Anti-HLA antibody ligation to HLA class I molecules expressed by endothelial cells stimulates tyrosine phosphorylation, inositol phosphate generation, and proliferation

Assessing the role of phosphorylated S6 ribosomal protein in the pathological diagnosis of pulmonary antibody-mediated rejection

BACKGROUND

Pulmonary antibody-mediated rejection is still a challenging diagnosis as C4d immunostaining has poor sensitivity. Previous studies have indicated that the phosphorylated S6 ribosomal protein, a component of the mammalian target of rapamycin (mTOR) pathway, is correlated with de novo donor-specific antibodies in lung transplantation. The objective of this study was to evaluate the phosphorylation of S6 ribosomal protein as a surrogate for antibody-mediated rejection diagnosis in lung transplant patients.

METHODS

This multicentre retrospective study analyzed transbronchial biopsies from 216 lung transplanted patients, 114 with antibody-mediated rejection and 102 without (19 with acute cellular rejection, 17 with ischemia/reperfusion injury, 18 with infection, and 48 without post-transplant complications). Immunohistochemistry was used to quantify phosphorylated S6 ribosomal protein expression in macrophages, endothelium, epithelium, and inter-pathologist agreement was assessed.

RESULTS

Median phosphorylated S6 ribosomal protein expression values were higher in antibody-mediated rejection cases than in controls for all cell components, with the highest sensitivity in macrophages (0.9) and the highest specificity in endothelial expression (0.8). The difference was mainly significant in macrophages compared to other post-lung transplantation complications. Inter-pathologist agreement was moderate for macrophages and endothelium, with higher agreement when phosphorylated S6 ribosomal protein expression was dichotomized into positive/negative. The inclusion of phosphorylated S6 ribosomal protein in the diagnostic algorithm could have increased antibody-mediated rejection certainty levels by 25%.

CONCLUSIONS

The study supports the role of the mTOR pathway in antibody-mediated rejection-related graft injury and suggests that tissue phosphorylation of S6 ribosomal protein could be a useful surrogate for a more accurate pathological diagnosis of lung antibody-mediated rejection.

On a Long and Winding Road: Alloantibodies in Organ Transplantation

Today we know that both the humoral and the cellular arm of the immune system are engaged in severe immunological challenges. A close interaction between B and T cells can be observed in most "natural" challenges, including infections, malignancies, and autoimmune diseases. The importance and power of humoral immunity are impressively demonstrated by the current coronavirus disease 2019 pandemic. Organ transplant rejection is a normal immune response to a completely "artificial" challenge. It took a long time before the multifaceted action of different immunological forces was recognized and a unified, generally accepted opinion could be formed. Here, we address prominent paradigms and paradigm shifts in the field of transplantation immunology. We identify several instances in which the transplant community missed a timely paradigm shift because essential, available knowledge was ignored. Moreover, we discuss key findings that critically contributed to our understanding of transplant immunology but sometimes developed with delay and in a roundabout way, as was the case with antibody-mediated rejection-a main focus of this article. These include the discovery of the molecular principles of histocompatibility, the recognition of the microcirculation as a key interface of immune damage, the refinement of alloantibody detection, the description of C4d as a footmark of endothelium-bound antibody, and last but not least, the developments in biopsy-based diagnostics beyond conventional morphology, which only now give us a glimpse of the enormous complexity and pathogenetic diversity of rejection.

Role of HLA-I Structural Variants and the Polyreactive Antibodies They Generate in Immune Homeostasis

Cell-surface HLA-I molecules consisting of β2-microglobulin (β2m) associated heavy chains (HCs), referred to as Face-1, primarily present peptides to CD8+ T-cells. HCs consist of three α-domains, with selected amino acid sequences shared by all alleles of all six isoforms. The cell-surface HLA undergoes changes upon activation by pathological conditions with the expression of β2m-free HCs (Face-2) resulting in exposure of β2m-masked sequences shared by almost all alleles and the generation of HLA-polyreactive antibodies (Abs) against them. Face-2 may homodimerize or heterodimerize with the same (Face-3) or different alleles (Face-4) preventing exposure of shared epitopes. Non-allo immunized males naturally carry HLA-polyreactive Abs. The therapeutic intravenous immunoglobulin (IVIg) purified from plasma of thousands of donors contains HLA-polyreactive Abs, admixed with non-HLA Abs. Purified HLA-polyreactive monoclonal Abs (TFL-006/007) generated in mice after immunizing with Face-2 are documented to be immunoregulatory by suppressing or activating different human lymphocytes, much better than IVIg. Our objectives are (a) to elucidate the complexity of the HLA-I structural variants, and their Abs that bind to both shared and uncommon epitopes on different variants, and (b) to examine the roles of those Abs against HLA-variants in maintaining immune homeostasis. These may enable the development of personalized therapeutic strategies for various pathological conditions.

CTOTC-08: A multicenter randomized controlled trial of rituximab induction to reduce antibody development and improve outcomes in pediatric lung transplant recipients

We conducted a randomized, placebo-controlled, double-blind study of pediatric lung transplant recipients, hypothesizing that rituximab plus rabbit anti-thymocyte globulin induction would reduce de novo donor-specific human leukocyte antigen antibodies (DSA) development and improve outcomes. We serially obtained clinical data, blood, and respiratory samples for at least one year posttransplant. We analyzed peripheral blood lymphocytes by flow cytometry, serum for antibody development, and respiratory samples for viral infections using multiplex PCR. Of 45 subjects enrolled, 34 were transplanted and 27 randomized to rituximab (n = 15) or placebo (n = 12). No rituximab-treated subjects versus five placebo-treated subjects developed de novo DSA with mean fluorescence intensity >2000. There was no difference between treatment groups in time to the primary composite outcome endpoint (death, bronchiolitis obliterans syndrome [BOS] grade 0-p, obliterative bronchiolitis or listing for retransplant). A post-hoc analysis substituting more stringent chronic lung allograft dysfunction criteria for BOS 0-p showed no difference in outcome (p = .118). The incidence of adverse events including infection and rejection episodes was no different between treatment groups. Although the study was underpowered, we conclude that rituximab induction may have prevented early DSA development in pediatric lung transplant recipients without adverse effects and may improve outcomes (Clinical Trials: NCT02266888).

Vascular Signaling in Allogenic Solid Organ Transplantation - The Role of Endothelial Cells

Graft rejection remains the major obstacle after vascularized solid organ transplantation. Endothelial cells, which form the interface between the transplanted graft and the host’s immunity, are the first target for host immune cells. During acute cellular rejection endothelial cells are directly attacked by HLA I and II-recognizing NK cells, macrophages, and T cells, and activation of the complement system leads to endothelial cell lysis. The established forms of immunosuppressive therapy provide effective treatment options, but the treatment of chronic rejection of solid organs remains challenging. Chronic rejection is mainly based on production of donor-specific antibodies that induce endothelial cell activation—a condition which phenotypically resembles chronic inflammation. Activated endothelial cells produce chemokines, and expression of adhesion molecules increases. Due to this pro-inflammatory microenvironment, leukocytes are recruited and transmigrate from the bloodstream across the endothelial monolayer into the vessel wall. This mononuclear infiltrate is a hallmark of transplant vasculopathy. Furthermore, expression profiles of different cytokines serve as clinical markers for the patient’s outcome. Besides their effects on immune cells, activated endothelial cells support the migration and proliferation of vascular smooth muscle cells. In turn, muscle cell recruitment leads to neointima formation followed by reduction in organ perfusion and eventually results in tissue injury. Activation of endothelial cells involves antibody ligation to the surface of endothelial cells. Subsequently, intracellular signaling pathways are initiated. These signaling cascades may serve as targets to prevent or treat adverse effects in antibody-activated endothelial cells. Preventive or therapeutic strategies for chronic rejection can be investigated in sophisticated mouse models of transplant vasculopathy, mimicking interactions between immune cells and endothelium.

