Morphologic and immunohistochemical findings in antibody-mediated rejection of the cardiac allograft

TLR7 Activation Accelerates Cardiovascular Pathology in a Mouse Model of Lupus

We report a novel model of lupus-associated cardiovascular pathology accelerated by the TLR7 agonist R848 in lupus-prone B6.Sle1.Sle2.Sle3 (TC) mice. R848-treated TC mice but not non-autoimmune C57BL/6 (B6) controls developed microvascular inflammation and myocytolysis with intracellular vacuolization. This histopathology was similar to antibody-mediated rejection after heart transplant, although it did not involve complement. The TC or B6 recipients of serum or splenocytes from R848-treated TC mice developed a reactive cardiomyocyte hypertrophy, which also presents spontaneously in old TC mice as well as in TC.Rag-/- mice that lack B and T cells. Each of these cardiovascular lesions correspond to abnormalities that have been reported in lupus patients. Lymphoid and non-lymphoid immune cells as well as soluble factors contribute to lupus-associated cardiovascular lesions in TC mice, which can now be dissected using this model with and without R848 treatment.

Ligation of HLA Class I Molecules Induces YAP Activation through Src in Human Endothelial Cells

Ab cross-linking of HLA class I (HLA I) molecules on the surface of endothelial cells (EC) triggers proliferative and prosurvival intracellular signaling, which is implicated in the process of chronic allograft rejection, also known as transplant vasculopathy. Despite the importance of Ab-mediated rejection in transplantation, the mechanisms involved remain incompletely understood. In this study, we examined the regulation of yes-associated protein (YAP) localization, phosphorylation, and transcriptional activity in human ECs challenged with Abs that bind HLA I. In unstimulated ECs, YAP localized mainly in the cytoplasm. Stimulation of these cells with Ab W6/32 induced marked translocation of YAP to the nucleus. The nuclear import of YAP was associated with a rapid decrease in YAP phosphorylation at Ser¹²⁷ and Ser³⁹⁷, sites targeted by LATS1/2 and with the expression of YAP-regulated genes, including connective tissue growth factor (CTGF), and cysteine-rich angiogenic inducer 61 (CYR61). Transfection of small interfering RNAs targeting YAP/TAZ blocked the migration of ECs stimulated by ligation of HLA I, indicating that YAP mediates the increase in EC migration induced by HLA I ligation. Treatment of intact ECs with Src family inhibitors induced cytoplasmic localization of YAP in unstimulated ECs and, strikingly, blocked the nuclear import of YAP induced by Ab-induced HLA I activation in these cells and the increase in the expression of the YAP-regulated genes CTGF and CYR61 induced by HLA I stimulation. Our results identify the Src/YAP axis as a key player in promoting the proliferation and migration of ECs that are critical in the pathogenesis of transplant vasculopathy.

Preservation of microvascular barrier function requires CD31 receptor-induced metabolic reprogramming

Endothelial barrier (EB) breaching is a frequent event during inflammation, and it is followed by the rapid recovery of microvascular integrity. The molecular mechanisms of EB recovery are poorly understood. Triggering of MHC molecules by migrating T-cells is a minimal signal capable of inducing endothelial contraction and transient microvascular leakage. Using this model, we show that EB recovery requires a CD31 receptor-induced, robust glycolytic response sustaining junction re-annealing. Mechanistically, this response involves src-homology phosphatase activation leading to Akt-mediated nuclear exclusion of FoxO1 and concomitant β-catenin translocation to the nucleus, collectively leading to cMyc transcription. CD31 signals also sustain mitochondrial respiration, however this pathway does not contribute to junction remodeling. We further show that pathologic microvascular leakage in CD31-deficient mice can be corrected by enhancing the glycolytic flux via pharmacological Akt or AMPK activation, thus providing a molecular platform for the therapeutic control of EB response.

Role of donor macrophages after heart and lung transplantation

Since the 1960s, heart and lung transplantation has remained the optimal therapy for patients with end-stage disease, extending and improving quality of life for thousands of individuals annually. Expanding donor organ availability and immunologic compatibility is a priority to help meet the clinical demand for organ transplant. While effective, current immunosuppression is imperfect as it lacks specificity and imposes unintended adverse effects such as opportunistic infections and malignancy that limit the health and longevity of transplant recipients. In this review, we focus on donor macrophages as a new target to achieve allograft tolerance. Donor organ-directed therapies have the potential to improve allograft survival while minimizing patient harm related to global suppression of recipient immune responses. Topics highlighted include the role of ontogenically distinct donor macrophage populations in ischemia-reperfusion injury and rejection, including their interaction with allograft-infiltrating recipient immune cells and potential therapeutic approaches. Ultimately, a better understanding of how donor intrinsic immunity influences allograft acceptance and survival will provide new opportunities to improve the outcomes of transplant recipients.

Missing self triggers NK cell-mediated chronic vascular rejection of solid organ transplants

Current doctrine is that microvascular inflammation (MVI) triggered by a transplant -recipient antibody response against alloantigens (antibody-mediated rejection) is the main cause of graft failure. Here, we show that histological lesions are not mediated by antibodies in approximately half the participants in a cohort of 129 renal recipients with MVI on graft biopsy. Genetic analysis of these patients shows a higher prevalence of mismatches between donor HLA I and recipient inhibitory killer cell immunoglobulin-like receptors (KIRs). Human in vitro models and transplantation of β2-microglobulin-deficient hearts into wild-type mice demonstrates that the inability of graft endothelial cells to provide HLA I-mediated inhibitory signals to recipient circulating NK cells triggers their activation, which in turn promotes endothelial damage. Missing self-induced NK cell activation is mTORC1-dependent and the mTOR inhibitor rapamycin can prevent the development of this type of chronic vascular rejection.

‘Missing self’ is a mode of natural killer (NK) cell activation aimed to detect the lack of HLA-I molecules on infected or neoplastic cells. Here, the authors show that mismatch between donor HLA-I and cognate receptors on recipient NK cells mediates microvascular inflammation-associated graft rejection, a pathology that is preventable by mTOR inhibition.

