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Ischemia-reperfusion injury is an inevitable event during organ transplantation and represents a primary risk factor for the development of early graft dysfunction in lung, heart, liver, and kidney transplant recipients. Recent studies have implicated recipient neutrophils as key mediators of this process and also have found that early innate immune responses after transplantation can ultimately augment adaptive alloimmunity and affect late graft outcomes. Here, we discuss signaling pathways involved in neutrophil recruitment and activation after ischemia-mediated graft injury in solid organ transplantation with an emphasis on lung allografts, which have been the focus of recent studies. These findings suggest novel therapeutic interventions that target ischemia-reperfusion injury-mediated graft dysfunction in transplant recipients.

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

Long-term survival of lung transplants lags behind other solid organs due to early onset of a fibrotic form of chronic rejection known as chronic lung allograft dysfunction (CLAD). Preventing CLAD is difficult as multiple immunologic and physiologic insults contribute to its development. Targeting fibroblast activation, which is the final common pathway leading to CLAD, offers the opportunity to ameliorate fibrosis irrespective of the initiating insult. Thy-1 is a surface glycoprotein that controls fibroblast differentiation and activation.To study the role of Thy-1 in CLAD, we utilized the minor antigen mismatched C57BL/6 (B6More severe CLAD was evident in B6Our findings suggest that the loss of Thy-1 on fibroblasts is a previously unrecognized cause of CLAD and its replacement may offer therapeutic applications for amelioration of this disease post-transplantation in the setting of infectious stress responses.

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

PDF file - 30K, Cytotoxicity of NK cells from C57BL/10 and Balb/c mice

PDF file - 36K, T cell depletion in AJ mice

PDF file - 25K, Cytotoxicity of neutrophils and monocytes/macrophages

PDF file - 21K, Strain-specific differences in Yac-1 cytotoxcity

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PDF file - 76K, Microsatellite mapping of the 129/SvEv.B6-NKC Rag2-/- mouse

PDF file - 47K, SNP rs13459098 in the Pas-1 locus is of 129 origin in 129/SvEv.B6-NKC Rag2-/- mice

The complement component C3 is a fundamental plasma protein for host defense, produced largely by the liver. However, recent work has demonstrated the critical importance of tissue-specific C3 expression in cell survival. Here, we analyzed the effects of local versus peripheral sources of C3 expression in a model of acute bacterial pneumonia induced by Pseudomonas aeruginosa . Whereas mice with global C3 deficiency had severe pneumonia-induced lung injury, those deficient only in liver-derived C3 remained protected, comparable to wild-type mice. Human lung transcriptome analysis showed that secretory epithelial cells, such as club cells, express high levels of C3 mRNA. Mice with tamoxifen-induced C3 gene ablation from club cells in the lung had worse pulmonary injury compared with similarly treated controls, despite maintaining normal circulating C3 levels. Last, in both the mouse pneumonia model and cultured primary human airway epithelial cells, we showed that stress-induced death associated with C3 deficiency parallels that seen in Factor B deficiency rather than C3a receptor deficiency. Moreover, C3-mediated reduction in epithelial cell death requires alternative pathway component Factor B. Thus, our findings suggest that a pathway reliant on locally derived C3 and Factor B protects the lung mucosal barrier.

BACKGROUND: Mechanical ventilation immediately after lung transplantation may impact the development of primary graft dysfunction (PGD), particularly in cases of donor-recipient size mismatch as ventilation is typically based on recipient rather than donor size. METHODS: We conducted a retrospective cohort study of adult bilateral lung transplant recipients at our center between January 2010 and January 2017. We defined donor-based lung protective ventilation (dLPV) as 6 to 8 ml/kg of donor ideal body weight and plateau pressure

Club cell secretory protein (CCSP), also known as secretoglobin 1A1 (gene name SCGB1A1), is one of the most abundant proteins in the lung, primarily produced by club cells of the distal airway epithelium. At baseline, CCSP is found in large concentrations in lung fluid specimens and can also be detected in the blood and urine. Obstructive lung diseases are generally associated with reduced CCSP levels, thought to be due to decreased CCSP production or club cell depletion. Conversely, several restrictive lung diseases have been found to have increased CCSP levels both in the lung and in the circulation, likely related to club cell dysregulation as well as increasedlung permeability. Recent studies demonstrate multiple mechanisms by which CCSP dampens acute and chronic lung inflammation. Given these anti-inflammatory effects, CCSP represents a novel potential therapeutic modality in lung disease.

