Immunity refers to a physiological response by which the immune system identifies self or non-self substances and eliminates antigenic foreign material through immune responses, thereby maintaining homeostasis. After transplantation, the recipient’s immune system recognizes antigens on the graft and mounts a response, while immune cells in the donor graft may also recognize antigens on the recipient tissues and generate a response. This specific immune response arising from interactions between the recipient's immune system and the donor graft is termed the transplant immune response, also referred to as transplant rejection.
The occurrence of rejection varies depending on the source and genetic background of the graft. Transplantations between non-identical individuals generally result in rejection. In clinical practice, most transplantations involve allogeneic grafts, and rejection is the primary obstacle to graft survival and functionality. Adaptive immunity plays a decisive role in transplantation immunity, primarily including T cell-mediated rejection (TCMR) and antibody-mediated rejection (AMR). Recent studies indicate that innate immunity also has an important role, with cells such as natural killer (NK) cells, monocytes/macrophages, dendritic cells, and granulocytes participating in the process.
Transplant Antigens
Transplant antigens, which elicit transplant rejection, include major histocompatibility antigens (MHC), minor histocompatibility antigens, and other antigens involved in the rejection process, such as ABO blood group antigens.
Major Histocompatibility Antigens (MHC)
Tissue compatibility, or histocompatibility, determines the degree to which donor and recipient tissues or organs are accepted in a transplant. The strongest transplantation antigens responsible for tissue compatibility are encoded by the major histocompatibility complex (MHC). In humans, the MHC is located on the short arm of chromosome 6. These molecules were initially identified on white blood cells using serology and are therefore also referred to as human leukocyte antigens (HLA).
MHC molecules are classified into three types, with Class I and Class II MHC molecules most relevant to transplantation:
- Class I molecules (HLA-A, HLA-B, HLA-C) are expressed on the surface of nearly all nucleated cells.
- Class II molecules (HLA-DR, HLA-DQ, HLA-DP) are typically expressed on the surface of antigen-presenting cells, including dendritic cells, macrophages, B cells, and other cells with antigen-presenting functions.
Due to extensive polymorphism in the MHC, differences between donor and recipient MHC molecules are the primary cause of acute rejection responses.
Minor Histocompatibility Antigens
Minor histocompatibility antigens induce weaker rejection responses. These include sex-linked antigens (e.g., H-Y antigens) and non-Y chromosome-linked antigens expressed on leukemia cells or normal cells. Peptides derived from these antigens contain allogeneic epitopes and are recognized by T cells in an MHC-restricted fashion.
Other Antigens Involved in Rejection
These include antigens such as ABO blood group antigens. ABO antigens are primarily expressed on the surface of red blood cells but are also present on vascular endothelial cells and tissue cells in organs such as the liver and kidneys. If the donor and recipient have incompatible blood types, blood group antibodies in the recipient’s circulation can bind to ABO antigens on the donor graft’s endothelial cells, triggering complement activation, endothelial cell damage, and intravascular coagulation, leading to hyperacute rejection.
Recognition of Transplant Antigens and Immune Response
The recognition of transplant antigens occurs through two pathways: direct and indirect recognition.
Direct recognition involves recipient alloreactive T cells directly recognizing donor antigen-presenting cells' MHC-peptide complexes and initiating an immune response.
Indirect recognition occurs when antigens from the donor graft are processed and presented by recipient antigen-presenting cells. These antigens are presented to recipient T cells as donor-derived peptides bound to the recipient’s MHC molecules, leading to their activation.
Direct recognition is believed to play a significant role in the early stages of acute rejection, while indirect recognition mechanisms contribute synergistically. In the later stages of acute rejection and in chronic rejection, indirect recognition becomes more dominant.
T cell-mediated rejection (TCMR) plays a central role in allogeneic transplant rejection. Multiple cell subsets contribute to graft damage:
CD4+ helper T cells facilitate the secretion of inflammatory cytokines, leading to their own proliferation and the recruitment of inflammatory cells such as monocytes/macrophages. This process causes delayed-type hypersensitivity inflammation and tissue damage.