Clinicopathological complexity in the application of the universal definition of myocardial infarction

A universal definition of myocardial infarction (UDMI) has been established, periodically updated, and refined over the past twenty years. The primary purpose of the UDMI is to bring uniformity and accuracy to clinical diagnosis. Herein, a review and analysis of the UDMI is presented with emphasis on clinicopathological correlation. Determination of the presence of myocardial injury is based on the detection of abnormal serum cardiac biomarkers, particularly cardiac troponin (cTn), and in the current fourth iteration of the UDMI, high sensitivity (hs)-cTn. Differentiation of myocardial infarction from other causes of myocardial injury requires the documentation of clinical evidence of myocardial ischemia. In this review, difficulties in applying the UDMI in actual practice are discussed, based on the experience and perspective of those of us who face these problems as part of our own practice of pathology. The complexity in application of the UDMI is highlighted by the presentation of five illustrative cases involving the differential diagnosis of myocardial injury and myocardial infarction due to atherothrombotic and nonatherothrombotic coronary artery disease. The cases include myocardial infarction due to severe coronary atherosclerosis, supply-demand mismatch, coronary artery dissection associated with an eosinophilic coronary periarteritis, and coronary thromboembolism, and a case with a differential diagnosis of myocarditis and myocardial infarction. These cases illustrate how pathological findings can contribute to more accurate application of the UDMI and how, when critically applied, the UDMI can be used to better characterize myocardial infarcts in clinical practice.

The Evolving Roles of Macrophages in Organ Transplantation

Organ transplantation is a life-saving strategy for patients with end-stage organ failure. Over the past few decades, organ transplantation has achieved an excellent success in short-term survival but only a marginal improvement in long-term graft outcomes. The pathophysiology of graft loss is multifactorial and remains incompletely defined. However, emerging evidence suggests macrophages as crucial mediators of acute and chronic allograft immunopathology. In this process, macrophage-mediated mobilization of first-line defenses, particularly phagocytosis and the release of acute inflammatory mediators, is important, but macrophages also launch adaptive alloimmune reactions against grafts through antigen processing and presentation, as well as providing costimulation. Additionally, crosstalk with other immune cells and graft endothelial cells causes tissue damage or fibrosis in transplanted organs, contributing to graft loss or tolerance resistance. However, some macrophages function as regulatory cells that are capable of suppressing allogeneic T cells, inhibiting DC maturation, inducing the differentiation of Tregs, and subsequently promoting transplant tolerance. This functional diversity of macrophages in organ transplantation is consistent with their heterogeneity. Although our knowledge of the detrimental or beneficial effects of macrophages on transplants has exponentially increased, the exact mechanisms controlling macrophage functions are not yet completely understood. Here, we review recent advances in our understanding of the multifaceted nature of macrophages, focusing on their evolving roles in organ transplantation and the mechanisms involved in their activation and function in allograft transplantation. We also discuss potential therapeutic options and opportunities to target macrophage to improve the outcomes of transplant recipients.

Critical Role of Macrophage FcγR Signaling and Reactive Oxygen Species in Alloantibody-Mediated Hepatocyte Rejection

Humoral alloimmunity negatively impacts both short- and long-term cell and solid organ transplant survival. We previously reported that alloantibody-mediated rejection of transplanted hepatocytes is critically dependent on host macrophages. However, the effector mechanism(s) of macrophage-mediated injury to allogeneic liver parenchymal cells is not known. We hypothesized that macrophage-mediated destruction of allogeneic hepatocytes occurs by cell-cell interactions requiring FcγRs. To examine this, alloantibody-dependent hepatocyte rejection in CD8-depleted wild-type (WT) and Fcγ-chain knockout (KO; lacking all functional FcγR) transplant recipients was evaluated. Alloantibody-mediated hepatocellular allograft rejection was abrogated in recipients lacking FcγR compared with WT recipients. We also investigated anti-FcγRI mAb, anti-FcγRIII mAb, and inhibitors of intracellular signaling (to block phagocytosis, cytokines, and reactive oxygen species [ROS]) in an in vitro alloantibody-dependent, macrophage-mediated hepatocytoxicity assay. Results showed that in vitro alloantibody-dependent, macrophage-mediated hepatocytotoxicity was critically dependent on FcγRs and ROS. The adoptive transfer of WT macrophages into CD8-depleted FcγR-deficient recipients was sufficient to induce alloantibody-mediated rejection, whereas adoptive transfer of macrophages from Fcγ-chain KO mice or ROS-deficient (p47 KO) macrophages was not. These results provide the first evidence, to our knowledge, that alloantibody-dependent hepatocellular allograft rejection is mediated by host macrophages through FcγR signaling and ROS cytotoxic effector mechanisms. These results support the investigation of novel immunotherapeutic strategies targeting macrophages, FcγRs, and/or downstream molecules, including ROS, to inhibit humoral immune damage of transplanted hepatocytes and perhaps other cell and solid organ transplants.

Xenoreactive antibodies and latent fibrin formation in VAD and cardiac transplant recipients can confound the detection and measurement of anti-AT1R antibodies

Autoantibodies to the angiotensin II type 1 receptor (AT1R) are thought to be important in antibody-mediated rejection (AMR), especially in the absence of anti-HLA antibodies. We used a variety of methods to examine the specificity of a commercially available kit designed to quantitate anti-AT1R antibodies. We found that fibrin formation in serum samples from patients awaiting cardiac transplantation with ventricular assist devices (VADs) can produce falsely elevated anti-AT1R values. In addition, absorption studies with a variety of cell lines with or without expression of human AT1R, and those that express xenoantigens, suggest that many of the antibodies detected in the AT1R test system are heterophilic and have reactivity to xenoantigens. Furthermore, we provide data that show that reactivity to the sialic acid Neu5Gc is a common finding among samples that are highest in anti-AT1R levels. We conclude that a common laboratory method for quantitation of anti-AT1R antibodies is nonspecific and overestimates the frequency of true positives. A reevaluation of the role that anti-AT1R antibodies play in allograft function and patient outcomes is warranted.

