Organ Transplantation and Graft Rejection

Biotechnology and Transplantation

Introduction to Organ Transplantation

  • Organ transplantation: The process of moving an organ from one body to another or from a donor site on the patient's own body.
  • Purpose: To replace a damaged or failing organ with a healthy, functioning one.
  • Organ Donors: Can be living or deceased (cadaveric).

Organs and Tissues that can be Transplanted

  • Organs: Heart, kidneys, liver, lungs, pancreas, penis, and intestine.
  • Tissues: Bones, tendons, cornea, heart valves, veins, arms, and skin.
  • Most Common: Kidneys are the most frequently transplanted organs worldwide.

Types of Grafts

  • Autograft:
    • Transfer of self-tissue from one site to another within the same individual.
    • The immune system does not respond due to genetic homology.
    • Uses:
      • Skin grafts
      • Bone marrow transplantation
      • Hair transplantation
  • Isograft:
    • Transplantation between identical twins.
  • Allograft:
    • Transfer of organs or tissue from one human to another.
    • Common due to the increasing number of people waiting for donor organs and tissues.
    • Has many applications.
  • Xenograft:
    • Xenotransplantation involves the transfer of tissue from one species to another (e.g., animal to human).
    • Provides a potential new source of organs for humans.
    • Examples: Heart, kidney, liver, or lung.

Graft Survival Rates

  • The following data represents graft survival rates in the USA (2006):
    • Kidney: 18,017 transplants, 71.9% 5-year survival
    • Liver: 6650 transplants, 67.4% 5-year survival
    • Heart: 2192 transplants, 71.5% 5-year survival
    • Pancreas: 1387 transplants, 53.2% 5-year survival
    • Lung: 1405 transplants, 46.3% 5-year survival
    • Cornea: ~40,000+ transplants, ~70% survival
    • Bone marrow: 15,000+ transplants, 40%/60% survival.
    • Source: Figure 14-45 Immunobiology, 7ed. (© Garland Science 2008).

Graft Rejection

  • Definition: An immunological response primarily mediated by T-cells.
  • Major Antigens Involved: MHC complex.
  • Minor Antigens: Minor antigens.

Types of Graft Rejection Responses

  • Single response (organ): Proliferation of host anti-graft cells leading to HVG (rejection).
  • Single response (bone marrow): Proliferation of graft anti-host cells leading to GVH (Graft-versus-Host).
  • Double response (organ): Immunosuppression, reciprocal clonal deletion, GVH, and HVG (rejection).
  • Double response (bone marrow): Reciprocal clonal deletion, GVH, unaltered bone marrow, HVG.

Specificity of Graft Rejection

  • Graft APCs migrate to host lymph node.
  • Present graft antigen to host T cells.
  • Host T cell activation.
  • Migration of activated anti-graft T cells to the grafted tissue, causing destruction.

Transplant Rejection Mechanisms

T Cell-Mediated Reactions
  • Involve both delayed-type hypersensitivity (DTH) and T cell-mediated cytotoxicity.
  • Host recognition of donor HLA occurs in two ways:
    • Indirect Recognition:
      • Host CD4+ T cells recognize donor HLA after processing and presentation by the host's APC.
      • This recognition activates DTH.
    • Direct Recognition:
      • Host T cells recognize HLA molecules on the surface of APCs of the donor.
      • Host T cells encounter donor dendritic cells within the grafted organ, or after these cells migrate to the draining lymph nodes.
      • Both host CD4+ and CD8+ T cells are involved in this reaction.
    • Direct Recognition:
      • Donor APC activates Host T cell resulting in CD4+ and CD8+ Cytotoxicity
      • Releases NO−,H<em>2O</em>2,O2−NO-, H<em>2O</em>2, O_2-
    • Indirect Recognition:
      • Uptake and Presentation of Donor Antigens/MHC by Host APC which activates Host T cell and B cell activation and T cell proliferation resulting in Humoral Rejection.
      • Releases NO−,H<em>2O</em>2,O2−NO-, H<em>2O</em>2, O_2-

Allorecognition: Indirect Pathways

  • Donor MHC molecules are shed and taken up and processed by host antigen-presenting cells (APCs).
  • Host APCs present donor class I-derived peptides via host class II molecules to CD4+ T cells.
  • CD4+ T cells produce IL-2, which activates CD8+ cytotoxic T cells.