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.

We tested whether the translocator protein (TSPO)-targeted positron emission tomography (PET) tracer, N-acetyl-N-(2-[11C]methoxybenzyl)-2-phenoxy-5-pyridinamine ([11C]PBR28), could distinguish macrophage dominant from neutrophilic inflammation better than 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) in mouse models of lung inflammation and assessed TSPO association with macrophages in lung tissue from the mouse models and in patients with chronic obstructive pulmonary disease (COPD). MicroPET imaging quantified [11C]PBR28 and [18F]FDG lung uptake in wild-type (Wt) C57BL/6J or heterozygous transgenic monocyte-deficient Wt/opT mice at 49 days after Sendai virus (SeV) infection, during macrophage-dominant inflammation, and in Wt mice at 3 days after SeV infection or 24 h after endotoxin instillation during neutrophilic inflammation. Immunohistochemical staining for TSPO in macrophages and neutrophils was performed using Mac3 and Ly6G for cell identification in mouse lung sections and CD68 and neutrophil elastase (NE) in human lung sections taken from explanted lungs from patients with COPD undergoing lung transplantation and donor lungs rejected for transplantation. Differences in tracer uptake among SeV-infected, endotoxin-treated, and uninfected/untreated control mice and in TSPO staining between neutrophils and macrophage populations in human lung sections were tested using analysis of variance. In Wt mice, [11C]PBR28 uptake (% injected dose/ml lung tissue) increased significantly with macrophage-dominant inflammation at 49 days (D49) after SeV infection compared to controls (p =

Postoperative atrial fibrillation (POAF) is the most common complication after cardiac surgery, and is associated with increased morbidity and mortality. Inflammation has been implicated as an etiology of POAF. Mitochondrial DNA (mtDNA) has been shown to initiate inflammation. This study analyzed inflammatory mechanisms of POAF by evaluating mtDNA, neutrophils, and cytokines/chemokines in the pericardial fluid and blood after cardiac surgery.Blood and pericardial fluid from patients who underwent coronary artery bypass or heart valve surgery, or both, were collected intraoperatively and at 4, 12, 24, and 48 hours postoperatively. Real-time polymerase chain reaction was used to quantify mtDNA in the pericardial fluid and blood. A Luminex (Luminex Corp, Austin, TX) assay was used to study cytokine and chemokine levels. Flow cytometry was used to analyze neutrophil infiltration and activation in the pericardial fluid.Samples from 100 patients were available for analysis. Postoperatively, mtDNA and multiple cytokine levels were higher in the pericardial fluid versus blood. Patients who had POAF had significantly higher levels of mtDNA in the pericardial fluid compared with patients who did not (P.001, area under the curve 0.74). There was no difference in the mtDNA concentration in the blood between the POAF group and non-POAF group (P = .897). Neutrophil concentration increased in the pericardial fluid over time from a baseline of 0.8% to 56% at 48 hours (P.01).The pericardial space has a high concentration of inflammatory mediators postoperatively. Mitochondrial DNA in the pericardial fluid was strongly associated with the development of POAF. This finding provides insight into a possible mechanism of inflammation that may contribute to POAF, and may offer novel therapeutic targets.

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+ CD11bhi 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.

Murine models of cardiac transplantation are frequently utilized to study ischemia-reperfusion injury, innate and adaptive immune responses after transplantation, and the impact of immunomodulatory therapies on graft rejection. Heterotopic cervical heart transplantation in mice was first described in 1991 using sutured anastomoses and subsequently modified to include cuff techniques. This modification allowed for improved success rates, and since then, there have been multiple reports that have proposed further technical improvements. However, translation into more widespread utilization remains limited due to the technical difficulty associated with graft anastomoses, which requires precision to achieve adequate length and caliber of the cuffs to avoid vascular anastomotic twisting or excessive tension, which can result in damage to the graft. The present protocol describes a modified technique for performing heterotopic cervical cardiac transplantation in mice which involves cuff placement on the recipient's common carotid artery and the donor's pulmonary artery in alignment with the direction of the blood flow.