Absence of evidence that respiratory viral infections influence pediatric lung transplantation outcomes: Results of the CTOTC-03 study

Based on reports in adult lung transplant recipients, we hypothesized that community-acquired respiratory viral infections (CARVs) would be a risk factor for poor outcome after pediatric lung transplant. We followed 61 pediatric lung transplant recipients for 2+ years or until they met a composite primary endpoint including bronchiolitis obliterans syndrome/obliterative bronchiolitis, retransplant, or death. Blood, bronchoalveolar lavage, and nasopharyngeal specimens were obtained with standard of care visits. Nasopharyngeal specimens were obtained from recipients with respiratory viral symptoms. Respiratory specimens were interrogated for respiratory viruses by using multiplex polymerase chain reaction. Donor-specific HLA antibodies, self-antigens, and ELISPOT reactivity were also evaluated. Survival was 84% (1 year) and 68% (3 years). Bronchiolitis obliterans syndrome incidence was 20% (1 year) and 38% (3 years). The primary endpoint was met in 46% of patients. CARV was detected in 156 patient visits (74% enterovirus/rhinovirus). We did not find a relationship between CARV recovery from respiratory specimens and the primary endpoint (hazard ratio 0.64 [95% confidence interval: 0.25-1.59], P = .335) or between CARV and the development of alloimmune or autoimmune humoral or cellular responses. These findings raise the possibility that the immunologic impact of CARV following pediatric lung transplant is different than that observed in adults.

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.

The Impact of HLA Class I-Specific Killer Cell Immunoglobulin-Like Receptors on Antibody-Dependent Natural Killer Cell-Mediated Cytotoxicity and Organ Allograft Rejection

Natural killer (NK) cells of the innate immune system are cytotoxic lymphocytes that play an important roles following transplantation of solid organs and hematopoietic stem cells. Recognition of self-human leukocyte antigen (HLA) class I molecules by inhibitory killer cell immunoglobulin-like receptors (KIRs) is involved in the calibration of NK cell effector capacities during the developmental stage, allowing the subsequent recognition and elimination of target cells with decreased expression of self-HLA class I (due to virus infection or tumor transformation) or HLA class I disparities (in the setting of allogeneic transplantation). NK cells expressing an inhibitory KIR-binding self-HLA can be activated when confronted with allografts lacking a ligand for the inhibitory receptor. Following the response of the adaptive immune system, NK cells can further destroy allograft endothelium by antibody-dependent cell-mediated cytotoxicity (ADCC), triggered through cross-linking of the CD16 Fc receptor by donor-specific antibodies bound to allograft. Upon recognizing allogeneic target cells, NK cells also secrete cytokines and chemokines that drive maturation of dendritic cells to promote cellular and humoral adaptive immune responses against the allograft. The cumulative activating and inhibitory signals generated by ligation of the receptors regulates mature NK cell killing of target cells and their production of cytokines and chemokines. This review summarizes the role of NK cells in allograft rejection and proposes mechanistic concepts that indicate a prominent role for KIR-HLA interactions in facilitating NK cells for Fc receptor-mediated ADCC effector function involved in antibody-mediated rejection of solid organ transplants.

Fcγ Receptors in Solid Organ Transplantation

In the current era, one of the major factors limiting graft survival is chronic antibody-mediated rejection (ABMR), whilst patient survival is impacted by the effects of immunosuppression on susceptibility to infection, malignancy and atherosclerosis. IgG antibodies play a role in all of these processes, and many of their cellular effects are mediated by Fc gamma receptors (FcγRs). These surface receptors are expressed by most immune cells, including B cells, natural killer cells, dendritic cells and macrophages. Genetic variation in FCGR genes is likely to affect susceptibility to ABMR and to modulate the physiological functions of IgG. In this review, we discuss the potential role played by FcγRs in determining outcomes in solid organ transplantation, and how genetic polymorphisms in these receptors may contribute to variations in transplant outcome.

Desensitization strategies in adult heart transplantation-Will persistence pay off?

Strategies are needed to enable successful heart transplantation in highly sensitized patients. Immunologic challenges from sensitization to human leukocyte antigen (HLA) reduce access to compatible donors, extend waiting times to transplant, and increase the risks of antibody-mediated rejection and cardiac allograft vasculopathy after transplant. The prime goal of desensitization is to increase access to transplantation through expansion of the donor organ pool. Existing therapies are directed at key components of the humoral immune response with newer biologically based regimens able to target plasma cells as the source of antibody production, as well as complement activation that has a central role in antibody-mediated injury. Despite the emergence of early promising results for these agents, a significant knowledge gap remains with the current data for desensitization, extrapolated mostly from non-heart solid-organ transplants and small observational studies. Notably, no approach has demonstrated significant and sustainable reductions in HLA antibody pre-transplant, and the ideal desensitization strategy remains elusive. In addition, clinical tools to evaluate the humoral response and efficacy of therapy are limited, focusing almost exclusively on HLA antibody detection. Importantly, desensitization is associated with significant costs and potential risks, and overall long-term outcomes and cost-effectiveness have not been sufficiently evaluated. Investigation is ongoing into the development of a clinically effective desensitization strategy in heart transplantation.

Positive crossmatch kidney transplant recipients treated with eculizumab: outcomes beyond 1 year

This study examined outcomes beyond 1 year in eculizumab-treated (EC) positive crossmatch kidney transplants (+XMKTx) compared to a historical control group. +XMKTx received desensitization with either plasma exchange (PE) alone (N = 48) or PE and EC (N = 30). EC, given for at least 1 month, was continued in the setting of persistently high DSA (B flow cytometric crossmatch [BFXM] >200) including: 4 weeks (n = 14); 9 weeks (n = 6), 6 months (n = 2), and 12 months (n = 8). All patients had at least 2 years follow-up. The incidence of acute clinical ABMR was lower in the EC group than controls (6.7% vs. 43.8% p < 0.01). Death-censored allograft survival was similar between groups. Chronic ABMR was the main cause of graft loss. On 1-year protocol biopsies, no differences were noted between EC and controls including: cg score >0, 26.7% versus 31.9% (p = 0.62), ptc score ≥ 2, 60.0% versus 60.0% (p = 1.00), or C4d + , 33.8% versus 13.5% (p = 0.08). A persistently high BFXM in EC-treated patients was associated with cg score >0 at 1 year, while EC appeared to protect against cg if the BFXM remained low. We conclude that despite decreasing acute clinical ABMR rates, EC treatment does not prevent chronic ABMR in recipients with persistently high BFXM after +XMKTx.

MHC class I signaling: new functional perspectives for an old molecule

Donor-specific antibodies are associated with refractory rejection episodes and poor allograft outcomes in solid organ transplantation. Our understanding of antibody-mediated allograft injury is expanding beyond complement deposition. In fact, unique mechanisms of alloantibodies are advancing our knowledge about transplant vasculopathy and antibody-mediated rejection. These include direct effects on the endothelium, resulting in the recruitment of leukocytes, chemokine and cytokine production, and stimulation of innate and adaptive alloresponses. These effects will be the focus of the following review.