Semi-Direct Antigen Presentation

  • Host CD8+ T cell recognizes allogeneic donor cell class I. Shed membrane with donor MHC is presented to Host APC which activates Host CD8+ cytotoxic cell.

Inflammatory Mediators in Graft Rejection

  • Cytokines: TNFßß, IFN-γ\gamma, IL-2, IL-4, IL-5, IL-6
  • Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)
  • Complement Activation: Vascular occlusion.
  • Cell-Mediated Cytotoxicity: Lytic damage to graft cells.
  • Cells Involved: CD4+ T helper cells (Th), CD8+ cytotoxic T cells (Tc), Natural Killer (NK) cells.

Types of Transplant Graft Rejection

  • Antibody-Mediated Rejection (AMR):
    • Hyperacute rejection
    • Acute or delayed AMR
  • Cellular Rejection
  • 'Chronic' Rejection

Mechanisms of Rejection

  1. Hyperacute Rejection:
    • Pre-formed anti-donor antibodies bind to graft endothelium immediately after transplantation.
    • Results in thrombosis, ischemic damage, and rapid graft failure.
  2. Acute Cellular Rejection:
    • T cells destroy graft parenchyma (and vessels) through cytotoxicity and inflammatory reactions.
  3. Acute Humoral Rejection:
    • Antibodies damage graft vasculature.
  4. Chronic Rejection:
    • Dominated by arteriosclerosis.
    • T cell reaction and secretion of cytokines induce proliferation of vascular smooth muscle cells.
    • Associated with parenchymal fibrosis.

Visual Representation of Graft Rejection Types

  • (A) Organ Transplantation: Shows the progression of rejection types; Hyperacute Rejection, Acute Vascular Rejection (Antibody-mediated Rejection), Acute Cellular Rejection, and Chronic Rejection. It also mentions Enhancement Complement Control and Accommodation.
  • (B) Tissue or Cell Transplantation: Focuses on Cellular Rejection, Enhancement, Complement Control?, and Accommodation?

Hyperacute Graft Rejection Details

  • Recipient has preexisting ABO antibodies due to:
    • Previous blood transfusions
    • ABO antigens also present on leukocytes, endothelial cells
  • During surgery, antibodies bind to endothelial vessels of the graft.
  • Immediate activation of complement and blood clotting.
  • Prevention: Can be prevented by cross-matching donor and recipient.

Chronic Rejection Details

  • Definition: Poorly defined term indicating chronic deterioration within the graft.
  • Occurs in some form in all organ allografts:
    • Kidney
    • Heart
    • Lung
    • Liver
  • May or may not be associated with recurrent cellular rejection episodes.
  • Alloantibody may or may not play a role.
  • Not prevented with current immunosuppressive drug therapies.

Graft Versus Host Reaction (GVHR)

  • When grafted tissue has mature T cells, they will attack host tissue, leading to GVHR.
  • Major problem for bone marrow transplant.
  • Methods to Overcome GVHR:
    • Treat bone marrow to deplete T cells.
    • Use autologous bone marrow.
    • Use umbilical cord blood.

Graft Versus Host Disease

  • Severe inflammatory disease.
  • Symptoms:
    • Rashes
    • Diarrhea
    • Liver disease

Mixed Leukocyte Culture

  • Purpose: To detect tissue incompatibilities.
  • Mix leukocytes from a potential donor with irradiated leukocytes from a potential recipient and vice versa.
  • If a mismatch occurs, donor leukocytes will proliferate and lyse host cells, and vice versa.

Immunosuppressive Drugs

  • Glucocorticosteroids:
    • Prednisone inhibits Macrophage activation
  • Small Molecule Drugs:
    • Azathioprine inhibits T-cell proliferation
  • Cytokine Inhibitors:
    • Cyclosporine, tacrolimus
  • Depleting Antibodies:
    • Rabbit polyclonal antilymphocyte globulin
    • Anti-CD52 (Campath-1h), anti-CD3
    • B cell depletion: anti-CD20
  • Non-Depleting Antibodies and Fusion Proteins:
    • Anti-CD25
    • CTLA4Ig fusion protein

Methods of Increasing Graft Survival

  1. Minimization of HLA disparity between the donor and the recipient by better HLA matching.
  2. Immunosuppressive therapy:
    • Drugs such as azathioprine, steroids, cyclosporine, antilymphocyte globulins, and monoclonal anti-T cell antibodies (e.g., monoclonal anti-CD3) are used.

