Transplant Immunobiology


Immunobiology

  1. Innate and Acquired Immunity

    1. Innate Immunity
      • older, nonspecific immune system that is broadly reactive against common components of pathogenic organisms (e.g. endotoxin)
      • also responsible for identifying altered or damaged tissue
      • effector cells include macrophages, monocytes, neutrophils, natural killer cells

    2. Acquired Immunity
      • purpose is specific recognition of foreign antigen and elimination of nonself
      • another purpose is to respond quickly to prior antigenic challenges
      • based on antigen receptors formed by germline rearrangement, which leads to highly specific binding interactions
      • mediated by T cells and B cells
      • each T cell or B cell recognizes only one particular antigen
      • 109 different clones of T cells and B cells exist in each individual

      1. T Cells
        • protect the cells of the body against alterations by mutation or viral infection (cellular immunity)
        • T cell receptors only recognize fragments of peptide antigens bound to MHC molecules
        • 2 main types of T cells: CD4 (T helper) and CD8 (T cytotoxic)
        • CD4 T cells recognize antigen bound to class II MHC molecules, which are found on antigen-presenting cells such as dendritic cells, macrophages, B cells
        • CD8 cells recognize antigen bound to Class I MHC molecules, which exist on all nucleated cells

      2. B Cells
        • provide protection against extracellular infectious organisms and foreign material
        • recognize antigen in its native unprocessed state
        • secrete antibodies to bind foreign molecules
        • material bound by antibody is then marked for destruction by phagocytic cells
        • bound antibody also activates a destructive enzymatic cascade (complement system)

      3. Amplification of the Immune Response
        • mediated by cytokines
        • IL-2 is produced by activated T cells and stimulates proliferation of activated T cells and B cells

  2. Major Histocompatibility Complex
    • called the HLA locus in humans, and is located on chromosome 6
    • produces cell surface proteins that are the principal antigenic determinants of graft rejection
    • antigens are grouped into 2 classes: class I and class II
    • many different alleles exist for each class I and class II molecule gene (polymorphism)
    • likelihood of any two random individuals expressing the same MHC antigens is extremely small

    1. Class I Molecules
      • occur on all nucleated cells in contact with blood
      • 3 major class I antigens: HLA-A, HLA-B, HLA-C
      • functional part of the class I molecule is the peptide binding groove, which is occupied by a native peptide
      • T cells are able to inspect and approve of ongoing protein synthesis
      • T cells bind but do not activate when encountering self MHC molecules presenting self-peptides
      • alteration in peptide content (e.g. by viral peptide synthesis or mutation) causes activation
      • in transplantation, T cells may be activated directly by the donor’s HLA molecules, or indirectly by APCs that have processed and presented the foreign antigen
      • class I molecules are bound only by cytotoxic CD8 cells

    2. Class II Molecules
      • located on specialized immune cells (antigen-presenting cells, macrophages, dendritic cells, B cells) and endothelial cells
      • consists of an alpha chain and a beta chain
      • 3 major class II antigens: HLA-DR, HLA-DP, HLA-DQ
      • functional part is the peptide-binding groove between the two polypeptide chains
      • proteins engulfed by phagocytic cells are degraded and then associated with class II molecules
      • this allows circulating foreign proteins to be presented to T cells
      • CD4 cells (T helper) bind class II molecules
      • CD4 cells are able to activate CD8 cells and B cells
      • when an inappropriate peptide is detected, CD4 cells release cytokines to recruit CD8 cells into the area
      • B cells release antibody to bind the offending peptide and aid in its clearance by phagocytic cells and the complement cascade
      • in transplants, an abnormal peptide or the foreign class II molecule itself can lead to T cell activation

    3. Genetics of HLA
      • unit of inheritance is the haplotype, which consists of one copy of chromosome 6 and one copy of each class I and class II locus
      • probability of a sibling being HLA-identical is 25%, haploidentical is 50%, and completely nonidentical is 25%
      • parents are haploidentical with their children

    4. HLA Matching
      • matching the recipient and donor as closely as possible with regard to HLA type reduces the risk of acute rejection
      • as immunosuppression has improved, the importance of HLA-matching has decreased

      1. Tissue Typing
        • the more antigenic the graft, the more vigorous the rejection response
        • antigenicity of the graft is determined by the degree of genetic disparity
        • historically, HLA disparity has been defined with the use of two biologic assays: the lymphocytotoxicity assay and the mixed lymphocyte reaction (MLC)
        • molecular techniques now exist for precise genotyping of an individual’s HLA

Transplant Antigen Recognition

  1. T Cell Activation
    • T cells can respond directly to intact allo-MHC molecules on the surface of the donor tissue
    • T cells may also encounter antigen-presenting cells that have phagocytosed fragmented allograft tissues and processed the antigens for expression with self-MHC
    • understanding T cell activation is the key to understanding the rational use of immunosuppressive agents to prevent acute rejection

