Preformed IgG antibodies against major histocompatibility complex class II antigens are major risk factors for high-grade cellular rejection in recipients of heart transplantation

Impacts of Pre-transplant Panel-Reactive Antibody on Post-transplantation Outcomes: A Study of Nationwide Heart Transplant Registry Data

BACKGROUND AND OBJECTIVES

The number of sensitized heart failure patients on waiting lists for heart transplantation (HTx) is increasing. Using the Korean Organ Transplantation Registry (KOTRY), a nationwide multicenter database, we investigated the prevalence and clinical impact of calculated panel-reactive antibody (cPRA) in patients undergoing HTx.

METHODS

We retrospectively reviewed 813 patients who underwent HTx between 2014 and 2021. Patients were grouped according to peak PRA level as group A: patients with cPRA ≤10% (n= 492); group B: patients with cPRA >10%, <50% (n=160); group C patients with cPRA ≥50% (n=161). Post-HTx outcomes were freedom from antibody-mediated rejection (AMR), acute cellular rejection, coronary allograft vasculopathy, and all-cause mortality.

RESULTS

The median follow-up duration was 44 (19-72) months. Female sex, re-transplantation, and pre-HTx renal replacement therapy were independently associated with an increased risk of sensitization (cPRA ≥50%). Group C patients were more likely to have longer hospital stays and to use anti-thymocyte globulin as an induction agent compared to groups A and B. Significantly more patients in group C had positive flow cytometric crossmatch and had a higher incidence of preformed donor-specific antibody (DSA) compared to groups A and B. During follow-up, group C had a significantly higher rate of AMR, but the overall survival rate was comparable to that of groups A and B. In a subgroup analysis of group C, post-transplant survival was comparable despite higher preformed DSA in a desensitized group compared to the non-desensitized group.

CONCLUSIONS

Patients with cPRA ≥50% had significantly higher incidence of preformed DSA and lower freedom from AMR, but post-HTx survival rates were similar to those with cPRA <50%. Our findings suggest that sensitized patients can attain comparable post-transplant survival to non-sensitized patients when treated with optimal desensitization treatment and therapeutic intervention.

Polyinosinic: polycytidylic acid induced inflammation enhances while lipopolysaccharide diminishes alloimmunity to platelet transfusion in mice

Introduction

Alloimmune responses against platelet antigens, which dominantly target the major histocompatibility complex (MHC), can cause adverse reactions to subsequent platelet transfusions, platelet refractoriness, or rejection of future transplants. Platelet transfusion recipients include individuals experiencing severe bacterial or viral infections, and how their underlying health modulates platelet alloimmunity is not well understood.

Methods

This study investigated the effect of underlying inflammation on platelet alloimmunization by modelling viral-like inflammation with polyinosinic-polycytidylic acid (poly(I:C)) or gram-negative bacterial infection with lipopolysaccharide (LPS), hypothesizing that underlying inflammation enhances alloimmunization. Mice were pretreated with poly(I:C), LPS, or nothing, then transfused with non-leukoreduced or leukoreduced platelets. Alloantibodies and allogeneic MHC-specific B cell (allo-B cell) responses were evaluated two weeks later. Rare populations of allo-B cells were identified using MHC tetramers.

Results

Relative to platelet transfusion alone, prior exposure to poly(I:C) increased the alloantibody response to allogeneic platelet transfusion whereas prior exposure to LPS diminished responses. Prior exposure to poly(I:C) had equivalent, if not moderately diminished, allo-B cell responses relative to platelet transfusion alone and exhibited more robust allo-B cell memory development. Conversely, prior exposure to LPS resulted in diminished allo-B cell frequency, activation, antigen experience, and germinal center formation and altered memory B cell responses.

Discussion

In conclusion, not all inflammatory environments enhance bystander responses and prior inflammation mediated by LPS on gram-negative bacteria may in fact curtail platelet alloimmunization.

Enhanced alloresponse to platelet transfusion due to immune dysregulation following ablative chemotherapy in mice

Introduction

Alloimmunization is common following platelet transfusion and can result in negative outcomes for recipients such as refractoriness to subsequent transfusions and rejection of transplants. Healthy people do not receive blood transfusions, and the diseases and therapies that result in a need to transfuse have significant impacts on the immunological environment to which these alloantigens are introduced. Ablative chemotherapies are common among platelet recipients and have potent immunological effects. In this study, we modeled the impact of chemotherapy on the alloresponse to platelet transfusion. As chemotherapies are generally regarded as immunosuppressive, we hypothesized that that they would result in a diminished alloresponse.

Methods

Mice were given a combination chemotherapeutic treatment of cytarabine and doxorubicin followed by transfusion of allogeneic platelets, and compared to controls given no treatment, chemotherapy alone, or transfusion alone. Alloantibody responses were measured 2 weeks after transfusion, and cellular responses and growth factors were monitored over time.

Results

Contrary to our hypothesis, we found that chemotherapy led to increased alloantibody responses to allogeneic platelet transfusion. This enhanced response was antigen-specific and was associated with increased CD4+ and CD8+ T cell responses. Chemotherapy led to rapid lymphocyte depletion followed by reconstitution, non-specific activation of transitional B cells with the highest levels of activation in the least mature subsets, and increased serum levels of B cell activating factor (BAFF).

Conclusion

These data suggest that ablative chemotherapy can increase the risk of alloimmunization and, if confirmed clinically, that additional measures to protect these patient populations may be warranted.

Current Desensitization Strategies in Heart Transplantation

Heart transplant candidates sensitized to HLA antigens wait longer for transplant, are at increased risk of dying while waiting, and may not be listed at all. The increasing prevalence of HLA sensitization and limitations of current desensitization strategies underscore the urgent need for a more effective approach. In addition to pregnancy, prior transplant, and transfusions, patients with end-stage heart failure are burdened with unique factors placing them at risk for HLA sensitization. These include homograft material used for congenital heart disease repair and left ventricular assist devices (LVADs). Moreover, these risks are often stacked, forming a seemingly insurmountable barrier in some cases. While desensitization protocols are typically implemented uniformly, irrespective of the mode of sensitization, the heterogeneity in success and post-transplant outcomes argues for a more tailored approach. Achieving this will require progress in our understanding of the immunobiology underlying the innate and adaptive immune response to these varied allosensitizing exposures. Further attention to B cell activation, memory, and plasma cell differentiation is required to establish methods that durably abrogate the anti-HLA antibody response before and after transplant. The contribution of non-HLA antibodies to the net state of sensitization and the potential implications for graft longevity also remain to be comprehensively defined. The aim of this review is to first bring forth select issues unique to the sensitized heart transplant candidate. The current literature on desensitization in heart transplantation will then be summarized providing context within the immune response. Building on this, newer approaches with therapeutic potential will be discussed emphasizing the importance of not only addressing the short-term pathogenic consequences of circulating HLA antibodies, but also the need to modulate alloimmune memory.

