Heart Transplant Rejection: How the Immune System Responds

Heart transplant rejection happens when the recipient’s immune system recognizes the donor heart as foreign and attacks it. Despite decades of progress in immunosuppressive drugs, rejection remains one of the most significant threats to graft survival and is a leading cause of death in the first year after transplant. The biology behind rejection is more complex than a simple on-off switch, though, involving different immune pathways that can damage the heart in distinct ways and on different timelines.

How the Immune System Turns on a Donor Heart

Your immune system is built to distinguish “self” from “not self.” A transplanted heart carries proteins on its cell surfaces that differ from yours, and those differences act like a red flag. White blood cells, particularly T cells, recognize these foreign surface proteins and mount an attack. Research in animal models has shown that CD4 T cells can drive rejection by responding to the donor’s tissue markers, leading to an infiltration of immune cells into the heart muscle, destruction of heart-muscle cells, and inflammation of blood vessels within the graft.

There are also situations where the recipient’s body produces antibodies against the donor heart. Antibody-mediated rejection (AMR) works differently from the T-cell-driven version. In AMR, antibodies bind to the inner lining of blood vessels in the graft and activate a cascade of proteins called complement, which punches holes in cells and triggers inflammation. AMR tends to be harder to treat than T-cell-driven rejection and responds poorly to conventional therapy.¹

The Three Timelines of Rejection

Rejection is not a single event. It falls into broad categories based on when it occurs and what drives it.

Hyperacute rejection: This happens within minutes to hours of surgery, when the recipient already has pre-formed antibodies against the donor. The antibodies attack the graft’s blood vessels almost immediately, causing widespread clotting and tissue death. Modern crossmatching tests before transplant have made hyperacute rejection rare, but it is almost always fatal to the graft when it occurs.
Acute rejection: This is the most closely monitored form and typically occurs within the first year, though it can happen at any point. Acute cellular rejection (ACR) is driven by T cells infiltrating the heart muscle. AMR involves antibodies and complement activation. Both can coexist, and both require treatment to prevent permanent damage.
Chronic rejection: Over months to years, immune-mediated damage to the coronary arteries of the donor heart leads to a condition called cardiac allograft vasculopathy (CAV). Unlike typical coronary artery disease, which creates irregular blockages in specific spots, CAV produces a diffuse thickening of artery walls that extends throughout the heart’s blood supply.² This gradually starves the heart muscle of blood flow and is a leading cause of late graft failure. Because the transplanted heart has no nerve connections, patients often do not feel the chest pain that would normally signal blocked arteries, making CAV especially insidious.

Why Some Patients Face Higher Risk Before Surgery Even Happens

Not everyone enters transplant with the same likelihood of rejection. One of the strongest pre-transplant predictors is the panel-reactive antibody (PRA) level, a blood test that measures how many donor tissue types the recipient’s immune system already has antibodies against. People develop these antibodies through previous blood transfusions, pregnancies, or prior transplants. A large study of over 8,000 heart transplant recipients found that roughly four out of five patients had a PRA of zero, meaning no pre-existing antibodies, and those patients had the best survival. As PRA levels climbed, both survival and the rate of rejection within the first year worsened significantly.³ Patients with PRA above 25% had the highest mortality risk. This is why transplant teams carefully screen for antibodies before surgery and may use desensitization protocols for highly sensitized patients waiting for a donor.

How Rejection Is Detected

The standard way to check for rejection has been the endomyocardial biopsy (EMB), in which a catheter is threaded through a vein into the right side of the heart and small tissue samples are snipped from the inner wall. Pathologists then examine the tissue under a microscope for signs of immune-cell infiltration or vascular damage. Biopsies have served as the gold standard for rejection surveillance for nearly four decades.⁴ In the first year after transplant, patients may undergo a dozen or more biopsies. The procedure is generally safe, but it is invasive, uncomfortable, and carries small risks of complications like puncturing the heart wall or causing abnormal heart rhythms.

Because of these drawbacks, researchers have spent years developing blood-based alternatives. Two have gained the most traction: gene-expression profiling and donor-derived cell-free DNA testing.

Gene-Expression Profiling

Gene-expression profiling (GEP) uses a blood draw to measure the activity levels of a panel of genes in the recipient’s white blood cells. The idea is that when rejection is brewing, certain immune-related genes ramp up in a detectable pattern. A landmark trial published in the New England Journal of Medicine compared a monitoring strategy based on GEP against routine biopsies in patients who were at least six months post-transplant and at low risk for rejection. The GEP-guided approach did not lead to worse outcomes and resulted in significantly fewer biopsies.⁵

That said, GEP is better at ruling rejection out than confirming it. The test has a high negative predictive value, meaning a low score reliably indicates the absence of rejection. But a high score does not necessarily mean rejection is present. The relatively low rate of rejection at any given time contributes to this imbalance: when something is uncommon, even a good test produces a lot of false alarms.⁶ Clinicians therefore use GEP scores alongside other clinical information rather than relying on them alone.