Three-dimensional echocardiography demonstrates a skewered left ventricular thrombus in a patient with a heart transplant

A left ventricular (LV) false tendon is a frequently visualized structure in echocardiography with unclear clinical significance. We present the case of a false tendon serving as a nidus for thrombus in a post-orthotopic heart transplantation patient. Three-dimensional transthoracic echocardiography (3DTTE) was utilized to visualize a LV mass and facilitate its identification as a thrombus as well as the surrounding structures. Using datasets from 3DTTE, the lack of ventricular wall attachment and circumferential formation of the thrombus around the false tendon was identified. Serial imaging demonstrated resolution of the thrombus with anticoagulation.

Outside-in HLA class I signaling regulates ICAM-1 clustering and endothelial cell-monocyte interactions via mTOR in transplant antibody-mediated rejection

Antibody-mediated rejection (AMR) resulting in transplant allograft vasculopathy (TAV) is the major obstacle for long-term survival of solid organ transplants. AMR is caused by donor-specific antibodies to HLA, which contribute to TAV by initiating outside-in signaling transduction pathways that elicit monocyte recruitment to activated endothelium. Mechanistic target of rapamycin (mTOR) inhibitors can attenuate TAV; therefore, we sought to understand the mechanistic underpinnings of mTOR signaling in HLA class I Ab-mediated endothelial cell activation and monocyte recruitment. We used an in vitro model to assess monocyte binding to HLA I Ab-activated endothelial cells and found mTOR inhibition reduced ezrin/radixin/moesin (ERM) phosphorylation, intercellular adhesion molecule 1 (ICAM-1) clustering, and monocyte firm adhesion to HLA I Ab-activated endothelium. Further, in a mouse model of AMR, in which C57BL/6. RAG1^-/- recipients of BALB/c cardiac allografts were passively transferred with donor-specific MHC I antibodies, mTOR inhibition significantly reduced vascular injury, ERM phosphorylation, and macrophage infiltration of the allograft. Taken together, these studies indicate mTOR inhibition suppresses ERM phosphorylation in endothelial cells, which impedes ICAM-1 clustering in response to HLA class I Ab and prevents macrophage infiltration into cardiac allografts. These findings indicate a novel therapeutic application for mTOR inhibitors to disrupt endothelial cell-monocyte interactions during AMR.

Complement-Mediated Enhancement of Monocyte Adhesion to Endothelial Cells by HLA Antibodies, and Blockade by a Specific Inhibitor of the Classical Complement Cascade, TNT003

Background

Antibody-mediated rejection (AMR) of most solid organs is characterized by evidence of complement activation and/or intragraft macrophages (C4d + and CD68+ biopsies). We previously demonstrated that crosslinking of HLA I by antibodies triggered endothelial activation and monocyte adhesion. We hypothesized that activation of the classical complement pathway at the endothelial cell surface by HLA antibodies would enhance monocyte adhesion through soluble split product generation, in parallel with direct endothelial activation downstream of HLA signaling.

Methods

Primary human aortic endothelial cells (HAEC) were stimulated with HLA class I antibodies in the presence of intact human serum complement. C3a and C5a generation, endothelial P-selectin expression, and adhesion of human primary and immortalized monocytes (Mono Mac 6) were measured. Alternatively, HAEC or monocytes were directly stimulated with purified C3a or C5a. Classical complement activation was inhibited by pretreatment of complement with an anti-C1s antibody (TNT003).

Results

Treatment of HAEC with HLA antibody and human complement increased the formation of C3a and C5a. Monocyte recruitment by human HLA antibodies was enhanced in the presence of intact human serum complement or purified C3a or C5a. Specific inhibition of the classical complement pathway using TNT003 or C1q-depleted serum significantly reduced adhesion of monocytes in the presence of human complement.

Conclusions

Despite persistent endothelial viability in the presence of HLA antibodies and complement, upstream complement anaphylatoxin production exacerbates endothelial exocytosis and leukocyte recruitment. Upstream inhibition of classical complement may be therapeutic to dampen mononuclear cell recruitment and endothelial activation characteristic of microvascular inflammation during AMR.

Valenzuela et al show that HLA antibody binding to human endothelial cells in vitro, triggered complement C3a and C5a deposition that mediated monocyte recruitment, and the salutary effects of inhibiting the classical complement pathway with an anti-C1s antibody. Supplemental digital content is available in the text.

Serum exosomal protein profiling for the non-invasive detection of cardiac allograft rejection

BACKGROUND

Exosomes are cell-derived circulating vesicles that play an important role in cell-cell communication. Exosomes are actively assembled and carry messenger RNAs, microRNAs and proteins. The "gold standard" for cardiac allograft surveillance is endomyocardial biopsy (EMB), an invasive technique with a distinct complication profile. The development of novel, non-invasive methods for the early diagnosis of allograft rejection is warranted. We hypothesized that the exosomal proteome is altered in acute rejection, allowing for a distinction between non-rejection and rejection episodes.

METHODS

Serum samples were collected from heart transplant (HTx) recipients with no rejection, acute cellular rejection (ACR) and antibody-mediated rejection (AMR). Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of serum exosome was performed using a mass spectrometer (Orbitrap Fusion Tribrid).

RESULTS

Principal component analysis (PCA) revealed a clustering of 3 groups: (1) control and heart failure (HF); (2) HTx without rejection; and (3) ACR and AMR. A total of 45 proteins were identified that could distinguish between groups (q < 0.05). Comparison of serum exosomal proteins from control, HF and non-rejection HTx revealed 17 differentially expressed proteins in at least 1 group (q < 0.05). Finally, comparisons of non-rejection HTx, ACR and AMR serum exosomes revealed 15 differentially expressed proteins in at least 1 group (q < 0.05). Of these 15 proteins, 8 proteins are known to play a role in the immune response. Of note, the majority of proteins identified were associated with complement activation, adaptive immunity such as immunoglobulin components and coagulation.