Ischemia reperfusion injury represents a common pathological condition that is triggered by the release of endogenous ligands. While neutrophils are known to play a critical role in its pathogenesis, the tissue-specific spatiotemporal regulation of ischemia-reperfusion injury is not understood. Here, using oxidative lipidomics and intravital imaging of transplanted mouse lungs that are subjected to severe ischemia reperfusion injury, we discovered that necroptosis, a nonapoptotic form of cell death, triggers the recruitment of neutrophils. During the initial stages of inflammation, neutrophils traffic predominantly to subpleural vessels, where their aggregation is directed by chemoattractants produced by nonclassical monocytes that are spatially restricted in this vascular compartment. Subsequent neutrophilic disruption of capillaries resulting in vascular leakage is associated with impaired graft function. We found that TLR4 signaling in vascular endothelial cells and downstream NADPH oxidase 4 expression mediate the arrest of neutrophils, a step upstream of their extravasation. Neutrophil extracellular traps formed in injured lungs and their disruption with DNase prevented vascular leakage and ameliorated primary graft dysfunction. Thus, we have uncovered mechanisms that regulate the initial recruitment of neutrophils to injured lungs, which result in selective damage to subpleural pulmonary vessels and primary graft dysfunction. Our findings could lead to the development of new therapeutics that protect lungs from ischemia reperfusion injury.

Monocytes play an important role in the regulation of alloimmune responses after heart transplantation (HTx). Recent studies have highlighted the importance of immunometabolism in the differentiation and function of myeloid cells. While the importance of glucose metabolism in monocyte differentiation and function has been reported, a role for fatty acid β-oxidation (FAO) has not been explored. Heterotopic HTx was performed using hearts from Balb/c donor mice implanted into C57Bl/6 recipient mice and treated with etomoxir (eto), an irreversible inhibitor of carnitine palmitoyltransferase 1 (Cpt1), a rate-limiting step of FAO, or vehicle control. FAO inhibition prolonged HTx survival, reduced early T cell infiltration/ activation and reduced dendritic cell (DC) and macrophage infiltration to heart allografts of eto-treated HTx recipients. ELISPOT demonstrated eto-treated HTx were less reactive to activated donor antigen presenting cells. FAO inhibition reduced monocyte-to-DC and monocyte-to-macrophage differentiation in vitro and in vivo. Further, FAO inhibition did not alter the survival of heart allografts when transplanted into Ccr2-deficient recipients, suggesting the effects of FAO inhibition on reduced immune cell infiltration and increased heart allograft survival were dependent on monocyte mobilization. Finally, we confirmed the importance of FAO on monocyte differentiation in vivo using conditional deletion of Cpt1a. Our findings demonstrate that targeting FAO attenuates alloimmunity after HTx, in part through impairing monocyte differentiation.

Tertiary lymphoid organs (TLOs) are collections of immune cells resembling secondary lymphoid organs (SLOs) that form in peripheral, non-lymphoid tissues in response to local chronic inflammation. While their formation mimics embryologic lymphoid organogenesis, TLOs form after birth at ectopic sites in response to local inflammation resulting in their ability to mount diverse immune responses. The structure of TLOs can vary from clusters of B and T lymphocytes to highly organized structures with B and T lymphocyte compartments, germinal centers, and lymphatic vessels (LVs) and high endothelial venules (HEVs), allowing them to generate robust immune responses at sites of tissue injury. Although our understanding of the formation and function of these structures has improved greatly over the last 30 years, their role as mediators of protective or pathologic immune responses in certain chronic inflammatory diseases remains enigmatic and may differ based on the local tissue microenvironment in which they form. In this review, we highlight the role of TLOs in the regulation of immune responses in chronic infection, chronic inflammatory and autoimmune diseases, cancer, and solid organ transplantation.

Despite standardized postoperative care, some lung transplant patients suffer multiple episodes of acute and chronic rejection while others avoid graft problems for reasons that are poorly understood. Using an established model of C57BL/10 to C57BL/6 minor antigen mismatched single lung transplantation, we now demonstrate that the recipient microbiota contributes to variability in the alloimmune response. Specifically, mice from the Envigo facility in Frederick, Maryland contain nearly double the number of CD4+ Foxp3+ regulatory T cells (Tregs ) than mice from the Jackson facility in Bar Harbor, Maine or the Envigo facility in Indianapolis, Indiana (18 vs 9 vs 7%). Lung graft recipients from the Maryland facility thus do not develop acute or chronic rejection. Treatment with broad-spectrum antibiotics decreases Tregs and increases both acute and chronic graft rejection in otherwise tolerant strains of mice. Constitutive depletion of regulatory T cells, using Foxp3-driven expression of diphtheria toxin receptor, leads to the development of chronic rejection and supports the role of Tregs in both acute and chronic alloimmunity. Taken together, our data demonstrate that the microbiota of certain individuals may contribute to tolerance through Treg -dependent mechanisms and challenges the practice of indiscriminate broad-spectrum antibiotic use in the perioperative period.

