Hyaluronan contributes to bronchiolitis obliterans syndrome and stimulates lung allograft rejection through activation of innate immunity

Metabolite-derived damage-associated molecular patterns in immunological diseases

Damage-associated molecular patterns (DAMPs) are typically derived from the endogenous elements of necrosis cells and can trigger inflammatory responses by activating DAMPs-sensing receptors on immune cells. Failure to clear DAMPs may lead to persistent inflammation, thereby contributing to the pathogenesis of immunological diseases. This review focuses on a newly recognized class of DAMPs derived from lipid, glucose, nucleotide, and amino acid metabolic pathways, which are then termed as metabolite-derived DAMPs. This review summarizes the reported molecular mechanisms of these metabolite-derived DAMPs in exacerbating inflammation responses, which may attribute to the pathology of certain types of immunological diseases. Additionally, this review also highlights both direct and indirect clinical interventions that have been explored to mitigate the pathological effects of these DAMPs. By summarizing our current understanding of metabolite-derived DAMPs, this review aims to inspire future thoughts and endeavors on targeted medicinal interventions and the development of therapies for immunological diseases.

Cyclic guanosine monophosphate-adenosine monophosphate synthetase/stimulator of interferon genes signaling aggravated corneal allograft rejection through neutrophil extracellular traps

The activation of innate immunity following transplantation has been identified as a crucial factor in allograft inflammation and rejection. However, the role of cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)/stimulator of interferon genes (STING) signaling-mediated innate immunity in the pathogenesis of allograft rejection remains unclear. Utilizing a well-established murine model of corneal transplantation, we demonstrated increased expression of cGAS and STING in rejected-corneal allografts compared with syngeneic (Syn) and normal (Nor) corneas, along with significant activation of the cGAS/STING pathway, as evidenced by the enhanced phosphorylation of TANK-binding kinase 1and interferon regulatory factor 3. Pharmacological and genetic inhibition of cGAS/STING signaling markedly delayed corneal transplantation rejection, resulting in prolonged survival time and reduced inflammatory infiltration. Furthermore, we observed an increase in the formation of neutrophil extracellular traps (NETs) in rejected allografts, and the inhibition of NET formation through targeting peptidylarginine deiminase 4 and DNase I treatment significantly alleviated immune rejection and reduced cGAS/STING signaling activity. Conversely, subconjunctival injection of NETs accelerated corneal transplantation rejection and enhanced the activation of the cGAS/STING pathway. Collectively, these findings demonstrate that NETs contribute to the exacerbation of allograft rejection via cGAS/STING signaling, highlighting the targeting of the NETs/cGAS/STING signaling pathway as a potential strategy for prolonging allograft survival.

Mitsugumin 53 mitigation of ischemia-reperfusion injury in a mouse model

OBJECTIVE

Primary graft dysfunction is often attributed to ischemia-reperfusion injury, and prevention would be a therapeutic approach to mitigate injury. Mitsugumin 53, a myokine, is a component of the endogenous cell membrane repair machinery. Previously, exogenous administration of recombinant human (recombinant human mitsugumin 53) protein has been shown to mitigate acute lung injury. In this study, we aimed to quantify a therapeutic benefit of recombinant human mitsugumin 53 to mitigate a transplant-relevant model of ischemia-reperfusion injury.

METHODS

C57BL/6J mice were subjected to 1 hour of ischemia (via left lung hilar clamp), followed by 24 hours of reperfusion. mg53-/- mice were administered exogenous recombinant human mitsugumin 53 or saline before reperfusion. Tissue, bronchoalveolar lavage, and blood samples were collected at death and used to quantify the extent of lung injury via histology and biochemical assays.

RESULTS

Administration of recombinant human mitsugumin 53 showed a significant decrease in an established biometric profile of lung injury as measured by lactate dehydrogenase and endothelin-1 in the bronchoalveolar lavage and plasma. Biochemical markers of apoptosis and pyroptosis (interleukin-1β and tumor necrosis factor-α) were also significantly mitigated, overall demonstrating recombinant human mitsugumin 53's ability to decrease the inflammatory response of ischemia-reperfusion injury. Exogenous recombinant human mitsugumin 53 administration showed a trend toward decreasing overall cellular infiltrate and neutrophil response. Fluorescent colocalization imaging revealed recombinant human mitsugumin 53 was effectively delivered to the endothelium.

CONCLUSIONS

These data demonstrate that recombinant human mitsugumin 53 has the potential to prevent or reverse ischemia-reperfusion injury-mediated lung damage. Although additional studies are needed in wild-type mice to demonstrate efficacy, this work serves as proof-of-concept to indicate the potential therapeutic benefit of mitsugumin 53 administration to mitigate ischemia-reperfusion injury.

Lymphatic vessel: origin, heterogeneity, biological functions, and therapeutic targets

Lymphatic vessels, comprising the secondary circulatory system in human body, play a multifaceted role in maintaining homeostasis among various tissues and organs. They are tasked with a serious of responsibilities, including the regulation of lymph absorption and transport, the orchestration of immune surveillance and responses. Lymphatic vessel development undergoes a series of sophisticated regulatory signaling pathways governing heterogeneous-origin cell populations stepwise to assemble into the highly specialized lymphatic vessel networks. Lymphangiogenesis, as defined by new lymphatic vessels sprouting from preexisting lymphatic vessels/embryonic veins, is the main developmental mechanism underlying the formation and expansion of lymphatic vessel networks in an embryo. However, abnormal lymphangiogenesis could be observed in many pathological conditions and has a close relationship with the development and progression of various diseases. Mechanistic studies have revealed a set of lymphangiogenic factors and cascades that may serve as the potential targets for regulating abnormal lymphangiogenesis, to further modulate the progression of diseases. Actually, an increasing number of clinical trials have demonstrated the promising interventions and showed the feasibility of currently available treatments for future clinical translation. Targeting lymphangiogenic promoters or inhibitors not only directly regulates abnormal lymphangiogenesis, but improves the efficacy of diverse treatments. In conclusion, we present a comprehensive overview of lymphatic vessel development and physiological functions, and describe the critical involvement of abnormal lymphangiogenesis in multiple diseases. Moreover, we summarize the targeting therapeutic values of abnormal lymphangiogenesis, providing novel perspectives for treatment strategy of multiple human diseases.

