Antibody and Complement Mediated Injury in Transplants Following Sensitization by Allogeneic Blood Transfusion

Romiplostim as a transfusion saving strategy in 20 patients after heart or lung transplantation: a single centre before-after pilot study

BACKGROUND

Thrombocytopenia is a common disorder after heart or lung transplantation. Platelet transfusion is often required to maintain haemostasis but represents a specific cause of morbidity and mortality in this setting including alloimmunisation and graft rejection.

STUDY DESIGN AND METHODS

As part of a health-care quality improvement project, in a single-centre before-after pilot study, the relevance of a platelet transfusion saving strategy based on romiplostim administration after transplantation was assessed in patients with platelet count <100 × 10⁹/L. Transfusions on days 28 and 90 were compared using propensity matched score for adjustment of demographic characteristics at baseline. The primary outcome was platelet transfusion until day 28 after transplantation.

RESULTS

Ninety-three patients were analysed (73 before vs. 20 after). The median [interquartile range] number of platelet concentrate was 1 [0;4.0] before versus 0.5 [0;2.0] in the after period, mean difference 0.5 confidence interval 95% [-0.7 to 1.7], p = 0.39. On day 28, median [interquartile range] red blood cell transfusion was significantly higher in the before versus the after period, 7 [2.0;13.5] versus 6 [1.5;8.5], mean difference 3.2 CI 95% [0.4-6.0], p = 0.02. At 6 months, the rate of patients with de novo anti-human leukocyte antigen alloimmunisation was 45% before versus 53% in the after period (p = 0.56). Deep venous thrombosis was detected in nine patients (12%) before versus seven patients (35%) in the after period (p = 0.04).

CONCLUSION

Romiplostim did not significantly reduce platelet transfusion after heart or lung transplantation. Its relevance and safety in a global transfusion strategy remains to be studied in this setting in a large randomised study.

Ventricular assist devices and increased blood product utilization for cardiac transplantation

BACKGROUND AND AIM OF STUDY

The purpose of this study was to examine whether blood product utilization, one-year cell-mediated rejection rates, and mid-term survival significantly differ for ventricular assist device (VAD patients compared to non-VAD (NVAD) patients following cardiac transplantation.

METHODS

From July 2004 to August 2011, 79 patients underwent cardiac transplantation at a single institution. Following exclusion of patients bridged to transplantation with VADs other than the HeartMate II® LVAD (n = 10), patients were stratified by VAD presence at transplantation: VAD patients (n = 35, age: 54.0 [48.0-59.0] years) vs. NVAD patients (n = 34, age: 52.5 [42.8-59.3] years). The primary outcomes of interest were blood product transfusion requirements, one-year cell-mediated rejection rates, and mid-term survival post-transplantation.

RESULTS

Preoperative patient characteristics were similar for VAD and NVAD patients. NVAD patients presented with higher median preoperative creatinine levels compared to VAD patients (1.3 [1.1-1.6] vs. 1.1 [0.9-1.4], p = 0.004). VAD patients accrued higher intraoperative transfusion of all blood products (all p ≤ 0.001) compared to NVAD patients. The incidence of clinically significant cell-mediated rejection within the first posttransplant year was higher in VAD compared to NVAD patients (66.7% vs. 33.3%, p = 0.02). During a median follow-up period of 3.2 (2.0, 6.3) years, VAD patients demonstrated an increased postoperative mortality that did not reach statistical significance (20.0% vs. 8.8%, p = 0.20).

CONCLUSIONS

During the initial era as a bridge to transplantation, the HeartMate II® LVAD significantly increased blood product utilization and one-year cell-mediated rejection rates for cardiac transplantation. Further study is warranted to optimize anticoagulation strategies and to define causal relationships between these factors for the current era of cardiac transplantation.

Characterization of transfusion-elicited acute antibody-mediated rejection in a rat model of kidney transplantation