Noninvasive biomarkers for prediction and diagnosis of heart transplantation rejection

For most patients with end-stage heart failure, heart transplantation is the treatment of choice. Allograft rejection is one of the major post-transplantation complications affecting graft outcome and survival. Recent advancements in science and technology offer an opportunity to integrate genomic and other omics-based biomarkers into clinical practice, facilitating noninvasive evaluation of allograft for diagnostic and prognostic purposes. Omics, including gene expression profiling (GEP) of blood immune cell components and donor-derived cell-free DNA (dd-cfDNA) are of special interest to researchers. Several studies have investigated levels of dd-cfDNA and miroRNAs in blood as potential markers for early detection of allograft rejection. One of the achievements in the field of transcriptomics is AlloMap, GEP of peripheral blood mononuclear cells (PBMC), which can identify 11 differentially expressed genes and help with detection of moderate and severe acute cellular rejection in stable heart transplant recipients. In recent years, the utilization of GEP of PBMC for identifying differentially expressed genes to diagnose acute antibody-mediated rejection and cardiac allograft vasculopathy has yielded promising results. Advancements in the field of metabolomics and proteomics as well as their potential implications have been further discussed in this paper.

Pathogen-reduced PRP blocks T-cell activation, induces Treg cells, and promotes TGF-β expression by cDCs and monocytes in mice

Alloimmunization against platelet-rich plasma (PRP) transfusions can lead to complications such as platelet refractoriness or rejection of subsequent transfusions and transplants. In mice, pathogen reduction treatment of PRP with UVB light and riboflavin (UV+R) prevents alloimmunization and appears to induce partial antigen-specific tolerance to subsequent transfusions. Herein, the in vivo responses of antigen-presenting cells and T cells to transfusion with UV+R-treated allogeneic PRP were evaluated to understand the cellular immune responses leading to antigen-specific tolerance. Mice that received UV+R-treated PRP had significantly increased transforming growth factor β (TGF-β) expression by CD11b+ CD4+ CD11cHi conventional dendritic cells (cDCs) and CD11bHi monocytes (P < .05). While robust T-cell responses to transfusions with untreated allogeneic PRP were observed (P < .05), these were blocked by UV+R treatment. Mice given UV+R-treated PRP followed by untreated PRP showed an early significant (P < .01) enrichment in regulatory T (Treg) cells and associated TGF-β production as well as diminished effector T-cell responses. Adoptive transfer of T-cell-enriched splenocytes from mice given UV+R-treated PRP into naive recipients led to a small but significant reduction of CD8+ T-cell responses to subsequent allogeneic transfusion. These data demonstrate that pathogen reduction with UV+R induces a tolerogenic profile by way of CD11b+ CD4+ cDCs, monocytes, and induction of Treg cells, blocking T-cell activation and reducing secondary T-cell responses to untreated platelets in vivo.

Profiling non-HLA antibody responses in antibody-mediated rejection following heart transplantation

Antibody-mediated rejection (AMR) driven by the development of donor-specific antibodies (DSA) directed against mismatched donor human leukocyte antigen (HLA) is a major risk factor for graft loss in cardiac transplantation. Recently, the relevance of non-HLA antibodies has become more prominent as AMR can be diagnosed in the absence of circulating DSA. Here, we assessed a single-center cohort of 64 orthotopic heart transplant recipients transplanted between 1994 and 2014. Serum collected from patients with ≥ pAMR1 (n = 43) and non-AMR (n = 21) were tested for reactivity against a panel of 44 non-HLA autoantigens. The AMR group had a significantly greater percentage of patients with elevated reactivity to autoantigens compared to non-AMR (P = .002) and healthy controls (n = 94, P < .0001). DSA-positive AMR patients exhibited greater reactivity to autoantigens compared to DSA-negative (P < .0001) and AMR patients with DSA and PRA > 10% were identified as the subgroup with significantly elevated responses. Reactivity to 4 antigens, vimentin, beta-tubulin, lamin A/C, and apolipoprotein L2, was significantly different between AMR and non-AMR patients. Moreover, increased reactivity to these antigens was associated with graft failure. These results suggest that antibodies to non-HLA are associated with DSA-positive AMR although their specific role in mediating allograft injury is not yet understood.

Impact of pretransplant antibodies on outcomes after heart transplantation

Purpose of review

Since the discovery of human leukocyte antigen (HLA) in the 1950s, there has been great interest in the role of antibodies in posttransplant rejection. The development of the lymphocyte toxicity test by Terasaki et al. in the 1960s was the first step toward understanding the role of antibodies in posttransplant rejection.

Recent findings

Subsequently, various organs have been transplanted and improving posttransplant outcomes have become a focus of research. In particular, methods to measure antibodies that affect posttransplant outcomes, including anti-HLA antibodies, and methods to desensitize patients from specific antibodies have been explored. One recent method for measuring antibodies is called the solid-phase assay, which uses purified HLA fixed to microbeads. This assay does not use donor lymphocytes and allows clinicians to test the reactivity of patient serum against a panel of antibodies. It has also enabled the identification of specific anti-HLA antibodies using a single HLA.

Summary

In addition to advances in methods to measure and analyze anti-HLA antibodies, the clinical impact of non-HLA antibodies has also received much attention recently.

A small allelic variant in donor class I MHC is sufficient to induce alloantibodies following transfusion of standard or pathogen-reduced platelets in mice

BACKGROUND AND OBJECTIVES

Alloimmunization targeting major histocompatibility (MHC) antigens is common following platelet transfusion. Pathogen reduction of platelets can block alloimmunization to MHC in mice and induce partial antigen-specific tolerance to subsequent transfusions. This study utilized small allelic variants to evaluate the relative contributions of class I and class II MHC to the alloresponse against untreated or pathogen-reduced platelets.

