Heart Transplantation Rejection

Etiology

HTR encompasses various etiologies depending on the timing and nature of rejection episodes. Early graft dysfunction, occurring within 24 hours posttransplant, can be categorized as primary or secondary:

• Primary graft dysfunction lacks a universally accepted definition. Still, it is typically characterized by severe ventricular dysfunction leading to cardiogenic shock requiring circulatory support, often without an obvious immune response from the recipient. Factors contributing to primary graft dysfunction include preexisting donor heart disease, reperfusion injury immediately after transplantation, and allograft injury during organ procurement, preservation, and implantation.[2]
• Secondary graft dysfunction arises from identifiable causes such as hyperacute graft rejection, increased pressure or volume load on the right ventricle, or undiagnosed recipient pulmonary hypertension.

Acute allograft rejection can manifest as cellular or antibody-mediated (humoral) rejection, influenced by risk factors like younger age, women donors or recipients, and higher human leukocyte antigen (HLA) mismatches.[3][4] Cardiac allograft vasculopathy presents as a chronic form of rejection associated with factors like elevated cholesterol levels, cytomegalovirus infection, insulin resistance, coronary artery disease in the donor or younger recipient, and a history of acute rejection. Recurrence of preexisting myocardial conditions or the development of new diseases like amyloidosis, sarcoidosis, giant cell myocarditis, hereditary hemochromatosis, and cardiac malignancies further contribute to allograft failure in HTx recipients. 

Epidemiology

HTx has progressed from a rudimentary experimental procedure to a standard treatment option for end-stage heart disease. A recent review focuses on the outcomes of HTx, particularly in the context of the new allocation system and the expansion of the donor pool to include donors with hepatitis C virus, donors after circulatory death, and other high-risk donors. This review also addresses desensitization and provides an update on posttransplant outcomes, particularly concerning primary graft dysfunction, acute cellular rejection, antibody-mediated rejection, cardiac allograft vasculopathy, and posttransplant complications such as malignancy and renal dysfunction.[5]

In October 2018, the United Network for Organ Sharing (UNOS) updated the donor heart allocation system, transitioning from a 3-tier to a 6-tier system. This change reduced the median waitlist time from 93 days in 2017 to 41 days in 2019. Additionally, the median distance traveled for organ procurement increased from 83 miles to 225 miles, which correspondingly led to an increase in donor ischemic time from a median of 3 hours to 3.5 hours.[6]

Regarding recipient factors, approximately 92% of HTx recipients at each center have a body mass index (BMI) of less than 35 kg/m². Higher recipient BMI is linked to poorer short- and long-term survival after HTx. For recipients with a BMI of 30 kg/m² or higher, donor hearts undersized to a predicted heart mass of 20% or less do not adversely impact short- and long-term survival after HTx. However, in clinical practice, undersized donor hearts are less frequently used for obese candidates.[7] While HTx is feasible for recipients over 70, with survival rates comparable to younger recipients, careful selection is crucial for these older candidates. Frailty within 6 months before HTx is associated with worse posttransplant survival.[8]

The heart donor sex distribution of 70% men and 30% women remained consistent from 1992 to 2018. While the overall effect of female donor gender alone does not seem to impact post-HTx survival, meta-analyses, and registry reports negatively indicate lower survival rates in gender-mismatched transplants, particularly for male recipients receiving female donor hearts.[9] The median age and BMI of donors have been increasing over time. Donors aged 55 and older are associated with reduced survival, with hazard ratios of 1.43 and 1.51, respectively.[10] The diabetic status of donors, whether insulin-dependent or not, does not affect recipient survival post-HTx.[11] Study results from single centers have shown that recipients carefully selected to receive allografts from donors with moderate coronary artery disease or coronary artery calcification can have similar short- and long-term survival rates and freedom from cardiac adverse events compared to other recipients.[11] However, longer donor ischemic times, especially those lasting 4 to 5 hours or more, are associated with higher short- and long-term mortality rates after HTx.[12]

According to ISHLT, 5074 HTxs were performed in 2015. Median survival for cardiac transplants performed between 1982 and June 2015 was 10.7 years for adult recipients and 16.1 years for pediatric recipients. Survival rates in adults posttransplant were 94.8%, 84.1%, and 72.3% after 1, 5, and 10 years, respectively. The rates of HTR have steadily declined with the use of immunosuppressive therapy. HTR rates after discharge to 1 year of follow-up have dropped from 30.5% in 2004 through 2006 to 24.1% in 2010 through 2015.[13]