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

Complement activation has been implicated in the pathogenesis of severe SARS-CoV-2 infection. However, it remains to be determined whether increased complement activation is a broad indicator of critical illness (and thus, no different in COVID-19). It is also unclear which pathways are contributing to complement activation in COVID-19, and if complement activation is associated with certain features of severe SARS-CoV-2 infection, such as endothelial injury and hypercoagulability. To address these questions, we investigated complement activation in the plasma from patients with COVID-19 prospectively enrolled at two tertiary care centers: Washington University School of Medicine (n=134) and Yale School of Medicine (n=49). We compared our patients to two non-COVID cohorts: (a) patients hospitalized with influenza (n=54), and (b) patients admitted to the intensive care unit (ICU) with acute respiratory failure requiring invasive mechanical ventilation (IMV, n=22). We demonstrate that circulating markers of complement activation are elevated in patients with COVID-19 compared to those with influenza and to patients with non-COVID-19 respiratory failure. Further, the results facilitate distinguishing those who are at higher risk of worse outcomes such as requiring ICU admission, or IMV. Moreover, the results indicate enhanced activation of the alternative complement pathway is most prevalent in patients with severe COVID-19 and is associated with markers of endothelial injury (i.e., angiopoietin-2) as well as hypercoagulability (i.e., thrombomodulin and von Willebrand factor). Our findings identify complement activation to be a distinctive feature of COVID-19, and provide specific targets that may be utilized for risk prognostication, drug discovery and personalized clinical trials., Increased complement activation is observed in COVID-19 compared to non-COVID respiratory illness, and is a key marker of organ failure.

We are entering 2021 with an expanding and effective COVID-19 vaccine armamentarium. Recent interim results from COVID-19 vaccine trials, including more than 80,000 participants worldwide, demonstrate remarkable efficacy and low rate of serious adverse events. Based on experience with other vaccines in transplant recipients and knowing the risk of severe COVID-19 in this population, we believe that COVID-19 vaccines provide potential benefit with minimal risk. We strongly support and encourage COVID-19 vaccination of our transplant recipients. ispartof: JOURNAL OF HEART AND LUNG TRANSPLANTATION vol:40 issue:3 pages:169-171 ispartof: location:United States status: published

We tested whether the translocator protein (TSPO)-targeted positron emission tomography (PET) tracer, N-acetyl-N-(2-[MicroPET imaging quantified [In Wt mice, [PET imaging with [

Summary Recent studies suggest that mitochondria can be transferred between cells to support the survival of metabolically compromised cells. However, whether intercellular mitochondria transfer occurs in white adipose tissue (WAT) or regulates metabolic homeostasis in vivo remains unknown. We found that macrophages acquire mitochondria from neighboring adipocytes in vivo and that this process defines a transcriptionally distinct macrophage subpopulation. A genome-wide CRISPR-Cas9 knockout screen revealed that mitochondria uptake depends on heparan sulfates (HS). High-fat diet (HFD)-induced obese mice exhibit lower HS levels on WAT macrophages and decreased intercellular mitochondria transfer from adipocytes to macrophages. Deletion of the HS biosynthetic gene Ext1 in myeloid cells decreases mitochondria uptake by WAT macrophages, increases WAT mass, lowers energy expenditure, and exacerbates HFD-induced obesity in vivo. Collectively, this study suggests that adipocytes and macrophages employ intercellular mitochondria transfer as a mechanism of immunometabolic crosstalk that regulates metabolic homeostasis and is impaired in obesity.

Recent discoveries demonstrate a critical role for circadian rhythms and sleep in immune system homeostasis. Both innate and adaptive immune responses - ranging from leukocyte mobilization, trafficking, and chemotaxis to cytokine release and T cell differentiation -are mediated in a time of day-dependent manner. The National Institutes of Health (NIH) recently sponsored an interdisciplinary workshop, "Sleep Insufficiency, Circadian Misalignment, and the Immune Response," to highlight new research linking sleep and circadian biology to immune function and to identify areas of high translational potential. This Review summarizes topics discussed and highlights immediate opportunities for delineating clinically relevant connections among biological rhythms, sleep, and immune regulation.