Everolimus inhibits anti-HLA I antibody-mediated endothelial cell signaling, migration and proliferation more potently than sirolimus

Antibody (Ab) crosslinking of HLA I molecules on the surface of endothelial cells triggers proliferative and pro-survival intracellular signaling, which is implicated in the process of chronic allograft rejection, also known as transplant vasculopathy (TV). The purpose of this study was to investigate the role of mammalian target of rapamycin (mTOR) in HLA I Ab-induced signaling cascades. Everolimus provides a tool to establish how the mTOR signal network regulates HLA I-mediated migration, proliferation and survival. We found that everolimus inhibits mTOR complex 1 (mTORC1) by disassociating Raptor from mTOR, thereby preventing class I-induced phosphorylation of mTOR, p70S6K, S6RP and 4E-BP1, and resultant class I-stimulated cell migration and proliferation. Furthermore, we found that everolimus inhibits class I-mediated mTORC2 activation (1) by disassociating Rictor and Sin1 from mTOR; (2) by preventing class I-stimulated Akt phosphorylation and (3) by preventing class I-mediated ERK phosphorylation. These results suggest that everolimus is more effective than sirolimus at antagonizing both mTORC1 and mTORC2, the latter of which is critical in endothelial cell functional changes leading to TV in solid organ transplantation after HLA I crosslinking. Our findings point to a potential therapeutic effect of everolimus in prevention of chronic Ab-mediated rejection.

Antibodies in transplantation: the effects of HLA and non-HLA antibody binding and mechanisms of injury

Until recently, allograft rejection was thought to be mediated primarily by alloreactive T cells. Consequently, immunosuppressive approaches focused on inhibition of T cell activation. While short-term graft survival has significantly improved and rejection rates have dropped, acute rejection has not been eliminated and chronic rejection remains the major threat to long-term graft survival. Increased attention to humoral immunity in experimental systems and in the clinic has revealed that donor specific antibodies (DSA) can mediate and promote acute and chronic rejection. Herein, we detail the effects of alloantibody, particularly HLA antibody, binding to graft vascular and other cells, and briefly summarize the experimental methods used to assess such outcomes.

HLA-B35 upregulates endothelin-1 and downregulates endothelial nitric oxide synthase via endoplasmic reticulum stress response in endothelial cells

The presence of the HLA-B35 allele has emerged as an important risk factor for the development of isolated pulmonary hypertension in patients with scleroderma, however the mechanisms underlying this association have not been fully elucidated. The goal of our study was to determine the molecular mechanisms that mediate the biological effects of HLA-B35 in endothelial cells (ECs). Our data demonstrate that HLA-B35 expression at physiological levels via adenoviral vector resulted in significantly increased endothelin-1 (ET-1) and a significantly decreased endothelial NO synthase (eNOS), mRNA, and protein levels. Furthermore, HLA-B35 greatly upregulated expression of chaperones, including heat shock proteins (HSPs) HSP70 (HSPA1A and HSPA1B) and HSP40 (DNAJB1 and DNAJB9), suggesting that HLA-B35 induces the endoplasmic reticulum (ER) stress and unfolded protein response in ECs. Examination of selected mediators of the unfolded protein response, including H chain binding protein (BiP; GRP78), C/Ebp homologous protein (CHOP; GADD153), endoplasmic reticulum oxidase, and protein disulfide isomerase has revealed a consistent increase of BiP expression levels. Accordingly, thapsigargin, a known ER stress inducer, stimulated ET-1 mRNA and protein levels in ECs. This study suggests that HLA-B35 could contribute to EC dysfunction via ER stress-mediated induction of ET-1 in patients with pulmonary hypertension.

Development of antibodies to human leukocyte antigen precedes development of antibodies to major histocompatibility class I-related chain A and are significantly associated with development of chronic rejection after human lung transplantation

The development of antibodies (Abs) to major histocompatibility (MHC) class I-related chain A (MICA) and human leukocyte antigen (HLA) and their role in the immunopathogenesis of chronic rejection (bronchiolitis obliterans syndrome [BOS]) after human lung transplantation (LTx) was analyzed. Sera from 80 LTx recipients were analyzed for anti-MICA and anti-HLA Abs using Luminex and flow PRA (panel reactive assay). Development of Abs either to MICA alone or MICA and HLA together significantly correlated (p < 0.01) with development of BOS. Kinetic analysis in the post-LTx period revealed that development of anti-HLA Abs (7.6 +/- 4.7 months) preceded the development of anti-MICA Abs (10.0 +/- 3.5 months). Abs to MICA alleles (*001 and *009) developed approximately 6 months after LTx and peak titers were present at the time of clinical diagnosis of BOS (16.3 +/- 2.7 months). The development of Abs to both MICA and HLA was strongly associated with the development of BOS thereby suggesting a synergistic effect. Furthermore, immune response to mismatched HLA can lead to development of Abs to other MHC related antigens expressed on the airway epithelial cells. Cumulatively, these immune responses contribute to the pathogenesis of chronic rejection following human LTx.

Effect of antibodies on endothelium

Patients developing posttransplant antibodies against HLA and non-HLA antigens expressed by the endothelium of the graft undergo more frequent episodes of rejection and have decreased long-term graft survival. Antibodies against the endothelium can alter/damage the cells of the graft through several mechanisms. Historically, antibodies were thought to elicit endothelial cell injury via complement-dependent mechanisms. New research has shown that antibodies can also contribute to the process of transplant rejection by stimulating proinflammatory and proproliferation signals. Antibody ligation leads to several functional alterations in EC including Weibel Palade body exocytosis, leukocyte recruitment, growth factor expression and cell proliferation. In contrast, under certain circumstances, antibodies may induce prosurvival signals and graft accommodation. The signaling events regulating accommodation vs. rejection appear to be influenced by the specificity and concentration of the anti-HLA antibody and the degree of molecular aggregation. Knowledge of the HLA and non-HLA antibody-mediated signaling pathways has the potential to identify new therapeutic targets to promote accommodation and prevent acute and chronic antibody-mediated rejection.

Human leukocyte antigen antibodies in chronic transplant vasculopathy-mechanisms and pathways

Transplant recipients exhibiting posttransplant antibodies are at a higher risk for acute and chronic antibody mediated rejection (AMR). The primary alloantigens recognized by antibodies in recipients with AMR are the highly polymorphic HLA class I and class II molecules expressed on the surface of the endothelial cells (ECs) of the graft. Traditionally, anti-HLA antibodies were thought to mediate graft injury through complement-dependent mechanisms. However, recent studies indicate that antibodies can also contribute to alterations in EC function through complement-independent mechanisms by transducing intracellular signals. Anti-HLA antibodies transduce signals that are both pro-inflammatory and pro-proliferative suggesting mechanistic roles in acute and chronic AMR.

RNA interference elucidates the role of focal adhesion kinase in HLA class I-mediated focal adhesion complex formation and proliferation in human endothelial cells

Ligation of class I molecules by anti-HLA Ab stimulates an intracellular signaling cascade resulting in endothelial cell (EC) survival and proliferation, and has been implicated in the process of chronic allograft rejection and transplant-associated vasculopathy. In this study, we used small interfering RNA blockade of focal adhesion kinase (FAK) protein to determine its role in class I-mediated organization of the actin cytoskeleton, cell survival, and cell proliferation in primary cultures of human aortic EC. Knockdown of FAK appreciably inhibited class I-mediated phosphorylation of Src at Tyr(418), p85 PI3K, and Akt at both Thr(308) and Ser(473) sites. FAK knockdown also reduced class I-mediated phosphorylation of paxillin at Try(118) and blocked class I-induced paxillin assembly into focal contacts. FAK small interfering RNA completely abrogated class I-mediated formation of actin stress fibers. Interestingly, FAK knockdown did not modify fibroblast growth factor receptor expression induced by class I ligation. However, FAK knockdown blocked HLA class I-stimulated cell cycle proliferation in the presence and absence of basic fibroblast growth factor. This study shows that FAK plays a critical role in class I-induced cell proliferation, cell survival, and focal adhesion assembly in EC and may promote the development of transplant-associated vasculopathy.