MATERIALS AND METHODS

C57BL/6 (B6) K^bm1 and B6 IA^bm12 mice with small variants in the class I K^b and class II IA^b alleles, respectively, were used as platelet donors for wild-type B6 recipients. Both untreated and pathogen-reduced platelet-rich plasma (PRP) transfusions were evaluated for immunogenicity by measuring antibody responses and ex vivo cytokine production.

RESULTS

Both the K^bm1 and IA^bm12 alleles induced antibody responses, though the response to K^bm1 was greater. Pathogen reduction blocked the antibody responses to IA^bm12 , but not to K^bm1 . Both the K^bm1 and IA^bm12 alleles primed ex vivo cytokine responses that were blocked with pathogen reduction, though responses to IA^bm12 were broader and larger (K^bm1 responses: IFN-γ, TNFα, and MIP-1β; IA^bm12 responses: IFN-γ, TNFα, IL-1β, IL-10, IL-13, and GM-CSF). Pathogen-reduced K^bm1 PRP did not appear to induce any tolerance to subsequent untreated K^bm1 PRP transfusions.

CONCLUSION

Minor allelic variants in both the class I and class II MHC are capable of inducing an alloresponse to transfusion. The K^bm1 PRP induced alloantibodies even with pathogen reduction and did not show signs of inducing the partial tolerance to subsequent transfusions observed with a larger MHC mismatch.

Anti-HLA Class II Antibodies Correlate with C-Reactive Protein Levels in Patients with Rheumatoid Arthritis Associated with Interstitial Lung Disease

The pathogenesis of Rheumatoid Arthritis (RA) is not fully understood, probably influenced by genetic and environmental factors. Interstitial Lung Disease (ILD) is an extra-articular manifestation of RA, which contributes significantly to morbidity and mortality. The identification of anti-HLA antibodies has been useful in the transplantation field; however, its contribution to autoimmune diseases as RA has not been fully studied. We aimed to determine the presence of anti-HLA antibodies in RA patients with and without ILD and its possible association with clinical and biochemical markers. One-hundred and forty-seven RA patients, of which 65 had ILD (RA-ILD group), were included. Sera samples for Anti-HLA Class II LABScreen panel-reactive antibodies (PRA) were analyzed. In both groups, women predominated, and lung function was worse in patients with ILD. The anti-CCP+ (UI/mL) was higher in the RA group in comparison to RA-ILD (p < 0.001). Expositional risk factors (tobacco smoking and biomass-burning smoke) were higher in RA-ILD patients. PRA+ was identified in ~25% RA-ILD patients, while ~29% in the RA group. The CRP levels have a positive correlation with the percentage of reactivity (%PRA, p = 0.02, r2 = 0.60) in the RA-ILD group. In conclusion, anti-HLA antibodies correlate with C-reactive protein levels in RA patients with ILD.

Pathogen reduction with riboflavin and ultraviolet light induces a quasi-apoptotic state in blood leukocytes

BACKGROUND

Alloimmunization to platelet-rich plasma (PRP) transfusions can cause adverse reactions such as platelet refractoriness or transplant rejection. Pathogen reduction treatment with ultraviolet light and riboflavin (UV + R) of allogeneic PRP was shown to reduce allogeneic antibody responses and confer partial antigen-specific immune tolerance to subsequent transfusions in mice. Studies have shown that UV + R was effective at both rapidly killing donor white blood cells (WBCs) and reducing their ability to stimulate an allogeneic response in vitro. However, the manner in which UV + R induces WBC death and its associated role in the immune response to treated PRP is unknown.

METHODS AND MATERIALS

This study evaluates whether UV + R causes WBC apoptosis by examining phosphatidylserine exposure on the plasma membrane, membrane asymmetry, caspase activity, and chromatin condensation by flow cytometry. The immunogenicity of WBCs killed with UV + R versus apoptotic or necrotic pathways was also examined in vivo.

RESULTS

WBCs after UV + R exhibited early apoptotic-like characteristics including phosphatidylserine exposure on the outer leaflet of the plasma membrane and loss of membrane asymmetry, but unlike canonical apoptotic cells, caspase activity and chromatin condensation were not apparent. However, in vivo studies demonstrated, unlike untreated or necrotic WBCs, both apoptotic WBCs and UV + R-treated WBCs failed to prime alloantibody responses to subsequent untreated transfusions.

CONCLUSION

Overall, the mechanism of WBC death following UV + R treatment shares some membrane characteristics of early apoptosis but is distinct from classic apoptosis. Despite these differences, UV + R-treated and apoptotic WBCs both offer some protection from alloimmunization.

Sensitization in Heart Transplantation: Emerging Knowledge: A Scientific Statement From the American Heart Association

Sensitization, defined as the presence of circulating antibodies, presents challenges for heart transplant recipients and physicians. When present, sensitization can limit a transplantation candidate’s access to organs, prolong wait time, and, in some cases, exclude the candidate from heart transplantation altogether. The management of sensitization is not yet standardized, and current therapies have not yielded consistent results. Although current strategies involve antibody suppression and removal with intravenous immunoglobulin, plasmapheresis, and antibody therapy, newer strategies with more specific targets are being investigated.

Allogeneic major histocompatibility complex antigens are necessary and sufficient for partial tolerance induced by transfusion of pathogen reduced platelets in mice

BACKGROUND AND OBJECTIVES

Alloimmunization is common following transfusion with platelet-rich plasma (PRP) and can cause complications such as platelet refractoriness or transplant rejection. It has previously been shown that pathogen reduction of PRP with riboflavin and UV light (UV+R) can protect against alloimmunization in mice and induce partial tolerance to subsequent transfusions.

MATERIALS AND METHODS

Using B6 H2^d congenic mice, this study evaluated the relative contributions of major histocompatibility complex (MHC) antigens and minor antigens to both the alloresponse to PRP transfusion and the partial tolerance induced by UV+R treatment.

RESULTS

Both total and MHC-specific alloantibody responses were highest when both MHC and minor antigens were mismatched, with lower alloantibody responses observed with MHC mismatch alone, demonstrating that allogeneic minor antigens can enhance the response to allogeneic MHC. There was a weak, but significant alloantibody response to minor antigens only. UV+R treatment protected against both major and minor antigen alloimmunization. Both allogeneic MHC and minor antigens primed an enhanced cytokine response ex vivo, though this was weaker with minor antigens, and both responses were blocked with UV+R treatment.

CONCLUSION

Allogeneic MHC is both necessary and sufficient to induce the partial tolerance associated with UV+R treatment.