Biometric Profiling to Quantify Lung Injury Through Ex Vivo Lung Perfusion Following Warm Ischemia

Standard physiologic assessment parameters of donor lung grafts may not accurately reflect lung injury or quality. A biometric profile of ischemic injury could be identified as a means to assess the quality of the donor allograft. We sought to identify a biometric profile of lung ischemic injury assessed during ex vivo lung perfusion (EVLP). A rat model of lung donation after circulatory death (DCD) warm ischemic injury with subsequent EVLP evaluation was utilized. We did not observe a significant correlation between the classical physiological assessment parameters and the duration of the ischemic. In the perfusate, solubilized lactate dehydrogenase (LDH) as well as hyaluronic acid (HA) significantly correlated with duration of ischemic injury and length of perfusion ( p < 0.05). Similarly, in perfusates, the endothelin-1 (ET-1) and Big ET-1 correlated ischemic injury ( p < 0.05) and demonstrated a measure of endothelial cell injury. In tissue protein expression, heme oxygenase-1 (HO-1), angiopoietin 1 (Ang-1), and angiopoietin 2 (Ang-2) levels were correlated with the duration of ischemic injury ( p < 0.05). Cleaved caspase-3 levels were significantly elevated at 90 and 120 minutes ( p < 0.05) demonstrating increased apoptosis. A biometric profile of solubilized and tissue protein markers correlated with cell injury is a critical tool to aid in the evaluation of lung transplantation, as accurate evaluation of lung quality is imperative and improved quality leads to better results. http://links.lww.com/ASAIO/B49.

JAK-STAT activation contributes to cytotoxic T cell-mediated basal cell death in human chronic lung allograft dysfunction

Chronic lung allograft dysfunction (CLAD) is the leading cause of death in lung transplant recipients. CLAD is characterized clinically by a persistent decline in pulmonary function and histologically by the development of airway-centered fibrosis known as bronchiolitis obliterans. There are no approved therapies to treat CLAD, and the mechanisms underlying its development remain poorly understood. We performed single-cell RNA-Seq and spatial transcriptomic analysis of explanted tissues from human lung recipients with CLAD, and we performed independent validation studies to identify an important role of Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling in airway epithelial cells that contributes to airway-specific alloimmune injury. Specifically, we established that activation of JAK-STAT signaling leads to upregulation of major histocompatibility complex 1 (MHC-I) in airway basal cells, an important airway epithelial progenitor population, which leads to cytotoxic T cell-mediated basal cell death. This study provides mechanistic insight into the cell-to-cell interactions driving airway-centric alloimmune injury in CLAD, suggesting a potentially novel therapeutic strategy for CLAD prevention or treatment.

The lymphatic vasculature in lung function and respiratory disease

The lymphatic vasculature maintains tissue homeostasis via fluid drainage in the form of lymph and immune surveillance due to migration of leukocytes through the lymphatics to the draining lymph nodes. Lymphatic endothelial cells (LECs) form the lymphatic vessels and lymph node sinuses and are key players in shaping immune responses and tolerance. In the healthy lung, the vast majority of lymphatic vessels are found along the bronchovascular structures, in the interlobular septa, and in the subpleural space. Previous studies in both mice and humans have shown that the lymphatics are necessary for lung function from the neonatal period through adulthood. Furthermore, changes in the lymphatic vasculature are observed in nearly all respiratory diseases in which they have been analyzed. Recent work has pointed to a causative role for lymphatic dysfunction in the initiation and progression of lung disease, indicating that these vessels may be active players in pathologic processes in the lung. However, the mechanisms by which defects in lung lymphatic function are pathogenic are understudied, leaving many unanswered questions. A more comprehensive understanding of the mechanistic role of morphological, functional, and molecular changes in the lung lymphatic endothelium in respiratory diseases is a promising area of research that is likely to lead to novel therapeutic targets. In this review, we will discuss our current knowledge of the structure and function of the lung lymphatics and the role of these vessels in lung homeostasis and respiratory disease.

Reprogramming alveolar macrophage responses to TGF-β reveals CCR2+ monocyte activity that promotes bronchiolitis obliterans syndrome

Bronchiolitis obliterans syndrome (BOS) is a major impediment to lung transplant survival and is generally resistant to medical therapy. Extracorporeal photophoresis (ECP) is an immunomodulatory therapy that shows promise in stabilizing BOS patients, but its mechanisms of action are unclear. In a mouse lung transplant model, we show that ECP blunts alloimmune responses and inhibits BOS through lowering airway TGF-β bioavailability without altering its expression. Surprisingly, ECP-treated leukocytes were primarily engulfed by alveolar macrophages (AMs), which were reprogrammed to become less responsive to TGF-β and reduce TGF-β bioavailability through secretion of the TGF-β antagonist decorin. In untreated recipients, high airway TGF-β activity stimulated AMs to express CCL2, leading to CCR2+ monocyte-driven BOS development. Moreover, we found TGF-β receptor 2-dependent differentiation of CCR2+ monocytes was required for the generation of monocyte-derived AMs, which in turn promoted BOS by expanding tissue-resident memory CD8+ T cells that inflicted airway injury through Blimp-1-mediated granzyme B expression. Thus, through studying the effects of ECP, we have identified an AM functional plasticity that controls a TGF-β-dependent network that couples CCR2+ monocyte recruitment and differentiation to alloimmunity and BOS.

Novel approaches for long-term lung transplant survival

Allograft failure remains a major barrier in the field of lung transplantation and results primarily from acute and chronic rejection. To date, standard-of-care immunosuppressive regimens have proven unsuccessful in achieving acceptable long-term graft and patient survival. Recent insights into the unique immunologic properties of lung allografts provide an opportunity to develop more effective immunosuppressive strategies. Here we describe advances in our understanding of the mechanisms driving lung allograft rejection and highlight recent progress in the development of novel, lung-specific strategies aimed at promoting long-term allograft survival, including tolerance.

Blocking hyaluronan synthesis alleviates acute lung allograft rejection

Lung allograft rejection results in the accumulation of low–molecular weight hyaluronic acid (LMW-HA), which further propagates inflammation and tissue injury. We have previously shown that therapeutic lymphangiogenesis in a murine model of lung allograft rejection reduced tissue LMW-HA and was associated with improved transplant outcomes. Herein, we investigated the use of 4-Methylumbelliferone (4MU), a known inhibitor of HA synthesis, to alleviate acute allograft rejection in a murine model of lung transplantation. We found that treating mice with 4MU from days 20 to 30 after transplant was sufficient to significantly improve outcomes, characterized by a reduction in T cell–mediated lung inflammation and LMW-HA content and in improved pathology scores. In vitro, 4MU directly attenuated activation, proliferation, and differentiation of naive CD4⁺ T cells into Th1 cells. As 4MU has already been demonstrated to be safe for human use, we believe examining 4MU for the treatment of acute lung allograft rejection may be of clinical significance.