14th International HLA and Immunogenetics Workshop: Report on understanding antibodies in transplantation

A session of the 14 International Histocompatibility Workshop brought together experts representing the major clinical protocols, clinical research, and basic research dealing with overcoming the barrier of alloantibody in transplantation and in understanding the mechanisms by which those antibodies exert their effect on a transplanted organ. This report is an integration of the presentations of those scientists.

Antibody to human leukocyte antigen triggers endothelial exocytosis

Although antibodies to HLA play a role in the pathogenesis of diseases processes such as rejection of transplanted organs, the precise mechanisms by which antibodies cause tissue injury are not completely understood. We hypothesized that antibodies to host tissues cause inflammation in part by activating endothelial exocytosis of granules that contain prothrombotic mediators such as von Willebrand Factor (VWF) and proinflammatory mediators such as P-selectin. To test this hypothesis, we treated human endothelial cells with murine monoclonal antibody W6/32 to HLA class I and then measured exocytosis by the release of VWF and the externalization of P-selectin. Antibody to HLA activates endothelial exocytosis in a dose-dependent manner over time. The biologically active complement split product, C5a, adds a slight but significant increase to antibody induction of exocytosis. Antibody to HLA alone or with C5a did not damage the cells. Cross-linking of HLA appears to play a role in the ability of antibody to activate exocytosis, because the W6/32 monovalent Fab fragment did not activate VWF release, but the bivalent Fab'2 was effective in triggering exocytosis. To explore the in vivo effects of antibody upon graft injury, we infused W6/32 Fab'2 antibody to human HLA into severe combined immunodeficient/beige mice that had been transplanted with human skin grafts. Antibody to HLA activated exocytosis and inflammation in human skin grafts. Our data show that antibody to host antigens can activate human endothelial cell exocytosis and leukocyte trafficking. By triggering vascular inflammation, antibody activation of exocytosis may play a role in transplant rejection.

HLA-B35 upregulates the production of endothelin-1 in HLA-transfected cells: a possible pathogenetic role in pulmonary hypertension

HLA-B35 is associated with an increased risk for developing isolated pulmonary hypertension (iPHT) in systemic sclerosis, but the mechanisms underlying this association have not been fully elucidated yet. Endothelin-1 (ET-1) is the main pathogenetic molecule implied in the development of iPHT; therefore, we sought to determine if ECV304 cells transfected with the HLA-B35 allele produce increased amounts of ET-1 after incubation with physiological concentrations of interleukin-1 beta (IL-1beta). ECV304 cells transfected with HLA-B*3501 and HLA-B*0801 polymorphic alpha chain or with pIRESneo2 were incubated with 100 U/ml of IL-1beta for 6, 12, 24, 36 and 48 h. ET-1 levels were determined using EIA kit (CAYMAN Chemical, Ann Arbor, MI) in supernatants from different cell cultures; the relative expression of the preproendothelin-1 (PPET-1) gene was also determined by reverse transcription-polymerase chain reaction. Cells expressing the HLA-B35 allele showed significantly increased levels of ET-1 at all the selected times compared with controls or HLA-B8-transfected cells. The relative expression of the PPET-1 gene was also increased in a proportionally direct manner. The HLA-B35 allele influences the production of ET-1 in HLA-B35-transfected ECV304 cells by promoting the expression of its precursor, PPET-1. Our results provide an explanation for the epidemiological association existing between iPHT and HLA-B35.

Anti-HLA Antibodies Can Induce Endothelial Cell Survival or Proliferation Depending on their Concentration

Patients exhibiting a humoral immune response to the transplanted organ are at increased risk of antibody-mediated rejection and development of transplant vasculopathy. Historically, antibodies were thought to elicit transplant rejection through complement mediated damage of the endothelium of the graft. More recently, studies from our laboratory and others have shown that antibody ligation of class I molecules on the surface of endothelial cells transduces signals resulting in functional changes including expression of cell survival proteins and cell proliferation. The intracellular events initiated by antibody ligation are dependent upon the degree of molecular aggregation and influenced by the concentration of the antibody and level of human leukocyte antigen (HLA) expression. Herein we describe our recent findings on the effect of molecular aggregation on the class I signaling pathway in human endothelial cells.

HLA-B35 influences the apoptosis rate in human peripheral blood mononucleated cells and HLA-transfected cells

Human leukocyte antigen (HLA) class I antigens can act as signal-transducing molecules that influence individual reactivity to external stimuli and the existence of haplotype-specific cell signal regulation has been suggested. In this article, we provide definite experimental evidence for the existence of a HLA-B35 haplotype-specific regulation of cell apoptosis in different experimental models. First, we demonstrated that HLA-B35, but not other HLA-class I antigens, was associated with an increased cell susceptibility to apoptosis in human peripheral mononuclear cells (PBMCs) exposed in vitro to thapsigargin. Second, we confirmed this association in human ECV 304 cells transfected with HLA-B35 or with HLA-B8, an antigen that did not appear to influence the apoptosis rate in the thapsigargin-treated PBMCs. Third, we confirmed the specific influence of HLA-B35 on cell apoptosis in non human cells (i.e., HLA-B35-transfected NIH3T3 murine fibroblasts). Our data show the existence of HLA-B35 haplotype-specific regulation of cell apoptosis and open new perspectives on the role of HLA class I genes in cell activation and disease susceptibility.

Regulation of ILT3 gene expression by processing of precursor transcripts in human endothelial cells

Immunoglobulin-like transcript (ILT)-3 is a transmembrane receptor, which belongs to the immunoglobulin superfamily. In previous studies, we showed that allospecific CD8+CD28- T suppressor cells (Ts) induce the expression of ILT3 in human endothelial cells (EC) rendering them tolerogenic. Using a polymerase chain reaction (PCR)-based approach, we now demonstrate by cell fractionation and sequencing studies that ILT3 precursor RNA is expressed and retained in nuclei of resting EC. Ts interaction with EC or exposure of EC to interleukin-10 (IL-10) and interferon alpha (IFN-alpha) triggers processing of ILT3 pre-mRNA. Western blot analysis showed that the expression of the mature ILT3 transcript is accompanied by production of ILT3 protein.

Utility of Peritransplant and Rescue Intravenous Immunoglobulin and Extracorporeal Immunoadsorption in Lung Transplant Recipients Sensitized to HLA Antigens

The role of anti-human leukocyte antigen (HLA) antibodies in lung transplantation is not fully clear. The presence of pretransplant third-party anti-HLA antibodies or the development of de novo anti-HLA antibodies has been associated with acute posttransplant complications, bronchiolitis obliterans syndrome (BOS), and early mortality in some studies. However, little has been reported regarding the utility of desensitization therapy in sensitized lung transplant recipients. For approximately 3 years, desensitization therapy consisting of intravenous immunoglobulin (IVIG) and, in most instances, extracorporeal immunoadsorption (ECI) has been administered peritransplant to lung transplant recipients at our institution with third-party anti-HLA antibodies or as rescue therapy to those who develop de novo anti-HLA antibodies. Notably, the administration of peritransplant desensitization therapy to these patients has been associated with improvement in several clinical parameters, including acute rejection and BOS. Furthermore, administration of rescue IVIG with or without ECI has been associated with an overall improvement in the rate of pulmonary function decline. Our experience suggests that desensitization therapy may be beneficial for lung transplant recipients with pretransplant or de novo anti-HLA antibodies. We discuss the appropriateness and clinical impact of IVIG and ECI in sensitized lung transplant recipients as well as cellular mechanisms that may contribute.

