Donor brain death exacerbates complement-dependent ischemia/reperfusion injury in transplanted hearts

MerTK-dependent efferocytosis by monocytic-MDSCs mediates resolution of post-lung transplant injury

Rationale

Patients with end stage lung diseases require lung transplantation (LTx) that can be impeded by ischemia-reperfusion injury (IRI) leading to subsequent chronic lung allograft dysfunction (CLAD) and inadequate outcomes.

Objectives

We examined the undefined role of MerTK (receptor Mer tyrosine kinase) on monocytic myeloid-derived suppressor cells (M-MDSCs) in efferocytosis (phagocytosis of apoptotic cells) to facilitate resolution of lung IRI.

Methods

Single-cell RNA sequencing of lung tissue and BAL from post-LTx patients was analyzed. Murine lung hilar ligation and allogeneic orthotopic LTx models of IRI were used with Balb/c (WT), cebpb -/- (MDSC-deficient), Mertk -/- or MerTK-CR (cleavage resistant) mice. Lung function, IRI (inflammatory cytokine and myeloperoxidase expression, immunohistology for neutrophil infiltration), and flow cytometry of lung tissue for efferocytosis of apoptotic neutrophils were assessed in mice.

Measurements and Main Results

A significant downregulation in MerTK-related efferocytosis genes in M-MDSC populations of CLAD patients compared to healthy subjects was observed. In the murine IRI model, significant increase in M-MDSCs, MerTK expression and efferocytosis was observed in WT mice during resolution phase that was absent in cebpb -/- Land Mertk -/- mice. Adoptive transfer of M-MDSCs in cebpb -/- mice significantly attenuated lung dysfunction, and inflammation leading to resolution of IRI. Additionally, in a preclinical murine orthotopic LTx model, increases in M-MDSCs were associated with resolution of lung IRI in the transplant recipients. In vitro studies demonstrated the ability of M-MDSCs to efferocytose apoptotic neutrophils in a MerTK-dependent manner.

Conclusions

Our results suggest that MerTK-dependent efferocytosis by M-MDSCs can significantly contribute to the resolution of post-LTx IRI.

Complement inhibition alleviates donor brain death-induced liver injury and posttransplant cascade injury by regulating phosphoinositide 3-kinase signaling

Brain death (BD) donors are the primary source of donor organs for liver transplantation. However, the effects of BD on donor livers and outcomes after liver transplantation remain unclear. Here, we explored the role of complement and the therapeutic effect of complement inhibition in BD-induced liver injury and posttransplantation injury in a mouse BD and liver transplantation model. For complement inhibition, we used complement receptor 2 (CR2)-Crry, a murine inhibitor of C3 activation that specifically targets sites of complement activation. In the mouse model, BD resulted in complement activation and liver injury in donor livers and a cascade liver injury posttransplantation, mediated in part through the C3a-C3aR (C3a receptor) signaling pathway, which was ameliorated by treatment with CR2-Crry. Treatment of BD donors with CR2-Crry improved graft survival, which was further improved when recipients received an additional dose of CR2-Crry posttransplantation. Mechanistically, we determined that complement inhibition alleviated BD-induced donor liver injury and posttransplant cascade injury by regulating phosphoinositide 3-kinase (PI3K) signaling pathways. Together, BD induced donor liver injury and cascade injury post-transplantation, which was mediated by complement activation products acting on PI3K signaling pathways. Our study provides an experimental basis for developing strategies to improve the survival of BD donor grafts in liver transplantation.

The impact of pre-existing hematologic disorders on morbidity and mortality following heart transplantation: Focus on early graft dysfunction

BACKGROUND

Heart transplantation (HT) is the gold standard therapy for advanced heart failure, providing excellent long-term outcomes. However, postoperative outcomes are limited by bleeding, infections, and primary graft dysfunction (PGD) that contribute to early mortality after HT. HT candidates with pre-existing hematologic disorders, bleeding, and clotting, may represent a higher risk population. We assessed the short- and long-term outcomes of patients with pre-existing hematologic disorders undergoing HT.

METHODS AND RESULTS

Medical records of all adult patients who received HT from January 2010 to December 2019 at our institution were retrospectively reviewed. Hematologic disorders were identified via chart review and adjudicated by a board-certified hematologist. Inverse probability weighting and multivariable models were used to adjust for potential pretransplant confounders. Four hundred and ninety HT recipients were included, of whom 29 (5.9%) had a hematologic disorder. Hematologic disorders were associated with severe PGD requiring mechanical circulatory support (aOR 3.15 [1.01-9.86]; p = .049), postoperative infections (aOR 2.93 [1.38-6.23]; p = .01), and 3-year acute cellular rejection (ACR) (≥1R/1B) (aSHR 2.06 [1.09-3.87]; p = .03). There was no difference in in-hospital mortality (aOR 1.23 [.20-7.58], p = .82) or 3-year mortality (aHR 1.58 [.49-5.12], p = .44).

CONCLUSIONS

Patients with hematologic disorders undergoing HT are at increased risk of severe PGD, postoperative infections, and ACR, while in-hospital and 3-year mortality remain unaffected.

Immune response associated with ischemia and reperfusion injury during organ transplantation

Background

Ischemia and reperfusion injury (IRI) is an ineluctable immune-related pathophysiological process during organ transplantation, which not only causes a shortage of donor organs, but also has long-term and short-term negative consequences on patients. Severe IRI-induced cell death leads to the release of endogenous substances, which bind specifically to receptors on immune cells to initiate an immune response. Although innate and adaptive immunity have been discovered to play essential roles in IRI in the context of organ transplantation, the pathway and precise involvement of the immune response at various stages has not yet to be elucidated.

Methods

We combined “IRI” and “organ transplantation” with keywords, respectively such as immune cells, danger signal molecules, macrophages, neutrophils, natural killer cells, complement cascade, T cells or B cells in PubMed and the Web of Science to search for relevant literatures.

Conclusion

Comprehension of the immune mechanisms involved in organ transplantation is promising for the treatment of IRI, this review summarizes the similarities and differences in both innate and adaptive immunity and advancements in the immune response associated with IRI during diverse organ transplantation.