Modulation of hyaluronan signaling as a therapeutic target in human disease

The extracellular matrix is an active participant, modulator and mediator of the cell, tissue, organ and organismal response to injury. Recent research has highlighted the role of hyaluronan, an abundant glycosaminoglycan constituent of the extracellular matrix, in many fundamental biological processes underpinning homeostasis and disease development. From this basis, emerging studies have demonstrated the therapeutic potential of strategies which target hyaluronan synthesis, biology and signaling, with significant promise as therapeutics for a variety of inflammatory and immune diseases. This review summarizes the state of the art in this field and discusses challenges and opportunities in what could emerge as a new class of therapeutic agents, that we term "matrix biologics".

Intragraft Hyaluronan Increases in Association With Acute Lung Transplant Rejection

Supplemental Digital Content is available in the text.

Background.

Acute perivascular rejection (AR) is common in lung recipients and increases the risk for chronic lung allograft dysfunction (CLAD). Hyaluronan (HA), an extracellular matrix constituent, accumulates in experimental AR and can act as an innate immune agonist, breaking tolerance and potentiating alloimmunity. We previously demonstrated HA accumulates in CLAD after human-lung transplantation. We sought to determine if HA accumulates in the bronchoalveolar lavage fluid (BALF) concurrent with AR in lung recipients.

Methods.

The cohort consisted of 126 first adult lung recipients at 5 transplant centers with a total of 373 BALF samples collected within the first posttransplant year. All samples were paired with a lung biopsy from the same bronchoscopy. BALF HA (ng/mL) was quantified by ELISA and log-transformed for analysis. Linear-mixed effect models, adjusted for potential confounders, were used to estimate the association between BALF HA concentration and the presence of AR on biopsy. The association between early posttransplant BALF HA levels and the development of CLAD was explored utilizing tertiles of maximum BALF HA level observed within the first 6 months of transplant.

Results.

In analyses adjusted for potential confounders, BALF HA concentration was significantly increased in association with AR (change in means on log-scale 0.31; 95% CI, 0.01-0.60; P = 0.044). When considered on the original scale (ng/mL), BALF HA concentrations were 1.36 times (36%) higher, on average, among samples with, versus without, AR. The cumulative incidence of CLAD was numerically higher in individuals in the highest tertiles of BALF HA level within the first 6 months after transplant, as compared with those in the lowest tertile; however, this difference was not statistically significant (P = 0.32).

Conclusions.

These results demonstrate accumulation of HA in clinical AR and suggest a mechanism by which innate and adaptive immune activation might interact in the development of AR and CLAD.

Innate immunity in lung transplantation

Innate immune pathways early after pulmonary transplantation have been shown to cause primary graft dysfunction (PGD) and also predispose to late graft failure. Recent studies in animal models have elucidated critical mechanisms governing such innate immune responses. Here, we discuss pathways of inflammatory cell death, triggers for sterile and infectious inflammation, and signaling cascades that mediate lung injury early after transplantation. These studies highlight potential avenues for lung-specific therapies early following lung transplantation to dampen innate immune responses and improve outcomes.

Protein Signatures of Remodeled Airways in Transplanted Lungs with Bronchiolitis Obliterans Syndrome Obtained Using Laser-Capture Microdissection

Bronchiolitis obliterans syndrome, a common form of chronic lung allograft dysfunction, is the major limitation to long-term survival after lung transplantation. The histologic correlate is progressive, fibrotic occlusion of small airways, obliterative bronchiolitis lesions, which ultimately lead to organ failure. The molecular composition of these lesions is unknown. In this sutdy, the protein composition of the lesions in explanted lungs from four end-stage bronchiolitis obliterans syndrome patients was analyzed using laser-capture microdissection and optimized sample preparation protocols for mass spectrometry. Immunohistochemistry and immunofluorescence were used to determine the spatial distribution of commonly identified proteins on the tissue level, and protein signatures for 14 obliterative bronchiolitis lesions were established. A set of 39 proteins, identified in >75% of lesions, included distinct structural proteins (collagen types IV and VI) and cellular components (actins, vimentin, and tryptase). Each respective lesion exhibited a unique composition of proteins (on average, n = 66 proteins), thereby mirroring the morphologic variation of the lesions. Antibody-based staining confirmed these mass spectrometry-based findings. The 14 analyzed obliterative bronchiolitis lesions showed variations in their protein content, but also common features. This study provides molecular and morphologic insights into the development of chronic rejection after lung transplantation. The protein patterns in the lesions were correlated to pathways of extracellular matrix organization, tissue development, and wound healing processes.

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.

Connective Tissue Growth Factor Is Overexpressed in Explant Lung Tissue and Broncho-Alveolar Lavage in Transplant-Related Pulmonary Fibrosis

Background

Connective tissue growth factor (CTGF) is an important mediator in several fibrotic diseases, including lung fibrosis. We investigated CTGF-expression in chronic lung allograft dysfunction (CLAD) and pulmonary graft-versus-host disease (GVHD).

Materials and Methods

CTGF expression was assessed by quantitative real-time polymerase chain reaction (qPCR) and immunohistochemistry in end-stage CLAD explant lung tissue (bronchiolitis obliterans syndrome (BOS), n=20; restrictive allograft syndrome (RAS), n=20), pulmonary GHVD (n=9). Unused donor lungs served as control group (n=20). Next, 60 matched lung transplant recipients (BOS, n=20; RAS, n=20; stable lung transplant recipients, n=20) were included for analysis of CTGF protein levels in plasma and broncho-alveolar lavage (BAL) fluid at 3 months post-transplant, 1 year post-transplant, at CLAD diagnosis or 2 years post-transplant in stable patients.

Results

qPCR revealed an overall significant difference in the relative content of CTGF mRNA in BOS, RAS and pulmonary GVHD vs. controls (p=0.014). Immunohistochemistry showed a significant higher percentage and intensity of CTGF-positive respiratory epithelial cells in BOS, RAS and pulmonary GVHD patients vs. controls (p<0.0001). BAL CTGF protein levels were significantly higher at 3 months post-transplant in future RAS vs. stable or BOS (p=0.028). At CLAD diagnosis, BAL protein content was significantly increased in RAS patients vs. stable (p=0.0007) and BOS patients (p=0.042). CTGF plasma values were similar in BOS, RAS, and stable patients (p=0.74).

Conclusions

Lung CTGF-expression is increased in end-stage CLAD and pulmonary GVHD; and higher CTGF-levels are present in BAL of RAS patients at CLAD diagnosis. Our results suggest a potential role for CTGF in CLAD, especially RAS, and pulmonary GVHD.

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.