In addition, these inflammatory cytokines activate CD8+ cytotoxic T cells, which directly damage graft endothelial and parenchymal cells by releasing perforin and granzyme.
Antibody-mediated rejection (AMR) is triggered when transplant antigens stimulate B cells to produce alloantibodies against donor-specific antigens. These antibodies cause endothelial cell damage and play a crucial role in the rejection process. AMR is a key factor influencing the long-term prognosis of transplanted organs such as kidneys, hearts, lungs, and livers.
Mechanisms and Classification of Clinical Rejection Reactions
Based on the direction of the immune attack, transplant rejection reactions can be categorized into two types. The first type is the host-versus-graft reaction (HvGR), which refers to the rejection response mounted by the recipient’s immune system against the donor's cells, tissues, or organs. This is the most common type in clinical practice. HvGR is typically classified into hyperacute rejection, acute rejection, and chronic rejection based on the timing of its occurrence. The second type is the graft-versus-host reaction (GvHR), which represents a rejection response initiated by immune cells within the graft against the recipient tissue.
Host-Versus-Graft Reaction
Hyperacute Rejection (HAR)
Hyperacute rejection occurs within minutes to hours after the restoration of blood flow to the graft in the recipient’s body and is a classic example of antibody-mediated rejection. HAR is usually caused by pre-existing, preformed antibodies in the recipient against donor antigens. These antibodies rapidly bind to endothelial cells within the graft and activate the complement system, directly damaging target cells. In parallel, coagulation cascades are activated, leading to widespread microthrombosis in the graft’s microvascular system. Clinically, HAR may present with intraoperative graft swelling, darkened coloration, reduced blood flow, softening of the graft, and loss of function.
Pathological features of HAR include marked edema, hemorrhage, and necrosis of organ parenchyma; thrombosis in capillaries and small vessels; infiltration of polymorphonuclear leukocytes (neutrophils); and fibrinoid necrosis in vessel walls. Once HAR occurs, it is often irreversible, with anti-rejection therapy generally proving ineffective. In such cases, graft removal is usually the only option. Prevention remains the key to managing HAR.
Acute Rejection (AR)
Acute rejection is the most common type of rejection in clinical settings and involves significant contributions from both T cell-mediated rejection (TCMR) and antibody-mediated rejection (AMR). In the past, acute rejection was thought to occur mainly within the first three months post-transplantation, but with the widespread use of potent immunosuppressive agents, it can now occur at any point after transplantation.
The typical clinical manifestations of acute rejection include fever, localized swelling and pain at the graft site, and impaired graft function. Currently, reliable biochemical or immunological markers for early diagnosis are lacking. Diagnosis relies heavily on graft biopsy, which typically reveals prominent inflammatory cell infiltration. Once acute rejection is confirmed, timely treatment is crucial. Standard interventions include high-dose corticosteroid therapy, anti-lymphocyte globulin, plasmapheresis, or adjustments to the immunosuppressive regimen, which are generally effective.
Chronic Rejection (CR)
Chronic rejection develops weeks, months, or even years after transplantation. Its exact mechanism remains unclear, but it is thought to involve repeated episodes of AMR and TCMR, alongside non-immune factors such as the toxicity of immunosuppressive drugs. Clinically, chronic rejection is characterized by a gradual decline in graft function.
The primary pathological hallmark of chronic rejection is thickening of the intimal layer of graft arteries, which leads to extensive graft ischemia, fibrosis, and eventual loss of function. Effective treatment options for chronic rejection are currently lacking, and it remains the leading cause of long-term graft failure.
Graft-Versus-Host Reaction (GvHR)
Graft-versus-host reaction is a rejection response initiated when specific lymphocytes within the graft recognize recipient antigens and mount an attack against the recipient. GvHR can be classified into acute GvHR and chronic GvHR. Graft-versus-host disease (GVHD) caused by GvHR can result in multi-organ failure and even death in the recipient.