The incidence of primary graft dysfunction varies from 20% to 40%. Results from a 6-year follow-up study published in 2018 reported a primary graft dysfunction incidence of 31% in posttransplant patients.[14] A similar study published in 2011 disclosed a primary graft dysfunction incidence rate of 23%.[15] Deaths due to acute allograft rejection reach up to 11% in the first 3 years after transplant. Nearly half of HTx recipients developing rejection after 7 years of transplantation have evidence of antibody-mediated rejection. The overall prevalence of cardiac allograft vasculopathy increases with time. Cardiac allograft vasculopathy is the leading cause of death between 1 and 3 years after transplantation and accounts for 17% of deaths after 3 years.[16][17]

Pathophysiology

Primary Graft Dysfunction

The "ischemic time," referring to the duration from clamping the donor heart to its implantation in the recipient, exposes it to various stresses. This period is critical as it impacts primary graft dysfunction, mainly influenced by the donor's age. The donor body experiences catecholamine stress during the time of brain death. This catecholamine surge increases oxygen demand, subsequently causing myocardial ischemia and desensitization of the myocardial beta-receptor transduction system, activating multiple proinflammatory mediators.[1] Hypothermic storage before implantation slows down the metabolic activity of allograft. Prolonged storage can lead to loss of normal aerobic metabolism, resulting in an anaerobic switch and lactic acidosis. Further, allograft reperfusion enhances calcium overload at the time of implantation, contributing to myocardial stunning.

Secondary Graft Dysfunction

This type of hyperacute transplant rejection results from either ABO blood type incompatibility or from preformed cytotoxic antibodies targeting major histocompatibility complex (MHC) antigens on the allograft.

Acute Allograft Rejection 

This type of rejection takes place by cellular or humoral (antibody-mediated) mechanisms:

• Acute cellular rejection
  • Major and minor histocompatibility antigens are not expressed equally among all individuals; this increases the potential of such proteins to act as alloantigens and activate alloimmunity by stimulating cytotoxic T cells. T cells respond to these donor antigens either directly or indirectly based on the method of antigen presentation. T cells can directly recognize donor MHC molecules on allograft or target when presented indirectly by recipient antigen-presenting cells (APC).[18] Interleukin-2 (IL-2), tumor necrosis factor-beta (TNF-beta), and interferon-gamma (IFN-gamma) all act as significant mediators during rejection.
• Acute humoral rejection 
  • Antibody-mediated humoral rejection is poorly understood. The antibody reacts to donor MHC antigens (HLA-I and II), leading to capillary endothelial changes. The deposition of immunoglobulin and complements within the myocardial capillary bed are detectable by immunofluorescence.[19]

Cardiac Allograft Vasculopathy

Endothelial damage in cardiac allograft vasculopathy can be immune- or nonimmune-mediated. Endothelial damage leads to mild intimal thickening before progressing to diffuse fibrous thickening of the intima.[20] More recent research has established the role of effector B-cells with cardiac allograft vasculopathy.[21]

Histopathology

Acute cellular rejection is a mononuclear inflammatory response infiltrating myocardial tissue with predominant lymphocytic cells. Immunohistologic assessment can confirm the presence of cluster of differentiation 4 (CD4) and CD8 positive T lymphocytes with high affinity to IL-2 receptors. Increased intercellular adhesion molecules with high MHC-II expression on cardiac myocytes are present. These findings should be distinguished from the Quilty effect, which carries no clinical significance. Quilty lesions extend to the endocardial surface and include significant B-lymphocytes, distinguishing them from acute cellular rejection.

Antibody-mediated rejection leads to intravascular macrophage accumulation with interstitial edema, hemorrhage, and neutrophilic infiltration in and around capillaries. The predominant feature of cardiac allograft vasculopathy is a diffuse, progressive thickening of the arterial intima that develops in the transplanted heart's epicardial and intramyocardial arteries.[1]

Histological Grading

ISHLT acute cellular rejection grading:

• Grade 0: No rejection
• Grade 1 R, mild: Interstitial and/or perivascular infiltrate with up to 1 focus of myocyte damage (grade 1A, 1B, and 2 in the 1990 system)
• Grade 2 R, moderate: 2 or more foci of infiltrates with associated myocyte damage (grade 3A in 1990 system)
• Grade 3 R, severe: Diffuse infiltrate with multifocal myocyte damage, with or without edema, hemorrhage, or vasculitis (grade 3B and 4 in the 1990 system)

Immunopathologic findings for acute antibody-mediated rejection include positive immunofluorescent staining for C4d, C3d, and antiHLA-DR or immunoperoxidase staining for C4d and CD68 (or C3d).