Overview and Challenges of Bronchiolar Disorders

Bronchiolar abnormalities are common and can occur in conditions that affect either the large airways or the more distal parenchyma. In this review, we focus on the diagnosis and management of primary bronchiolar disorders, or conditions in which bronchiolitis is the predominant pathologic process, including constrictive bronchiolitis, follicular bronchiolitis, acute bronchiolitis, respiratory bronchiolitis, and diffuse panbronchiolitis. Due to the nature of abnormalities in the small airway, clinical and physiological changes in bronchiolitis can be subtle, making diagnosis challenging. Primary bronchiolar disorders frequently present with progressive dyspnea and cough that can be out of proportion to imaging and physiologic studies. Pulmonary function tests may be normal, impaired in an obstructive, restrictive, or mixed pattern, or have an isolated decrease in diffusion capacity. High-resolution computed tomography scan is an important diagnostic tool that may demonstrate one or more of the following three patterns: 1) solid centrilobular nodules, often with linear branching opacities (i.e., "tree-in-bud" pattern); 2) ill-defined ground glass centrilobular nodules; and 3) mosaic attenuation on inspiratory images that is accentuated on expiratory images, consistent with geographic air trapping. Bronchiolitis is often missed on standard transbronchial lung biopsies, as the areas of small airway involvement can be patchy. Fortunately, many patients can be diagnosed with a combination of clinical suspicion, inspiratory and expiratory high-resolution computed tomography scans, and pulmonary function testing. Joint consultation of clinicians with both radiologists and pathologists (in cases where histopathology is pursued) is critical to appropriately assess the clinical-radiographic-pathologic context in each individual patient.

Untangling Local Pro-Inflammatory, Reparative, and Regulatory Damage-Associated Molecular-Patterns (DAMPs) Pathways to Improve Transplant Outcomes

Detrimental inflammatory responses after solid organ transplantation are initiated when immune cells sense pathogen-associated molecular patterns (PAMPs) and certain damage-associated molecular patterns (DAMPs) released or exposed during transplant-associated processes, such as ischemia/reperfusion injury (IRI), surgical trauma, and recipient conditioning. These inflammatory responses initiate and propagate anti-alloantigen (AlloAg) responses and targeting DAMPs and PAMPs, or the signaling cascades they activate, reduce alloimmunity, and contribute to improved outcomes after allogeneic solid organ transplantation in experimental studies. However, DAMPs have also been implicated in initiating essential anti-inflammatory and reparative functions of specific immune cells, particularly Treg and macrophages. Interestingly, DAMP signaling is also involved in local and systemic homeostasis. Herein, we describe the emerging literature defining how poor outcomes after transplantation may result, not from just an over-abundance of DAMP-driven inflammation, but instead an inadequate presence of a subset of DAMPs or related molecules needed to repair tissue successfully or re-establish tissue homeostasis. Adverse outcomes may also arise when these homeostatic or reparative signals become dysregulated or hijacked by alloreactive immune cells in transplant niches. A complete understanding of the critical pathways controlling tissue repair and homeostasis, and how alloimmune responses or transplant-related processes disrupt these will lead to new immunotherapeutics that can prevent or reverse the tissue pathology leading to lost grafts due to chronic rejection.

Lymphatic drainage from bronchus-associated lymphoid tissue in tolerant lung allografts promotes peripheral tolerance

Tertiary lymphoid organs are aggregates of immune and stromal cells including high endothelial venules and lymphatic vessels that resemble secondary lymphoid organs and can be induced at nonlymphoid sites during inflammation. The function of lymphatic vessels within tertiary lymphoid organs remains poorly understood. During lung transplant tolerance, Foxp3+ cells accumulate in tertiary lymphoid organs that are induced within the pulmonary grafts and are critical for the local downregulation of alloimmune responses. Here, we showed that tolerant lung allografts could induce and maintain tolerance of heterotopic donor-matched hearts through pathways that were dependent on the continued presence of the transplanted lung. Using lung retransplantation, we showed that Foxp3+ cells egressed from tolerant lung allografts via lymphatics and were recruited into donor-matched heart allografts. Indeed, survival of the heart allografts was dependent on lymphatic drainage from the tolerant lung allograft to the periphery. Thus, our work indicates that cellular trafficking from tertiary lymphoid organs regulates immune responses in the periphery. We propose that these findings have important implications for a variety of disease processes that are associated with the induction of tertiary lymphoid organs.

Bacterial products in donor airways prevent the induction of lung transplant tolerance

Although postoperative bacterial infections can trigger rejection of pulmonary allografts, the impact of bacterial colonization of donor grafts on alloimmune responses to transplanted lungs remains unknown. Here, we tested the hypothesis that bacterial products present within donor grafts at the time of implantation promote lung allograft rejection. Administration of the toll-like receptor 2 (TLR2) agonist Pam₃ Cys₄ to Balb/c wild-type grafts triggered acute cellular rejection after transplantation into B6 wild-type recipients that received perioperative costimulatory blockade. Pam₃ Cys₄ -triggered rejection was associated with an expansion of CD8⁺ T lymphocytes and CD11c⁺ CD11b^hi MHC (major histocompatibility complex) class II⁺ antigen-presenting cells within the transplanted lungs. Rejection was prevented when lungs were transplanted into TLR2-deficient recipients but not when MyD88-deficient donors were used. Adoptive transfer of B6 wild-type monocytes, but not T cells, following transplantation into B6 TLR2-deficient recipients restored the ability of Pam₃ Cys₄ to trigger acute cellular rejection. Thus, we have demonstrated that activation of TLR2 by a bacterial lipopeptide within the donor airways prevents the induction of lung allograft tolerance through a process mediated by recipient-derived monocytes. Our work suggests that donor lungs harboring bacteria may precipitate an inflammatory response that can facilitate allograft rejection.

Advances in Human Lung Transplantation

Lung transplantation improves survival and quality of life in patients with advanced pulmonary disease. Over the past several decades, the volume of lung transplants has grown substantially, with increasing transplantation of older and acutely ill individuals facilitated by improved utilization and preservation of available donor organs. Other advances include improvements in the diagnosis and mechanistic understanding of frequent post-transplant complications, such as primary graft dysfunction, acute rejection, and chronic lung allograft dysfunction (CLAD). CLAD occurs as a result of the host immune response to the allograft and is the principal factor limiting long-term survival after lung transplantation. Two distinct clinical phenotypes of CLAD have emerged, bronchiolitis obliterans syndrome and restrictive allograft syndrome, and this distinction has enabled further understanding of underlying immune mechanisms. Building on these advances, ongoing studies are exploring novel approaches to diagnose, prevent, and treat CLAD. Such studies are necessary to improve long-term outcomes for lung transplant recipients.