Animal models of antibody-mediated rejection (ABMR) may provide important evidence supporting proof of concept. We elicited donor-specific antibodies (DSA) by transfusion of donor blood (Brown Norway RT1(n) ) into a complete mismatch recipient (Lewis RT1(l) ) 3 weeks prior to kidney transplantation. Sensitized recipients had increased anti-donor splenocyte IgG1, IgG2b and IgG2c DSA 1 week after transplantation. Histopathology was consistent with ABMR characterized by diffuse peritubular capillary C4d and moderate microvascular inflammation with peritubular capillaritis + glomerulitis > 2. Immunofluorescence studies of kidney allograft tissue demonstrated a greater CD68/CD3 ratio in sensitized animals, primarily of the M1 (pro-inflammatory) phenotype, consistent with cytokine gene analyses that demonstrated a predominant T helper (TH )1 (interferon-γ, IL-2) profile. Immunoblot analyses confirmed the activation of the M1 macrophage phenotype as interferon regulatory factor 5, inducible nitric oxide synthase and phagocytic NADPH oxidase 2 were significantly up-regulated. Clinical biopsy samples in sensitized patients with acute ABMR confirmed the dominance of M1 macrophage phenotype in humans. Despite the absence of tubulitis, we were unable to exclude the effects of T cell-mediated rejection. These studies suggest that M1 macrophages and TH 1 cytokines play an important role in the pathogenesis of acute mixed rejection in sensitized allograft recipients.

Role of complement and NK cells in antibody mediated rejection

Despite extensive research on T cells and potent immunosuppressive regimens that target cellular mediated rejection, few regimens have been proved to be effective on antibody-mediated rejection (AMR), particularly in the chronic setting. C4d deposition in the graft has been proved to be a useful marker for AMR; however, there is an imperfect association between C4d and AMR. While complement has been considered as the main player in acute AMR, the effector mechanisms in chronic AMR are still debated. Recent studies support the role of NK cells and direct effects of antibody on endothelium cells in a mechanism suggesting the presence of a complement-independent pathway. Here, we review the history, currently available systems and progress in experimental animal research. Although there are consistent findings from human and animal research, transposing the experimental results from rodent to human has been hampered by the differences in endothelial functions between species. We briefly describe the findings from patients and compare them with results from animals, to propose a combined perspective.

Antibodies, isotypes and complement in allograft rejection

PURPOSE OF REVIEW

Classical complement activation is a key step in the process of antibody-mediated rejection. Emphasizing novel diagnostic strategies, this study will discuss recent studies highlighting the particular relevance of alloantibodies with complement-fixing ability.

RECENT FINDINGS

Reinforcing the pivotal role of complement, numerous studies have shown tight associations of capillary C4d deposition, a 'footprint' of alloantibody-triggered complement activation, with the occurrence of allograft injury. Distribution patterns of immunoglobulin isotypes or subclasses, which strongly differ in their ability to activate complement, may not adequately reflect the actual pathogenetic relevance of detected allosensitization. This fact may be explained by the finding that other variables, such as antibody-binding density or a synergism of antibodies against different epitopes of the same antigen, may contribute to complement activation. An attractive approach to distinguish between complement-fixing and presumably less harmful noncomplement-fixing alloreactivities could be the detection of C4d deposition in vitro. Applying such techniques, recent studies have shown that human leukocyte antigen reactivity with C4d-fixing ability, in contrast to noncomplement-fixing sensitization, may strongly predict antibody-mediated rejection and inferior graft survival.

SUMMARY

Considering the pivotal role of complement, technologies that uncover the complement-fixing ability of alloantibodies may be of particular interest for the selective detection of deleterious sensitization.

Biomarkers of heart transplant rejection: the good, the bad, and the ugly!

Acute cellular rejection (ACR), antibody-mediated rejection (AMR), and cardiac allograft vasculopathy (CAV) are important limitations for the long-term survival of heart transplant recipients. Although much progress has been made in reducing ACR with modern immunosuppressive treatments and continuous biopsy surveillance, there is still a long way to go to better understand and treat AMR, to enable early detection of patients at risk of CAV, and to reduce the development and sustained progression of this irreversible disease that permanently compromises graft function. This review considers the advances made in ACR detection and treatment allowing a more prolonged survival and the risk factors leading to endothelial injury, dysfunction, inflammation, and subsequent CAV, as well as new treatment modalities for CAV. The review also evaluates the controversies around the definition, pathogenesis, and treatment of AMR. To date, much progress is still needed to significantly reduce post-transplant complications and increase graft and patient survival.

Sensitized recipients exhibit accelerated but not hyperacute rejection of vascularized composite tissue allografts

BACKGROUND

Currently, the donor-recipient matching process for vascularized composite tissue allotransplantation (VCTA) closely follows the standard practices for solid organ transplantation. Sensitization is considered a contraindication to VCTA. However, the role of sensitization in VCTA rejection is largely unstudied.

METHODS

Major histocompatibility-mismatched ACI (RT1) donors and Wistar Furth (WF) (RT1) recipients were used to determine whether sensitization would lead to hyperacute rejection in VCTA as in other organs, such as kidneys. WF rats were presensitized to ACI antigens by skin transplantation and received heterotopic osteomyocutaneous VCTA flaps. Kidney transplants served as controls.

