONCLUSIONS

Our findings validate bronchoalveolar lavage fluid exosomal shuttle RNA as a source for understanding the pathophysiology of AR and for biomarker discovery in lung transplantation.

Altered Exosomal RNA Profiles in Bronchoalveolar Lavage from Lung Transplants with Acute Rejection

RATIONALE

The mechanism by which acute allograft rejection leads to chronic rejection remains poorly understood despite its common occurrence. Exosomes, membrane vesicles released from cells within the lung allograft, contain a diverse array of biomolecules that closely reflect the biologic state of the cell and tissue from which they are released. Exosome transcriptomes may provide a better understanding of the rejection process. Furthermore, biomarkers originating from this transcriptome could provide timely and sensitive detection of acute cellular rejection (AR), reducing the incidence of severe AR and chronic lung allograft dysfunction and improving outcomes.

OBJECTIVES

To provide an in-depth analysis of the bronchoalveolar lavage fluid exosomal shuttle RNA population after lung transplantation and evaluate for differential expression between acute AR and quiescence.

METHODS

Serial bronchoalveolar lavage specimens were ultracentrifuged to obtain the exosomal pellet for RNA extraction, on which RNA-Seq was performed.

MEASUREMENTS AND MAIN RESULTS

AR demonstrates an intense inflammatory environment, skewed toward both innate and adaptive immune responses. Novel, potential upstream regulators identified offer potential therapeutic targets.

CONCLUSIONS

Our findings validate bronchoalveolar lavage fluid exosomal shuttle RNA as a source for understanding the pathophysiology of AR and for biomarker discovery in lung transplantation.

Staphylococcus via an interaction with the ELR+ CXC chemokine ENA-78 is associated with BOS

Staphylococcus aureus is the most commonly isolated gram-positive bacterium after lung transplantation (LT) and has been associated with poor posttransplant outcomes, but its effect on bronchiolitis obliterans syndrome (BOS) and death in the context of the allograft inflammatory environment has not been studied. A three-state Cox semi-Markovian model was used to determine the influence of allograft S. aureus and the ELR+ CXC chemokines on the survival rates and cause-specific hazards for movement from lung transplant (State 1) to BOS (State 2), from transplant (State 1) to death (State 3), and from BOS (State 2) to death (State 3). Acute rejection, pseudomonas pneumonia, bronchoalveolar lavage fluid (BALF) CXCL5 and its interaction with S. aureus all increased the likelihood of transition from transplant to BOS. Transition to death from transplant was facilitated by pseudomonas infection and single lung transplant. Movement from BOS to death was affected by the interaction between aspergillus, pseudomonas and CXCL5, but not S. aureus. S. aureus isolation had state specific effects after LT and only in concert with elevated BALF CXCL5 concentrations did it augment the risk of BOS. Pseudomonas and elevated BALF concentrations of CXCL5 continued as significant risk factors for BOS and death after BOS in lung transplantation.

Control of tissue-localized immune responses by human regulatory T cells

Treg cells control immune responses to self and nonharmful foreign antigens. Emerging data from animal models indicate that Treg cells function in both secondary lymphoid organs and tissues, and that these different microenvironments may contain specialized subsets of Treg cells with distinct mechanisms of action. The design of therapies for the restoration of tissue-localized immune homeostasis is dependent upon understanding how local immune responses are influenced by Treg cells in health versus disease. Here we review the current state of knowledge about human Treg cells in four locations: the skin, lung, intestine, and joint. Despite the distinct biology of these tissues, there are commonalities in the biology of their resident Treg cells, including phenotypic and functional differences from circulating Treg cells, and the presence of cytokine-producing (e.g. IL-17(+)) FOXP3(+) cells. We also highlight the challenges to studying tissue Treg cells in humans, and opportunities to use new technologies for the detailed analysis of Treg cells at the single-cell level. As emerging biological therapies are increasingly targeted toward tissue-specific effects, it is critical to understand their potential impact on local immune regulation.

Preventive azithromycin treatment reduces noninfectious lung injury and acute graft-versus-host disease in a murine model of allogeneic hematopoietic cell transplantation