Complement-targeting therapeutics for ischemia-reperfusion injury in transplantation and the potential for ex vivo delivery

Organ shortages and an expanding waitlist have led to increased utilization of marginal organs. All donor organs are subject to varying degrees of IRI during the transplant process. Extended criteria organs, including those from older donors and organs donated after circulatory death are especially vulnerable to ischemia-reperfusion injury (IRI). Involvement of the complement cascade in mediating IRI has been studied extensively. Complement plays a vital role in the propagation of IRI and subsequent recruitment of the adaptive immune elements. Complement inhibition at various points of the pathway has been shown to mitigate IRI and minimize future immune-mediated injury in preclinical models. The recent introduction of ex vivo machine perfusion platforms provides an ideal window for therapeutic interventions. Here we review the role of complement in IRI by organ system and highlight potential therapeutic targets for intervention during ex vivo machine preservation of donor organs.

Modulating donor mitochondrial fusion/fission delivers immunoprotective effects in cardiac transplantation

Early insults associated with cardiac transplantation increase the immunogenicity of donor microvascular endothelial cells (ECs), which interact with recipient alloreactive memory T cells and promote responses leading to allograft rejection. Thus, modulating EC immunogenicity could potentially alter T cell responses. Recent studies have shown modulating mitochondrial fusion/fission alters immune cell phenotype. Here, we assess whether modulating mitochondrial fusion/fission reduces EC immunogenicity and alters EC-T cell interactions. By knocking down DRP1, a mitochondrial fission protein, or by using the small molecules M1, a fusion promoter, and Mdivi1, a fission inhibitor, we demonstrate that promoting mitochondrial fusion reduced EC immunogenicity to allogeneic CD8⁺ T cells, shown by decreased T cell cytotoxic proteins, decreased EC VCAM-1, MHC-I expression, and increased PD-L1 expression. Co-cultured T cells also displayed decreased memory frequencies and Ki-67 proliferative index. For in vivo significance, we used a novel murine brain-dead donor transplant model. Balb/c hearts pretreated with M1/Mdivi1 after brain-death induction were heterotopically transplanted into C57BL/6 recipients. We demonstrate that, in line with our in vitro studies, M1/Mdivi1 pretreatment protected cardiac allografts from injury, decreased infiltrating T cell production of cytotoxic proteins, and prolonged allograft survival. Collectively, our data show promoting mitochondrial fusion in donor ECs mitigates recipient T cell responses and leads to significantly improved cardiac transplant survival.

A clinically relevant sheep model of orthotopic heart transplantation 24 h after donor brainstem death

Background

Heart transplantation (HTx) from brainstem dead (BSD) donors is the gold-standard therapy for severe/end-stage cardiac disease, but is limited by a global donor heart shortage. Consequently, innovative solutions to increase donor heart availability and utilisation are rapidly expanding. Clinically relevant preclinical models are essential for evaluating interventions for human translation, yet few exist that accurately mimic all key HTx components, incorporating injuries beginning in the donor, through to the recipient. To enable future assessment of novel perfusion technologies in our research program, we thus aimed to develop a clinically relevant sheep model of HTx following 24 h of donor BSD.

Methods

BSD donors (vs. sham neurological injury, 4/group) were hemodynamically supported and monitored for 24 h, followed by heart preservation with cold static storage. Bicaval orthotopic HTx was performed in matched recipients, who were weaned from cardiopulmonary bypass (CPB), and monitored for 6 h. Donor and recipient blood were assayed for inflammatory and cardiac injury markers, and cardiac function was assessed using echocardiography. Repeated measurements between the two different groups during the study observation period were assessed by mixed ANOVA for repeated measures.

Results

Brainstem death caused an immediate catecholaminergic hemodynamic response (mean arterial pressure, p = 0.09), systemic inflammation (IL-6 - p = 0.025, IL-8 - p = 0.002) and cardiac injury (cardiac troponin I, p = 0.048), requiring vasopressor support (vasopressor dependency index, VDI, p = 0.023), with normalisation of biomarkers and physiology over 24 h. All hearts were weaned from CPB and monitored for 6 h post-HTx, except one (sham) recipient that died 2 h post-HTx. Hemodynamic (VDI - p = 0.592, heart rate - p = 0.747) and metabolic (blood lactate, p = 0.546) parameters post-HTx were comparable between groups, despite the observed physiological perturbations that occurred during donor BSD. All p values denote interaction among groups and time in the ANOVA for repeated measures.

Conclusions

We have successfully developed an ovine HTx model following 24 h of donor BSD. After 6 h of critical care management post-HTx, there were no differences between groups, despite evident hemodynamic perturbations, systemic inflammation, and cardiac injury observed during donor BSD. This preclinical model provides a platform for critical assessment of injury development pre- and post-HTx, and novel therapeutic evaluation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40635-021-00425-4.

In Situ Pre-Treatment of Vascularized Composite Allografts With a Targeted Complement Inhibitor Protects Against Brain Death and Ischemia Reperfusion Induced Injuries

Introduction

Donor brain death (BD) is an unavoidable component of vascularized composite allograft (VCA) transplantation and a key contributor to ischemia-reperfusion injury (IRI). Complement is activated and deposited within solid organ grafts as a consequence of BD and has been shown to exacerbate IRI, although the role of BD and complement in VCA and the role it plays in IRI and VCA rejection has not been studied.

Methods

BD was induced in Balb/c donors, and the VCA perfused prior to graft procurement with UW solution supplemented with or without CR2-Crry, a C3 convertase complement inhibitor that binds at sites of complement activation, such as that induced on the endothelium by induction of BD. Following perfusion, donor VCAs were cold stored for 6 hours before transplantation into C57BL/6 recipients. Donor VCAs from living donors (LD) were also procured and stored. Analyses included CR2-Crry graft binding, complement activation, toxicity, injury/inflammation, graft gene expression and survival.

Results

Compared to LD VCAs, BD donor VCAs had exacerbated IRI and rejected earlier. Following pretransplant in-situ perfusion of the donor graft, CR2-Crry bound within the graft and was retained post-transplantation. CR2-Crry treatment significantly reduced complement deposition, inflammation and IRI as compared to vehicle-treated BD donors. Treatment of BD donor VCAs with CR2-Crry led to an injury profile not dissimilar to that seen in recipients of LD VCAs.