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.

Antibody-mediated rejection (AMR) is a principal cause of acute and chronic failure of lung allografts. However, mechanisms mediating this oftentimes fatal complication are poorly understood. Here, we show that Foxp3+ T cells formed aggregates in rejection-free human lung grafts and accumulated within induced bronchus-associated lymphoid tissue (BALT) of tolerant mouse lungs. Using a retransplantation model, we show that selective depletion of graft-resident Foxp3+ T lymphocytes resulted in the generation of donor-specific antibodies (DSA) and AMR, which was associated with complement deposition and destruction of airway epithelium. AMR was dependent on graft infiltration by B and T cells. Depletion of graft-resident Foxp3+ T lymphocytes resulted in prolonged interactions between B and CD4+ T cells within transplanted lungs, which was dependent on CXCR5-CXCL13. Blockade of CXCL13 as well as inhibition of the CD40 ligand and the ICOS ligand suppressed DSA production and prevented AMR. Thus, we have shown that regulatory Foxp3+ T cells residing within BALT of tolerant pulmonary allografts function to suppress B cell activation, a finding that challenges the prevailing view that regulation of humoral responses occurs peripherally. As pulmonary AMR is largely refractory to current immunosuppression, our findings provide a platform for developing therapies that target local immune responses.

Objective— Aortic arch transplants have advanced our understanding of processes that contribute to progression and regression of atherosclerotic plaques. To characterize the dynamic behavior of monocytes and macrophages in atherosclerotic plaques over time, we developed a new model of cervical aortic arch transplantation in mice that is amenable to intravital imaging. Approach and Results— Vascularized aortic arch grafts were transplanted heterotropically to the right carotid arteries of recipient mice using microsurgical suture techniques. To image immune cells in atherosclerotic lesions during regression, plaque-bearing aortic arch grafts from B6 ApoE-deficient donors were transplanted into syngeneic CX 3 CR1 GFP reporter mice. Grafts were evaluated histologically, and monocytic cells in atherosclerotic plaques in ApoE-deficient grafts were imaged intravitally by 2-photon microscopy in serial fashion. In complementary experiments, CCR2 + cells in plaques were serially imaged by positron emission tomography using specific molecular probes. Plaques in ApoE-deficient grafts underwent regression after transplantation into normolipidemic hosts. Intravital imaging revealed clusters of largely immotile CX 3 CR1 + monocytes/macrophages in regressing plaques that had been recruited from the periphery. We observed a progressive decrease in CX 3 CR1 + monocytic cells in regressing plaques and a decrease in CCR2 + positron emission tomography signal during 4 months. Conclusions— Cervical transplantation of atherosclerotic mouse aortic arches represents a novel experimental tool to investigate cellular mechanisms that contribute to the remodeling of atherosclerotic plaques.

Immunological requirements for rejection and tolerance induction differ between various organs. While memory CD8+ T cells are considered a barrier to immunosuppression-mediated acceptance of most tissues and organs, tolerance induction after lung transplantation is critically dependent on central memory CD8+ T lymphocytes. Here we demonstrate that costimulation blockade-mediated tolerance after lung transplantation is dependent on programmed cell death 1 (PD-1) expression on CD8+ T cells. In the absence of PD-1 expression, CD8+ T cells form prolonged interactions with graft-infiltrating CD11c+ cells; their differentiation is skewed towards an effector memory phenotype and grafts are rejected acutely. These findings extend the notion that requirements for tolerance induction after lung transplantation differ from other organs. Thus, immunosuppressive strategies for lung transplant recipients need to be tailored based on the unique immunological properties of this organ.