CONCLUSIONS

Characterizing of circulating exosomal proteome in different cardiac disease states reveals unique protein expression patterns indicative of the respective pathologies. Our data suggest that HTx and allograft rejection alter the circulating exosomal protein content. Exosomal protein analysis could be a novel approach to detect and monitor acute transplant rejection and lead to the development of predictive and prognostic biomarkers.

Association of genetic polymorphisms of macrophage inhibitory factor (MIF) and B-cell activating factor (BAFF) with the detection of donor specific antibodies in kidney allograft recipients

The posttransplant development of donor specific antibodies (DSA) initiates the antibody mediated rejection (AMR), which is associated with the increased rate of graft loss. One of the characteristics of AMR is the infiltration of innate immune system including macrophages, monocytes, neutrophils or NK cells. Macrophage inhibitory factor (MIF) and B-cell activating factor (BAFF) are well known cytokines that are associated with the activation of the innate immune system which can damage kidney allograft. In this article, the association of the genetic polymorphisms of MIF and BAFF with the development of DSA including Class I and II in kidney transplant patients is investigated. A total of 231 renal transplant patients between 2008 and 2012 at St. Vincent Medical Center, CA were studied in a retrospective study design. DSA were determined by Luminex technology, and single nucleotide polymorphisms (SNP) of MIF and BAFF were determined by the real time PCR based on 5' nuclease allelic discrimination assay. The genetic polymorphisms of MIF rs1007888 (C/T) was associated with increased risk of positive DSA detection (p=0.04) after transplantation, and consistently significant after 1year (p=0.016). Furthermore, the presence of C allele were associated with the increased risk of Class I DSA detection (OR 1.816, CI 1.141-2.889, p=0.011). Also, genetic polymorphisms of BAFF rs12583006 were associated with the increased risk of Class II DSA detection (p=0.033). In conclusion, the genetic polymorphisms of MIF and BAFF may increase the risk of posttransplant development of DSA. This result suggests the association between the development of posttransplant DSA and the activation of innate immune system.

Antibody-mediated rejection across solid organ transplants: manifestations, mechanisms, and therapies

Solid organ transplantation is a curative therapy for hundreds of thousands of patients with end-stage organ failure. However, long-term outcomes have not improved, and nearly half of transplant recipients will lose their allografts by 10 years after transplant. One of the major challenges facing clinical transplantation is antibody-mediated rejection (AMR) caused by anti-donor HLA antibodies. AMR is highly associated with graft loss, but unfortunately there are few efficacious therapies to prevent and reverse AMR. This Review describes the clinical and histological manifestations of AMR, and discusses the immunopathological mechanisms contributing to antibody-mediated allograft injury as well as current and emerging therapies.

Targeting the Monocyte-Macrophage Lineage in Solid Organ Transplantation

There is an unmet clinical need for immunotherapeutic strategies that specifically target the active immune cells participating in the process of rejection after solid organ transplantation. The monocyte–macrophage cell lineage is increasingly recognized as a major player in acute and chronic allograft immunopathology. The dominant presence of cells of this lineage in rejecting allograft tissue is associated with worse graft function and survival. Monocytes and macrophages contribute to alloimmunity via diverse pathways: antigen processing and presentation, costimulation, pro-inflammatory cytokine production, and tissue repair. Cross talk with other recipient immune competent cells and donor endothelial cells leads to amplification of inflammation and a cytolytic response in the graft. Surprisingly, little is known about therapeutic manipulation of the function of cells of the monocyte–macrophage lineage in transplantation by immunosuppressive agents. Although not primarily designed to target monocyte–macrophage lineage cells, multiple categories of currently prescribed immunosuppressive drugs, such as mycophenolate mofetil, mammalian target of rapamycin inhibitors, and calcineurin inhibitors, do have limited inhibitory effects. These effects include diminishing the degree of cytokine production, thereby blocking costimulation and inhibiting the migration of monocytes to the site of rejection. Outside the field of transplantation, some clinical studies have shown that the monoclonal antibodies canakinumab, tocilizumab, and infliximab are effective in inhibiting monocyte functions. Indirect effects have also been shown for simvastatin, a lipid lowering drug, and bromodomain and extra-terminal motif inhibitors that reduce the cytokine production by monocytes–macrophages in patients with diabetes mellitus and rheumatoid arthritis. To date, detailed knowledge concerning the origin, the developmental requirements, and functions of diverse specialized monocyte–macrophage subsets justifies research for therapeutic manipulation. Here, we will discuss the effects of currently prescribed immunosuppressive drugs on monocyte/macrophage features and the future challenges.

Donor-Specific Antibodies Are Produced Locally in Ectopic Lymphoid Structures in Cardiac Allografts

Cardiac allograft vasculopathy (CAV) is a transplant pathology, limiting graft survival after heart transplantation. CAV arteries are surrounded by ectopic lymphoid structures (ELS) containing B cells and plasma cells. The aim of this study was to characterize the antigenic targets of antibodies produced in ELS. Coronary arteries and surrounding epicardial tissue from 56 transplant recipients were collected during autopsy. Immunofluorescence was used to identify antibody-producing plasma cells. Immunoglobulin levels in tissue lysates were measured by enzyme-linked immunosorbent assay and analyzed for donor-specific HLA antibodies by Luminex assay. Cytokine and receptor expression levels were quantified using quantitative polymerase chain reaction. Plasma cells in ELS were polyclonal and produced IgG and/or IgM antibodies. In epicardial tissue, IgG (p < 0.05) and IgM levels were higher in transplant patients with larger ELS than smaller ELS. In 4 of 21 (19%) patients with ELS, donor-specific HLA type II antibodies were detected locally. Cytokine and receptor expression (CXCR3, interferon γ and TGF-β) was higher in large ELS in the epicardial tissue than in other vessel wall layers, suggesting active recruitment and proliferation of T and B lymphocytes. ELS exhibited active plasma cells producing locally manufactured antibodies that, in some cases, were directed against the donor HLA, potentially mediating rejection with major consequences for the graft.