Conclusion

Pre-coating a VCA with CR2-Crry in a clinically relevant treatment paradigm provides localized, and therefore minimally immunosuppressive, protection from the complement-mediated effects of BD induced exacerbated IRI.

Renal Delivery of Pharmacologic Agents During Machine Perfusion to Prevent Ischaemia-Reperfusion Injury: From Murine Model to Clinical Trials

Donor organ shortage still remains a serious obstacle for the access of wait-list patients to kidney transplantation, the best treatment for End-Stage Kidney Disease (ESKD). To expand the number of transplants, the use of lower quality organs from older ECD or DCD donors has become an established routine but at the price of increased incidence of Primary Non-Function, Delay Graft Function and lower-long term graft survival. In the last years, several improvements have been made in the field of renal transplantation from surgical procedure to preservation strategies. To improve renal outcomes, research has focused on development of innovative and dynamic preservation techniques, in order to assess graft function and promote regeneration by pharmacological intervention before transplantation. This review provides an overview of the current knowledge of these new preservation strategies by machine perfusions and pharmacological interventions at different timing possibilities: in the organ donor, ex-vivo during perfusion machine reconditioning or after implementation in the recipient. We will report therapies as anti-oxidant and anti-inflammatory agents, senolytics agents, complement inhibitors, HDL, siRNA and H2S supplementation. Renal delivery of pharmacologic agents during preservation state provides a window of opportunity to treat the organ in an isolated manner and a crucial route of administration. Even if few studies have been reported of transplantation after ex-vivo drugs administration, targeting the biological pathway associated to kidney failure (i.e. oxidative stress, complement system, fibrosis) might be a promising therapeutic strategy to improve the quality of various donor organs and expand organ availability.

Risk Factors for Severe Primary Graft Dysfunction in Infants Following Heart Transplant

Background

Previous studies suggest that infant heart transplant (HT) recipients are at higher risk of developing severe primary graft dysfunction (PGD) than older children. We sought to identify risk factors for developing severe PGD in infant HT recipients.

Methods and Results

We identified all HT recipients aged <1 year in the United States during 1996 to 2015 using the Organ Procurement and Transplant Network database. We linked their data to ELSO (Extracorporeal Life Support Organization) registry data to identify those with severe PGD, defined by initiation of extracorporeal membrane oxygenation support for PGD within 2 days following HT. We used multivariable logistic regression to assess risk factors for developing severe PGD. Of 1718 infants analyzed, 600 (35%) were <90 days old and 1079 (63%) had congenital heart disease. Overall, 134 (7.8%) developed severe PGD; 95 (71%) were initiated on extracorporeal membrane oxygenation support on the day of HT, 34 (25%) the next day, and 5 (4%) the following day. In adjusted analysis, recipient congenital heart disease, extracorporeal membrane oxygenation, or biventricular assist device support at transplant, recipient blood type AB, donor‐recipient weight ratio <0.9, and graft ischemic time ≥4 hours were independently associated with developing severe PGD whereas left ventricular assist device support at HT was not. One‐year graft survival was 48% in infants with severe PGD versus 87% without severe PGD.

Conclusions

Infant HT recipients with severe PGD have poor graft survival. Although some recipient‐level risk factors are nonmodifiable, avoiding modifiable risk factors may mitigate further risk in infants at high risk of developing severe PGD.

Novel biomarkers useful in surveillance of graft rejection after heart transplantation

Heart transplantation (HTx) is considered the gold-standard therapy for the treatment of advanced heart failure (HF). The long-term survival in HTx is hindered by graft failure which represents one of the major limitations of the long-term efficacy of HTx. Endomyocardial biopsy (EMB) and the evaluation of donor-specific antibodies (DSA) are currently considered the essential diagnostic tools for surveillance of graft rejection. Recently, new molecular biomarkers (including cell-free DeoxyriboNucleic Acid, exosomes, gene profiling microarray, nanostring, reverse transcriptase multiplex ligation-dependent probe amplification, proteomics and immune profiling by quantitative multiplex immunofluorescence) provide useful information on mechanisms of graft rejection. The ambitious role of a similar change of perspective is aimed at a better and longer graft preservation.

Brain death-induced lung injury is complement dependent, with a primary role for the classical/lectin pathway

In brain-dead donors immunological activation occurs, which deteriorates donor lung quality. Whether the complement system is activated and which pathways are herein involved, remain unknown. We aimed to investigate whether brain death (BD)-induced lung injury is complement dependent and dissected the contribution of the complement activation pathways. BD was induced and sustained for 3 hours in wild-type (WT) and complement deficient mice. C3^-/- mice represented total complement deficiency, C4^-/- mice represented deficiency of the classical and lectin pathway, and factor properdin (P)^-/- mice represented alternative pathway deficiency. Systemic and local complement levels, histological lung injury, and pulmonary inflammation were assessed. Systemic and local complement levels were reduced in C3^-/- mice. In addition, histological lung injury and inflammation were attenuated, as corroborated by influx of neutrophils and gene expressions of interleukin (IL)-6, IL-8-like KC, TNF-α, E-selectin, and MCP-1. In C4^-/- mice, complement was reduced on both systemic and local levels and histological lung injury and inflammatory status were ameliorated. In P^-/- mice, histological lung injury was attenuated, though systemic and local complement levels, IL-6 and KC gene expressions, and neutrophil influx were not affected. We demonstrated that BD-induced lung injury is complement dependent, with a primary role for the classical/lectin activation pathway.