Increased alloreactive and autoreactive antihuman leucocyte antigen antibodies associated with systemic lupus erythematosus and rheumatoid arthritis

Objectives

Rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) disproportionately affect women during and following childbearing years. Antihuman leucocyte antigen (HLA) alloantibody responses are common in healthy parous women, and as these diseases are both linked with HLA and immune dysregulation, we sought to evaluate anti-HLA antibodies in RA and SLE.

Methods

Anti-HLA antibodies were measured among parous SLE cases (n=224), parous RA cases (n=202) and healthy parous controls (n=239) and compared with each other as well as with nulliparous female and male controls. Antibody specificities were identified and compared against subject HLA types to determine autoreactivity versus alloreactivity. The association of anti-HLA antibodies with clinical outcomes was evaluated.

Results

Levels and frequencies of anti-HLA antibodies were significantly higher among parous females with SLE (52%) or RA (46%) compared with controls (26%), and anti-HLA antibodies were also found among nulliparous females and males with SLE and RA. Autoreactive anti-HLA antibodies were observed among SLE and RA antibody-positive subjects, but not healthy controls, with the highest frequency of autoreactive anti-HLA antibodies found in the SLE subjects. Higher levels of anti-HLA antibodies were associated with nephritis among the nulliparous SLE cases (p<0.01). The presence of anti-class I HLA antibodies was associated with younger age at diagnosis among both the RA and SLE nulliparous cases.

Conclusions

Both autoreactive and alloreactive anti-HLA antibodies were found at high levels in RA and SLE subjects. These occurred even in the absence of alloexposure, particularly among SLE subjects and may be linked with disease severity.

Donor-specific HLA-DQ antibodies may contribute to poor graft outcome after heart transplantation

BACKGROUND

HLA-DQ donor-specific antibodies (DSA) are implicated in allograft dysfunction after renal and lung transplantation. Limited data exists on the impact of HLA-DQ antibodies on heart transplant patients.

OBJECTIVE

To investigate the impact of DSA formation on allograft function and outcomes in heart transplant patients.

DESIGN

Retrospective cohort study.

SETTING

Collating post-transplantation patient data from computerized database in a tertiary hospital in Riyadh, Saudi Arabia from January 2006 to October 2014.

PATIENTS AND METHODS

We excluded recipients with positive preoperative complement-dependent-cytotoxicity crossmatch grafts and those with preformed DSA. Anti-HLA antibodies were identified using Luminex-based assay in sera collected before transplantation with a routine endomyocardial biopsy the first year and then annually.

MAIN OUTCOME MEASURES

Primary outcome measures were all-cause mortality, development of antibody mediated rejection, treated acute cellular rejection (ACR) and cardiac allograft vasculopathy (CAV).

SAMPLE SIZE

127 patients.

RESULTS

DSA formation occurred in 43/127 (34%), with 33/43 (77%) targeting HLA-DQ antigens alone (n=7) or in combination with -DR, -A or B antibodies (n=26). Most (76%) were male and the mean (SD) age was 36 (14) years. Ten patients developed -A, -B or -DR antibodies without -DQ antibodies also present. Treated ACR (P=.011), reduced left ventricular ejection fraction (P less than .001), CAV development (P=.003), and all-cause mortality (P=.01) were all significantly more prevalent in the DSA-positive cohort.

CONCLUSION

HLA-DQ donor-specific antibodies were the most common type detected and may play a significant role in poor outcomes post-cardiac transplantation. This emphasizes the importance of HLA-DQ matching and monitoring for DSA formation in order to minimize post-transplantation immunological risk.

LIMITATIONS

Retrospective design comes with inherent biases, results from single institute, with a particularly young cohort.

CONFLICT OF INTEREST

None.

Reduced MHC alloimmunization and partial tolerance protection with pathogen reduction of whole blood

BACKGROUND

Allogeneic blood transfusion can result in an immune response against major histocompatibility complex (MHC) antigens, potentially complicating future transfusions or transplants. We previously demonstrated that pathogen reduction of platelet-rich plasma (PRP) with riboflavin and ultraviolet light (UV+R) can prevent alloimmunization in mice. A similar pathogen-reduction treatment is currently under development for the treatment of whole blood using riboflavin and a higher dose of UV light. We sought to determine the effectiveness of this treatment in the prevention of alloimmunization.

STUDY DESIGN AND METHODS

BALB/cJ mice were transfused with untreated or UV+R-treated, allogeneic C57Bl/6J whole blood with or without leukoreduction. Mice were evaluated for donor-specific antibodies, ex vivo splenocyte cytokine responses, and changes in the frequency of regulatory T (T_reg ) cells.

RESULTS

UV+R treatment blocked cytokine priming and reduced anti-MHC alloantibody responses to transfused whole blood. Leukoreduction reduced alloantibody levels in both the untreated and UV+R-treated groups. Mice transfused with UV+R-treated whole blood had reduced alloantibody and cytokine responses when subsequently transfused with untreated blood from the same donor type. This reduction in responses was not associated with increased T_reg cells.

CONCLUSIONS

Pathogen reduction of whole blood with UV+R significantly reduces, but does not eliminate, the alloimmune response. Exposure to UV+R-treated whole blood transfusion does appear to induce tolerance to alloantigens, resulting in reduced anti-MHC alloantibody and cytokine responses to subsequent exposures to the same alloantigens. This tolerance does not appear to be driven by an increase in T_reg cells.

C1q-binding anti-HLA antibodies do not predict platelet transfusion failure in Trial to Reduce Alloimmunization to Platelets study participants

BACKGROUND

In the Trial to Reduce Alloimmunization to Platelets (TRAP) study, 101 of 530 subjects became clinically refractory (CR) to platelets (PLTs) without lymphocytotoxicity assay (LCA)-detectable anti-HLA antibodies. The LCA only detects complement-binding antibodies and is less sensitive than newer assays. Utilizing a more sensitive bead-based assay that does not distinguish between complement-binding versus non-complement-binding antibodies, we have previously shown that while many LCA-negative (LCA-) patients do have anti-HLA antibodies, these low- to moderate-level antibodies do not predict refractoriness. As complement can contribute to PLT rejection, we assessed if previously undetected complement-binding antibodies account for refractoriness among LCA- patients.