Complement C5 Inhibition Reduces T Cell-Mediated Allograft Vasculopathy Caused by Both Alloantibody and Ischemia Reperfusion Injury in Humanized Mice

Allograft vasculopathy (AV) is characterized by diffuse stenoses in the vasculature of solid organ transplants. Previously, we developed two humanized models showing that alloantibody and ischemia reperfusion injury (IRI) exacerbated T cell-mediated AV in human arterial xenografts in vivo. Herein we examined a causal role for terminal complement activation in both settings. IRI, in contrast to alloantibody, elicited widespread membrane attack complex (MAC) assembly throughout the vessel wall. Both alloantibody and IRI caused early (24 h) and robust endothelial cell (EC) activation localized to regions of intimal MAC deposition, indicated by increases in nuclear factor kappa B (NF-κB)-inducing kinase, an MAC-dependent activator of noncanonical NF-kB, VCAM-1 expression and Gr-1+ neutrophil infiltration. Endothelial cell activation by alloantibody was inhibited by antimouse C5 mAb, but not by anti-C5a mAb or by control mAb, implicating MAC as the primary target of anti-C5 mAb. Antimouse C5 mAb significantly reduced alloantibody- and IRI-enhanced T cell infiltration and AV-like changes, including neointimal hyperplasia as well as intraluminal thrombosis in a subset of IRI-treated arterial grafts. These results indicate that increased AV lesion formation in response to either alloantibody or IRI is dependent on complement C5 activation and, accordingly, inhibition of this pathway may attenuate AV.

Many de novo donor-specific antibodies recognize β2 -microglobulin-free, but not intact HLA heterodimers

Solid‐phase single antigen bead (SAB) assays are standard of care for detection and identification of donor‐specific antibody (DSA) in patients who receive solid organ transplantation (SOT). While several studies have documented the reproducibility and sensitivity of SAB testing for DSA, there are little data available concerning its specificity. This study describes the identification of antibodies to β₂‐microglobulin‐free human leukocyte antigen (β₂‐m‐fHLA) heavy chains on SAB arrays and provides a reassessment of the clinical relevance of DSA testing by this platform. Post‐transplant sera from 55 patients who were positive for de novo donor‐specific antibodies on a SAB solid‐phase immunoassay were tested under denaturing conditions in order to identify antibodies reactive with β₂‐m‐fHLA or native HLA (nHLA). Antibodies to β₂‐m‐fHLA were present in nearly half of patients being monitored in the post‐transplant period. The frequency of antibodies to β₂‐m‐fHLA was similar among DSA and HLA antigens that were irrelevant to the transplant (non‐DSA). Among the seven patients with clinical or pathologic antibody‐mediated rejection (AMR), none had antibodies to β₂‐m‐fHLA exclusively; thus, the clinical relevance of β₂‐m‐fHLA is unclear. Our data suggests that SAB testing produces false positive reactions due to the presence of β₂‐m‐fHLA and these can lead to inappropriate assignment of unacceptable antigens during transplant listing and possibly inaccurate identification of DSA in the post‐transplant period.

Alloantibody Generation and Effector Function Following Sensitization to Human Leukocyte Antigen

Allorecognition is the activation of the adaptive immune system to foreign human leukocyte antigen (HLA) resulting in the generation of alloantibodies. Due to a high polymorphism, foreign HLA is recognized by the immune system following transplant, transfusion, or pregnancy resulting in the formation of the germinal center and the generation of long-lived alloantibody-producing memory B cells. Alloantibodies recognize antigenic epitopes displayed by the HLA molecule on the transplanted allograft and contribute to graft damage through multiple mechanisms, including (1) activation of the complement cascade resulting in the formation of the MAC complex and inflammatory anaphylatoxins, (2) transduction of intracellular signals leading to cytoskeletal rearrangement, growth, and proliferation of graft vasculature, and (3) immune cell infiltration into the allograft via FcγR interactions with the FC portion of the antibody. This review focuses on the generation of HLA alloantibody, routes of sensitization, alloantibody specificity, and mechanisms of antibody-mediated graft damage.

Practical Applications in Immunohistochemistry: Evaluation of Rejection and Infection in Organ Transplantation

CONTEXT

-Immunohistochemical analysis of tissue biopsy specimens is a crucial tool in diagnosis of both rejection and infection in patients with solid organ transplants. In the past 15 years, the concept of antibody-mediated rejection has been refined, and diagnostic criteria have been codified in renal, heart, pancreas, and lung allografts (with studies ongoing in liver, small intestine, and composite grafts), all of which include immunoanalysis for the complement split product C4d.

OBJECTIVES

-To review the general concepts of C4d biology and immunoanalysis, followed by organ-allograft-specific data, and interpretative nuances for kidney, pancreas, and heart, with discussion of early literature for lung and liver biopsies. Additionally, practical applications and limitations of immunostains for infectious organisms (Polyomavirus, Adenoviridae [adenovirus], and the herpes virus family, including Herpes simplex virus, Cytomegalovirus, Human herpes virus 8, and Epstein-Barr virus) are reviewed in the context of transplant recipients.

DATA SOURCES

-Our experience and published primary and review literature.

CONCLUSIONS

-Immunohistochemistry continues to have an important role in transplant pathology, most notably C4d staining in assessment of antibody-mediated rejection and assessment of viral pathogens in tissue. In all facets of transplant pathology, correlation of morphology with special studies and clinical data is critical, as is close communication with the transplant team.

The divergent roles of macrophages in solid organ transplantation

PURPOSE OF REVIEW

This review summarizes the phenotype and function of macrophages in the context of solid organ transplantation and will focus on fundamental insights into their paradoxical pro-inflammatory versus suppressive function. We will also discuss the therapeutic potential of regulatory macrophages in tolerance induction.

RECENT FINDINGS

Macrophages are emerging as an essential element of solid organ transplantation. Macrophages are involved in the pathogenesis of ischemia reperfusion injury, as well as both acute and chronic rejection, exacerbating injury through secretion of inflammatory effectors and by amplifying adaptive immune responses. Notably, not all responses associated with macrophages are deleterious to the graft, and graft protection can in fact be conferred by macrophages. This has been attributed to the presence of macrophages with tissue-repair capabilities, as well as the effects of regulatory macrophages.