The severity of GvHR primarily depends on the degree of HLA compatibility between the donor and recipient. It is also strongly associated with differences in minor histocompatibility antigens. GvHR is most commonly observed after hematopoietic stem cell transplantation and small bowel transplantation.
Prevention and Management of Rejection Reactions
The main strategies for preventing and managing transplant rejection reactions include pre-transplant tissue typing, recipient preconditioning, the use of immunosuppressive drugs, and post-transplant immunological monitoring.
Tissue Typing
ABO Blood Group Testing
Typically, the ABO blood types of the donor and recipient should be identical or compatible. In special circumstances, transplants with incompatible ABO blood types may be performed, but the risk of rejection reactions is increased.
HLA Typing
HLA typing focuses on loci relevant to transplantation, primarily HLA-A, HLA-B, HLA-DR, and HLA-DQ. Donors with a higher degree of HLA matching are preferred. HLA-DR and HLA-DQ loci are considered most critical for the long-term survival of grafts in kidney and heart transplantation, followed by HLA-B, with HLA-A having a lesser impact.
Panel Reactive Antibody (PRA) and Donor-Specific Antibody (DSA)
PRA testing is used to detect pre-existing HLA antibodies in the recipient. PRA levels reflect the extent of sensitization to allo-HLA antigens and are closely associated with the incidence of rejection and graft survival rates. Sensitization can occur as a result of prior transplants, pregnancies, or blood transfusions, which induce the formation of HLA antibodies. DSA detection has even greater clinical significance.
Lymphocytotoxic Cross-Match Test
This test involves using the donor’s live lymphocytes as antigens, to which the recipient’s serum is added. Under the action of complement, antigen-antibody binding and complement activation occur. A positive cross-match (>10%) is a contraindication for organ transplantation, particularly for kidney and heart transplants.
Recipient Preconditioning
Preconditioning of the recipient aims to prevent or reduce rejection reactions. This approach is primarily applied in cases such as ABO-incompatible organ transplantation. Preconditioning methods include:
- Plasmapheresis to remove pre-existing specific antibodies from the recipient’s blood.
- Use of anti-CD20 monoclonal antibody, which depletes B lymphocytes and suppresses antibody-mediated rejection.
- Intravenous administration of high-dose intravenous immunoglobulin (IVIG) to neutralize antibodies.
Use of Immunosuppressive Drugs
The treatment of acute rejection reactions in clinical practice includes both prophylactic (baseline) treatment and rescue treatment. Baseline treatment involves the use of immunosuppressive drugs to effectively prevent rejection. Since immune responses begin as soon as graft blood flow is restored, immunosuppressive drug dosages are high during the perioperative period, a phase known as the induction phase. Gradual tapering to maintenance levels occurs afterward to prevent acute rejection, in the maintenance phase. Immunosuppressive therapy is typically lifelong.
If acute rejection occurs, rescue therapy involves high-dose corticosteroid pulse therapy or the use of anti-lymphocyte/anti-thymocyte globulins, along with adjustments to the immunosuppressive protocol to reverse the rejection.
Immunosuppressive drugs are generally classified into induction drugs and maintenance drugs.
Induction Drugs
Induction drugs are primarily anti-lymphocyte immunoglobulins, including polyclonal and monoclonal antibodies:
- Polyclonal antibodies, such as anti-lymphocyte globulin or anti-thymocyte globulin, are immunoglobulins extracted from animal sera following sensitization with human lymphocytes or thymocytes. These antibodies exert direct cytotoxic effects on lymphocytes, leading to their lysis. Polyclonal antibodies are mainly used during the induction phase and for reversing steroid-refractory rejection.
- Monoclonal antibodies primarily target:
- Interleukin-2 receptor (IL-2R): Monoclonal antibodies selectively target IL-2R and are mainly used for induction therapy.
- CD20: Anti-CD20 monoclonal antibodies bind specifically to the CD20 antigen on B cells, leading to B cell lysis. This action inhibits antibody-mediated immune responses and helps prevent rejection mediated by antibodies.