RESULTS

Production of anti-donor antibody was detected in WF recipients after rejection of the ACI skin grafts. Sensitized WF rats rejected VCTA grafts from ACI rats significantly faster (P<0.05) than unsensitized recipients, but not hyperacutely. Rejection in the sensitized recipients was not prevented by immunosuppression with FK506 and mycophenolate mofetil. In contrast, kidney allografts from ACI rats were hyperacutely rejected within 30 min by sensitized recipients. To confirm the role of antibody-mediated rejection in the sensitized recipients, serum from presensitized rats was adoptively transferred into naïve WF rats. Hyperacute rejection occurred only in transplanted kidneys but not VCTA. Histologic examination of tissues from acceleratedly rejected VCTA showed dense lymphocytic infiltrates, and no antibody deposition.

CONCLUSIONS

VCTA are rejected in an accelerated fashion but not hyperacutely in the presence of allosensitization and preformed anti-donor antibody. The rejection of VCTA in sensitized recipients is mainly cell mediated and differs mechanistically from that for renal transplants.

Mechanisms involved in antibody- and complement-mediated allograft rejection

Antibody-mediated rejection has become critical clinically because this form of rejection is usually unresponsive to conventional anti-rejection therapy, and therefore, it has been recognized as a major cause of allograft loss. Our group developed experimental animal models of vascularized organ transplantation to study pathogenesis of antibody- and complement-mediated endothelial cell injury leading to graft rejection. In this review, we discuss mechanisms of antibody-mediated graft rejection resulting from activation of complement by C1q- and MBL (mannose-binding lectin)-dependent pathways and interactions with a variety of effector cells, including macrophages and monocytes through Fcgamma receptors and complement receptors.

Antibody-mediated rejection: emergence of animal models to answer clinical questions

Decades of experiments in small animals had tipped the balance of opinion away from antibodies as a cause of transplant rejection. However, clinical experience, especially with sensitized patients, has convinced basic immunologists of the need to develop models to investigate mechanisms underlying antibody-mediated rejection (AMR). This resurgent interest has resulted in several new rodent models to investigate antibody-mediated mechanisms of heart and renal allograft injury, but satisfactory models of chronic AMR remain more elusive. Nevertheless, these new studies have begun to reveal many insights into the molecular and pathological sequelae of antibody binding to the allograft endothelium. In addition, complement-independent and complement-dependent effects of antibodies on endothelial cells have been identified in vitro. As small animal models become better defined, it is anticipated that they will be more widely used to answer further questions concerning mechanisms of antibody-mediated tissue injury as well as to design therapeutic interventions.

Immunological challenges of cardiac transplantation: the need for better animal models to answer current clinical questions

INTRODUCTION

In the last decade, two advances have shifted attention from cellular rejection to antibody-mediated rejection (AMR) of cardiac transplants. First, more sensitive diagnostic tests for detection of AMR have been developed. Second, improvements in immunosuppression have made severe acute cellular rejection uncommon, but have had less effect on AMR.

DISCUSSION

Antibodies can contribute to graft rejection by activation of complement, by activation of vascular endothelial and smooth muscle cells, and by activation of neutrophils, macrophages or natural killer cells. Because acute rejection is a risk factor for chronic rejection in all types of organ transplants, it is has been proposed that AMR can cause chronic rejection.

CONCLUSION

Small animal models need to be developed to gain further insights into AMR and the role of antibodies in chronic graft arteriopathy. This article reviews the current clinical data and existing mouse models for AMR.

Indirect CD4+ TH1 response, antidonor antibodies and diffuse C4d graft deposits in long-term recipients conditioned by donor antigens priming

Priming of recipients by DST induces long-term survival of mismatched allografts in adult rats. Despite these recipients developing inducible T regulatory cells able to transfer long-term graft survival to a secondary host, a state of chronic rejection is also observed. We revisited the molecular donor MHC targets of the cellular response in acute rejection and analyzed the cellular and humoral responses in recipients with long-term graft survival following transplantation. We found three immunodominant peptides, all derived from LEW.1W RT1.D(u) molecules to be involved in acute rejection of grafts from unmodified LEW.1A recipients. Although the direct pathway of allorecognition was reduced in DST-treated recipients, the early CD4+ indirect pathway response to dominant peptides was almost unimpaired. We also detected early and sustained antidonor class I and II antibody subtypes with diffuse C4d deposits on graft vessels. Finally, long-term accepted grafts displayed leukocyte infiltration, endarteritis and fibrosis, which evolved toward vascular narrowing at day 100. Altogether, these data suggest that the chronic graft lesions developed in long-term graft recipients are the result of progressive humoral injury associated with a persisting indirect T helper response. These features may represent a useful model for understanding and manipulating chronic active antibody-mediated rejection in human.