STUDY DESIGN AND METHODS

Samples from 169 LCA- (69 CR, 100 non-CR) and 20 LCA-positive (LCA+; 10 CR, 10 non-CR) subjects were selected from the TRAP study serum repository. Anti-Class I HLA immunoglobulin (Ig)G and C1q-binding antibodies were measured in serum or plasma with bead-based detection assays. Levels of C1q-binding antibodies were compared between CR and non-CR subjects and correlated with corrected count increments (CCIs).

RESULTS

While some of the LCA- subjects had detectable C1q-binding anti-Class I HLA antibodies, and some LCA+ subjects did not, levels were significantly higher among LCA+ subjects. C1q-binding anti-Class I HLA antibody levels did not differ significantly between CR and non-CR among either the LCA- or the LCA+ subjects. Furthermore, there was no significant correlation observed between CCIs and either C1q-binding or any anti-HLA IgG antibodies.

CONCLUSIONS

This work confirms that low- to moderate-level anti-Class I antibodies do not drive PLT rejection, suggesting a role for antibody-independent mechanisms.

Reduced alloimmunization in mice following repeated transfusion with pathogen-reduced platelets

BACKGROUND

Allogeneic transfusion can result in alloimmunization, leading to platelet (PLT) refractoriness and rejection of solid organ transplants. Previously we demonstrated that pathogen reduction using UV light and riboflavin (UV + R) eliminates the immunogenicity of white blood cells (WBCs) in vitro, blocks alloimmunization from transfusion in mice, and results in reduced ex vivo cytokine responses to subsequent untreated transfusions. We sought to determine if repeated transfusion with pathogen-reduced PLT-rich plasma (PRP) would eventually cause breakthrough alloimmunization or enhanced tolerance.

STUDY DESIGN AND METHODS

BALB/cJ mice were transfused weekly for 2, 4, or 8 weeks with C57Bl/6J PRP that was either untreated or pathogen reduced with UV + R and leukoreduced or not. Alloimmunization was determined by measuring donor antibody levels, ex vivo cytokine responses, and 24-hour donor PLT recovery. The role of donor antibodies in PLT refractoriness was also assessed by transfer of diluted immune sera into naïve recipients.

RESULTS

Donor antibody levels increased with the number of transfusions, but levels were significantly reduced using either UV + R or leukoreduction, and combining UV + R and leukoreduction gave the best protection. Priming of ex vivo cytokine responses required WBCs and remained suppressed with repeated UV + R-treated transfusion. PLT recovery was reduced with UV + R in naïve mice, and multiply transfused mice had poor PLT recovery even when antibody levels were relatively low. Approximately 1/100 dose of serum from a multiply transfused mouse was sufficient for complete rejection of donor PLTs.

CONCLUSIONS

Pathogen reduction significantly reduces alloimmunization in repeatedly transfused mice and combined with leukoreduction provides a high level of protection from alloimmunization.

Detecting the humoral alloimmune response: we need more than serum antibody screening

Whereas many techniques exist to detect HLA antibodies in the sera of immunized individuals, assays to detect and quantify HLA-specific B cells are only just emerging. The need for such assays is becoming clear, as in some patients, HLA-specific memory B cells have been shown to be present in the absence of the accompanying serum HLA antibodies. Because HLA-specific B cells in the peripheral blood of immunized individuals are present at only a very low frequency, assays with high sensitivity are required. In this review, we discuss the currently available methods to detect and/or quantify HLA-specific B cells, as well as their promises and limitations. We also discuss scenarios in which quantification of HLA-specific B cells may be of additional value, besides classical serum HLA antibody detection.

Antibody-mediated rejection as a contributor to previously unexplained early liver allograft loss

We analyzed 60 patients with idiopathic early allograft loss (defined as death or retransplantation at <90 days) to determine the relative contribution of preformed donor-specific human leukocyte antigen alloantibodies (DSAs) to this endpoint, and we defined strict criteria for the diagnosis of antibody-mediated rejection (AMR) in liver allografts. The inclusion criteria encompassed the availability of a pretransplant serum sample and both postreperfusion and follow-up tissue specimens for a blinded, retrospective re-review of histology and complement component 4d (C4d) staining. AMR was diagnosed on the basis of the presence of all 4 of the following strict criteria: (1) DSAs in serum, (2) histopathological evidence of diffuse microvascular injury/microvasculitis consistent with antibody-mediated injury, (3) diffuse C4d staining in the portal microvasculature with or without staining in the sinusoids or central veins in at least 1 sample, and (4) the exclusion of other causes of a similar type of injury. Patients thought to be experiencing definite AMR on the basis of routine histopathology alone showed the highest levels of DSA sensitization. Forty percent of patients with pretransplant DSAs with a pattern of bead saturation after serial dilutions developed AMR. Another multiparous female developed what appeared to be a strong recall response, which resulted in combined AMR and acute cellular rejection (ACR) causing graft failure. A contribution of DSAs to allograft failure could not be excluded for 3 additional patients who received marginal grafts. In conclusion, liver allograft recipients with preformed DSAs with a high mean fluorescence intensity despite dilution seem to be at risk for clinically significant allograft injury and possibly for loss from AMR, often in combination with ACR.

Ventricular-assist devices for the treatment of chronic heart failure

The role of ventricular-assist devices in the management of end-stage heart failure is growing. Initially developed as a 'bridge to transplantation', they are now implanted permanently in patients who need cardiac replacement but are not candidates for cardiac transplantation ('destination therapy'). Furthermore, observations from expert centers indicate that a significant proportion of patients under long-term mechanical assistance can be weaned from mechanical circulatory support after significant functional recovery of their native heart ('bridge to recovery'). This review discusses the emerging roles of mechanical circulatory support and their direct implications in clinical practice. Evolution of devices, important aspects of candidate selection, challenging issues in the management of ventricular-assist device patients (infection, device malfunction, anticoagulation-thromboembolic complications, psychosocial issues and cost) and ongoing research targeting sustained myocardial recovery are discussed.

Antibody depletion for the treatment of crossmatch-positive pediatric heart transplant recipients

Sensitization to HLA is a risk factor for adverse outcomes after heart transplantation. Requiring a negative prospective CM results in longer waiting times and increased waitlist mortality. We report outcomes in a cohort of sensitized children who underwent transplant despite a positive CDC CM+ using a protocol of antibody depletion at time of transplant, followed by serial IVIG administration. All patients <21 yrs old who underwent heart transplantation at Boston Children's Hospital from 1/1998 to 1/2011 were included. We compared freedom from allograft loss, allograft rejection, and serious infection between CM+ and CM- recipients. Of 134 patients in the cohort, 33 (25%) were sensitized prior to transplantation and 12 (9%) received a CM+ heart transplant. Serious infection in the first post-transplant year was more prevalent in the CM+ patients compared with CM- patients (50% vs. 16%; p = 0.005), as was HD-AMR (50% vs. 2%; p < 0.001). There was no difference in freedom from allograft loss or any rejection. At our center, children transplanted despite a positive CM had acceptable allograft survival and risk of any rejection, but a higher risk of HD-AMR and serious infection.