Antibody-mediated rejection grading:

• Grade 0: Negative histologic and immunopathologic findings
• Grade 1: Presence of positive histologic and immunopathologic findings
• Grade 2: Presence of both histologic and immunopathologic findings
• Grade 3: Presence of severe histologic plus immunopathologic findings

History and Physical

Patients with a history of HTx require comprehensive evaluations, including thorough history taking and physical examinations. Reviewing medication histories and ensuring adherence to immunosuppressant therapies during patient intake is crucial. New onset of ventricular dysfunction, whether systolic, diastolic, or mixed, should prompt consideration of transplant rejection, with timing providing important diagnostic clues. Common symptoms indicating rejection include orthopnea, shortness of breath, paroxysmal nocturnal dyspnea, syncope, palpitations, nausea, weight gain, edema, arrhythmias like atrial flutter, oliguria, and hypotension. Physical exams may reveal signs such as elevated jugular venous pressure, additional heart sounds on auscultation, and peripheral edema, all of which can be indicative of heart failure.

Evaluation

When signs of heart failure are noted in an HTx patient, suggesting possible rejection, the workup typically involves several diagnostic steps to assess the function and condition of the transplanted heart. Here’s an outline of the workup process:

• Laboratory tests
  • Cardiac biomarkers
    • Assesses for myocardial injury (eg, troponin)
  • Complete blood count
    • Evaluate for signs of infection
  • Complete metabolic panel
    • Evaluate possible electrolyte disturbances, kidney dysfunction, and liver failure
  • Serologic testing
    • Assesses for specific antibodies (eg, anti-HLA antibodies) associated with rejection
• Imaging studies
  • Echocardiography
    • Assesses cardiac function, including systolic and diastolic parameters, and detects any structural abnormalities or fluid around the heart
  • Cardiac magnetic resonance imaging (MRI)
    • Particularly useful for assessing myocardial tissue characteristics, detecting myocardial edema (T2-weighted imaging), and evaluating for structural abnormalities
  • Radionuclide imaging
    • Assess myocardial perfusion or inflammation
• Endomyocardial biopsy (EMB)
  • Considered the gold standard for diagnosing rejection
  • Typically performed via right heart catheterization
  • Histologic assessment of myocardial tissue for signs of cellular or antibody-mediated rejection

In routine patient follow-up after HTx, HTR is commonly diagnosed through surveillance EMBs. These biopsies are typically performed weekly for the first 4 weeks, biweekly for the next 6 weeks, monthly for 3 to 4 months, and then every 3 months until the first year. After the first year, routine biopsies have not shown added benefits.[22] Chi NH et al suggest performing biopsies at 3 years due to the low rejection rate.[23]

Diagnosis of HTR is primarily based on histologic findings from biopsies. However, up to 20% of cases may show no abnormalities on biopsy, known as biopsy-negative rejection. Noninvasive monitoring methods such as troponin measurement, Doppler echocardiography, cardiac MRI, radiolabeled lymphocyte imaging, antimyosin antibodies, or annexin-V are useful in these cases.[24][25][26] T2-weighted cardiac MRI has proven effective in detecting early myocardial edema.[27] Gene expression profiling has also emerged as a viable alternative to EMB, showing non-inferior and safe results, as demonstrated by the E-IMAGE study.[28] Antibody-mediated rejection typically presents with histologic findings and serum antibodies against HLA class I and II antigens. Investigating nonHLA antibodies like antiendothelial, antivimentin, and antiMICA/MICB is also crucial when HLA antibodies are absent.

In organ transplantation, biopsy remains the gold standard for diagnosing rejection; however, its invasive nature limits its routine and repeatable use in clinical settings. Targeted ultrasound imaging offers a practical alternative for physicians, allowing them to assess key target expressions in the allograft with a comprehensive overview. Contrast-enhanced ultrasound is a well-tolerated, noninvasive imaging technique. The most commonly used ultrasound contrast agents are microbubbles, which are stabilized by a shell composed of lipids, proteins, or polymers. Additionally, specific disease markers can be detected and quantified by labeling these microbubbles with target materials. Various intravascular targets have been identified for microbubble targeting, including integrins, selectins, and cell adhesion molecules.[29]

During the first 5 years, cardiac allograft vasculopathy surveillance is performed with annual coronary angiography in patients with normal kidney functions. Annual dobutamine stress echocardiography for patients with significant kidney disease is necessary. After 5 years, yearly dobutamine stress echocardiography with or without coronary angiography should be done based on the risk status of the recipient. Intravascular ultrasound should be performed when angiographic evidence is insufficient. Compared to coronary angiography, coronary CT angiography has offered a safer and equally accurate diagnostic approach for cardiac allograft vasculopathy.[30]

Treatment / Management

Treatment with immunosuppressant therapy posttransplant has significantly reduced rates of rejection. Immunosuppressive therapy usually consists of steroids, antiproliferative therapy such as cyclosporine, sirolimus/tacrolimus, and antimetabolites like azathioprine, mycophenolate mofetil, and rapamycin. The thirty-fourth HTx consensus from the ISHLT registry reported a lower rejection rate when treated with tacrolimus-based immunosuppression compared to recipients receiving cyclosporine. Treatment strategy depends on the type of rejection. Recent ISHLT guidelines continue the previous immunosuppression guidelines.[1]