When tissue is the issue: A histological review of chronic lung allograft dysfunction

Although chronic lung allograft dysfunction (CLAD) remains the major life-limiting factor following lung transplantation, much of its pathophysiology remains unknown. The discovery that CLAD can manifest both clinically and morphologically in vastly different ways led to the definition of distinct subtypes of CLAD. In this review, recent advances in our understanding of the pathophysiological mechanisms of the different phenotypes of CLAD will be discussed with a particular focus on tissue-based and molecular studies. An overview of the current knowledge on the mechanisms of the airway-centered bronchiolitis obliterans syndrome, as well as the airway and alveolar injuries in the restrictive allograft syndrome and also the vascular compartment in chronic antibody-mediated rejection is provided. Specific attention is also given to morphological and molecular markers for early CLAD diagnosis or histological changes associated with subsequent CLAD development. Evidence for a possible overlap between different forms of CLAD is presented and discussed. In the end, "tissue remains the (main) issue," as we are still limited in our knowledge about the actual triggers and specific mechanisms of all late forms of posttransplant graft failure, a shortcoming that needs to be addressed in order to further improve the outcome of lung transplant recipients.

Sterile inflammation in thoracic transplantation

The life-saving benefits of organ transplantation can be thwarted by allograft dysfunction due to both infectious and sterile inflammation post-surgery. Sterile inflammation can occur after necrotic cell death due to the release of endogenous ligands [such as damage-associated molecular patterns (DAMPs) and alarmins], which perpetuate inflammation and ongoing cellular injury via various signaling cascades. Ischemia-reperfusion injury (IRI) is a significant contributor to sterile inflammation after organ transplantation and is associated with detrimental short- and long-term outcomes. While the vicious cycle of sterile inflammation and cellular injury is remarkably consistent amongst different organs and even species, we have begun understanding its mechanistic basis only over the last few decades. This understanding has resulted in the developments of novel, yet non-specific therapies for mitigating IRI-induced graft damage, albeit with moderate results. Thus, further understanding of the mechanisms underlying sterile inflammation after transplantation is critical for identifying personalized therapies to prevent or interrupt this vicious cycle and mitigating allograft dysfunction. In this review, we identify common and distinct pathways of post-transplant sterile inflammation across both heart and lung transplantation that can potentially be targeted.

FK506 induces lung lymphatic endothelial cell senescence and downregulates LYVE-1 expression, with associated decreased hyaluronan uptake

BACKGROUND

Therapeutic lymphangiogenesis in an orthotopic lung transplant model has been shown to improve acute allograft rejection that is mediated at least in part through hyaluronan drainage. Lymphatic vessel endothelial hyaluronan receptor (LYVE-1) expressed on the surface of lymphatic endothelial cells plays important roles in hyaluronan uptake. The impact of current immunosuppressive therapies on lung lymphatic endothelial cells is largely unknown. We tested the hypothesis that FK506, the most commonly used immunosuppressant after lung transplantation, induces lung lymphatic endothelial cell dysfunction.

METHODS

Lung lymphatic endothelial cells were cultured in vitro and treated with FK506. Telomerase activity was measured using the TRAP assay. Protein expression of LYVE-1 and senescence markers p21 and β-galactosidase was assessed with western blotting. Matrigel tubulation assay were used to investigate the effects of FK506 on TNF-α-induced lymphangiogenesis. Dual luciferase reporter assay was used to confirm NFAT-dependent transcriptional regulation of LYVE-1. Flow cytometry was used to examine the effects of FK506 on LYVE-1 in precision-cut-lung-slices ex vivo and on hyaluronan uptake in vitro.

RESULTS

In vitro, FK506 downregulated telomerase reverse transcriptase expression, resulting in decreased telomerase activity and subsequent induction of p21 expression and cell senescence. Treatment with FK506 decreased LYVE-1 mRNA and protein levels and resulted in decreased LEC HA uptake. Similar result showing reduction of LYVE-1 expression when treated with FK506 was observed ex vivo. We identified a putative NFAT binding site on the LYVE-1 promoter and cloned this region of the promoter in a luciferase-based reporter construct. We showed that this NFAT binding site regulates LYVE-1 transcription, and mutation of this binding site blunted FK506-dependent downregulation of LYVE-1 promoter-dependent transcription. Finally, FK506-treated lymphatic endothelial cells show a blunted response to TNF-α-mediated lymphangiogenesis.

CONCLUSION

FK506 alters lymphatic endothelial cell molecular characteristics and causes lymphatic endothelial cell dysfunction in vitro and ex vivo. These effects of FK506 on lymphatic endothelial cell may impair the ability of the transplanted lung to drain hyaluronan macromolecules in vivo. The implications of our findings on the long-term health of lung allografts merit more investigation.

IL-17A Contributes to Lung Fibrosis in a Model of Chronic Pulmonary Graft-versus-host Disease

BACKGROUND

Chronic pulmonary graft-versus-host disease (cpGVHD) after hematopoietic cell transplant (HCT) manifests as progressive airway and parenchymal lung fibrosis. On the basis of our prior data, mice that undergo allogeneic HCT with Tbet-knockout donors (AlloTbet) have increased lung Th17 cells and IL-17A and develop fibrosis resembling human cpGVHD. The role of IL-17A in posttransplant pulmonary fibrosis remains incompletely understood. We hypothesized that IL-17A is necessary for development of murine cpGVHD in this model.

METHODS

AlloTbet mice received weekly intraperitoneal anti-IL-17A or IgG (200 μg/mouse) starting 2 weeks post-HCT and were sacrificed after week 5. Histologic airway and parenchymal fibrosis were semiquantitatively graded in a blinded fashion. Lung cells and proteins were measured by flow cytometry, ELISA, and multicytokine assays.

RESULTS

Anti-IL-17A modestly decreased airway and parenchymal lung fibrosis, along with a striking reduction in pulmonary neutrophilia, IL-6, MIP-1α, MIP-1β, CXCL1, and CXCL5 in AlloTbet mice. Additionally, anti-IL-17A decreased CCL2, inflammatory monocytes and macrophages, and Th17 cells.

CONCLUSIONS

In the setting of murine AlloHCT with Tbet donors, IL-17A blockade decreases fibrotic features of cpGVHD. This may be mediated by the observed reduction in neutrophils or specific lung monocyte and macrophage populations or alternatively via a direct effect on fibroblasts. Collectively, our results further suggest that anti-IL-17A strategies could prove useful in preventing alloimmune-driven fibrotic lung diseases.

Is There a Role for Natural Antibodies in Rejection Following Transplantation?