Maintenance Drugs
Maintenance therapy utilizes several classes of immunosuppressive drugs:
- Glucocorticoids: Commonly used glucocorticoids include prednisone, prednisolone, and methylprednisolone. These drugs suppress monocytes/macrophages, neutrophils, T cells, and B cells. Glucocorticoids are often combined with antiproliferative agents and/or calcineurin inhibitors for baseline therapy, and they are also the first-line treatment for acute rejection. However, due to their numerous side effects, lower doses with gradual tapering to a minimal effective dose or discontinuation are currently preferred.
- Antiproliferative Agents:
- Azathioprine inhibits the synthesis of DNA, significantly suppressing T cell proliferation.
- Mycophenolate mofetil (MMF) relatively specifically inhibits T and B cell proliferation and suppresses antibody production. MMF is commonly used in clinical practice as a maintenance therapy.
- Calcineurin Inhibitors (CNIs): CNIs are fundamental to maintenance immunotherapy and include cyclosporine (CsA) and tacrolimus (TAC).
- CsA binds to cyclophilin in T cells, and this complex interferes with the calcineurin–calmodulin complex, preventing the expression of cytokines necessary for T cell activation (e.g., IL-2). This inhibits T cell activation and proliferation.
- TAC binds to intracellular receptors and inhibits T cell activation/proliferation via mechanisms similar to CsA.
- Mammalian Target of Rapamycin (mTOR) Inhibitors: Drugs such as sirolimus (rapamycin) and everolimus inhibit the downstream signaling pathway of IL-2 receptors, arresting the cell cycle in the G1 and S phases to exert immunosuppressive effects. mTOR inhibitors also possess anti-tumor properties and have emerged as multifunctional immunomodulators with both immunosuppressive and anti-tumor effects.
An ideal immunosuppressive therapy aims to prevent graft rejection while minimizing adverse effects on the recipient’s immune system and toxic side effects. The basic principle of immunosuppressive drug use involves combination therapy to reduce the dosage and adverse effects of individual drugs while enhancing synergistic effects.
The most commonly used triple therapy regimen combines a CNI (CsA or TAC) with a glucocorticoid and an antiproliferative agent (e.g., MMF). Depending on specific circumstances, this can be modified into dual or quadruple therapy. In general, transplant recipients require lifelong maintenance immunosuppressive therapy. However, a small subset of patients may, after an extended period, reduce immunosuppressive doses to minimal levels or even discontinue therapy altogether, achieving a state known as “clinical tolerance” or “operational tolerance.”
Post-Transplant Immunological Monitoring
Commonly used clinical monitoring indicators include blood concentrations of immunosuppressive drugs, donor-specific antibodies, absolute counts and percentages of lymphocyte subsets, lymphocyte functionality, and levels of immunological molecules. Physiological changes in graft function serve as crucial indicators for assessing the occurrence and severity of rejection.
Transplantation Tolerance
Transplantation tolerance refers to an immune state in which the recipient's immune system does not mount a rejection response to the graft in the absence of any immunosuppressive drugs, while maintaining normal immune responses to other antigens, thereby allowing long-term graft survival. Inducing graft-specific immune tolerance in recipients represents an ideal strategy to overcome rejection reactions and remains one of the most challenging topics in transplantation immunology research.
Based on mechanisms, transplantation tolerance can be classified into central immune tolerance and peripheral immune tolerance. Central immune tolerance arises when T cells and B cells encounter self or exogenous antigens during their differentiation and maturation in central immune organs. Peripheral immune tolerance, on the other hand, refers to antigen-specific immune tolerance that occurs during the maturation and immune response of T cells and B cells in peripheral lymphoid organs.
Methods to induce immune tolerance include promoting the formation of allogeneic chimerism, blocking costimulatory pathways to induce anergy in reactive T cells, depletion of T and B cells, and the induction or adoptive transfer of immunosuppressive cells. Significant progress has been made in both animal experiments and clinical applications. Further exploration into the mechanisms underlying the development of immune tolerance holds important theoretical value and clinical significance for the advancement of organ transplantation science.