SUMMARY

The explosion of new information on the role of macrophages in solid organ transplantation has opened up new avenues of research and the possibility of therapeutic intervention. However, the role of myeloid cells in graft rejection, resolution of rejection and tissue repair remains poorly understood. A better understanding of plasticity and regulation of monocyte polarization is vital for the development of new therapies for the treatment of acute and chronic transplant rejection.

An Anti-C1s Monoclonal, TNT003, Inhibits Complement Activation Induced by Antibodies Against HLA

Antibody-mediated rejection (AMR) of solid organ transplants (SOT) is characterized by damage triggered by donor-specific antibodies (DSA) binding donor Class I and II HLA (HLA-I and HLA-II) expressed on endothelial cells. While F(ab')2 portions of DSA cause cellular activation and proliferation, Fc regions activate the classical complement cascade, resulting in complement deposition and leukocyte recruitment, both hallmark features of AMR. We characterized the ability of an anti-C1s monoclonal antibody, TNT003, to inhibit HLA antibody (HLA-Ab)-induced complement activation. Complement deposition induced by HLA-Ab was evaluated using novel cell- and bead-based assays. Human aortic endothelial cells (HAEC) were cultured with HLA-Ab and human complement; production of activated complement proteins was measured by flow cytometry. Additionally, C3d deposition was measured on single antigen beads (SAB) mixed with HLA-Ab and human complement. TNT003 inhibited HLA-Ab mediated complement deposition on HAEC in a concentration-dependent manner; C3a, C4a and C5a anaphylatoxin production was also diminished by TNT003. Finally, TNT003 blocked C3d deposition induced by Class I (HLAI-Ab)- and Class II (HLAII-Ab)-specific antibodies on SAB. These data suggest TNT003 may be useful for modulating the effects of DSA, as TNT003 inhibits complement deposition and split product formation generated by HLA-I/II-Ab in vitro.

Biomarkers of acute rejection following cardiac transplantation

Cardiac transplantation can be a life-saving treatment for selected patients with heart failure. However, despite advances in immunosuppressive therapy, acute allograft rejection remains a significant cause of morbidity and mortality. The current 'gold standard' for rejection surveillance is endomyocardial biopsy, which aims to identify episodes of rejection prior to development of clinical manifestations. This is an invasive technique with a risk of false-positive and false-negative results. Consequently, a wide variety of noninvasive alternatives have been investigated for their potential role as biomarkers of rejection. This article reviews the evidence behind proposed alternatives such as imaging techniques, electrophysiological parameters and peripheral blood markers, and highlights the potential future role for biomarkers in cardiac transplantation as an adjunct to biopsy.

Monocyte recruitment by HLA IgG-activated endothelium: the relationship between IgG subclass and FcγRIIa polymorphisms

It is currently unclear which donor specific HLA antibodies confer the highest risk of antibody-mediated rejection (AMR) and allograft loss. In this study, we hypothesized that two distinct features (HLA IgG subclass and Fcγ receptor [FcγR] polymorphisms) which vary from patient to patient, influence the process of monocyte trafficking to and macrophage accumulation in the allograft during AMR in an interrelated fashion. Here, we investigated the contribution of human IgG subclass and FcγR polymorphisms in monocyte recruitment in vitro by primary human aortic endothelium activated with chimeric anti-HLA I human IgG1 and IgG2. Both subclasses triggered monocyte adhesion to endothelial cells, via a two-step process. First, HLA I crosslinking by antibodies stimulated upregulation of P-selectin on endothelium irrespective of IgG subclass. P-selectin-induced monocyte adhesion was enhanced by secondary interactions of IgG with FcγRs, which was highly dependent upon subclass. IgG1 was more potent than IgG2 through differential engagement of FcγRs. Monocytes homozygous for FcγRIIa-H131 adhered more readily to HLA antibody-activated endothelium compared with FcγRIIa-R131 homozygous. Finally, direct modification of HLA I antibodies with immunomodulatory enzymes EndoS and IdeS dampened recruitment by eliminating antibody-FcγR binding, an approach that may have clinical utility in reducing AMR and other forms of antibody-induced inflammation.

Antibodies to HLA Molecules Mimic Agonistic Stimulation to Trigger Vascular Cell Changes and Induce Allograft Injury

Human leukocyte antigen (HLA)-induced signaling in endothelial and smooth muscle cells causes dramatic cytoskeletal rearrangement, increased survival, motility, proliferation, adhesion molecule and chemokine expression, and adhesion of leukocytes. These mechanisms are directly related to endothelial activation, neointimal proliferation, and intragraft accumulation of leukocytes during antibody-mediated rejection (AMR) and chronic rejection. Clustering of HLA by ligands in trans, such as in antigen-presenting cells at the immune synapse, triggers physiological functions analogous to HLA antibody-induced signaling in vascular cells. Emerging evidence has revealed previously unknown functions for HLA beyond antigen presentation, including association with coreceptors in cis to permit signal transduction, and modulation of intracellular signaling downstream of other receptors that may be relevant to HLA signaling in the graft vasculature. We discuss the literature regarding HLA-induced signaling in vascular endothelial and smooth muscle cells, as well as under endogenous biological conditions, and how such signaling relates to functional changes and pathological mechanisms during graft injury.

Decreased circulating and neutrophil mediated VEGF-A165 release in stable long-term cardiac transplant recipients

Background

Vascular endothelial growth factor (VEGF) may play a role on the allograft remodelling following cardiac transplantation (CTx). We measured the circulating levels of VEGF-A₁₆₅ concomitantly with the proinflammatory (Interleukin-8; IL-8), anti-inflammatory (IL-1 receptor antagonist; IL-1RA) and their release from neutrophils of CTx recipients.