Sensitive solid-phase detection of donor-specific antibodies as an aid highly relevant to improving allograft outcomes

Transplant recipients who have had sensitizing events such as pregnancies, blood transfusions and previous transplants often develop antibodies directed against human leukocyte antigen (HLA)-molecules of the donor tissue. These pre-formed donor-specific antibodies (DSA) represent a high risk of organ failure as a consequence of antibody-mediated hyper-acute or acute allograft rejection. As a first assay to detect DSA, the complement-dependent lymphocytotoxicity assay (CDC) was established more than 40 years ago. However, this assay is characterized by several drawbacks such as a low sensitivity and a high susceptibility to various artificial factors generally not leading to valid and reliable outcomes under several circumstances that are reviewed in this article. Furthermore, only those antibodies that exert complement-fixing activity are detected. As a consequence, novel procedures that act independently of the complement system and that do not represent functional assays were generated in the format of solid phase assays (SPAs) (bead- or ELISA-based). In this article, we review the pros and cons of these sensitive SPA in comparison with the detection of DSA through the use of the traditional methods such as CDC and flow cytometric analyses. Potential drawbacks of the alternative methodological approaches comprising high background reactivity, susceptibility to environmental factors and the possible influence of subjective operators' errors concerning the interpretation of the results are summarized and critically discussed for each method. We provide a forecast on the future role of SPAs reliably excluding highly deleterious DSA, thus leading to an improved graft survival.

Platelet transfusions are not associated with increased morbidity or mortality in cardiac surgery

PURPOSE

To determine the independent relationship between leukoreduced platelet transfusions and adverse events in cardiac surgery.

METHODS

In this observational study, detailed baseline and perioperative data were prospectively collected on consecutive patients who underwent cardiac surgery at a single institution from 1999 to 2004. The independent associations of platelet transfusion with clinical outcomes (low output syndrome, myocardial infarction, stroke, renal failure, sepsis, and death) were determined by multivariable logistic regression analysis and propensity score case-control analysis.

RESULTS

Of the 11,459 patients analyzed, 2,174 (19%) received (leukoreduced) platelets - 1,408 received 5 U, 471 received 10 U, 140 received 15 U, and 155 received 20 or more units. Although all measured adverse event rates were higher in those who received platelets, in neither the logistic regression analyses nor the propensity score analyses was there any association between platelet transfusion and any of the adverse events.

CONCLUSIONS

Transfusion of leukoreduced platelets in cardiac surgery is not associated with adverse clinical outcomes when adjustments are made for important confounders.

Occurrence of Fatal and Nonfatal Adverse Outcomes after Heart Transplantation in Patients with Pretransplant Noncytotoxic HLA Antibodies

HLA antibodies (HLA ab) in transplant candidates have been associated with poor outcome. However, clinical relevance of noncytotoxic antibodies after heart transplant (HT) is controversial. By using a Luminex-based HLA screening, we retested pretransplant sera from HT recipients testing negative for cytotoxic HLA ab and for prospective crossmatch. Out of the 173 consecutive patients assayed (52 ± 13y; 16% females; 47% ischemic etiology), 32 (18%) showed pretransplant HLA ab, and 12 (7%) tested positive against both class I and class II HLA. Recipients with any HLA ab had poorer survival than those without (65 ± 9 versus 82 ± 3%; P = 0.02), accounting for a doubled independent mortality risk (P = 0.04). In addition, HLA-ab detection was associated with increased prevalence of early graft failure (35 versus 15%; P = 0.05) and late cellular rejection (29 versus 11%; P = 0.03). Of the subgroup of 37 patients suspected for antibody mediated rejection (AMR), the 9 with pretransplant HLA ab were more likely to display pathological AMR grade 2 (P = 0.04). By an inexpensive, luminex-based, HLA-screening assay, we were able to detect non-cytotoxic HLA ab predicting fatal and nonfatal adverse outcomes after heart transplant. Allocation strategies and desensitization protocols need to be developed and prospectively tested in these patients.

The use of the calculated panel-reactive antibody and virtual crossmatch in heart transplantation

PURPOSE OF REVIEW

Alloantibodies to human leukocyte antigens (HLAs) in patients awaiting heart transplantation are associated with prolonged wait time to transplant, increased risk of posttransplant rejection and cardiac allograft vasculopathy, and decreased survival. Solid-phase assays to determine antibody presence have allowed for the development of a calculated panel-reactive antibody to denote unacceptable antigens. The virtual crossmatch allows for the comparison of recipient HLA antibodies to prospective donor HLA antigens to safely match a patient to an appropriate donor without a prospective crossmatch.

RECENT FINDINGS

Expansion of the donor pool and decreased waiting time for heart transplant may be impacted by further assessment of the functional status of alloantibodies and novel means for desensitization. Sensitized patients who receive left ventricular assist device (LVAD) as a bridge to cardiac transplant appear to have similar postoperative 1-year outcomes compared with nonsensitized patients.

SUMMARY

Antibody sensitization poses an additional hurdle to patients awaiting heart transplantation. Functional antibody assessment, placement of a LVAD as bridge to cardiac transplant, and novel means of desensitization may impact a sensitized patient's ability to safely undergo heart transplantation.

Immunologic considerations in heart transplantation for congenital heart disease

Children and adults with congenital heart disease (CHD) can require interventions that result in immunologic alterations that are different than those seen in patients with cardiomyopathies. Patients with CHD can be exposed to heart surgeries, blood products, valved and non-valved allograft tissue, and mechanical circulatory support, all of which can alter the immunologic status of these patients. This change in immunologic status is most commonly manifested as the development of anti-human leukocyte antigen (HLA) antibodies. This review will delineate a) the causes of anti-HLA anti-body production (often referred to as allosensitization); b) preventive strategies for anti-HLA antibody production before transplantation; c) treatment strategies for those patients who develop anti-HLA antibodies before transplantation; d) consequences of HLA allosensitization after transplantation; and e) treatment of HLA allosensitization and antibody-mediated rejection after transplantation.