Anti-HLA class I antibodies activate endothelial cells and promote chronic rejection

Transplant recipients exhibiting a humoral immune response to the allograft demonstrate lower graft survival and increased risk for the development of chronic rejection and transplant arteriosclerosis. Our studies suggest that anti-HLA class I antibodies (Ab) play an important role in controlling endothelial cell (EC) function by binding to class I molecules on the surface of the EC and transducing intracellular signals. Anti-HLA Ab exhibit two primary effector functions: stimulation of cell proliferation and up-regulation of cell survival genes. Importantly, the intracellular events initiated by class I ligation appear to be influenced by the concentration of the Ab. High-titered anti-HLA Ab stimulate cell proliferation whereas low-titered Ab activate the PI3K/Akt pathway and promote expression of cell survival proteins including Bcl-2 and Bcl-xL. Anti-HLA class I Ab may contribute to the process of chronic allograft rejection by promoting EC survival and proliferation.

Vascular Endothelial Cells Evade Apoptosis Triggered by Human Leukocyte Antigen-DR Ligation Mediated by Allospecific Antibodies

BACKGROUND

Human leukocyte antigen (HLA)-DR ligation mediates cell death of antigen-presenting cells (APC), including mature B cells, macrophages, and dendritic cells. This study investigates the apoptotic effects of HLA class II ligation mediated by anti-HLA antibodies on activated human vascular graft endothelial cells (ECs).

METHODS

HLA class II expression was examined by flow cytometry using a panel of HLA-typed vascular ECs isolated from transplant donors and compared with that of B lymphocytes. The apoptotic effects of anti-HLA-DR monoclonal antibodies (mAbs) were investigated using viability assays, DNA content analysis, and annexin-V labeling. Intracellular signaling pathways mediated by HLA-DR ligation on ECs were examined by Western blotting.

RESULTS

Even with optimal stimulation, the expression of HLA-DR on interferon (IFN)-gamma-treated ECs was quantitatively lower (3-5-fold) than that on B cells. Whereas anti-HLA-DR monomorphic mAbs induced apoptosis of B cells (approximately 22%), no significant apoptosis of IFN-gamma-activated (DR-positive) ECs ( < 5%), collected from the same donor, was observed under the same conditions. Similarly, specific polymorphic anti-HLA-DR11 or -DR16 antibodies were unable to induce EC apoptosis. Nevertheless, antibody-binding to HLA-DR on ECs is sufficient to induce intracellular signaling, as evident in the modulation of tyrosine phosphorylation and protein kinase (PK)C-alpha/beta and PKB/Akt activation. Our results suggest that HLA-DR ligation induces both common and divergent signaling events in ECs and B cells.

CONCLUSION

Collectively, our data suggest that, in contrast with professional APC, graft ECs evade apoptosis mediated by HLA-DR ligation, not as a result of moderate HLA-DR expression but rather as a result of a specific signaling pathway.

HLA class I signal transduction is dependent on Rho GTPase and ROK

Chronic rejection is the major limitation to long-term allograft survival. HLA class I signaling pathways have been implicated in this process because ligation of class I molecules by anti-HLA antibodies (Ab) initiates intracellular signals in smooth muscle cells (SMC) and endothelial cells (EC) that synergize with growth factor receptors to elicit cell survival and proliferation. Anti-HLA Ab mediate cell proliferation and survival through a focal adhesion kinase dependent pathway that requires the integrity of the actin cytoskeleton. In this study, we investigated the role of Rho and Rho-kinase (ROK) in class I signal transduction. We show that class I ligation results in activation of Rho and increased stress fiber formation. In addition, inhibitors of Rho GTPase and ROK block HLA class I-mediated tyrosyl phosphorylation of paxillin and FAK, central elements of the focal adhesion signaling complex. These results suggest that HLA class I-induced signaling in EC is dependent on Rho GTPase and ROK.

Anti-HLA class I antibody-mediated activation of the PI3K/Akt signaling pathway and induction of Bcl-2 and Bcl-xL expression in endothelial cells

Anti-human leukocyte antigen (HLA) antibodies (Ab) have long been implicated in the process of acute and chronic allograft rejection, yet their mechanism(s) of action is not well understood. The aim of this study was to determine whether ligation of HLA class I molecules by anti-HLA Ab on the surface of human endothelial cells (EC) activates the PI3 Kinase (PI3K)/Akt signaling pathway and downstream target proteins of the cell death apparatus. We report that Ab ligation of major histocompatibility complex (MHC) class I molecules on the surface of EC triggers phosphorylation of Akt, PI3K, and recruitment of PI3K and Akt into a signaling unit with focal adhesion kinase. Signaling through class I also stimulated phosphorylation of Bad and upregulated expression of Bcl-2 and Bcl-xL. Pretreatment of EC with the PI3K inhibitor wortmannin blocked class I-mediated expression of Bcl-2, but not Bcl-xL, suggesting a role for the PI3K/Akt signaling pathway in regulation of class I-induced Bcl-2 expression. The intracellular events initiated by class I ligation were influenced by the concentration of the anti-HLA Ab with the lowest tested concentrations of Ab stimulating the highest level of Akt phosphorylation, Bcl-xL and Bcl-2 expression. Consistent with the in vitro experiments, analysis of biopsy samples from heart transplant recipients with evidence of Ab-mediated rejection exhibited increased Bcl-2 expression on the vascular endothelium. These results suggest that exposure of the graft endothelium to low concentrations of anti-HLA Ab may promote cell survival by transducing signals resulting in upregulation of cell survival genes.

Effect of maternal anti-HPA-1a antibodies and polyclonal IVIG on the activation status of vascular endothelial cells

Maternal anti-HPA-1a antibodies can cause severe fetal and neonatal alloimmune thrombocytopenia (FNAIT), complicated by intracranial haemorrhage (ICH). Antenatal treatment with maternal intravenous immunoglobulin (IVIG) seems to protect against ICH even when thrombocytopenia persists. The aim of this study was to investigate if anti-HPA-1a antibodies and IVIG potentially affect vascular endothelial cells (ECs) in order to identify susceptibility for ICH. Human umbilical cord endothelial cells (HUVEC) were incubated with anti-HPA-1a antibodies with or without polyclonal IVIG and evaluated for EC activation. Maternal sera with anti-HPA-1a antibodies affected neither the EC expression of intracellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1) and tissue factor (TF) nor the release of van Willebrand factor (vWF) or interleukin (IL)-8 nor the integrity of ECs. Maternal sera obtained after IVIG treatment and polyclonal IVIG decrease constitutive and cytokine-induced ICAM-1 and VCAM-1 expression on ECs. The results show that maternal anti-HPA-1a antibodies cause no activation or damage of ECs in this model. The clinical relevance of the de-activating properties of IVIG on EC activation with respect to ICH deserves further investigation.