Donor-Derived Cell-Free DNA

When cells are damaged or die, they release fragments of their DNA into the bloodstream. After a heart transplant, some of those fragments come from the donor heart. If the immune system is actively attacking the graft, more donor cells die, and the percentage of donor-derived cell-free DNA (dd-cfDNA) in the blood rises. A validation study found that dd-cfDNA had an area under the curve of 0.92 for detecting acute rejection, and at a threshold of 0.25%, the test had a negative predictive value of 99%. The researchers estimated that monitoring with dd-cfDNA could have safely eliminated about four out of every five biopsies.⁷ The test also appeared to detect rejection earlier than the biopsy schedule did.

A separate multicenter study using a commercially standardized version of the test confirmed that dd-cfDNA levels were significantly higher during acute rejection episodes and during antibody-mediated rejection, with levels roughly three times higher in AMR compared to patients without it.⁸ Because these tests are non-invasive and repeatable, many transplant centers are incorporating dd-cfDNA into their surveillance programs, though biopsies remain necessary for ambiguous results or when AMR is suspected and tissue-level confirmation is needed.

Cardiac MRI

Imaging is gaining ground too. Cardiac MRI techniques like T1 and T2 mapping can detect swelling and scarring in the heart muscle that accompanies rejection. One study found that T1 mapping values above a specific threshold, combined with measures of how the heart muscle deforms during contraction, achieved perfect sensitivity and negative predictive value for identifying moderate-to-severe rejection.⁹ Importantly, T1 values dropped after successful treatment, suggesting the technique can also track response to therapy. Research is still working out how MRI fits into routine monitoring. Tissue-level changes detectable by MRI mapping may appear earlier in the rejection process than changes in overall heart-muscle function, which means MRI could eventually help catch rejection at an earlier, more treatable stage.¹⁰

Preventing Rejection With Medications

Nearly every heart transplant recipient takes immunosuppressive drugs for life. The standard maintenance regimen involves three types of medication: a calcineurin inhibitor (such as tacrolimus or cyclosporine), an antimetabolite agent (such as mycophenolate), and a corticosteroid.¹¹ Each targets a different part of the immune response, and using them in combination allows lower doses of each, which helps limit side effects. Calcineurin inhibitors, for instance, are effective at suppressing T-cell activation but can damage the kidneys over time. The balance between enough immunosuppression to prevent rejection and not so much that it causes infections or organ damage is one of the central challenges of post-transplant care.

In the first few days after surgery, patients also receive induction therapy, often with potent antibodies that deplete or block T cells, to provide intense immunosuppression during the period when rejection risk is highest. Over time, maintenance doses are typically reduced, though the drugs are never fully stopped.

Treating Rejection When It Happens

When a biopsy or blood test confirms acute cellular rejection, the first-line treatment is usually high-dose intravenous corticosteroids, often given as pulses of methylprednisolone over several consecutive days.¹² Most mild-to-moderate episodes respond to this approach. For cases that are steroid-resistant or recurrent, transplant teams escalate to more aggressive antibody-based therapies such as anti-thymocyte globulin (ATG) or OKT3, which directly deplete the T cells driving the attack. These drugs carry higher risks, including infections and infusion reactions, so they are reserved for more dangerous situations.

AMR is trickier. Because the damage is driven by antibodies and complement rather than by T cells alone, steroid pulses are less effective. Treatment strategies for AMR typically involve a combination of approaches aimed at removing existing antibodies from the blood (plasmapheresis), suppressing new antibody production, and blocking complement activation. Responses to AMR treatment are less predictable, and the condition is associated with worse long-term outcomes.

The Cytomegalovirus Connection

Cytomegalovirus (CMV) has a peculiar relationship with heart transplant rejection that goes beyond the obvious concern of infection in an immunosuppressed patient. CMV appears to actively promote rejection. Research has shown that prolonged antiviral treatment to suppress even low-level, symptomless CMV infection reduced both the rate of acute rejection episodes and the development of cardiac allograft vasculopathy.¹³ The likely mechanism is that CMV stimulates the immune system in ways that cross-react with the graft, essentially fanning the flames of rejection even when the virus is not causing obvious illness. This finding has made aggressive CMV prevention a standard part of post-transplant care, with many centers prescribing extended courses of antiviral medication.