Antibody-mediated rejection continues to hinder long-term survival of solid organ allografts. Natural antibodies (Nabs) with polyreactive and autoreactive properties have recently emerged as potential contributors to antibody-mediated graft rejection. This review discusses Nabs, their functions in health and disease, their significance in rejection following kidney, heart, and lung transplantation, and their implication in serum reactivity to key antigens associated with rejection. Finally, potential effector mechanisms of Nabs in the context of transplantation are explored.

The role of innate immunity in the long-term outcome of lung transplantation

Long-term survival after lung transplantation remains suboptimal due to chronic lung allograft dysfunction (CLAD), a progressive scarring process affecting the graft. Although anti-donor alloimmunity is central to the pathogenesis of CLAD, its underlying mechanisms are not fully elucidated and it is neither preventable nor treatable using currently available immunosuppression. Recent evidence has shown that innate immune stimuli are fundamental to the development of CLAD. Here, we examine long-standing assumptions and new concepts linking innate immune activation to late lung allograft fibrosis.

Amphiregulin contributes to airway remodeling in chronic allograft dysfunction after lung transplantation

Chronic lung allograft dysfunction (CLAD), a condition of excess matrix deposition and airways fibrosis, limits survival after lung transplantation. Amphiregulin (Areg) is an epidermal growth factor receptor (EGFR) ligand suggested to regulate airway injury and repair. We sought to determine whether Areg expression increases in CLAD, localize the cellular source of Areg induction in CLAD, and assess its effects on airway matrix deposition. Lung fluid Areg protein was quantified in patients with or without CLAD. In situ hybridization was performed to localize Areg and EGFR transcript in CLAD and normal lung tissue. Expression of hyaluronan, a matrix constituent that accumulates in CLAD, was measured in Areg-exposed bronchial epithelial cells in the presence or absence of an EGFR inhibitor. We demonstrated that lung fluid Areg protein was significantly increased in CLAD in a discovery and replication cohort. Areg and EGFR transcripts were abundantly expressed within CLAD tissue, localized to basally distributed airway epithelial cells overlying fibrotic regions. Areg-exposed bronchial epithelial cells increased hyaluronan and hyaluronan synthase expression in an EGFR-dependent manner. Collectively, these novel observations suggest that Areg contributes to airway remodeling and CLAD. Moreover these data implicate a role for EGFR signaling in CLAD pathogenesis, suggesting novel therapeutic targets.

Hyaluronan Fragmentation During Inflammatory Pathologies: A Signal that Empowers Tissue Damage

The mechanisms that modulate the response to tissue injury are not fully understood. Abnormalities in the repair response are associated with a variety of chronic disease states characterized by inflammation, followed subsequently by excessive ECM deposition. As cell-matrix interactions are able to regulate cellular homeostasis, modification of ECM integrity appears to be an unspecific factor in promoting the onset and progression of inflammatory diseases. Evidence is emerging to show that endogenous ECM molecules supply signals to damage tissues and cells in order to promote further ECM degradation and inflammation progression. Several investigations have been confirmed that HA fragments of different molecular sizes exhibit different biological effects and responses. In fact, the increased deposition of HA into the ECM is a strong hallmark of inflammation processes. In the context of inflammatory pathologies, highly polymerized HA is broken down into small components, which are able to exacerbate the inflammatory response by inducing the release of various detrimental mediators such as reactive oxygen species, cytokines, chemokines and destructive enzymes and by facilitating the recruitment of leukocytes. However, strategies involving the modulation of the HA fragment with specific receptors on cell surface could represent different promising effects for therapeutic scope. This review will focus on the inflammation action of small HA fragments in recent years obtained by in vivo reports.

SP-A and TLR4 localization in lung tissue of SM-exposed patients

INTRODUCTION

Long-term pulmonary complications are one of the major long-term consequences of sulfur mustard (SM) exposure. Toll-like receptor 4 (TLR4) involves in the pathogenesis of several pulmonary disorders. Surfactant protein-A (SP-A) regulates LPS-induced TLR4 localization and activation responses. However, the intensity and significance of TLR4 and SP-A expression by lung cells in SM-exposed patients is not clear.

METHODS

The gene expression of TLR4 (through real-time PCR) and TLR4 and SP-A positive cells and alveolar type II cells, as SP-A producers, (using IHC) were assessed in formalin fixed paraffin embedded (FFPE) specimens from SM-exposed (n = 17), and non-SM exposed individuals (n = 12).

RESULTS

TLR4 gene expression did not change between study groups. However, its cell surface presentation was significantly reduced in SM-exposed patients and particularly in which with constrictive bronchiolitis compared with the control group (P < 0.001 and P = 0.002, respectively). Frequency of alveolar type II cells was lower in the case group rather than the control group while the number of SP-A positive cells did not alter.

CONCLUSIONS

These findings suggest that reduced TLR4 cell surface presentation may have anti-inflammatory function and SP-A may have a critical role in regulation of inflammatory responses in SM-exposed patients. Further investigation on other possible mechanisms involved in TLR4 internalization maybe help to illustrate the modulatory or inflammatory activity of TLR4 in these patients.

Hyaluronan and LYVE-1 and allograft function in lung transplantation recipients

Hyaluronan (HA) is associated with innate immune response activation and may be a marker of allograft dysfunction in lung transplant recipients. This was a prospective, single center study comparing levels of bronchioalveolar lavage (BAL) and serum HA and the HA immobilizer LYVE-1 in lung transplant recipients with and without acute cellular rejection (ACR). Chronic lung allograft dysfunction (CLAD)-free survival was also evaluated based on HA and LYVE-1 levels. 78 recipients were enrolled with a total of 115 diagnostic biopsies and 1.5 years of median follow-up. Serum HA was correlated with BAL HA (r = 0.25, p = 0.01) and with serum LYVE-1 (r = 0.32, p = 0.002). There was significant variation in HA and LYVE-1 over time, regardless of ACR status. Levels of serum HA (median 74.7 vs 82.7, p = 0.69), BAL HA (median 149.4 vs 134.5, p = 0.39), and LYVE-1 (mean 190.2 vs 183.8, p = 0.72) were not associated with ACR. CLAD-free survival was not different in recipients with any episode of elevated serum HA (HR = 1.5, 95% CI = 0.3–7.7, p = 0.61) or BAL HA (HR = 0.94, 95% CI = 0.2–3.6, p = 0.93). These results did not differ when stratified by bilateral transplant status. In this small cohort, serum HA, BAL HA, and LYVE-1 levels are not associated with ACR or CLAD-free survival in lung transplant recipients.