Heart Transplantation From Brain Dead Donors: A Systematic Review of Animal Models

Despite advances in mechanical circulatory devices and pharmacologic therapies, heart transplantation (HTx) is the definitive and most effective therapy for an important proportion of qualifying patients with end-stage heart failure. However, the demand for donor hearts significantly outweighs the supply. Hearts are sourced from donors following brain death, which exposes donor hearts to substantial pathophysiological perturbations that can influence heart transplant success and recipient survival. Although significant advances in recipient selection, donor and HTx recipient management, immunosuppression, and pretransplant mechanical circulatory support have been achieved, primary graft dysfunction after cardiac transplantation continues to be an important cause of morbidity and mortality. Animal models, when appropriate, can guide/inform medical practice, and fill gaps in knowledge that are unattainable in clinical settings. Consequently, we performed a systematic review of existing animal models that incorporate donor brain death and subsequent HTx and assessed studies for scientific rigor and clinical relevance. Following literature screening via the U.S National Library of Medicine bibliographic database (MEDLINE) and Embase, 29 studies were assessed. Analysis of included studies identified marked heterogeneity in animal models of donor brain death coupled to HTx, with few research groups worldwide identified as utilizing these models. General reporting of important determinants of heart transplant success was mixed, and assessment of posttransplant cardiac function was limited to an invasive technique (pressure-volume analysis), which is limitedly applied in clinical settings. This review highlights translational challenges between available animal models and clinical heart transplant settings that are potentially hindering advancement of this field of investigation.

Review: Ischemia Reperfusion Injury-A Translational Perspective in Organ Transplantation

Thanks to the development of new, more potent and selective immunosuppressive drugs together with advances in surgical techniques, organ transplantation has emerged from an experimental surgery over fifty years ago to being the treatment of choice for many end-stage organ diseases, with over 139,000 organ transplants performed worldwide in 2019. Inherent to the transplantation procedure is the fact that the donor organ is subjected to blood flow cessation and ischemia during harvesting, which is followed by preservation and reperfusion of the organ once transplanted into the recipient. Consequently, ischemia/reperfusion induces a significant injury to the graft with activation of the immune response in the recipient and deleterious effect on the graft. The purpose of this review is to discuss and shed new light on the pathways involved in ischemia/reperfusion injury (IRI) that act at different stages during the donation process, surgery, and immediate post-transplant period. Here, we present strategies that combine various treatments targeted at different mechanistic pathways during several time points to prevent graft loss secondary to the inflammation caused by IRI.

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.

Targeted donor complement blockade after brain death prevents delayed graft function in a nonhuman primate model of kidney transplantation

Delayed graft function (DGF) in renal transplant is associated with reduced graft survival and increased immunogenicity. The complement-driven inflammatory response after brain death (BD) and posttransplant reperfusion injury play significant roles in the pathogenesis of DGF. In a nonhuman primate model, we tested complement-blockade in BD donors to prevent DGF and improve graft survival. BD donors were maintained for 20 hours; kidneys were procured and stored at 4°C for 43-48 hours prior to implantation into ABO-compatible, nonsensitized, MHC-mismatched recipients. Animals were divided into 3 donor-treatment groups: G1 - vehicle, G2 - rhC1INH+heparin, and G3 - heparin. G2 donors showed significant reduction in classical complement pathway activation and decreased levels of tumor necrosis factor α and monocyte chemoattractant protein 1. DGF was diagnosed in 4/6 (67%) G1 recipients, 3/3 (100%) G3 recipients, and 0/6 (0%) G2 recipients (P = .008). In addition, G2 recipients showed superior renal function, reduced sC5b-9, and reduced urinary neutrophil gelatinase-associated lipocalin in the first week posttransplant. We observed no differences in incidence or severity of graft rejection between groups. Collectively, the data indicate that donor-management targeting complement activation prevents the development of DGF. Our results suggest a pivotal role for complement activation in BD-induced renal injury and postulate complement blockade as a promising strategy for the prevention of DGF after transplantation.

Emphysema-associated Autoreactive Antibodies Exacerbate Post-Lung Transplant Ischemia-Reperfusion Injury

Chronic obstructive pulmonary disease-associated chronic inflammation has been shown to lead to an autoimmune phenotype characterized in part by the presence of lung autoreactive antibodies. We hypothesized that ischemia-reperfusion injury (IRI) liberates epitopes that would facilitate preexisting autoantibody binding, thereby exacerbating lung injury after transplant. We induced emphysema in C57BL/6 mice through 6 months of cigarette smoke (CS) exposure. Mice with CS exposure had significantly elevated serum autoantibodies compared with non-smoke-exposed age-matched (NS) mice. To determine the impact of a full preexisting autoantibody repertoire on IRI, we transplanted BALB/c donor lungs into NS or CS recipients and analyzed grafts 48 hours after transplant. CS recipients had significantly increased lung injury and immune cell infiltration after transplant. Immunofluorescence staining revealed increased IgM, IgG, and C3d deposition in CS recipients. To exclude confounding alloreactivity and confirm the role of preexisting autoantibodies in IRI, syngeneic Rag1^-/- (recombination-activating protein 1-knockout) transplants were performed in which recipients were reconstituted with pooled serum from CS or NS mice. Serum from CS-exposed mice significantly increased IRI compared with control mice, with trends in antibody and C3d deposition similar to those seen in allografts. These data demonstrate that pretransplant CS exposure is associated with increased IgM/IgG autoantibodies, which, upon transplant, bind to the donor lung, activate complement, and exacerbate post-transplant IRI.

New Techniques for Optimization of Donor Lungs/Hearts

Injuries sustained by donor heart and lung allografts during the transplantation process are multiple and cumulative. Optimization of allograft function plays an essential role in short- and long-term outcomes after transplantation. Therapeutic targets to prevent or attenuate injury are present in the donor, the preservation process, during transplantation, and in postoperative management of the recipient. The newest and most promising methods of optimizing donor heart and lung allografts are found in alternative preservation strategies, which enable functional assessment of donor organs and provide a modality to initiate therapies for injured allografts or prevent injury during reperfusion in recipients.

Controversies in the Postoperative Management of the Critically Ill Heart Transplant Patient

Heart transplant recipients are susceptible to a number of complications in the immediate postoperative period. Despite advances in surgical techniques, mechanical circulatory support (MCS), and immunosuppression, evidence supporting optimal management strategies of the critically ill transplant patient is lacking on many fronts. This review identifies some of these controversies with the aim of stimulating further discussion and development into these gray areas.

Donor Urinary C5a Levels Independently Correlate With Posttransplant Delayed Graft Function

BACKGROUND

Accumulating evidence implicates the complement cascade as pathogenically contributing to ischemia-reperfusion injury and delayed graft function (DGF) in human kidney transplant recipients. Building on observations that kidney injury can initiate in the donor before nephrectomy, we tested the hypothesis that anaphylatoxins C3a and C5a in donor urine before transplantation associate with risk of posttransplant injury.