Predicting kidney graft failure by HLA antibodies: a prospective trial

HLA antibodies have been shown to be associated with late graft loss of organ transplants in prior studies. Recently they were even shown to appear years BEFORE rejection. (1) An international cooperative study of 4763 patients from 36 centers was undertaken to determine the frequency of HLA antibodies in patients with functional transplants. The overall frequency of HLA antibodies among kidney transplant recipients was 20.9%; 19.3% in the liver, 22.8% in the heart, and 14.2% in the lung. Patients treated with CsA-MMF had significantly lower antibodies (9.8%) than those treated with CsA-Aza (18.1%) (0.00008). (2) Second, a prospective trial was performed in 23 kidney transplant centers to determine whether HLA antibodies could predict failures within 1 year. Among the 2278 patients followed up, 91 grafts failed and 34 patients died. Of 500 patients who had HLA antibodies, 6.6% failed compared with 3.3% among 1778 patients without antibodies (p = 0.0007). Among 244 patients who made de novo antibodies, 8.6% failed compared with 3.0% failures among 1421 patients who did not make antibodies (p = 0.00003). Death occurred in 1.5% of patients and was not associated with antibodies. Thus, after 1 year in this prospective trial, patients with HLA antibodies had graft failure at a significantly higher rate than those without antibodies.

Non-HLA-type endothelial cell reactive alloantibodies in pre-transplant sera of kidney recipients trigger apoptosis

The vascular endothelium of transplanted organs represents an important target for allograft-directed immune responses. Although HLA antigens expressed on graft endothelial cells (EC) can become targets of the host immune response, the role of other, non-HLA-encoded EC antigens has been proposed but is still unclear. The aim of this study was to investigate the presence of and to characterize anti-EC antibodies (AECA) in 57 kidney transplant recipients according to their HLA-immunization status. Flow cytometry in pretransplant sera was used to detect AECA reactive with surface antigens on ABO and HLA-typed primary cultures of arterial ECs, stimulated or not with tumor necrosis factor-alpha (TNFalpha) or interferon-gamma (IFNgamma). FACS analysis revealed the presence of AECA in 47% of HLA-sensitized (PRA = 10%: mostly IgG) vs. 16.0% in nonsensitized patients (PRA < 10%) (p < 0.02). No significant correlation was found between the presence of AECA and acute rejection occurrence and graft outcome. Non-HLA reactive AECA are directed against TNFalpha- and IFNgamma-inducible membrane molecule(s), and react with two predominant antigens of approximately 35 kDa and approximately 50 kDa expressed on ECs but not on B cells. Binding of AECA decreases in vitro EC viability by 50-60% by promoting EC apoptosis, as demonstrated by DNA fragmentation assays.

Is presensitization relevant to liver transplantation outcome?

The impact of anti-HLA antibodies and crossmatch (CM) on liver transplantation (LT) outcome is still controversial. In this retrospective study we analyzed LT outcome according to pretransplant pre-formed anti-HLA antibodies and CM status. Serum anti-HLA antibodies were screened by ELISA assay, utilizing One Lambda antigen tray-mixed (LAT-M). CMs were performed by the complement dependent cytotoxicity test using Dithiotreitol treated sera. Anti-HLA antibodies were studied in 80 recipients; 56/80 had positive LAT-M tests (PLAT-M), whereas the remaining 24 recipients tested negative for both classes I and II (NLAT-M). Rejection episodes were more frequent in PLAT-M compared with NLAT-M group in post-LT intervals of <1 week (p = 0.05), 1 week-3 months (p = 0.035), and 3-12 months (p = 0.076). Graft and patient survival rates were better, albeit not significantly, in the NLAT-M compared with PLAT-M recipients. CM status was investigated in 62/80 recipients, 18/62 recipients had positive CM (PCM), and 44 had negative CM (NCM). Five of 18 PCM recipients (28%) experienced early graft loss compared with 1/44 (2%) with NCM (p = 0.006). Rejection episodes were more frequent within first 3 months post-LT in PCM recipients compared with NCM (p = 0.015). One-year graft survival rate was better in NCM, compared with PCM recipients (graft loss of 2/44 vs 5/18). NCM PLAT-M had a higher incidence of rejection episodes compared with the NCM NLAT-M group (p = 0.031). The presence of anti-HLA antibodies suggests a deleterious effect on LT outcome, and was associated with an increased incidence of early graft loss and rejection episodes.

Ligation of HLA class I molecules on endothelial cells induces phosphorylation of Src, paxillin, and focal adhesion kinase in an actin-dependent manner

The development of chronic rejection is the major limitation to long-term allograft survival. HLA class I Ags have been implicated to play a role in this process because ligation of class I molecules by anti-HLA Abs stimulates smooth muscle cell and endothelial cell proliferation. In this study, we show that ligation of HLA class I molecules on the surface of human aortic endothelial cells stimulates phosphorylation of Src, focal adhesion kinase, and paxillin. Signaling through class I stimulated Src phosphorylation and mediated fibroblast growth factor receptor (FGFR) translocation to the nucleus. In contrast, Src kinase activity was not involved in class I-mediated transfer of FGFR from cytoplasmic stores to the cell surface. Inhibition of Src protein kinase activity blocked HLA class I-stimulated tyrosine phosphorylation of paxillin and focal adhesion kinase. Furthermore, HLA class I-mediated phosphorylation of the focal adhesion proteins and FGFR expression was inhibited by cytochalasin D and latrunculin A, suggesting a role for the actin cytoskeleton in the signaling process. These findings indicate that anti-HLA Abs have the capacity to transduce activation signals in endothelial cells that may promote the development of chronic rejection.

Indirect recognition and antibody production against a single mismatched HLA-A2-transgenic molecule precede the development of obliterative airway disease in murine heterotopic tracheal allografts

BACKGROUND

Previous studies have implicated the allogeneic immune response in the development of obliterative bronchiolitis after lung transplantation. However, the progression of specific pathogenic events leading to this form of chronic allograft dysfunction have not been well characterized. We used a murine tracheal transplantation model in which a single mismatched HLA-A2-transgenic molecule is indirectly recognized by the recipient CD4(+) T cells to show that obliterative airway disease (OAD) that developed in these allografts was preceded by indirect recognition of the HLA-A2 molecule and subsequent development of anti-HLA-A2 antibodies.

METHODS

Tracheas from HLA-A2(+) C57BL/6 mice were heterotopically transplanted into C57BL/6 mice. Allograft histopathology as well as anti-HLA-A2 T-cell proliferative responses and anti-HLA-A2 antibody development were determined at days 5, 10, 20, and 28 after transplantation.

RESULTS

All of the HLA-A2(+) tracheal allografts transplanted into C57BL/6 recipients demonstrated complete development of OAD by day 20. Spleen cells from the mice that underwent transplantation demonstrated significant proliferation against HLA-A2(+) cells by day 5. Indirect recognition of HLA-A2-derived peptides by spleen cells from allograft recipients was also higher on days 5 and 10 as compared with irrelevant peptides derived from HLA-A1, HLA-A3, and HLA-B44. Allograft recipients showed detectable levels of anti-HLA-A2 antibodies by day 5 and full development of anti-HLA-A2 antibodies by day 20.

CONCLUSION

These results show that sensitization of CD4+ T cells against the mismatched HLA-A2 alloantigen precedes the development of anti-HLA antibodies as well as OAD, suggesting an important role for alloreactive CD4(+) T-cell activation and alloantibody development in the immunopathogenesis of OAD.