General Immunosuppressive Therapy

  • Most immunosuppressive treatments are nonspecific, slowing the proliferation of activated lymphocytes, as well as any dividing non-immune cells (e.g., epithelial cells of the gut or bone-marrow hematopoietic stem cells).
  • Complications: Patients on long-term immunosuppressive therapy are at increased risk of cancer, hypertension, and metabolic bone disease.

Immune Tolerance to Allografts

  • An allograft may be accepted in two general cases:
    1. When cells or tissue are grafted to a so-called privileged site that is sequestered from immune-system surveillance.
    2. When a state of tolerance has been induced biologically, usually by previous exposure to the antigens of the donor in a manner that causes immune tolerance rather than sensitization in the recipient.

Fetus as an Allograft

  • A fetus can be considered an allograft within the mother's body.
  • Strain A and Strain B mates, skin graft fetus survives but is rejected after birth if mother does not become immunized with father’s Ags.

Why is Fetus Not Rejected?

  • Placenta acts as a barrier or filter.
  • It filters anti-MHC Abs.
  • Trophoblast (outermost layer of fetal tissue) is in direct contact with maternal blood.
  • Trophoblast expresses weak or no MHC.
  • Progesterone (hormone) acts as immunosuppressive.
  • Placenta expresses FasL.
  • Spontaneous abortions are sometimes triggered by a maternal immune response against the fetus.
  • Secretion of inhibitory cytokines (IL10, TGFßß, IL4).
  • Outer layer of placenta does not express MHC.
  • Placenta expresses FasL and secretes a substance that depletes tryptophan, therefore inhibiting T cell stimulation.
  • Fetal blood rich in Alpha feto protein is also immunosuppressive.
  • Mucopolysaccharide barrier rich in Sialic acid surrounds Trophoblasts
  • Uterine epithelium and trophoblast secrete cytokines that suppresses TH1
  • Tolerance of paternal MHC antigens

Fetal-Maternal Interface

  • Fetal Side:
    • Chorionic Villi comprises of Syncytiotrophoblast (IDO, PDL1)
    • Villous cytotrophoblast:(Complement regulatory proteins; DAF and MCP)
    • Blood vessels, stroma
  • Maternal Side:
    • Decidua (PDL1, IDO, FasL)
    • Treg (PDL1, CTLA-4)
    • uNK (KIR)
  • Interface:
    • Trophoblast giant cell (FasL, PDL1, Qa-2 (homologue of HLA-G)
    • Syncytiotrophoblast (IDO, Crry)
    • Treg (PDL1, CTLA-4)
    • Decidua (Treg, uNK cells, FasL)
    • Extravillous trophoblast (HLA-G, HLA-E, HLA-C, FasL, IDO, PDL1)
    • Column cytotrophoblast (PDL1)

The Fetus as an Allograft That Is Typically Not Rejected

  • Fetus is detected as mothers generate antibodies against father’s MHC proteins.
  • Placenta sequesters fetus from maternal T cells.
  • Trophoblast is major protective layer
    • Does not express MHC I and II
    • Expression of non-classical MHC molecules that bind to inhibitory NK cell receptors
    • Active tryptophan depletion
    • Secretion of inhibitory cytokines (IL10, TGFßß, IL4)

Ethical Aspects of Organ Transplantation

  • Organs for sale! - Raises ethical concerns regarding commodification of the human body.

MHC Polymorphism

  • The high degree of polymorphism in MHC genes is a factor in transplantation.

Ethical Cases

  • Case #1 (Anissa):
    • A 17-year-old with leukemia needs a bone marrow transplant, but there are no suitable donors.
    • Her parents decide to have another child in the hope that the infant will be a tissue match (25% chance).
    • Ethical Questions: Is it ethically right to conceive a child for the purpose of generating tissue for transplantation? If the infant is a tissue match, is it right for the parents to decide for the infant?
  • Case #2 (Mrs. Simpatico):
    • Mrs. Simpatico, a nurse, cared for Joseph, a 30-year-old patient who died.
    • The hospital policy requires nurses to ask the families of all dead patients for organ donations.
    • Mrs. Simpatico does not ask for organ donation because she believes the family needs comfort and not decisions at that moment.
    • The nursing supervisor reprimands Mrs. Simpatico and warns her.
    • Ethical Questions: Is the hospital’s policy good? Was it right for Mrs. Simpatico to make an exception in this case?