RETRACTED: Chronic cardiac allograft rejection in mice is alleviated by inhibition of CCR5 in combination with cyclosporine A

The chemokine receptor CCR5 plays important roles in acute allograft rejection. In this study, we examined the inhibition of CCR5 in combination with the treatment with cyclosporine A (CsA) in chronic rejection in cardiac transplantation. Forty-five transplant recipients were randomized to three groups. Recipients in group A were treated with anti-CCR5 mAb and CsA, mice in group B were given anti-CCR5 mAb alone, and animals in group C were administered with only CsA. On day 45 after transplantation, the allografts were harvested and examined by immunohistologic technique and PT-PCR methods. Allografts treated with anti-CCR5 mAb and CsA showed significantly prolonged survival (44.73 ± 0.258 days, P < 0.01) as compared with CsA-treated group (37.00 ± 2.04 days). Treatment with anti-CCR5 mAb plus CsA significantly inhibited the progression of cardiac allograft vasculopathy. Our findings demonstrated that anti-CCR5 mAb in combination with CsA can prolong the survival of allograft through their cardio-protective and immunomodulative properties. Thus, combined administration of anti-CCR5 mAb and CsA may become a new therapeutic approach for the prevention of cardiac graft failure that has not been obviated by conventional immunosuppressive agents.

Critical role of effector macrophages in mediating CD4-dependent alloimmune injury of transplanted liver parenchymal cells

Despite the recognition that humoral rejection is an important cause of allograft injury, the mechanism of Ab-mediated injury to allograft parenchyma is not well understood. We used a well-characterized murine hepatocellular allograft model to determine the mechanism of Ab-mediated destruction of transplanted liver parenchymal cells. In this model, allogeneic hepatocytes are transplanted into CD8-deficient hosts to focus on CD4-dependent, alloantibody-mediated rejection. Host serum alloantibody levels correlated with in vivo allospecific cytotoxic activity in CD8 knockout hepatocyte rejector mice. Host macrophage depletion, but not CD4(+) T cell, NK cell, neutrophil, or complement depletion, inhibited in vivo allocytotoxicity. Recipient macrophage deficiency delayed CD4-dependent hepatocyte rejection and inhibited in vivo allocytotoxicity without influencing alloantibody production. Furthermore, hepatocyte coincubation with alloantibody and macrophages resulted in Ab-dependent hepatocellular cytotoxicity in vitro. These studies are consistent with a paradigm of acute humoral rejection in which CD4(+) T cell-dependent alloantibody production results in the targeting of transplanted allogeneic parenchymal cells for macrophage-mediated cytotoxic immune damage. Consequently, strategies to eliminate recipient macrophages during CD4-dependent rejection pathway may prolong allograft survival.

Treatment with riboflavin and ultraviolet light prevents alloimmunization to platelet transfusions and cardiac transplants

BACKGROUND

Functional leukocytes in blood transfusions can cause alloimmunization. Previous studies have shown that exposure of platelet concentrates to riboflavin and light (Mirasol PRT treatment) causes irreparable modification of nucleic acids. This treatment does not interfere with platelet function but does inhibit a wide range of immunological functions of leukocytes present in platelet concentrates. The current study evaluated the ability of Mirasol treatment to prevent alloimmunization by platelet transfusions in rats.

METHODS

Lewis rats received eight transfusions of untreated or Mirasol-treated platelets containing leukocytes from DA rats. Animals were subsequently challenged with a heart transplant under cyclosporine treatment.

RESULTS

Mirasol treatment caused apoptosis of the leukocytes as measured by annexin V and CD45 staining. Complement split products were deposited on the apoptotic bodies in the platelet pack. The primary and secondary immunoglobulin (Ig) M and IgG responses in rats that received Mirasol-treated platelets were almost completely abolished compared to animals that received untreated platelets. Untreated platelet transfusions elicited strong IgG responses that were associated with rapid rejection of subsequent heart transplants. Rejected hearts contained macrophage infiltrates and C4d deposits. In contrast, no grafts were rejected by recipients transfused with Mirasol-treated platelets. Macrophage infiltrates and C4d deposits were decreased in these grafts. Recipients that were presensitized to untreated platelets were capable of producing a memory response to Mirasol-treated platelets that caused accelerated rejection of subsequent transplants.