Methods

Eighteen CTx recipients aged 49.6 ± 3.1 years, being transplanted for 145 ± 20 months were age-matched to 35 healthy control (HC) subjects. Concomitantly to plasma assessment, circulating neutrophils were isolated, purified and stimulated by vehicle (PBS), N-formyl-Met-Leu-Phe (fMLP, 10⁻⁷ M), bacterial lipopolysaccharide (LPS, 1 μg/mL), or tumour necrosis factor alpha (TNF-α, 10 ng/mL).

Results

Compared with HC, CTx recipients exhibited a decrease (−80%) in plasmatic levels of VEGF-A₁₆₅ (225 ± 42 (HC) vs 44 ± 10 pg/mL (CTx); (p < 0.001). There were no differences in the levels of IL-8 and IL-1RA. Under basal or stimulated conditions, neutrophils from CTx patients exhibited a marked decrease ranging from −30 to −88% on their capacity to release VEGF-A₁₆₅, IL-8 and IL-1RA upon stimulation.

Conclusions

Long-term CTx recipients exhibit a marked reduction in the circulating levels of VEGF-A₁₆₅, as well as neutrophil-mediated release of VEGF-A₁₆₅, IL-1RA and IL-8 compared to healthy volunteers. The mechanisms and physiological impacts of these findings deserve additional investigations.

The perfect storm: HLA antibodies, complement, FcγRs, and endothelium in transplant rejection

The pathophysiology of antibody-mediated rejection (AMR) in solid organ transplants is multifaceted and predominantly caused by antibodies directed against polymorphic donor human leukocyte antigens (HLAs). Despite the clearly detrimental impact of HLA antibodies (HLA-Abs) on graft function and survival, the prevention, diagnosis, and treatment of AMR remain a challenge. The histological manifestations of AMR reflect the signatures of HLA-Ab-triggered injury, specifically endothelial changes, recipient leukocytic infiltrate, and complement deposition. We review the interconnected mechanisms of HLA-Ab-mediated injury that might synergize in a 'perfect storm' of inflammation. Characterization of antibody features that are critical for effector functions may help to identify HLA-Abs that are more likely to cause rejection. We also highlight recent advances that may pave the way for new, more effective therapies.

Inflammatory cell burden and phenotype in endomyocardial biopsies with antibody-mediated rejection (AMR): a multicenter pilot study from the AECVP

This multicenter case-controlled pilot study evaluated myocardial inflammatory burden (IB) and phenotype in endomyocardial biopsies (EMBs) with and without pathologic antibody-mediated rejection (pAMR). Sixty-five EMBs from five European heart transplant centers were centrally reviewed as positive (grade 2, n = 28), suspicious (grade 1, n = 7) or negative (n = 30) for pAMR. Absolute counts of total, intravascular (IV) and extravascular (EV) immunophenotyped mononuclear cells were correlated with pAMR grade, capillary C4d deposition, donor specific antibody (DSA) status and acute cellular rejection (ACR). In pAMR+ biopsies, equivalent number of IV CD3+ T lymphocytes (23 ± 4/0.225 mm(2) ) and CD68+ macrophages (21 ± 4/0.225 mm(2) ) were seen. IB and cell phenotype correlated with pAMR grade, C4d positivity and DSA positivity (p < 0.0001). High numbers of IV T lymphocytes were associated with low grade ACR (p = 0.002). In late-occurring AMR EV plasma cells occurring in 34% of pAMR+ EMBs were associated with higher IB. The IB in AMR correlated with pAMR+, C4d positivity and DSA positivity. In pAMR+ equivalent numbers of IV T lymphocytes and macrophages were found. The presence of plasma cells was associated with a higher IB and occurrence of pAMR late after transplantation.

Phosphorylated S6 kinase and S6 ribosomal protein are diagnostic markers of antibody-mediated rejection in heart allografts

BACKGROUND

Anti-MHC Class I alloantibodies have been implicated in the processes of acute and chronic rejection. These antibodies (Ab) bind to endothelial cells (EC) and transduce signals leading to the activation of cell survival and proliferation pathways, including Src, FAK and mTOR, as well as downstream targets ERK, S6 kinase (S6K) and S6 ribosomal protein (S6RP). We tested the hypothesis that phosphorylation of S6K, S6RP and ERK in capillary endothelium may serve as an adjunct diagnostic tool for antibody-mediated rejection (AMR) in heart allografts.

METHODS

Diagnosis of AMR was based on histology or immunoperoxidase staining of paraffin-embedded tissue, consistent with 2013 ISHLT criteria. Diagnosis of acute cellular rejection (ACR) was based on ISHLT criteria. Endomyocardial biopsies from 67 heart transplant recipients diagnosed with acute rejection [33 with pAMR, 18 with ACR (15 with Grade 1R, 3 with Grade ≥2R), 16 with pAMR and ACR (13 with 1R and 3 with ≥2R)] and 40 age- and gender-matched recipients without rejection were tested for the presence of phosphorylated forms of ERK, S6RP and S6K by immunohistochemistry.

RESULTS

Immunostaining of endomyocardial biopsies with evidence of pAMR showed a significant increase in expression of p-S6K and p-S6RP in capillary EC compared with controls. A weaker association was observed between pAMR and p-ERK.

CONCLUSIONS

Biopsies diagnosed with pAMR often showed phosphorylation of S6K and S6RP, indicating that staining for p-S6K and p-S6RP is useful for the diagnosis of AMR. Our findings support a role for antibody-mediated HLA signaling in the process of graft injury.

Macrophages in solid organ transplantation

Macrophages are highly plastic hematopoietic cells with diversified functions related to their anatomic location and differentiation states. A number of recent studies have examined the role of macrophages in solid organ transplantation. These studies show that macrophages can induce allograft injury but, conversely, can also promote tissue repair in ischemia-reperfusion injury and acute rejection. Therapeutic strategies that target macrophages to improve outcomes in solid organ transplant recipients are being examined in preclinical and clinical models. In this review, we discuss the role of macrophages in different types of injury and rejection, with a focus on macrophage-mediated tissue injury, specifically vascular injury, repair and remodeling. We also discuss emerging macrophage-centered therapeutic opportunities in solid organ transplantation.