Noninfectious lung injury and acute graft-versus-host disease (GVHD) after allogeneic hematopoietic cell transplantation (allo-HCT) are associated with significant morbidity and mortality. Azithromycin is widely used in allogeneic HCT recipients for pulmonary chronic GVHD, although current data appear controversial. We induced GVHD and noninfectious lung injury in lethally irradiated B6D2F1 mice by transplanting bone marrow and splenic T cells from allogeneic C57BL/6 mice. Experimental groups were treated with oral azithromycin starting on day 14 until the end of week 6 or week 14 after transplantation. Azithromycin treatment resulted in improved survival and decreased lung injury; the latter characterized by improved pulmonary function, reduced peribronchial and perivascular inflammatory cell infiltrates along with diminished collagen deposition, and a decrease in lung cytokine and chemokine expression. Azithromycin also improved intestinal GVHD but did not affect liver GVHD at week 6 early after transplantation. At week 14, azithromycin decreased liver GVHD but had no effect on intestinal GVHD. In vitro, allogeneic antigen-presenting cell (APC)- dependent T cell proliferation and cytokine production were suppressed by azithromycin and inversely correlated with relative regulatory T cell (Treg) expansion, whereas no effect was seen when T cell proliferation occurred APC independently through CD3/CD28-stimulation. Further, azithromycin reduced alloreactive T cell expansion but increased Treg expansion in vivo with corresponding downregulation of MHC II on CD11c(+) dendritic cells. These results demonstrate that preventive administration of azithromycin can reduce the severity of acute GVHD and noninfectious lung injury after allo-HCT, supporting further investigation in clinical trials.

Central memory CD8+ T lymphocytes mediate lung allograft acceptance

Memory T lymphocytes are commonly viewed as a major barrier for long-term survival of organ allografts and are thought to accelerate rejection responses due to their rapid infiltration into allografts, low threshold for activation, and ability to produce inflammatory mediators. Because memory T cells are usually associated with rejection, preclinical protocols have been developed to target this population in transplant recipients. Here, using a murine model, we found that costimulatory blockade-mediated lung allograft acceptance depended on the rapid infiltration of the graft by central memory CD8+ T cells (CD44(hi)CD62L(hi)CCR7+). Chemokine receptor signaling and alloantigen recognition were required for trafficking of these memory T cells to lung allografts. Intravital 2-photon imaging revealed that CCR7 expression on CD8+ T cells was critical for formation of stable synapses with antigen-presenting cells, resulting in IFN-γ production, which induced NO and downregulated alloimmune responses. Thus, we describe a critical role for CD8+ central memory T cells in lung allograft acceptance and highlight the need for tailored approaches for tolerance induction in the lung.

Increased numbers of circulating CD8 effector memory T cells before transplantation enhance the risk of acute rejection in lung transplant recipients

The effector and regulatory T cell subpopulations involved in the development of acute rejection episodes in lung transplantation remain to be elucidated. Twenty-seven lung transplant candidates were prospectively monitored before transplantation and within the first year post-transplantation. Regulatory, Th17, memory and naïve T cells were measured in peripheral blood of lung transplant recipients by flow cytometry. No association of acute rejection with number of peripheral regulatory T cells and Th17 cells was found. However, effector memory subsets in acute rejection patients were increased during the first two months post-transplant. Interestingly, patients waiting for lung transplant with levels of CD8⁺ effector memory T cells over 185 cells/mm³ had a significant increased risk of rejection [OR: 5.62 (95% CI: 1.08-29.37), p=0.04]. In multivariate analysis adjusted for age and gender the odds ratio for rejection was: OR: 5.89 (95% CI: 1.08-32.24), p=0.04. These data suggest a correlation between acute rejection and effector memory T cells in lung transplant recipients. The measurement of peripheral blood CD8⁺ effector memory T cells prior to lung transplant may define patients at high risk of acute lung rejection.

Bronchiolitis obliterans syndrome: the Achilles' heel of lung transplantation

Lung transplantation is a therapeutic option for patients with end-stage pulmonary disorders. Unfortunately, chronic lung allograft dysfunction (CLAD), most commonly manifest as bronchiolitis obliterans syndrome (BOS), continues to be highly prevalent and is the major limitation to long-term survival. The pathogenesis of BOS is complex and involves alloimmune and nonalloimmune pathways. Clinically, BOS manifests as airway obstruction and dyspnea that are classically progressive and ultimately fatal; however, the course is highly variable, and distinguishable phenotypes may exist. There are few controlled studies assessing treatment efficacy, but only a minority of patients respond to current treatment modalities. Ultimately, preventive strategies may prove more effective at prolonging survival after lung transplantation, but their remains considerable debate and little data regarding the best strategies to prevent BOS. A better understanding of the risk factors and their relationship to the pathological mechanisms of chronic lung allograft rejection should lead to better pharmacological targets to prevent or treat this syndrome.

Bronchoalveolar lavage as a tool to predict, diagnose and understand bronchiolitis obliterans syndrome

Bronchiolitis obliterans syndrome (BOS), a condition of irreversible small airway fibrosis, is the principal factor limiting long-term survival after lung transplantation. Bronchoscopy and bronchoalveolar lavage (BAL), techniques central to lung transplant clinical practice, provide a unique opportunity to interrogate the lung allograft during BOS development and identify potential disease mechanisms or biomarkers. Over the past 20 years, numerous studies have evaluated the BAL cellular composition, cytokine profiles and protein constituents in lung transplant recipients with BOS. To date, however, no summative evaluation of this literature has been reported. We developed and applied objective criteria to qualitatively rank the strength of associations between BAL parameters and BOS in order to provide a comprehensive and systematic assessment of the literature. Our analysis indicates that several BAL parameters, including neutrophil count, interleukin-8, alpha defensins and MMP-9, demonstrate highly replicable associations with BOS. Additionally, we suggest that considerable opportunity exists to increase the knowledge gained from BAL analyses in BOS through increased sample sizes, covariant adjustment and standardization of the BAL technique. Further efforts to leverage analysis of BAL constituents in BOS may offer great potential to provide additional in-depth and mechanistic insights into the pathogenesis of this complex disease.