METHODS

We evaluated the effects of C3a and C5a in donor urine on outcomes of 469 deceased donors and their corresponding 902 kidney recipients in a subset of a prospective cohort study.

RESULTS

We found a threefold increase of urinary C5a concentrations in donors with stage 2 and 3 acute kidney injury (AKI) compared donors without AKI (P < 0.001). Donor C5a was higher for the recipients with DGF (defined as dialysis in the first week posttransplant) compared with non-DGF (P = 0.002). In adjusted analyses, C5a remained independently associated with recipient DGF for donors without AKI (relative risk, 1.31; 95% confidence interval, 1.13-1.54). For donors with AKI, however, urinary C5a was not associated with DGF. We observed a trend toward better 12-month allograft function for kidneys from donors with C5a concentrations in the lowest tertile (P = 0.09). Urinary C3a was not associated with donor AKI, recipient DGF, or 12-month allograft function.

CONCLUSIONS

Urinary C5a correlates with the degree of donor AKI. In the absence of clinical donor AKI, donor urinary C5a concentrations associate with recipient DGF, providing a foundation for testing interventions aimed at preventing DGF within this high-risk patient subgroup.

Complement Therapeutics in the Multi-Organ Donor: Do or Don't?

Over the last decade, striking progress has been made in the field of organ transplantation, such as better surgical expertise and preservation techniques. Therefore, organ transplantation is nowadays considered a successful treatment in end-stage diseases of various organs, e.g. the kidney, liver, intestine, heart, and lungs. However, there are still barriers which prevent a lifelong survival of the donor graft in the recipient. Activation of the immune system is an important limiting factor in the transplantation process. As part of this pro-inflammatory environment, the complement system is triggered. Complement activation plays a key role in the transplantation process, as highlighted by the amount of studies in ischemia-reperfusion injury (IRI) and rejection. However, new insight have shown that complement is not only activated in the later stages of transplantation, but already commences in the donor. In deceased donors, complement activation is associated with deteriorated quality of deceased donor organs. Of importance, since most donor organs are derived from either brain-dead donors or deceased after circulatory death donors. The exact mechanisms and the role of the complement system in the pathophysiology of the deceased donor have been underexposed. This review provides an overview of the current knowledge on complement activation in the (multi-)organ donor. Targeting the complement system might be a promising therapeutic strategy to improve the quality of various donor organs. Therefore, we will discuss the complement therapeutics that already have been tested in the donor. Finally, we question whether complement therapeutics should be translated to the clinics and if all organs share the same potential complement targets, considering the physiological differences of each organ.

Complement and Transplantation: From New Mechanisms to Potential Biomarkers and Novel Treatment Strategies

The complement system, traditionally considered a component of innate immunity, is now recognized as a crucial mediator of the adaptive immune response in solid organ transplantation. Preclinical and early human trials have demonstrated the importance of complement effector mechanisms in driving allograft injury during specific antigraft immune responses, including ischemia-reperfusion injury, T-cell-mediated rejection, and antibody-mediated rejection, as well as a potential role for complement-derived risk stratification biomarkers. These data support the need for further testing of complement inhibitors in solid organ transplant recipients.

Donor pretreatment with nebulized complement C3a receptor antagonist mitigates brain-death induced immunological injury post-lung transplant

Donor brain death (BD) is an inherent part of lung transplantation (LTx) and a key contributor to ischemia-reperfusion injury (IRI). Complement activation occurs as a consequence of BD in other solid organ Tx and exacerbates IRI, but the role of complement in LTx has not been investigated. Here, we investigate the utility of delivering nebulized C3a receptor antagonist (C3aRA) pretransplant to BD donor lungs in order to reduce post-LTx IRI. BD was induced in Balb/c donors, and lungs nebulized with C3aRA or vehicle 30 minutes prior to lung procurement. Lungs were then cold stored for 18 hours before transplantation into C57Bl/6 recipients. Donor lungs from living donors (LD) were removed and similarly stored. At 6 hours and 5 days post-LTx, recipients of BD donor lungs had exacerbated IRI and acute rejection (AR), respectively, compared to recipients receiving LD lungs, as determined by increased histopathological injury, immune cells, and cytokine levels. A single pretransplant nebulized dose of C3aRA to the donor significantly reduced IRI as compared to vehicle-treated BD donors, and returned IRI and AR grades to that seen following LD LTx. These data demonstrate a role for complement inhibition in the amelioration of IRI post-LTx in the context of donor BD.

Acute and chronic phagocyte determinants of cardiac allograft vasculopathy

Post-transplant immunosuppression has reduced the incidence of T cell-mediated acute rejection, yet long-term cardiac graft survival rates remain a challenge. An important determinant of chronic solid organ allograft complication is accelerated vascular disease of the transplanted graft. In the case of cardiac allograft vasculopathy (CAV), the precise cellular etiology remains inadequately understood; however, histologic evidence hints at the accumulation and activation of innate phagocytes as a causal contributing factor. This includes monocytes, macrophages, and immature dendritic cell subsets. In addition to crosstalk with adaptive T and B immune cells, myeloid phagocytes secrete paracrine signals that directly activate fibroblasts and vascular smooth muscle cells, both of which contribute to fibrous intimal thickening. Though maladaptive phagocyte functions may promote CAV, directed modulation of myeloid cell function, at the molecular level, holds promise for tolerance and prolonged cardiac graft function.

Acute Cardiac Complications in Critical Brain Disease

Acute cardiac complications in critical brain disease should be understood as a clinical condition representing an intense brain-heart crosstalk and might mimic ischemic heart disease. Two main entities (neurogenic stunned myocardium [NSM] and stress cardiomyopathy) have been better characterized in the neurocritically ill patients and they portend worse clinical outcomes in these cases. The pathophysiology of NSM remains elusive. However, significant progress has been made on the early identification of neurocardiac compromise following acute critical brain disease. Effective prevention and treatment interventions are yet to be determined.