    1. T Cell Binding
      • a single interaction with an MHC molecule is not sufficient to cause T cell activation
      • T cell must register multiple receptor/ligand interactions with the same antigen before a threshold of activation is reached (signal 1) - transient encounters are not sufficient
      • additional costimulatory pathways (signal 2) are required for T cell activation – additional T cell receptors must bind to specific ligands on the antigen presenting cell surface

      T cell two signal activation
    2. Intracellular Signaling
      • repetitive binding signals eventually result in the T cell receptor becoming internalized, where it binds to immunophilin in the cytoplasm
      • immunophilin stimulates calcineurin, which in turn activates the cytokine transcription factor NF-AT
      • activated NF-AT then translocates to the nucleus where it initiates transcription of IL-2
      • IL-2 is then released and binds to the T cell in an autocrine loop
      • IL-2 binding stimulates the T cell to undergo cell division and replication

      Overview of T cell activation
  2. T Cell Amplification
    • once activation occurs, cytokines, particularly IL-2 and interferon-γ, recruit other T cells into the response
    • B cell activation also is mediated through cytokine secretion
    • cytokines are responsible for the systemic symptoms of fever and malaise associated with severe graft rejection

Clinical Rejection Syndromes

  1. Hyperacute Rejection
    • caused by presensitization of the recipient to a donor antigen
    • develops within the first minutes to hours following graft reperfusion
    • exposure is usually from a prior transplant, transfusion, or pregnancy
    • preformed antibodies bind to donor endothelial cells, initiating complement-mediated lysis and a procoagulant state, resulting in immediate graft thrombosis
    • no treatment exists
    • can be avoided in 99.5% of transplants by proper ABO matching and a negative transplant antigen crossmatch assay

  2. Acute Rejection
    • caused primarily by T cells
    • usually occurs within the first 6 months after transplant
    • inevitable result of allotransplants unless immunosuppression against T cells is used
    • to initiate acute rejection, T cells bind donor antigen directly or after phagocytosis of donor tissue
    • this leads to T cell activation and massive infiltration of the graft by T cells, leading to organ destruction
    • incidence of acute rejection declines with decreasing MHC disparity
    • only kidneys can be preserved long enough to allow organ allocation to the recipient to be the most closely matched to the MHC of the donor
    • treatment of acute rejection leads to successful restoration of graft function in 90 to 95 percent of cases
    • prompt recognition is imperative, and monitoring must be intense, especially during the first year after transplant
    • unexplained graft dysfunction should prompt biopsy and evaluation for the lymphocytic infiltration and parenchymal necrosis characteristic of acute rejection
    • liver acute rejection is also characterized by eosinophilic infiltration of the graft

  3. Chronic Rejection
    • poorly understood
    • onset is insidious, occurring over months to years
    • it is untreatable, since the pathophysiology is undefined
    • increased immunosuppression is not effective in reversing or retarding the progression
    • distinguished from acute rejection by biopsy (parenchymal replacement by fibrous tissue with a relatively sparse lymphocytic infiltrate)
    • requires retransplantation

Immunosuppression

  1. General Principles
    • no immunosuppressive intervention is allograft-specific
    • all interventions do so at the expense of a vital defense network
    • rational, selective use of several immunosuppressive agents acting through different synergistic pathways is required to successfully prevent rejection without completely removing the body’s defenses
    • immunosuppression is extremely intense in the early postop period (induction immunosuppression)
    • induction therapy involves deletion of the T cell response completely and cannot be maintained indefinitely without lethal consequences
    • maintenance immunosuppression is used to prevent acute rejection for the life of the patient
    • rescue agents are immunosuppressants used to reverse an acute rejection episode

    1. Corticosteroids
      • a mainstay of virtually all immunosuppression induction and maintenance regimens
      • in combination with other agents, they significantly improve graft survival
      • high doses are used as a rescue agent to treat acute rejection
      • mechanism of action has not been completely elucidated
      • functional effect is to depress all T cell responses
      • total blood lymphocyte count decreases within 6 hours of corticosteroid administration
      • steroids inhibit the production of T cell proinflammatory cytokines and so prevent the primary mechanism by which lymphocytes amplify their responsiveness
      • steroids inhibit both chemotaxis and phagocytosis by macrophages and neutrophils
      • many acute side effects (glucose intolerance, poor wound healing, salt and water retention, CNS effects)
      • many chronic side effects (Cushing’s syndrome, cataracts, muscle wasting, osteoporosis)
      • current trend in transplantation is to lower the dose of steroids used and add other immunosuppressive agents
      • patients who have survived a year without a rejection episode may be considered for withdrawal of steroids