Primary Graft Dysfunction

Primary graft dysfunction is treated with high-dose inotropes to improve ventricular function. An escalation algorithm can be employed to manage primary graft dysfunction. This approach begins with the initiation of inotropic support, followed by temporary mechanical circulatory support, which may include the placement of a ventricular assist device and, ultimately, retransplantation as a last resort.[5] Hyperacute rejection is treatable with plasmapheresis, corticosteroids, and intravenous immunoglobulin.(B3)

Acute Cellular Rejection

The treatment strategy usually involves oral or intravenous steroids, antithymocyte globulin (ATG), and murine monoclonal antibody (OKT3). Steroids inhibit the production of IL-1, IL-2, IL-6, TNF-alpha, and IFN-gamma. ATG prepared from immunized rabbits or horses causes cell death by complement-mediated lysis. OKT3 is a murine monoclonal antibody that inhibits T-cell function by binding to CD3 antigen. Selection among these options is dictated based on the hemodynamic status of the recipient and the histologic severity of rejection.

Hemodynamic compromise is defined by the presence of 1 or more of the following:

• Reduction in cardiac output (less than 4.0 L/min) or cardiac index (less than 2.0 L/min per m²)
• A decrease in pulmonary artery saturation (less than 50%)
• Elevation in the pulmonary artery to capillary wedge pressure (PCWP)

Histology-based treatment for acute cellular rejection:

• Recipients with grade 1R rejection (grade 1A,1B, and 2 in the 1990 system) do not require treatment unless hemodynamically compromised. Low-dose steroids are helpful in such cases. For patients with hemodynamic compromise, pulse-dose steroids orally or intravenously have shown a significant response.
• Grade 2R rejection (grade 3A in the 1990 system) is treated the same way as grade 1R rejection with hemodynamic compromise. An oral pulse steroid (3-5 mg/kg for 3-5 days) or 500 to 1000 mg/day of intravenous methylprednisolone can be used.[31] Repeat biopsies are obtained weekly for 2 weeks to verify resolution. A repeat pulse dose steroid can be attempted in the event of persistent rejection.
• Grade 2R rejection with hemodynamic compromise, grade 3R rejection, and steroid-resistant rejection episodes are treated with either ATG or OKT3 antibodies. The usual dose of OKT3 is 5 mg/day intravenously for 10 to 14 days. Cyclosporine and mycophenolate are continued at pretreatment doses if therapeutic levels have been achieved. Other options include switching immunosuppressive therapy from cyclosporine to tacrolimus. Recipients treated with OKT3 antibodies should have levels of CD3-positive cells checked before and 3 to 5 days after the initiation of therapy.

Antibiotic, antifungal, and antiviral prophylaxis are conventional adjunct therapies for patients treated with high-dose steroids or anti-lymphocyte therapy.

Antibody-Mediated Rejection

Antibody-mediated rejection is hemodynamically more severe compared to acute cellular rejection and has an association with a worse prognosis. Improved outcomes with plasmapheresis in combination with corticosteroids and ATG or OKT3 antibodies have undergone study. Treatment with the CD20 monoclonal antibody rituximab has shown some promise. Recurrent or resistant rejection despite 2 to 3 courses of OKT3 or ATG requires alternative approaches. These include photopheresis, total lymphoid irradiation, and immunosuppressive regimen changes.

Differential Diagnosis

The differential diagnoses for HTR include:

• Primary graft dysfunction
• Secondary graft dysfunction
• Acute allograft rejection
• Cardiac allograft vasculopathy
• Amyloidosis
• Sarcoidosis
• Giant cell myocarditis
• Hereditary hemochromatosis
• Lymphoproliferative disorders such as non-Hodgkin lymphoma

Prognosis

After HTx, the 1-year survival rate is nearly 90%, reflecting significant improvement in median survival rates. Patients requiring extracorporeal membrane oxygenation support before HTx generally have poorer prognoses. Acute allograft rejection accounts for 10% of deaths within the first 3 years, while cardiac allograft vasculopathy incidence rises steadily post-transplant. Malignancy becomes the leading cause of mortality from 5 years post-HTx. Approximately 2% to 4% of HTx recipients undergo repeat retransplantation, with generally poorer outcomes than initial HTx.

Complications

The complications that can manifest with HTR are as follows: 

• Repeated EMB can cause tricuspid valve regurgitation [32]
• Graft failure
• Atrial arrhythmia
• Lymphoproliferative malignancy
• Cardiac allograft vasculopathy
• Acute graft rejection
• Increased risk of secondary infections
• Serum sickness due to ATG
• Acute myocardial infarction
• Repeat HTx
• Death