Acute Cardiac Complications in Critical Brain Disease

Acute cardiac complications in critical brain disease should be understood as a clinical condition representing an intense brain-heart crosstalk and might mimic ischemic heart disease. Two main entities (neurogenic stunned myocardium [NSM] and stress cardiomyopathy) have been better characterized in the neurocritically ill patients and they portend worse clinical outcomes in these cases. The pathophysiology of NSM remains elusive. However, significant progress has been made on the early identification of neurocardiac compromise following acute critical brain disease. Effective prevention and treatment interventions are yet to be determined.

Tissue-targeted complement therapeutics

Complement activation contributes to the pathogenesis of numerous inflammatory and autoimmune diseases. Therapeutic complement inhibitors have proven effective in several of these diseases and have now entered clinical use. Complement activation has multiple different biologic effects, however, and the currently available drugs can have undesirable side-effects, such as an increased risk of infection. Several different complement inhibitors have been developed that bind to target molecules, thereby concentrating the drug at a specific anatomic site. This approach appears to be both more effective than untargeted drugs and to have fewer side effects. In this article we review different targeting strategies that have been developed and the evidence supporting the use and benefits of targeted drugs.

A prospective randomized, controlled trial of eculizumab to prevent ischemia-reperfusion injury in pediatric kidney transplantation

Ischemia-reperfusion injury has multiple effects on a transplanted allograft, including delayed or impaired graft function, compromised long-term survival, and an association with an increased incidence of rejection. Eculizumab, a monoclonal antibody blocking terminal complement activation, has been postulated to be an effective agent in the prevention or amelioration of IRI. We performed a single-center prospective, randomized controlled trial involving 57 pediatric kidney transplant recipients between 2012 and 2016. The immunosuppressive protocol included two doses of alemtuzumab; half of the patients were randomized to receive a single dose of eculizumab prior to transplantation. Maintenance immunosuppression was based on a combination of low-dose tacrolimus and mycophenolate, without steroids. Eculizumab-treated patients had a significantly better early graft function, less arteriolar hyalinosis and chronic glomerulopathy on a protocol biopsies taken on day 30, 1 year, and 3 years after transplantation. In the eculizumab group, four non-vaccinated children lost their grafts during the course of a flu-like infection. Eculizumab is associated with better early graft function and improved graft morphology; however, there was an unacceptably high number of early graft losses among the eculizumab-treated children. While a promising strategy, the best approach to complement inhibition remains to be established.

Role of biobanks in transplantation

The establishment of bio-banks together with high throughput technologies, such as genomics, transcriptomics and proteomics has opened new frontiers in biomarker discovery and the development of systems biology approaches to identifying key pathways that could be exploited to improve outcomes of solid organ transplantation. One of the major challenges in organ donation has been the lack of access to large scale well characterised material to facilitate projects that aim to characterise injury to donor organs and identify biomarkers. This may have hampered research in the field of organ donation by not allowing researchers to materials of high quality and lower pre-analytical variability. We describe in this manuscript the need for bio-banks in organ donation, research opportunities and the particular challenges in establishing such an initiative.

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We address:

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The main challenges in transplantation.

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Underpinning cellular processes of injury and repair.

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The role of biobanks can be used in transplantation.

Sensitivity to perioperative ischemia/reperfusion injury in male and female donor myocardium

Many functions of the cardiovascular apparatus are affected by gender. The aim of our study was find out whether markers of cell death present in the donor myocardium differ in male and female hearts. The study involved 81 patients undergoing heart transplantation from September 2010 to January 2013. Patients were divided into two groups: male allograft (n=49), and female allograft (n=32). Two types of myocardial cell death were analyzed. High-sensitive cardiac troponin T as a necrosis marker and protein bcl-2, caspase 3 and TUNEL as apoptosis markers were measured. We observed a significantly higher level of high-sensitive cardiac troponin T after correcting for predicted ventricular mass in female donors before transplantation as well as in the female allograft group after transplantation throughout the monitored period (P=0.011). There were no differences in apoptosis markers (bcl-2, caspase 3, TUNEL) between male and female hearts before transplantation. Both genders showed a significant increase of TUNEL-positive myocytes one week after transplantation without differences between the groups. Moreover, there were no differences in caspase 3 and bcl-2 expression between the two groups. Our results demonstrated the presence of necrotic and apoptotic cell death in human heart allografts. High-sensitive cardiac troponin T adjusted for predicted ventricular mass as a marker of myocardial necrosis was higher in female donors, and this gender difference was even more pronounced after transplantation.

Complement in Kidney Transplantation

The complement system is considered to be an important part of innate immune system with a significant role in inflammation processes. The activation can occur through classical, alternative, or lectin pathway, resulting in the creation of anaphylatoxins C3a and C5a, possessing a vast spectrum of immune functions, and the assembly of terminal complement cascade, capable of direct cell lysis. The activation processes are tightly regulated; inappropriate activation of the complement cascade plays a significant role in many renal diseases including organ transplantation. Moreover, complement cascade is activated during ischemia/reperfusion injury processes and influences delayed graft function of kidney allografts. Interestingly, complement system has been found to play a role in both acute cellular and antibody-mediated rejections and thrombotic microangiopathy. Therefore, complement system may represent an interesting therapeutical target in kidney transplant pathologies.

Complement in renal transplantation: The road to translation

Renal transplantation is the treatment of choice for patients with end-stage renal disease. The vital role of the complement system in renal transplantation is widely recognized. This review discusses the role of complement in the different phases of renal transplantation: in the donor, during preservation, in reperfusion and at the time of rejection. Here we examine the current literature to determine the importance of both local and systemic complement production and how complement activation contributes to the pathogenesis of renal transplant injury. In addition, we dissect the complement pathways involved in the different phases of renal transplantation. We also review the therapeutic strategies that have been tested to inhibit complement during the kidney transplantation. Several clinical trials are currently underway to evaluate the therapeutic potential of complement inhibition for the treatment of brain death-induced renal injury, renal ischemia-reperfusion injury and acute rejection. We conclude that it is expected that in the near future, complement-targeted therapeutics will be used clinically in renal transplantation. This will hopefully result in improved renal graft function and increased graft survival.

Clinical Factors Implicated in Primary Graft Dysfunction After Heart Transplantation: A Single-center Experience

BACKGROUND

Primary graft dysfunction (PGD) is a frequent complication after cardiac transplantation and remains one of the leading causes of mortality in these patients. The objective of this case-control study is to identify donor and surgical procedure's factors associated with PGD, and further guide possible strategies to prevent PGD.