Ligation of major histocompatibility complex class I antigens (MHC-I) prevents apoptosis induced by Fas or SAPK/JNK activation in T-lymphoma cells

Early apoptosis in Jurkat T-lymphoma cells was induced by agonistic anti-Fas Ab or by anisomycin which activates the stress kinases SAPK/JNK. Apoptosis was inhibited by ligation of major histocompatibility complex class I antigens (MHC-I). MHC-I ligation induced upregulation of the anti-apoptotic Bcl-2 protein and stabilized the mitochondrial membrane potential (Deltapsim). MHC-I ligation also prevented downregulation of Bcl-2 and destabilization of Deltapsim induced by anti-Fas Ab treatment or anisomycin exposure. Studies on three different Jurkat cell mutants deficient for src p56(lck), ZAP-70 kinase, or TCR/CD3 gamma-chain showed that the cells undergo apoptosis after Fas ligation. Anisomycin exposure induced apoptosis in the src p56(lck)-deficient cell line but not in the two other mutant cell lines. Simultaneous cross-linking of MHC-I and Fas ligation inhibited apoptosis in the ZAP-70 kinase and the TCR/CD3 gamma-chain mutants, but did not protect the src p56(lck)-deficient cells. Similarly, MHC-I ligation did not protect anisomycin-treated src p56(lck)-deficient cells against apoptosis. These data suggest that MHC-I-induced inhibition of apoptosis depends on intact src p56(lck) activity, but not on major secondary messenger molecules associated with TCR signaling. Overall the results support the idea that signal transduction by MHC-I molecules is involved in homeostatic processes of importance for T-cell survival and death.

Determination of the frequency of HLA antibody secreting B-lymphocytes in alloantigen sensitized individuals

Sera from prospective transplant patients are usually screened for HLA antibodies prior to transplantation, but presently available tests do not permit quantification of the humoral alloantigen directed response. We adapted a culture system for isolated human B-lymphocytes to assay the secretion of HLA-antibodies on a single cell basis. B-cell supernatants were screened for HLA antibodies by complement dependent cytotoxicity. The assay assigns precursor frequencies for HLA-alloantibody secreting B-lymphocytes (BCPFs), and simultaneously allows for dissection of the humoral alloantigen directed response into its monoclonal components. The lymphocytes of 15 HLA-seropositive multiparous women that were used to validate the assay, were found to contain HLA-BCPFs ranging from 0 to 123 per 10(6) B-lymphocytes (mean: 43 +/- 45 per 10(6) B-lymphocytes). The HLA-specificities of antibodies in the B-cell supernatants were in agreement with serum specificities. Genuine HLA reactivity of B-cell supernatants was confirmed using an ELISA with purified HLA class I antigens. When applied to lymphocytes of patients on transplant waiting lists, the present assay may enable the unraveling of serum specificities in their components, thus supplementing HLA antibody serum screening data.

ACTIVATION OF HUMAN AIRWAY EPITHELIAL CELLS BY NON-HLA ANTIBODIES DEVELOPED AFTER LUNG TRANSPLANTATION: A POTENTIAL ETIOLOGICAL FACTOR FOR BRONCHIOLITIS OBLITERANS SYNDROME1

BACKGROUND

The main cause of morbidity and mortality after lung transplantation (LT) is bronchiolitis obliterans syndrome (BOS). Anti-HLA antibodies development after LT has been shown to play an important role in BOS pathogenesis. However, the nature of non-HLA antibodies developed after LT and their role in BOS pathogenesis have not been determined.

METHODS

Sera from 16 BOS+ patients and 11 BOS- patients were collected at 12, 24, 36, and 48 months after LT. Anti-HLA class I and class II antibodies were absorbed with pooled human platelets and pooled human lymphoblastoid cell lines, respectively. Then, the presence of non-HLA antibodies against several cell lines from different origin was determined by flow cytometric analysis. Antibody-positive samples were tested for induction of proliferation and growth factor production in two selected airway epithelial cell (AEC) lines.

RESULTS

Five of 16 BOS+ patients (31.2%) and 0 of 11 BOS- patients (0%) developed anti-AEC antibodies after LT (P=0.05). No reactivity against endothelial cells, lymphocytes, monocytes, or granulocytes was detected. Further analysis of two selected sera demonstrated the development of reactivity against a 60-kDa antigen expressed by 60% of AEC lines and only 12% of cell lines from other tissues. Antibody binding to this antigen induced intracellular Ca++ influx, tyrosine phosphorylation, proliferation, and up-regulation of transforming growth factor-beta and heparin-binding epidermal growth factor mRNA transcription in AECs.

CONCLUSIONS

These results indicate that anti-AEC antibodies may play a role in the immunopathogenesis of BOS in the absence of anti-HLA antibodies.

Anti-HLA antibodies after solid organ transplantation

We have cited more than 23 studies showing that de novo development of anti-HLA antibodies is associated with increased acute and chronic rejection and decreased graft survival in kidney, heart, lung, liver, and corneal transplants. Antibodies to both HLA class I and class II antigens seem to be detrimental. Antibodies of the IgG isotype and possibly the IgM isotype were clinically relevant. Most studies showed that donor-specific antibodies were associated with rejection and graft loss. Therefore, HLA antibodies provide a clinical readout for patient alloreactivity that may have the ability to distinguish graft dysfunction due to immunologic and nonimmunologic causes. Antibody may act as a critical trigger for rejection of allografts and may serve as an early indicator of a slowly smoldering chronic rejection that is not manifested at a given time by biochemical measures such as serum creatinine levels. The effectiveness of various drugs on chronic rejection should be evaluable by their effects on HLA antibody production. We predict that recently developed ELISA and flow cytometry techniques using purified HLA antigen will increase the clinical relevance of posttransplantation HLA antibody monitoring by (1) allowing the detection of low levels of donor antibody; (2) easily distinguishing the isotype and target (HLA class I or class II) of the antibodies; and (3) correlating the antibody with specific graft pathology.

Anti-HLA antibody binding to hla class I molecules induces proliferation of airway epithelial cells: a potential mechanism for bronchiolitis obliterans syndrome

OBJECTIVE

Development of anti-HLA antibodies is associated with development of bronchiolitis obliterans syndrome after lung transplantation. We sought to determine the mechanism by which anti-HLA antibodies affect the development of bronchiolitis obliterans syndrome. We postulated that anti-HLA antibodies bind to the donor lung epithelium and stimulate phosphorylation and proliferation.

METHODS

The A549 lung epithelial carcinoma cell line was cultured in serum-deficient medium to produce static growth. Then the cells were treated with anti-HLA sera from lung transplant recipients, pooled anti-HLA serum from highly sensitized patients, or normal human serum. The cells were also treated with the W6/32 mouse anti-HLA class I monoclonal antibody or control mouse IgG. Tritiated thymidine uptake was determined at 24, 48, and 72 hours. In parallel experiments the cells were treated as described above, and the levels of tyrosine phosphorylation were determined by Western blot analysis.

RESULTS

Cells treated with anti-HLA serum or the W6/32 monoclonal antibody exhibited significantly greater proliferation and tyrosine phosphorylation of proteins of approximately 170, 130, 110, and 70 kd compared with cells treated with normal human serum or mouse IgG, respectively.

CONCLUSIONS

These data indicate that anti-HLA antibodies have the ability to stimulate airway epithelial cell proliferation and that they may play an important role in the development of bronchiolitis obliterans syndrome. Prevention of HLA sensitization and immunosuppression with agents capable of blocking indirect antigen presentation and the humoral immune response against the allograft may be pivotal in preventing the development of bronchiolitis obliterans syndrome after lung transplantation.

Susceptibility of lung transplants to preformed donor-specific HLA antibodies as detected by flow cytometry

BACKGROUND

Preformed anti-HLA antibodies are known to have the potential to induce early graft damage in organ transplant recipients. However, in lung transplant recipients, little information exists about the significance of preformed antibodies directed to either class I or class II HLA antigens.

METHODS

A two-color flow cytometry cross-match was performed in 92 consecutive lung transplant recipients using serum obtained immediately before transplantation. The presence of preformed antibodies was correlated with the incidence of severe graft dysfunction manifested as pulmonary infiltrates and severe hypoxemia with onset in the first few hours after transplantation.