Origin and Consequences of Necroinflammation

When cells undergo necrotic cell death in either physiological or pathophysiological settings in vivo, they release highly immunogenic intracellular molecules and organelles into the interstitium and thereby represent the strongest known trigger of the immune system. With our increasing understanding of necrosis as a regulated and genetically determined process (RN, regulated necrosis), necrosis and necroinflammation can be pharmacologically prevented. This review discusses our current knowledge about signaling pathways of necrotic cell death as the origin of necroinflammation. Multiple pathways of RN such as necroptosis, ferroptosis, and pyroptosis have been evolutionary conserved most likely because of their differences in immunogenicity. As the consequence of necrosis, however, all necrotic cells release damage associated molecular patterns (DAMPs) that have been extensively investigated over the last two decades. Analysis of necroinflammation allows characterizing specific signatures for each particular pathway of cell death. While all RN-pathways share the release of DAMPs in general, most of them actively regulate the immune system by the additional expression and/or maturation of either pro- or anti-inflammatory cytokines/chemokines. In addition, DAMPs have been demonstrated to modulate the process of regeneration. For the purpose of better understanding of necroinflammation, we introduce a novel classification of DAMPs in this review to help detect the relative contribution of each RN-pathway to certain physiological and pathophysiological conditions.

Role of Toll-like Receptor 4 Expressed by Fibroblasts in Allograft Fibrosis in Mouse Orthotopic Tracheal Transplantation

Development of chronic lung allograft dysfunction involves various alloimmune-independent insults including those mediated by Toll-like receptor (TLR) signaling, which is known to activate alloimmune responses. We hypothesized that TLR signaling may also contribute to the activation of fibroblasts and promoting allograft airway fibrosis. Mouse orthotopic tracheal transplants were conducted between major histocompatibility complex (MHC)-mismatched Balb/c donor and wild-type C3H or C3H-derived TLR4 mutant recipients (nonfunctional TLR4). Immunohistochemistry on day 21 showed significantly smaller alpha-smooth muscle actin (α-SMA)-positive areas in TLR4 mutant recipients than wild-type recipients (P = .01). No difference was found for CD3+ T-cell infiltration. Proliferation of alloreactive T cells derived from the recipient spleen showed no difference between TLR4 mutant and wild-type recipients in a mixed lymphocyte reaction. The effect of TLR4 signaling was examined in primary pulmonary fibroblast cultures both with lipopolysaccharide (LPS) and transforming growth factor (TGF)-β1. Stimulation with LPS significantly increased expression of α-SMA mRNA in wild-type fibroblasts cultured with TGF-β1 compared with the control without LPS (P = .001). Taken together, these findings suggest disruption of TLR signaling leads to reduced activation of fibroblasts without affecting T-cell infiltration and proliferation in this model. TLR4-mediated activation of fibroblasts may be a potentially important mechanism of allograft remodeling.

The histomorphological spectrum of restrictive chronic lung allograft dysfunction and implications for prognosis

Chronic lung allograft dysfunction continues to be the main contributor to poor long-term allograft survival after lung transplantation. The restrictive phenotype of chronic lung allograft dysfunction carries a particularly poor prognosis. Little is known about the pathogenetic mechanisms involved in restrictive chronic lung allograft dysfunction. In this study, we performed histomorphological and immunohistochemical analysis of restrictive chronic lung allograft dysfunction lungs. Explant lung tissue from 21 restrictive chronic lung allograft dysfunction patients was collected and histopathologic patterns of rejection, fibrosis and vascular changes were scored after routine histochemical stains and additional immunohistochemistry for endothelial markers and C4d. In all, 75% of cases showed evidence of acute cellular rejection; lymphocytic bronchiolitis was absent in most lungs, whereas in 55% there was obliterative bronchiolitis. Almost half of the cases showed a pattern consistent with pleuroparenchymal fibro-elastosis (n=10), and a subset showed nonspecific interstitial pneumonia (n=5) or irregular emphysema (n=5). Fibrinous alveolar exudates were frequently seen in association with fibrosis (n=6), but no diffuse alveolar damage was found. Evidence of microvascular damage was present in most cases. An emphysematous pattern of fibrosis was associated with a better survival (P=0.0030), whereas fibrinous exudates were associated with a worse survival (P=0.0007). In addition to the previously described nonspecific interstitial pneumonia and pleuroparenchymal fibro-elastosis patterns in restrictive chronic lung allograft dysfunction, we are the first to describe a pattern of fibrosis-induced subpleural/paraseptal emphysema. This pattern confers a better survival, whereas fibrinous exudates are associated with a worse survival. We believe that our findings offer a pathogenetic theory for pleuroparenchymal fibro-elastosis in restrictive chronic lung allograft dysfunction, and show that restrictive chronic lung allograft dysfunction is an increasingly heterogeneous disease with presumably different mechanisms of subpattern formation.

Bronchiolitis obliterans syndrome susceptibility and the pulmonary microbiome

BACKGROUND

Lung transplantation outcomes remain complicated by bronchiolitis obliterans syndrome (BOS), a major cause of mortality and retransplantation for patients. A variety of factors linking inflammation and BOS have emerged, meriting further exploration of the microbiome as a source of inflammation. In this analysis, we determined features of the pulmonary microbiome associated with BOS susceptibility.

METHODS

Bronchoalveolar lavage (BAL) samples were collected from 25 patients during standard of care bronchoscopies before BOS onset. Microbial DNA was isolated from BAL fluid and prepared for metagenomics shotgun sequencing. Patient microbiomes were phenotyped using k-means clustering and compared to determine effects on BOS-free survival.

RESULTS

Clustering identified 3 microbiome phenotypes: Actinobacteria dominant (AD), mixed, and Proteobacteria dominant. AD microbiomes, distinguished by enrichment with Gram-positive organisms, conferred reduced odds and risks for patients to develop acute rejection and BOS compared with non-AD microbiomes. These findings were independent of treatment models. Microbiome findings were correlated with BAL cell counts and polymorphonuclear cell percentages.

CONCLUSIONS

In some populations, features of the microbiome may be used to assess BOS susceptibility. Namely, a Gram-positive enriched pulmonary microbiome may predict resilience to BOS.

Hyaluronan interactions with innate immunity in lung biology

Lung disease is a leading cause of morbidity and mortality worldwide. Innate immune responses in the lung play a central role in the pathogenesis of lung disease and the maintenance of lung health, and thus it is crucial to understand factors that regulate them. Hyaluronan is ubiquitous in the lung, and its expression is increased following lung injury and in disease states. Furthermore, hyaladherins like inter-α-inhibitor, tumor necrosis factor-stimulated gene 6, pentraxin 3 and versican are also induced and help form a dynamic hyaluronan matrix in injured lung. This review synthesizes present knowledge about the interactions of hyaluronan and its associated hyaladherins with the lung immune system, and the implications of these interactions for lung biology and disease.