METHODS

Retrospective analysis of the medical records of patients who underwent cardiac transplantation at Memorial Hermann Hospital at Texas Medical Center between October 2012 and February 2015.

RESULTS

The study population included 99 patients, of which 18 developed PGD. Univariate analysis of donor characteristics revealed opioid use (P = .049) and death owing to anoxia (P = .021) were associated with PGD. The recipient/donor blood type match AB/A was significantly associated with PGD (P = .031). Time from brain death to aortic cross clamp (TBDACC) of ≥3 and ≥5 days were also found to be associated with PGD (P = .0011 and .0003, respectively). Multivariate analysis confirmed that patients with a time from brain death to aortic cross clamp ≥3 and ≥5 days had lesser odds of developing PGD (odds ratio, 0.098 [P = .0026] and OR, 0.092 [P = .0017], respectively].

CONCLUSIONS

Our study showed that a longer time from brain death to aortic cross clamp was associated with lower odds of developing PGD. Therefore, postponing heart procurement for a few days after brain death seems to be beneficial in preventing PGD.

Complement-mediated inflammation and injury in brain dead organ donors

The importance of the complement system in renal ischemia-reperfusion injury and acute rejection is widely recognized, however its contribution to the pathogenesis of tissue damage in the donor remains underexposed. Brain-dead (BD) organ donors are still the primary source of organs for transplantation. Brain death is characterized by hemodynamic changes, hormonal dysregulation, and immunological activation. Recently, the complement system has been shown to be involved. In BD organ donors, complement is activated systemically and locally and is an important mediator of inflammation and graft injury. Furthermore, complement activation can be used as a clinical marker for the prediction of graft function after transplantation. Experimental models of BD have shown that inhibition of the complement cascade is a successful method to reduce inflammation and injury of donor grafts, thereby improving graft function and survival after transplantation. Consequently, complement-targeted therapeutics in BD organ donors form a new opportunity to improve organ quality for transplantation. Future studies should further elucidate the mechanism responsible for complement activation in BD organ donors.

Targeting the Innate Immune Response to Improve Cardiac Graft Recovery after Heart Transplantation: Implications for the Donation after Cardiac Death

Heart transplantation (HTx) is the ultimate treatment for end-stage heart failure. The number of patients on waiting lists for heart transplants, however, is much higher than the number of available organs. The shortage of donor hearts is a serious concern since the population affected by heart failure is constantly increasing. Furthermore, the long-term success of HTx poses some challenges despite the improvement in the management of the short-term complications and in the methods to limit graft rejection. Myocardial injury occurs during transplantation. Injury initiated in the donor as result of brain or cardiac death is exacerbated by organ procurement and storage, and is ultimately amplified by reperfusion injury at the time of transplantation. The innate immune system is a mechanism of first-line defense against pathogens and cell injury. Innate immunity is activated during myocardial injury and produces deleterious effects on the heart structure and function. Here, we briefly discuss the role of the innate immunity in the initiation of myocardial injury, with particular focus on the Toll-like receptors and inflammasome, and how to potentially expand the donor population by targeting the innate immune response.

Complement-here, there and everywhere, but what about the transplanted organ?

The part of the innate immune system that communicates and effectively primes the adaptive immune system was termed "complement" by Ehrlich to reflect its complementarity to antibodies having previously been described as "alexine" (i.e protective component of serum) by Buchner and Bordet. It has been established that complement is not solely produced systemically but may have origin in different tissues where it can influence organ specific functions that may affect the outcome of transplanted organs. This review looks at the role of complement in particular to kidney transplantation. We look at current literature to determine whether blockade of the peripheral or central compartments of complement production may prevent ischaemic reperfusion injury or rejection in the transplanted organ. We also review new therapeutics that have been developed to inhibit components of the complement cascade with varying degrees of success leading to an increase in our understanding of the multiple triggers of this complex system. In addition, we consider whether biomarkers in this field are effective markers of disease or treatment.

Using an Integrated -Omics Approach to Identify Key Cellular Processes That Are Disturbed in the Kidney After Brain Death

In an era where we are becoming more reliant on vulnerable kidneys for transplantation from older donors, there is an urgent need to understand how brain death leads to kidney dysfunction and, hence, how this can be prevented. Using a rodent model of hemorrhagic stroke and next-generation proteomic and metabolomic technologies, we aimed to delineate which key cellular processes are perturbed in the kidney after brain death. Pathway analysis of the proteomic signature of kidneys from brain-dead donors revealed large-scale changes in mitochondrial proteins that were associated with altered mitochondrial activity and morphological evidence of mitochondrial injury. We identified an increase in a number of glycolytic proteins and lactate production, suggesting a shift toward anaerobic metabolism. Higher amounts of succinate were found in the brain death group, in conjunction with increased markers of oxidative stress. We characterized the responsiveness of hypoxia inducible factors and found this correlated with post-brain death mean arterial pressures. Brain death leads to metabolic disturbances in the kidney and alterations in mitochondrial function and reactive oxygen species generation. This metabolic disturbance and alteration in mitochondrial function may lead to further cellular injury. Conditioning the brain-dead organ donor by altering metabolism could be a novel approach to ameliorate this brain death-induced kidney injury.

Cardiac Dysfunction After Neurologic Injury: What Do We Know and Where Are We Going?

Recent literature has implicated severe neurologic injuries, such as aneurysmal subarachnoid hemorrhage, as a cause of cardiac dysfunction, impaired hemodynamic function, and poor outcomes. Mechanistic links between the brain and the heart have been explored in detail over the past several decades, and catecholamine excess, neuroendocrine dysfunction, and unchecked inflammation all likely contribute to the pathophysiologic process. Although cardiac dysfunction has also been described in other disease paradigms, including septic shock and thermal injury, there is likely a common underlying pathophysiology. In this review, we will examine the pathophysiology of cardiac dysfunction after neurologic injury, discuss the evidence surrounding cardiac dysfunction after different neurologic injuries, and suggest future research goals to gain knowledge and improve outcomes in this patient population.

Effects of complement activation on allograft injury

PURPOSE OF REVIEW

To summarize the current knowledge regarding mechanisms linking the complement system to transplant injury, highlighting findings reported since 2013.