    2. Antiproliferative Agents
      1. Azathioprine
        • purine analog
        • cleaved in the liver to form the active compound, 6-mercaptopurine
        • prevents RNA and DNA synthesis
        • inhibits the replication of T and B cells
        • major side effects include bone marrow suppression (leukopenia) and liver toxicity
        • has largely been replaced by mycophenolate mofetil

      2. Mycophenolate Mofetil (MMF)
        • more specific purine antimetabolite
        • prevents a critical step in RNA and DNA synthesis
        • MMF exploits a critical difference between lymphocytes and other cells to produce relatively selective immunosuppressive effects
        • blocks the proliferative response of both T cells and B cells
        • causes less bone marrow suppression than azathioprine
        • is not nephrotoxic or hepatotoxic, but GI side effects (diarrhea, nausea, bloating) can be disabling
        • teratogenic in pregnant females

    3. Calcineurin Inhibitors
      1. Cyclosporine
        • produced by a fungus
        • its introduction in 1983 revolutionized transplantation, especially cardiac and liver transplantation
        • T cell specific
        • binds with high affinity to cyclophilin in the cell cytoplasm, which inhibits calcineurin and prevents it from activating the transcription-regulating factor NF-AT
        • this prevents transcription of the IL-2 gene and other genes critical for T cell activation
        • works solely as a maintenance agent; it is ineffective as a rescue agent
        • most significant side effect is nephrotoxicity
        • other side effects include hypertension, tremor and other neurotoxicities, hyperkalemia, hirsutism, hepatotoxicity

      2. Tacrolimus (FK-506)
        • produced by a fungus
        • like cyclosporine, it blocks the effects of NF-AT, prevents cytokine transcription, and arrests T cell activation
        • has a different intracellular target than cyclosporine (FK-binding protein) and is one hundred times more potent in inhibiting IL-2 production
        • main role is as a maintenance agent, but it has shown promise as a rescue agent
        • side effect profile is similar to that of cyclosporine with regard to renal and hepatic toxicity
        • extremely effective in liver transplantation and has largely replaced cyclosporine
        • high rate of posttransplant lymphoproliferative disorders in children

    4. Mammalian Target of Rapamycin Inhibitors
      1. Sirolimus (Rapamycin)
        • structurally similar to tacrolimus, but does not inhibit calcineurin
        • impairs signal transduction by the IL-2 receptor, thus inhibiting T cell and B cell proliferation
        • nephrotoxicity and neurotoxicity have not been observed
        • hepatic artery thrombosis following liver transplant is a major side effect

    5. Monoclonal Antibodies
      1. Antilymphocyte Globulin (ALG)
        • polyclonal antibodies produced by injecting human thymocytes into different species (horse, rabbit)
        • T cells and B cells are eliminated through complement-mediated lysis and opsonin-induced phagocytosis
        • most commonly used for induction of immunosuppression or treatment of steroid-resistant rejection
        • used in kidney, kidney/pancreas, and intestinal transplants, but rarely in liver transplants
        • severe thrombocytopenia is a major side effect
        • causes an increase in viral reactivation and viral infections (CMV, EBV)
        • chills, fever, skin rash occur in 15 to 20 percent of patients - these can be ameliorated by pretreatment with steroids, antihistamines, and antipyretics

      2. Anti-IL2 Receptor Antibodies (basiliximab)
        • blocks IL2 binding to its receptor
        • used during induction

    6. Inhibition of T Cell Activation
      1. Belatacept
        • blocks T cell costimulation pathways
        • has an increased risk of posttransplant lymphoproliferative disorder, especially in recipients who are EBV-seronegative pretransplant
        • given monthly by IV

  2. Complications of Immunosuppression
    1. Infection
      • most common cause of mortality in transplant patients
      • most infections are by opportunistic organisms
      • Candida and Aspergillus species are the most common cause of fungal infections
      • Pneumocystis Carinii, a protozoan, is a frequent cause of pulmonary infection
      • Cytomegalovirus is the most common viral offender

    2. Malignancy
      • incidence of virally-mediated tumors is significantly increased in transplant patients
      • rate not high enough to contraindicate transplantation
      • incidence of squamous cell skin cancers, Kaposi’s sarcoma, non-Hodgkin’s lymphoma, cancer of the liver, anus, and cervix are five times as frequent as compared to the general population







References

  1. Schwartz, 10th ed., pgs 321 - 334
  2. Sabiston, 20th ed., pgs 598 - 630
  3. UpToDate. Transplant Immunobiology. John Vela, MD, FACP, FRCP. March 2021. Pgs 1 – 38
  4. UpToDate. Liver Transplantation in Adults. Norman L. Sussman, MD, John M. Vierling, MD, FACP. Oct 07, 2020. Pgs 1 – 42