Lymphatic Vessels: The Next Frontier in Lung Transplant

Lymphatic vessels are essential for the uptake of fluid, immune cells, macromolecules, and lipids from the interstitial space. During lung transplant surgery, the pulmonary lymphatic vessel continuum is completely disrupted, and, as a result, lymphatic drainage function is severely compromised. After transplantation, the regeneration of an effective lymphatic drainage system plays a crucial role in maintaining interstitial fluid balance in the lung allograft. In the meantime, these newly formed lymphatic vessels are commonly held responsible for the development of immune responses leading to graft rejection, because they are potentially capable of transporting antigen-presenting cells loaded with allogeneic antigens to the draining lymph nodes. However, despite remarkable progress in the understanding of lymphatic biology, there is still a paucity of consistent evidence that demonstrates the exact impacts of lymphatic vessels on lung graft function. In this review, we examine the current literature related to roles of lymphatic vessels in the pathogenesis of lung transplant rejection.

Microvascular injury after lung transplantation

PURPOSE OF REVIEW

Airway microvessel injury following transplantation has been implicated in the development of chronic rejection. This review focuses on the most recent developments in the field describing preclinical and clinical findings that further implicate the loss of microvascular integrity as an important pathological event in the evolution of irreversible fibrotic remodeling.

RECENT FINDINGS

When lungs are transplanted, the airways appear vulnerable from the perspective of perfusion. Two vascular systems are lost, the bronchial artery and the lymphatic circulations, and the remaining vasculature in the airways expresses donor antigens susceptible to alloimmune-mediated injury via innate and adaptive immune mechanisms. Preclinical studies indicate the importance of hypoxia-inducible factor-1α in mediating microvascular repair and that hypoxia-inducible factor-1α can be upregulated to bolster endogenous repair.

SUMMARY

Airway microvascular injury is a feature of lung transplantation that limits short-term and long-term organ health. Although some problems are attributable to a missing bronchial artery circulation, another significant issue involves alloimmune-mediated injury to transplant airway microvessels. For a variety of reasons, bronchial artery revascularization surgery at the time of transplantation has not been widely adopted, and the current best hope for this era may be new medical approaches that offer protection against immune-mediated vascular injury or that promote microvascular repair.

Immune responses to bioengineered organs

PURPOSE OF REVIEW

Organ donation in the United States registered 9079 deceased organ donors in 2015. This high percentage of donations allowed organ transplantation in 29 851 recipients. Despite increasing numbers of transplants performed in comparison with previous years, the numbers of patients that are in need for a transplant increase every year at a higher rate. This reveals that the discrepancy between the demand and availability of organs remains fundamental problem in organ transplantation.

RECENT FINDINGS

Development of bioengineered organs represents a promising approach to increase the pool of organs for transplantation. The technology involves obtaining complex three-dimensional scaffolds that support cellular activity and functional remodeling though tissue recellularization protocols using progenitor cells. This innovative approach integrates cross-thematic approaches from specific areas of transplant immunology, tissue engineering and stem cell biology, to potentially manufacture an unlimited source of donor organs for transplantation.

SUMMARY

Although bioengineered organs are thought to escape immune recognition, the potential immune reactivity toward each of its components has not been studied in detail. Here, we summarize the host immune response toward different progenitor cells and discuss the potential implications of using nonself biological scaffolds to develop bioengineered organs.

Innate immune mechanisms in transplant allograft vasculopathy

PURPOSE OF REVIEW

Allograft vasculopathy is the leading cause of late allograft loss following solid organ transplantation. Ischemia reperfusion injury and donor-specific antibody-induced complement activation confer heightened risk for allograft vasculopathy via numerous innate immune mechanisms, including MyD88, high-mobility group box 1 (HMGB1), and complement-induced noncanonical nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling.

RECENT FINDINGS

The role of MyD88, a signal adaptor downstream of the Toll-like receptors (TLR), has been defined in an experimental heart transplant model, which demonstrated that recipient MyD88 enhanced allograft vasculopathy. Importantly, triggering receptor on myeloid receptor 1, a MyD88 amplifying signal, was present in rejecting human cardiac transplant biopsies and enhanced the development of allograft vasculopathy in mice. HMGB1, a nuclear protein that activates Toll-like receptors, also enhanced the development of allograft vasculopathy. Complement activation elicits assembly of membrane attack complexes on endothelial cells which activate noncanonical NF-κB signaling, a novel complement effector pathway that induces proinflammatory genes and potentiates endothelial cell-mediated alloimmune T-cell activation, processes which enhance allograft vasculopathy.

SUMMARY

Innate immune mediators, including HMGB1, MyD88, and noncanonical NF-κB signaling via complement activation contribute to allograft vasculopathy. These pathways represent potential therapeutic targets to reduce allograft vasculopathy after solid organ transplantation.

Lymphatic Changes in Respiratory Diseases: More than Just Remodeling of the Lung?

Advances in our ability to identify lymphatic endothelial cells and differentiate them from blood endothelial cells have led to important progress in the study of lymphatic biology. Over the past decade, preclinical and clinical studies have shown that there are changes to the lymphatic vasculature in nearly all lung diseases. Efforts to understand the contribution of lymphatics and their growth factors to disease initiation, progression, and resolution have led to seminal findings establishing critical roles for lymphatics in lung biology spanning from the first breath after birth to asthma, tuberculosis, and lung transplantation. However, in other diseases, it remains unclear if lymphatics are part of the overall lung remodeling process or real contributors to disease pathogenesis. The goal of this Translational Review is to highlight some of the advances in our understanding of the role(s) of lymphatics in lung disease and shed light on the critical needs and unanswered questions that might lead to novel translational applications.

Danger signals in regulating the immune response to solid organ transplantation

Endogenous danger signals, or damage-associated molecular patterns (DAMPs), are generated in response to cell stress and activate innate immunity to provide a pivotal mechanism by which an organism can respond to damaged self. Accumulating experimental and clinical data have established the importance of DAMPs, which signal through innate pattern recognition receptors (PRRs) or DAMP-specific receptors, in regulating the alloresponse to solid organ transplantation (SOT). Moreover, DAMPs may incite distinct downstream cellular responses that could specifically contribute to the development of allograft fibrosis and chronic graft dysfunction. A growing understanding of the role of DAMPs in directing the immune response to transplantation has suggested novel avenues for the treatment or prevention of allograft rejection that complement contemporary immunosuppression and could lead to improved outcomes for solid organ recipients.