CONCLUSIONS

Transfusions of platelets that were treated with riboflavin and ultraviolet light prevented presensitization to transplants. However, Mirasol-treated platelets were immunogenic in presensitized recipients.

The involvement of FcR mechanisms in antibody-mediated rejection

BACKGROUND

Antibody-mediated rejection is characterized by macrophage margination against vascular endothelium. The potential interactions triggered by antibodies between endothelial cells (EC) and macrophages have not been examined thoroughly in transplants. We used in vivo and in vitro models of antibody-mediated rejection.

METHODS

Passive transfer of monoclonal alloantibodies (Allo-mAbs) to donor major histocompatibility complex-class I antigens was used to restore acute rejection of B10.A (H-2a) hearts to C57BL/6 (H-2b) immunoglobulin knockout (IgKO) recipients. Intragraft cytokine mRNA expression was measured by real-time polymerase chain reaction. In vitro, mouse EC were cultured in the presence of Allo-mAbs to donor major histocompatibility complex class I antigens and mononuclear cells. Levels of cytokines in culture supernatants were determined in enzyme-linked immunosorbent assay.

RESULTS

Expression of MCP-1, IL-6 and IL-1alpha mRNA was higher in rejecting transplants from recipients treated with Allo-mAbs compared to non-rejecting transplants. EC sensitized with Allo-mAbs produced high levels of MCP-1 and KC. The addition of macrophages to sensitized EC stimulated high levels of IL-6 in addition to MCP-1, KC, Rantes, and TIMP-1. The levels of MCP-1 and IL-6 were significantly lower in co-cultures of EC sensitized with IgG1 Allo-mAbs in the presence of mononuclear cells from Fcgamma-Receptor III KO (FcgammaRIII-KO) graft recipients compared to co-cultures with wild-type cells. The levels of both cytokines were also lower in co-cultures of EC stimulated with F(ab')2 fragments of antibody.

CONCLUSIONS

Our findings indicate that IgG1 Allo-mAbs to major histocompatibility complex class I antigens can augment graft injury by stimulating EC to produce MCP-1 and by activating mononuclear cells through their Fc receptors.

Synergistic deposition of C4d by complement-activating and non-activating antibodies in cardiac transplants

The role of non-complement-activating alloantibodies in humoral graft rejection is unclear. We hypothesized that the non-complement-activating alloantibodies synergistically activate complement in combination with complement-activating antibodies. B10.A hearts were transplanted into immunoglobulin knock out (Ig-KO) mice reconstituted with monoclonal antibodies to MHC class I antigens. In allografts of unreconstituted Ig-KO recipients, no C4d was detected. Similarly, reconstitution with IgG1 or low dose IgG2b alloantibodies did not induce C4d deposition. However, mice administered with a low dose of IgG2b combined with IgG1 had heavy linear deposits of C4d on vascular endothelium. C4d deposits correlated with decreased graft survival. To replicate this synergy in vitro, mononuclear cells from B10.A mice were incubated with antibodies to MHC class I antigens followed by incubation in normal mouse serum. Flow cytometry revealed that both IgG2a and IgG2b synergized with IgG1 to deposit C4d. This synergy was significantly decreased in mouse serum deficient in mannose binding lectin (MBL) and in serum deficient in C1q. Reconstitution of MBL-A/C knock out (MBL-KO) serum with C1q-knock out (C1q-KO) serum reestablished the synergistic activity. This suggests a novel role for non-complement-activating alloantibodies and MBL in humoral rejection.

New concepts of complement in allorecognition and graft rejection

In transplantation, activation of complement has largely been equated to antibody-mediated rejection, but complement is also important in recognition of apoptotic and necrotic cells as well as in modifying antigen presentation to T cells and B cells. As a part of the innate immune system, complement is one of the first responses to injury, and it can determine the direction and magnitude of the subsequent responses. Consequently, the effects of complement in allorecognition and graft rejection are increased when organs are procured from cadaver donors because these organs sustain a series of stresses from brain death, prolonged life support, ischemia and finally reperfusion that initiate proinflammatory processes and tissue injury. In addition, these organs are transplanted to patients, who frequently have been sensitized to histocompatibility antigens as the result of transfusions, pregnancies or transplants. Complement activation generates a series of biologically active effector molecules that can modulate graft rejection by directly binding to the graft or by modifying the response of macrophages, T and B cells of the recipient. However, complement is regulated and the process of regulation produces split products that can decrease as well as increase immune responses. Small animal models have been developed to test these variables. The guide for evaluating results from these models remains clinical findings because there are significant differences between the rodent and human complement systems.