Interaction between Pseudomonas and CXC chemokines increases risk of bronchiolitis obliterans syndrome and death in lung transplantation

RATIONALE

Pseudomonas aeruginosa is the most commonly isolated gram-negative bacterium after lung transplantation and has been shown to up-regulate glutamic acid-leucine-arginine-positive (ELR(+)) CXC chemokines associated with bronchiolitis obliterans syndrome (BOS), but the effect of pseudomonas on BOS and death has not been well defined.

OBJECTIVES

To determine if the influence of pseudomonas isolation and ELR(+) CXC chemokines on the subsequent development of BOS and the occurrence of death is time dependent.

METHODS

A three-state model was developed to assess the likelihood of transitioning from lung transplant (state 1) to BOS (state 2), from transplant (state 1) to death (state 3), and from BOS (state 2) to death (state 3). This Cox semi-Markovian approach determines state survival rates and cause-specific hazards for movement from one state to another.

MEASUREMENTS AND MAIN RESULTS

The likelihood of transition from transplant to BOS was increased by acute rejection, CXCL5, and the interaction between pseudomonas and CXCL1. The pseudomonas effect in this transition was due to infection rather than colonization. Movement from transplant to death was facilitated by pseudomonas infection and single lung transplant. Transition from BOS to death was affected by the length of time in state 1 and by the interactions between any pseudomonas isolation and CXCL5 and aspergillus, either independently or in combination.

CONCLUSIONS

Our model demonstrates that common post-transplantation events drive movement from one post-transplantation state to another and influence outcomes differently depending upon when after transplantation they occur. Pseudomonas and the ELR(+) CXC chemokines may interact to negatively influence lung transplant outcomes.

Lung transplantation affects expression of the chemokine receptor type 4 on specific T cell subsets

Alloreactive T cells that infiltrate the graft after lung transplantation (LTx) play a role in chronic rejection. Chemokines such as thymus and activation-regulated chemokine (TARC), macrophage-derived chemokine (MDC) and monocyte chemotactic protein-1 (MCP-1) are produced locally in the lung and attract T cells via chemokine receptor 4 (CCR4). In a TARC gradient, cells expressing CCR4(++) migrate more efficiently than CCR4(+) -expressing cells. In this study, we compared the CCR4 expression of T cells in blood from 20 lung transplant recipients to healthy controls. We then examined whether CCR4 expression is associated with the occurrence of chronic rejection. The CCR4(++) expression was decreased on CD4 T cells from LTx patients (P < 0·0001) when compared to healthy controls. The analysis of CD4 T cell subsets showed that this decrease was present on central memory, effector memory and terminally differentiated T cells (P = 0·0007, P < 0·0001 and P = 0·05, respectively), while a trend was found for naive CD4 T cells (P = 0·06). Also, the expression of CCR4(+) on regulatory T cells (T(regs) ) was decreased in LTx patients when compared to healthy controls (P = 0·02). Interestingly, the CCR4(++) expression on CD4 effector memory T cells was decreased in patients developing chronic rejection sometimes more than a year before the clinical diagnosis when compared to patients who did not (P = 0·04). The analysis of CD8 T cell subsets only showed the CCR4(+) expression to be increased significantly on effector memory and terminally differentiated CD8 T cells (P = 0·02, P = 0·03, respectively) in LTx patients, but no relation was found in chronic rejection. In conclusion, the expression of CCR4 on T cell subsets was altered after LTx and appears to be related to chronic rejection.

Do CD4+ Foxp3+ Treg cells correlate with transplant outcomes: a systematic review on recipients of solid organ transplantation

Regulatory T cells (Tregs) are considered to be critical for the induction of transplant tolerance. Tregs counts were measured in blood, biopsy and urine sample after transplantation in many studies. Although not unanimous, some studies have suggested that Tregs is associated with better outcome and can also serve as an immune marker to predict the individual risk of rejection and identify tolerant patients. In this study, we systematically reviewed the correlation between Tregs and transplant outcomes, identifying if Tregs can predict transplant rejection and tolerance. A total of 22 articles were included and assessed, the results showed that Tregs in recipients are helpful to maintain a stable graft function, reduce acute/chronic rejection rate. And the Tregs in graft and urine, rather than in PBL, may have a better diagnostic value for transplant outcomes. However, since the low quality of included studies, results may be influenced by bias. More high quality studies with bigger sample size are still needed in future.