Cardiac antibody-mediated rejection

Many factors limit short- and long-term survival after pediatric heart transplantation. Historically, attention had been directed toward T-cell responses and acute cellular rejection. Presence of pretransplant antibodies against HLA is associated with increased donor wait times and poor post-transplant outcomes. Therapies aimed to mitigate circulating antibodies include plasmapheresis, protein A immunoadsorption columns, intravenous immune globulin, rituximab, and bortezomib. The negative effects of B cells, HLA antibodies, and AMR and potential interventions are the focus of this review article.

Plasma Exchange and Immunoadsorption of Patients with Thoracic Organ Transplantation

Primary organ failure after transplantation (TX) remains a serious complication and leads to a high percentage of lethality. It is known, however, that the speed of rejection and tissue destruction depends on 3 main factors: antibody titer, the ability of the tissue to repair itself, and immunosuppressive measures. Especially with evidence for antibodies against human leukocyte antigen (HLA-ab), the immunological risk of persistent and acute episodes of rejection increases. The role of non-HLA-ab in rejection episodes is often underestimated and should be studied further. Antibody-mediated rejection (AMR) is still an unsolved problem in thoracic organ TX. An essential pillar of antihumoral therapy are the extracorporeal procedures like plasmapheresis (PP), therapeutic plasma exchange (TPE), and immunoadsorption (IA), because only they have the ability to remove preformed or de novo developed antibodies quickly and effectively. The quick removal of antibodies and other plasma factors through TPE or IA remains an effective and supportive method for treating AMR and allows the TX despite preformed antibodies. The pertinent literature does not disclose, however, how often and for how long treatment should be administered. It is known, that repeated treatment cycles with adequately processed plasma volume must be used to overcome redistribution of pathological antibodies. Based on our experience in heart transplant recipients with compromised graft function due to non-HLA-ab and HLA-ab, IA seems to be more effective.

Heart transplantation in adults with end-stage congenital heart disease

Residual abnormalities in cardiac structure and function predispose adults with congenital heart disease to late-onset heart failure and its complications. Evaluation of this population requires collaboration between adult congenital and heart failure specialists. In addition to assessing heart transplant eligibility, clinicians must balance the risks of premature listing against progressive heart failure and increased waiting list mortality. Following heart transplantation, adults with congenital heart disease have higher mortality due to an increased risk of bleeding, infection and donor right heart failure secondary to pulmonary hypertension. Concerns relating to increased early mortality should be balanced against superior long-term survival in adult congenital heart disease patients surviving beyond the first year after heart transplantation.

How to deal with presensitized candidates for heart transplantation?

PURPOSE OF REVIEW

With increasing utilization of assist devices and adoption of calculated panel-reactive antibody (cPRA), the number of presensitized patients being listed for heart transplantation is increasing. An effort to standardize identification and management of such patients is paramount and recently initiated in the heart transplant community. The current review describes the basic concepts of presensitization and details the most relevant work including the latest advancements in this area.

RECENT FINDINGS

More sensitive techniques in identifying presensitized patients have posed challenges in understanding the clinical relevance and implications of such testing. cPRA has been shown to benefit presensitized heart transplant patients. De-sensitization strategies have never been studied in a large clinical trial setting but a combination of plasmapheresis and intravenous immunoglobulin has been shown to be beneficial in small studies. Long-term positive outcomes of de-sensitization have been recently reported. Newer agents like alemtuzumab, bortezomib and complement inhibitors have been reported in case reports and series with promising results as de-sensitization strategy.

SUMMARY

Data specific to strategies and therapies in heart transplantation are sparse and most knowledge stems from other organ transplantation. Consensus efforts to standardize care and also advance research in this area were initiated recently with hope for improving care for these patients.

HLA antibodies in pediatric heart transplantation

We have analyzed the impact of anti-HLA antibodies present in the patients' circulation prior and/or following heart transplantation in a population of 108 pediatric recipients. Anti-HLA class I and class II antibodies were monitored by traditional CDC using donor and panel T and B lymphocytes and by SPA for detection of DSA. There was a highly significant correlation between the development of AMR and presence of CDC- or SPA-detected DSA. However, the fraction of the transplant population which remained AMR-free was much higher among patients with SPA-detected compared to CDC-detected DSA. Furthermore, long-term graft survival was negatively affected only by cytotoxic, complement-fixing anti-HLA class I antibodies developing following transplantation. Anti-HLA class I or class II antibodies detected by SPA had no effect on long-term survival rates.

Pre- and posttransplantation allosensitization in heart allograft recipients: major impact of de novo alloantibody production on allograft survival

The involvement of humoral response in allograft rejection has been suggested by both immunologic and histochemistry studies. In the present study, we explored the role of alloantibodies in a large cohort of heart allograft recipients followed for 15 years. Sequential samples of sera were obtained from 950 recipients of heart allografts before and after transplantation at the time when protocol endomyocardial biopsies were performed. The presence of anti-human leukocyte antigen (HLA) antibodies was investigated using complement mediated cytotoxicity and solid phase assay (SPA). Our data reveal an inverse correlation between the development of alloantibodies after transplantation and heart allograft survival. The 15-year graft survival was highest in patients who never developed alloantibodies (70%) or who displayed them only before transplantation (71%); graft survival in recipients who showed antibodies both before and after transplantation (56%), or only after transplantation (47%), was lower. The deleterious effect of antibodies on graft survival started 8 years after transplantation, suggesting that the production of de novo antibodies may have been triggered by some later event. We found that patients with de novo antibodies appearing more than 1 year after transplantation had the poorest survival. Furthermore, the development of de novo antibodies was preceded in 76% of these patients by cellular rejection grade 3 or higher, according to the International Society for Heart Transplantation (ISHT) grading criteria. Development of antibody-mediated rejection (AMR) had a significant negative impact on graft survival (16% in AMR(+) vs 63% in AMR(-) patients, p = 0.0008). Of the 23 patients with AMR, 21 displayed cytotoxic donor-specific antibodies (DSA) at the time of diagnosis, and in 18 of these cases SPA showed that they were directed against the donors' HLA. The data demonstrate that the detection of alloantibodies permits a better definition of AMR in heart allograft recipients. Identification of patients at risk for developing AMR is of great importance for early treatment of rejection episodes.