RECENT FINDINGS

Building upon the documentation that complement activation is a pathogenic mediator of posttransplant ischemia-reperfusion injury, emerging evidence from animal models indicates that blocking either the classical or lectin pathways attenuates ischemia-reperfusion injury. Immune cell-derived and locally activated complement, including intracellular C3, positively modulates alloreactive T-cell activation and expansion, whereby simultaneously inhibiting regulatory T-cell induction and function, and together promoting transplant rejection. Although alloantibody-initiated complement activation directly injures target cells, complement-dependent signals activate endothelial cells to facilitate T-cell-dependent inflammation. Complement activation within allografts contributes to progressive chronic injury and fibrosis.

SUMMARY

The complement cascade, traditionally considered to be relevant to transplantation only as an effector mechanism of antibody-initiated allograft injury, is now understood to damage the allograft through multiple mechanisms. Complement activation promotes posttransplant ischemia-reperfusion injury, formation and function of alloantibody, differentiation and function of alloreactive T cells, and contributes to chronic progressive allograft failure. The recognition that complement affects transplant injury at many levels provides a foundation for targeting complement as a therapy to prolong transplant survival and improve patient health.

Biomarkers of cellular apoptosis and necrosis in donor myocardium are not predictive of primary graft dysfunction

Primary graft dysfunction (PGD) is a life-threatening complication among heart transplant recipients and a major cause of early mortality. Although the pathogenesis of PGD is still unclear, ischemia/reperfusion injury has been identified as a predominant factor. Both necrosis and apoptosis contribute to the loss of cardiomyocytes during ischemia/reperfusion injury, and this loss of cells can ultimately lead to PGD. The aim of our prospective study was to find out whether cell death, necrosis and apoptosis markers present in the donor myocardium can predict PGD. The prospective study involved 64 consecutive patients who underwent orthotopic heart transplantation at our institute between September 2010 and January 2013. High-sensitive cardiac troponin T (hs-cTnT) as a marker of minor myocardial necrosis was detected from arterial blood samples before the donor's pericardium was opened. Apoptosis (caspase-3, active + pro-caspase-3, bcl-2, TUNEL) was assessed from bioptic samples taken from the right ventricle prior graft harvesting. In our study, 14 % of transplant recipients developed PGD classified according to the standardized definition proposed by the ISHLT Working Group. We did not find differences between the groups in regard to hs-cTnT serum levels. The mean hs-cTnT value for the PGD group was 57.4+/-22.9 ng/l, compared to 68.4+/-10.8 ng/l in the group without PGD. The presence and severity of apoptosis in grafted hearts did not differ between grafts without PGD and hearts that subsequently developed PGD. In conclusion, our findings did not demonstrate any association between measured myocardial cell death, necrosis or apoptosis markers in donor myocardium and PGD in allograft recipients. More detailed investigations of cell death signaling pathways in transplanted hearts are required.

Inflammatory triggers of acute rejection of organ allografts

Solid organ transplantation is a vital therapy for end stage diseases. Decades of research have established that components of the adaptive immune system are critical for transplant rejection, but the role of the innate immune system in organ transplantation is just emerging. Accumulating evidence indicates that the innate immune system is activated at the time of organ implantation by the release of endogenous inflammatory triggers. This review discusses the nature of these triggers in organ transplantation and also potential mediators that may enhance inflammation resolution after organ implantation.

Targeting pathogenic postischemic self-recognition by natural IgM to protect against posttransplantation cardiac reperfusion injury

BACKGROUND

Natural IgM antibodies represent a class of innate pattern recognition receptors that recognize danger-associated molecular patterns expressed on stressed or dying cells. They play important roles in tissue homeostasis by disposing of prenecrotic cells and suppressing inflammation. However, ischemic insult leads to a pathogenic level of IgM binding and complement activation, resulting in inflammation and injury. We investigate the role of self-reactive IgM in the unique setting of transplantation where the donor organ undergoes both cold and warm ischemia and global ischemic insult.

METHODS AND RESULTS

By transplanting hearts from wild-type donor mice into antibody-deficient mice reconstituted with specific self-reactive IgM monoclonal antibodies, we identified neoepitopes expressed after transplantation and demonstrated a key role for IgM recognition of these epitopes in graft injury. With this information, we developed and characterized a therapeutic strategy that exploited the postischemia recognition system of natural antibodies. On the basis of neoepitope identification, we constructed an anti-annexin IV single-chain antibody (scFv) and an scFv linked to Crry, an inhibitor of C3 activation (scFv-Crry). In an allograft transplantation model in which recipients contain a full natural antibody repertoire, both constructs blocked graft IgM binding and complement activation and significantly reduced graft inflammation and injury. Furthermore, scFv-Crry specifically targeted to the transplanted heart and, unlike complement deficiency, did not affect immunity to infection, an important consideration for immunosuppressed transplant recipients.

CONCLUSIONS

We identified pathophysiologically important epitopes expressed within the heart after transplantation and described a novel translatable strategy for targeted complement inhibition that has several advantages over currently available approaches.

Control of innate immunological mechanisms as a route to drug minimization

PURPOSE OF REVIEW

Much research in transplantation focuses on treatments for rejection and induction of tolerance. Recent evidence has shown that initial inflammation induced by innate immune effectors after transplantation has a key role in modulating adaptive immune responses that cause organ rejection. Here, we describe the role of the innate immune system, particularly the complement activation pathways, and how they influence adaptive immune responses post-transplantation and current strategies, which are under development to block these innate pathways.

RECENT FINDINGS

Anaphylatoxins and their respective receptors are proving to be important in T-cell-mediated immunity and make attractive targets for therapies designed to promote tolerance in solid organ transplantation. Additionally, regulators of complement activation are currently being tested in clinical trials, with improvements in drug delivery.

SUMMARY

Preventing ischaemia-reperfusion injury in transplanted organs significantly reduces immune activation and promotes graft survival. Research into the mechanisms of complement activation in both native organ ischaemia and transplantation models detail emerging roles for complement intermediates that can serve as targets for intervention, with the aim of reducing early post-transplant inflammation, reducing the intensity of immunosuppressive regimens, leading to prolonged graft survival.