Lung transplant recipients receive lifelong immunosuppression that includes a calcineurin inhibitor (tacrolimus or cyclosporine), an anti-proliferative agent (mycophenolate or azathioprine) and corticosteroids. They are also known to be at increased risk for a broad spectrum of adverse hematologic events. This includes relatively common complications such as neutropenia and other cytopenias, which occur in over half of patients, as well as rarer events such as de novo hematopoietic malignancy. Certain patterns of clonal hematopoiesis, the expansion of unique somatic clones in hematopoietic stem cells, are known to be associated with cytotoxic therapy and the development of hematologic malignancies. We hypothesized that the stress of transplantation and subsequent immunosuppression provides an environment whereby a unique pattern of clonal hematopoiesis emerges. To evaluate this hypothesis, we designed a custom panel of 59 genes using an error-corrected sequencing assay capable of detecting variant allele frequency (VAF) as low as 0.01. We characterized the overall burden and distribution of clonal hematopoiesis and compared differences among lung transplant recipients (n=73) and age-matched healthy controls (n=19). Clonal hematopoiesis was identified in 51/73 (69.9%) of lung transplant recipients. This was significantly higher than the frequency in the age-matched healthy controls (6/19,31.6% p=0.0071). Additionally, 27/51 (52.9%) of patients with clonal hematopoiesis had multiple variants. The increase in clonal hematopoiesis was mainly due to mutations in DNA damage response (DDR) genes (ATM, PPM1D, SRCAP or TP53), with 29/73 (39.7%) of lung transplant recipients carrying one or more mutation compared with 1/19 (5.3%) of age-matched healthy controls (5.3%, p=0.029). No significant difference in the frequency of clonal hematopoiesis due to non-DDR genes was also observed (48% vs. 26.3%, p=0.121). We first evaluated the relationship between clonal hematopoiesis and lung transplant indication. This is relevant, since interstitial lung disease (ILD), a common indication for lung transplantation, is associated with telomeropathies which are also linked to bone marrow failure and clonal hematopoiesis. However, the frequency of DDR clonal hematopoiesis in ILD patients (17/39, 44%) was similar to that observed in patients with COPD (9/20, 45%). We next investigated the association of immunosuppression with clonal hematopoiesis. Overall, 30/48 patients on mycophenolate (MPA) and 13/19 patients on azathioprine (AZA) had at least 1 clonal hematopoiesis mutation. We found no significant difference in overall clonal hematopoiesis frequency among patients on MPA vs. AZA. However, the frequency of DDR clonal hematopoiesis was significantly higher in patients on AZA (OR 4.04, 95% CI 1.22-13.38, p=0.022) than in patients on MPA. Moreover, when we assessed clonal hematopoiesis burden via Poisson regression, it was increased in patients receiving AZA (1.68, 95% CI 1.08-2.59, p=0.020) when compared to patients on MPA. All lung transplant recipients were maintained on tacrolimus but one, so associations with type of calcineurin inhibitor could not be assessed. Finally, we assessed clonal hematopoiesis in recipients who developed neutropenia (n=24) and found no significant association. To the best of our knowledge, we report the first evidence of increased clonal hematopoiesis in a solid organ transplant population. Our data indicate that azathioprine therapy is associated with the expansion of hematopoietic clones carrying variants in DDR genes. However, azathioprine therapy often represents a failure of MPA therapy, typically due to either hematologic or gastrointestinal toxicity. Therefore, the association noted might reflect MPA intolerance rather than azathioprine therapy. Nevertheless, prior studies show that treatment with genotoxic agents, such as chemotherapy or radiation therapy, provide a fitness advantage to hematopoietic stem/progenitor cells carrying DDR gene variants. Whether azathioprine vs. MPA therapy confers a similar fitness advantage is currently under investigation. Further investigation also is warranted to determine if the presence of clonal hematopoiesis influences lung transplant outcomes. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Outcomes after lung transplant lag behind those of other solid-organ transplants. A better understanding of the pathways that contribute to rejection and tolerance after lung transplant will be required to develop new therapeutic strategies that take into account the unique immunological features of lungs. Mechanistic immunological investigations in an orthotopic transplant model in the mouse have shed new light on immune responses after lung transplant. Here, we highlight that interactions between immune cells within pulmonary grafts shape their fate. These observations set lungs apart from other organs and help provide the conceptual framework for the development of lung-specific immunosuppression.

Neutrophils are often viewed as nonspecialized effector cells whose presence is a simple indicator of tissue inflammation. There is new evidence that neutrophils exist in subsets and have specialized effector functions that include extracellular trap generation and the stimulation of angiogenesis. The application of intravital imaging to transplanted organs has revealed novel requirements for neutrophil trafficking into graft tissue and has illuminated direct interactions between neutrophils and other leukocytes that promote alloimmunity. Paradoxically, retaining some neutrophilia may be important to induce or maintain tolerance. Neutrophils can stimulate anti-inflammatory signals in other phagocytes and release molecules that inhibit T cell activation. In this article, we will review the available evidence of how neutrophils regulate acute and chronic inflammation in transplanted organs and discuss the possibility of targeting these cells to promote tolerance.