RESULTS

Six patients (6.5%) had low-level anti-donor IgG antibodies detected by flow cytometry, four against T and two against B lymphocytes. Three patients (50%) developed severe graft dysfunction with pulmonary infiltrates and hypoxemia. Two patients responded to treatment, but the third, who had an antibody highly specific for HLA-DR11, died at 48 hr after transplant. Results of histopathologic studies in this patient are consistent with hyperacute rejection and support a pathogenic role of these antibodies. In contrast, of 86 (93.5%) cases with a negative flow cytometry cross-match, only 4 (5%) had severe but reversible early graft dysfunction with pulmonary infiltrates and hypoxemia, attributed to ischemia-reperfusion injury (P<0.005).

CONCLUSIONS

Class II, and perhaps class I HLA antibodies at relatively low concentrations represent a risk factor for severe early pulmonary graft dysfunction, with the potential to progress to hyperacute rejection and death.

Alloantibody-Mediated Class I Signal Transduction in Endothelial Cells and Smooth Muscle Cells: Enhancement by IFN-γ and TNF-α

Chronic rejection is the major limiting factor to long term survival of solid organ allografts. The hallmark of chronic rejection is transplant atherosclerosis, which is characterized by the intimal proliferation of smooth muscle cells, endothelial cells, and fibroblasts, leading to vessel obstruction, fibrosis, and eventual graft loss. The mechanism of chronic rejection is poorly understood, but it is suspected that the associated vascular changes are a result of anti-HLA Ab-mediated injury to the endothelium and smooth muscle of the graft. In this study we have investigated whether anti-HLA Abs, developed by transplant recipients following transplantation, are capable of transducing signals via HLA class I molecules, which stimulate cell proliferation. In this report we show that ligation of class I molecules with Abs to distinct HLA-A locus and HLA-B locus molecules results in increased tyrosine phosphorylation of intracellular proteins and induction of fibroblast growth factor receptor expression on endothelial and smooth muscle cells. Treatment of cells with IFN-γ and TNF-α up-regulated MHC class I expression and potentiated anti-HLA Ab-induced fibroblast growth factor receptor expression. Engagement of class I molecules also stimulated enhanced proliferative responses to basic fibroblast growth factor, which augmented endothelial cell proliferation. These findings support a role for anti-HLA Abs and cytokines in the transduction of proliferative signals, which stimulate the development of myointimal hyperplasia associated with chronic rejection of human allografts.

HLA Class I-Mediated Induction of Cell Proliferation Involves Cyclin E-Mediated Inactivation of Rb Function and Induction of E2F Activity

Chronic rejection of transplanted organs is manifested as atherosclerosis of the blood vessels of the allograft. HLA class I Ags have been implicated to play a major role in this process, since signaling via HLA class I molecules can induce the proliferation of aortic endothelial as well as smooth muscle cells. In this study, we show that HLA class I-mediated induction of cell proliferation correlates with inactivation of the Rb protein in the T cell line Jurkat as well as human aortic endothelial cells. HLA class I-mediated inactivation of Rb can be inhibited specifically by neutralizing Abs to basic fibroblast growth factor (bFGF), suggesting a role for FGF receptors in the signaling process. Signaling through HLA class I molecules induced cyclin E-associated kinase activity within 4 h in quiescent endothelial cells, and appeared to mediate the inactivation of Rb. A cdk2 inhibitor, Olomoucine, as well as a dominant-negative cdk2 construct prevented HLA class I-mediated inactivation of Rb; in contrast, dominant-negative cdk4 and cdk6 constructs had no effect. Furthermore, there was no increase in cyclin D-associated kinase activity upon HLA class I ligation, suggesting that cyclin E-dependent kinase activity mediates Rb inactivation, leading to E2F activation and cell proliferation.

Development of ELISA-detected anti-HLA antibodies precedes the development of bronchiolitis obliterans syndrome and correlates with progressive decline in pulmonary function after lung transplantation

BACKGROUND

Development of anti-HLA antibodies after lung transplantation (LT) is thought to play an important role in the etiology of bronchiolitis obliterans syndrome (BOS). However, a cause-effect relationship between anti-HLA antibodies and BOS has not been established. This study was conducted to determine the temporal relationship between the development of anti-HLA antibodies and BOS after LT, and to determine the antigenic specificity of the antibodies developed in BOS patients.

METHODS

Sera from 15 BOS+ LT patients and 12 BOS- LT patients were obtained before LT and collected again at 6, 12, 24, 36, and 48 months after LT. Anti-HLA antibodies were detected by the PRA-STAT ELISA system and by complement-dependent cytotoxicity assays. Anti-HLA reactivity was further characterized by flow cytometry and absorption/elution with human platelets.

RESULTS

When analyzed by ELISA, 10 of 15 BOS+ patients developed anti-HLA antibodies, whereas 0 of 12 BOS- patients developed anti-HLA antibodies (P<0.001). When analyzed by complement-dependent cytotoxicity, only 2 of 15 BOS+ patients developed anti-HLA antibodies and 1 of 12 BOS- patients developed anti-HLA antibodies (P = 0.99). There was a significant difference of 20.1 months between the time of anti-HLA antibody detection and the time of BOS diagnosis (P = 0.005). A progressive decrease in pulmonary function correlated with a progressive increase in the anti-HLA reactivity 36 months after LT. The anti-HLA reactivity was directed to one of the donor HLA class I antigens and to other unrelated HLA class I antigens. No anti-HLA reactivity was found against HLA class II molecules.

CONCLUSIONS

Our study indicates that anti-HLA class I antibodies play an important role in the pathogenesis of BOS and that monitoring of anti-HLA class I antibody development by a highly sensitive assay such as the PRA-STAT ELISA after LT can provide an early identification of an important subset of LT patients with an increased risk of developing BOS.

Endothelial cell activation by sera containing HLA antibodies is mediated by interleukin-1

BACKGROUND

It has been suggested that antibodies which are associated with chronic pathological conditions such as chronic rejection and autoimmune diseases have the capacity to activate endothelial cells by induction and up-regulation of adhesion molecules. It has also been suggested that HLA antibodies formed by patients awaiting transplantation can activate endothelial cells. These antibodies include HLA and those that bind to endothelial cells.

METHODS

We have further investigated this phenomenon using monoclonal antibodies against HLA class I determinants and sera from aortic valve graft recipients, containing strong HLA antibodies. The effect of 24-hr incubation of antibodies/serum with human umbilical vein endothelial cells (HUVECs) on adhesion molecule expression was measured by flow cytometry.

RESULTS

HLA monoclonal antibodies had no effect on ICAM-1 expression on HUVECs. Five of 31 (16%) patients' sera caused strong up-regulation of adhesion molecules (ICAM-1, vascular cell adhesion molecule-1, and E-selectin) but this did not correlate with HLA specificity, IgG, or IgM binding to HUVECs. The activity, found in whole serum and IgG-depleted fractions was inhibited by neutralizing antibodies against interleukin (IL)-1beta and tumor necrosis factor-alpha. Examination of patient sera for presence of IL-1beta demonstrated high levels of IL-1beta in all five sera (range, 30 -500 U/ml) as well as in samples from an additional three patients.

CONCLUSION

The ability to activate endothelial cells detected in our patient sera was caused by cytokines and not antibody. Our observation that addition of cytokines to sera before separation into large and low molecular weight fractions demonstrated retention of cytokines in both fractions may be a confounding issue when investigating endothelial cell activation by patients' sera.