Why Skipping Medications Is So Dangerous

Immunosuppressive drugs only work if you take them consistently, and non-adherence is a well-recognized problem, especially as years pass and patients begin to feel healthy. A systematic review of adherence in adult heart transplant recipients found that inadequate adherence to agents like tacrolimus places patients at risk for acute rejection, chronic rejection, development of donor-specific antibodies, and progressive scarring that can lead to graft failure.¹⁴

A five-year follow-up study of heart transplant patients who were identified as non-compliant with their medication found that they had nearly five times the rate of late acute rejection compared to compliant patients, and a markedly higher rate of retransplantation.¹⁵ The reasons people stop taking their drugs are varied: side effects like weight gain and tremor, the cost of medications, mental health challenges, or simply the fatigue of a complex daily regimen that stretches indefinitely. Transplant programs increasingly recognize that supporting adherence through reminders, simplified regimens, and mental health services is as important as choosing the right drugs in the first place.

Rejection in Children

Pediatric heart transplant recipients face many of the same rejection threats as adults, but with some additional complications. Children are more prone to infections, partly because their immune systems are still developing and partly because the immunosuppressive doses relative to their body size can be harder to calibrate. Medication compliance is a particular challenge in adolescents, who may resist the restrictions and side effects of lifelong drug therapy during a period when fitting in with peers feels paramount. Problems affecting long-term outcomes in pediatric recipients include acute cellular rejection, antibody-mediated rejection, CAV, infections, kidney damage from immunosuppressive drugs, and post-transplant lymphoproliferative disorder, a type of abnormal cell growth linked to Epstein-Barr virus in immunosuppressed patients.¹⁶ Meticulous monitoring and strategies to support medication compliance, especially during the teenage years, are critical for improving outcomes in this group.

Xenotransplantation and the Rejection Problem

The shortage of human donor hearts has driven researchers to explore animal-to-human transplantation, or xenotransplantation, most prominently using genetically modified pigs. Pig hearts are a reasonable size match for humans, and gene-editing technology now allows scientists to knock out pig genes that trigger the human immune response and add human genes that help the pig tissue look less foreign.

In 2022, a 57-year-old man with end-stage heart failure who was not eligible for a conventional transplant received a heart from a pig with ten genetic modifications. His immunosuppression regimen was built around blocking a protein called CD40, rather than using a standard calcineurin inhibitor-based approach. The xenograft initially functioned well without apparent rejection, though the patient ultimately died about two months later from causes that included suspected pig virus reactivation and other complications.¹⁷

A subsequent study transplanted hearts from similarly engineered pigs into two brain-dead human recipients and monitored them for 66 hours. Both hearts functioned immediately after transplant, and detailed analysis found no evidence of cellular or antibody-mediated rejection, and no signs of pig viruses spreading to the recipients.¹⁸ These are early steps, but they suggest that with enough genetic editing and targeted immunosuppression, the hyperacute rejection that once made cross-species transplantation impossible can be overcome. The long-term rejection problem, whether pig hearts will develop the equivalent of CAV or trigger delayed antibody responses, remains an open question.

Emerging Strategies to Retrain the Immune System

Rather than simply suppressing the immune system with drugs, some researchers are pursuing the more ambitious goal of teaching it to tolerate the graft. One promising avenue involves regulatory T cells (Tregs), a specialized subset of immune cells whose job is to calm the immune response and prevent it from attacking the body’s own tissues. The idea is to expand a patient’s own Tregs outside the body and then infuse them back in, essentially reinforcing the brakes on the immune system rather than just cutting the accelerator.

A clinical trial underway in pediatric heart transplant recipients is testing Tregs derived from thymic tissue, which is routinely removed during heart surgery in children. The thymus is where Tregs are naturally generated, so it offers a source of these cells in very high purity and with strong suppressive function. Early-phase work has shown the approach is safe, though clinical effectiveness has yet to be established.¹⁹ If Treg therapy proves effective, it could allow patients to reduce or even stop conventional immunosuppressive drugs, avoiding their long-term toxicities.

Organ preservation technology is also advancing in ways that may affect rejection. Traditional cold storage limits donor hearts to roughly four to six hours between removal and implantation, which restricts how far a heart can travel and how well it can be matched to a recipient. Newer machine perfusion systems keep the heart warm and supplied with oxygenated blood during transport, extending the viable preservation window. Evidence is accumulating that these systems reduce graft dysfunction, which in turn may lower the inflammatory triggers that contribute to early rejection.²⁰ By allowing better donor-recipient matching across greater distances, machine perfusion could also reduce immunological mismatches that predispose to rejection in the first place.