Antibody-mediated graft injury: complement-dependent and complement-independent mechanisms

PURPOSE OF REVIEW

Antibody-mediated rejection (AMR) is emerging as the leading cause of chronic rejection and allograft failure. Traditionally, the mechanisms of graft injury mediated by donor-specific antibodies beyond complement activation were not well appreciated. However, an evolving paradigm of Fc-independent antibody functions, along with clinical recognition of C4d-negative AMR, has increased awareness of the action of antibodies leading to endothelial activation and dysfunction.

RECENT FINDINGS

Herein, we address current clinical trends, including the signature of microvascular inflammation in biopsies of grafts undergoing AMR, the prevalence of antibodies to human leukocyte antigen class II DQ locus (HLA-DQ) and non-HLA targets, and the functional characterization of HLA immunoglobulin G (IgG) subclasses and complement-fixing capacity. We also discuss recent experimental evidence revealing new mechanisms of endothelial and smooth muscle cell activation by HLA antibodies, which may contribute to vascular inflammation and chronic rejection. Finally, we touch upon novel discoveries of the interplay between antibodies, the complement system, and CD4 T-cell-mediated alloimmunity.

SUMMARY

The current literature suggests that, although complement-fixing antibodies may have some prognostic value for graft outcome, complement-independent mechanisms of graft injury are increasingly relevant. Therapeutic strategies, which target endothelial activation induced by antibodies may ameliorate vascular inflammation and mononuclear cell infiltration characteristic of AMR.

SAHA, an HDAC inhibitor, attenuates antibody-mediated allograft rejection

BACKGROUND

Antibody-mediated rejection (AMR) is gaining increasing recognition as a critical causative factor contributing to graft loss in organ transplantation. However, current therapeutic options for prevention and treatment of AMR are very limited and ineffective. The impact of epigenetic modification in B-cell function and its involvement in AMR is still yet to be explored.

METHODS

The impacts of suberoylanilide hydroxamic acid (SAHA), a histone deacetylase inhibitor, on isolated murine B-cell viability, proliferation, apoptosis, expression of surface marker, and secretion of immunoglobulin and interleukin-10 were investigated. In vivo, a murine cardiac transplant model was used to evaluate the effect of SAHA on splenic B-cell subsets and on AMR in Rag1(-/-) recipient mice after reconstitution of allostimulated B cells.

RESULTS

SAHA possesses capability to repress B-cell function. Specifically, SAHA is potent to decrease the viability of isolated B cells by inducing apoptosis. SAHA was also found capable of suppressing the expression of B-cell costimulatory molecules and, as a result, addition of SAHA into the cultures attenuated B-cell proliferation and immunoglobulin secretion. In line with these results, administration of SAHA significantly suppressed AMR in Rag1(-/-) recipient mice after reconstitution of allostimulated B cells along with enhanced cardiac allograft survival time. Mechanistic studies revealed that SAHA promotes B-cell secretion of interleukin-10.

CONCLUSIONS

Our data support that SAHA could be a promising immunosuppressive agent with potential beneficial effect on prevention and treatment of AMR.

HLA class I antibodies trigger increased adherence of monocytes to endothelial cells by eliciting an increase in endothelial P-selectin and, depending on subclass, by engaging FcγRs

Ab-mediated rejection (AMR) of solid organ transplants is characterized by intragraft macrophages. It is incompletely understood how donor-specific Ab binding to graft endothelium promotes monocyte adhesion, and what, if any, contribution is made by the Fc region of the Ab. We investigated the mechanisms underlying monocyte recruitment by HLA class I (HLA I) Ab-activated endothelium. We used a panel of murine mAbs of different subclasses to crosslink HLA I on human aortic, venous, and microvascular endothelial cells and measured the binding of human monocytic cell lines and peripheral blood monocytes. Both anti-HLA I murine (m)IgG1 and mIgG2a induced endothelial P-selectin, which was required for monocyte adhesion to endothelium irrespective of subclass. mIgG2a but not mIgG1 could bind human FcγRs. Accordingly, HLA I mIgG2a but not mIgG1 treatment of endothelial cells significantly augmented recruitment, predominantly through FcγRI, and, to a lesser extent, FcγRIIa. Moreover, HLA I mIgG2a promoted firm adhesion of monocytes to ICAM-1 through Mac-1, which may explain the prominence of monocytes during AMR. We confirmed these observations using human HLA allele-specific mAbs and IgG purified from transplant patient sera. HLA I Abs universally elicit endothelial exocytosis leading to monocyte adherence, implying that P-selectin is a putative therapeutic target to prevent macrophage infiltration during AMR. Importantly, the subclass of donor-specific Ab may influence its pathogenesis. These results imply that human IgG1 and human IgG3 should have a greater capacity to trigger monocyte infiltration into the graft than IgG2 or IgG4 due to enhancement by FcγR interactions.

Blockade of p-selectin is sufficient to reduce MHC I antibody-elicited monocyte recruitment in vitro and in vivo

Donor-specific HLA antibodies significantly lower allograft survival, but as yet there are no satisfactory therapies for prevention of antibody-mediated rejection. Intracapillary macrophage infiltration is a hallmark of antibody-mediated rejection, and macrophages are important in both acute and chronic rejection. The purpose of this study was to investigate the Fc-independent effect of HLA I antibodies on endothelial cell activation, leading to monocyte recruitment. We used an in vitro model to assess monocyte binding to endothelial cells in response to HLA I antibodies. We confirmed our results in a mouse model of antibody-mediated rejection, in which B6.RAG1(-/-) recipients of BALB/c cardiac allografts were passively transferred with donor-specific MHC I antibodies. Our findings demonstrate that HLA I antibodies rapidly increase intracellular calcium and endothelial presentation of P-selectin, which supports monocyte binding. In the experimental model, donor-specific MHC I antibodies significantly increased macrophage accumulation in the allograft. Concurrent administration of rPSGL-1-Ig abolished antibody-induced monocyte infiltration in the allograft, but had little effect on antibody-induced endothelial injury. Our data suggest that antagonism of P-selectin may ameliorate accumulation of macrophages in the allograft during antibody-mediated rejection.