The long-term outcome of treated sensitized patients who undergo heart transplantation

BACKGROUND

Sensitized patients prior to heart transplantation are reportedly at risk for hyperacute rejection and for poor outcome after heart transplantation. It is not known whether the reduction of circulating antibodies pre-transplant alters post-transplant outcome.

METHODS AND RESULTS

Between July 1993 and July 2003, we reviewed 523 heart transplant patients of which 95 had pre-transplant panel reactive antibody (PRAs) >10%; 21/95 were treated pre-transplant for circulating antibodies. These 21 patients had PRAs > 10% (majority 50-100%) and were treated with combination therapy including plasmapheresis, intravenous gamma globulin and rituximab to reduce antibody counts. The 74 untreated patients with PRAs > 10% (untreated sensitized group) and those patients with PRAs < 10% (control group) were used for comparison. Routine post-transplant immunosuppression included triple-drug therapy. After desensitization therapy, circulating antibody levels pre-transplant decreased from a mean of 70.5 to 30.2%, which resulted in a negative prospective donor-specific crossmatch and successful heart transplantation. Compared to the untreated sensitized group and the control group, the treated sensitized group had similar five-yr survival (81.1% and 75.7% vs. 71.4%, respectively, p = 0.523) and freedom from cardiac allograft vasculopathy (74.3% and 72.7% vs. 76.2%, respectively, p = 0.850).

CONCLUSION

Treatment of sensitized patients pre-transplant appears to result in acceptable long-term outcome after heart transplantation.

Novel solid phase-based ELISA assays contribute to an improved detection of anti-HLA antibodies and to an increased reliability of pre- and post-transplant crossmatching

Antibodies directed against HLA antigens of a given organ donor represent the dominating reason for hyper-acute or acute allograft rejections. In order to select recipients without donor-specific antibodies, a standard crossmatch (CM) procedure, the complement-dependent cytotoxicity assay (CDC), was developed. This functional assay strongly depends on the availability of isolated vital lymphocytes of a given donor. However, the requirements of the donor’s material may often not be fulfilled, so that the detection of the antibodies directed against HLA molecules is either impaired or becomes completely impossible. To circumvent the disadvantages of the CDC procedure, enzyme-linked immunosorbent assay (ELISA)-based and other solid phase-based ELISA-related techniques have been designed to reliably detect anti-HLA antibodies in recipients. Due to the obvious advantages of these novel technologies, when compared with the classical CDC assay, there is an urgent need to implement them as complementary methods or even as a substitution for the conventional CDC crossmatch that is currently being applied by all tissue typing laboratories.

B cells help alloreactive T cells differentiate into memory T cells

B cells are recognized as effector cells in allograft rejection that are dependent upon T cell help to produce alloantibodies causing graft injury. It is not known if B cells can also help T cells differentiate into memory cells in the alloimmune response. We found that in B-cell-deficient hosts, differentiation of alloreactive T cells into effectors was intact whereas their development into memory T cells was impaired. To test if B cell help for T cells was required for their continued differentiation into memory T cells, activated T cells were sorted from alloimmunized mice and transferred either with or without B cells into naïve adoptive hosts. Activated T cells cotransferred with B cells gave rise to more memory T cells than those transferred without B cells and upon recall, mediated accelerated rejection of skin allografts. Cotransfer of B cells led to increased memory T cells by enhancing activated CD4 T-cell proliferation and activated CD8 T-cell survival. These results indicate that B cells help alloreactive T-cell differentiation, proliferation and survival to generate optimal numbers of functional memory T cells.

Monitoring the operationally tolerant liver allograft recipient

PURPOSE OF REVIEW

A fundamental goal in transplantation is the establishment of allograft function without ongoing immunosuppression. Robust allograft tolerance has been established in experimental transplantation models, whereas clinical operational tolerance has been described most frequently following human liver transplantation.

RECENT FINDINGS

Clinical assessment of tolerance has been limited to laboratory evaluation of organ function. Additional tools include graft monitoring through biopsy and blood sampling for biomarker analysis. Current biomarkers under assessment in recent years include dendritic cell subsets, regulatory T cells, antidonor antibodies, and gene polymorphisms. Emerging microarray analysis that is being prospectively validated will also be reviewed. A further tool in the characterization of the tolerant patient will be the accurate enrollment of such patients into a multicenter registry that will prospectively follow the natural history of the patient withdrawn from immunosuppression and help facilitate the entry of interested patients to mechanistic and immune monitoring trials. The International Solid Organ Transplant Tolerance Registry (www.transplant-tolerance.org) will be briefly described.

SUMMARY

Effective biomarker characterization of the operationally tolerant liver allograft recipient would allow earlier, well tolerated, prospective drug withdrawal with the goal of extending the potential benefits of drug minimization to an increasing number of patients in a more predictable fashion.

Decline in rejection in the first year after pediatric cardiac transplantation: a multi-institutional study

BACKGROUND

Rejection is a major cause of morbidity and mortality after pediatric heart transplantation (HTx). Survival after pediatric HTx has improved over time, but whether there has been an era-related improvement in the occurrence of allograft rejection is unknown.

METHODS

The Pediatric Heart Transplant Study (PHTS) database was queried for patients who underwent HTx from January 1993 to December 2005 to determine the incidence of rejection and identify factors associated with the first episode of rejection in the first year after HTx.

RESULTS

Data were reviewed in 1,852 patients from 36 centers. The incidence of rejection declined over 13 years at a rate of -2.58 +/- 0.41 (p < 0.001) from approximately 60% to 40% (p < 0.001). The mean number of episodes of rejection also significantly fell at a rate of -0.05 +/- 0.01 per patient/year from 1.19 to 0.66 (p < 0.001). The incidence of rejection with hemodynamic compromise and death from rejection did not change. Multivariate analysis for the risk of a first rejection episode demonstrated decreased risk of rejection with later year of HTx (odds ratio [OR], 0.88; 95% confidence interval [CI], 0.85-0.91; p < 0.001) and use of mechanical support (OR, 0.65; 95% CI, 0.42-0.99; p = 0.046). Increased risk of rejection was associated with positive donor-specific crossmatch (OR, 1.85; 95% CI, 1.18-2.88; p = 0.007) and older recipient age (OR, 1.05; 95% CI, 1.02-1.07; p < 0.001).

CONCLUSIONS

Although the overall incidence and prevalence of rejection has substantially decreased over time in pediatric HTx recipients in the first year after HTx, the rate of rejection with hemodynamic compromise or death from rejection remains unchanged.