Transplantation

Transplantation: Overview and Definitions

• Purpose of Transplantation:
  • Treatment of choice for end-stage organ failure.
  • Commonly addressed organs include:
  • Kidneys
  • Liver
  • Heart
  • Lungs
  • Cornea
  • Pancreas
  • Also used for curing haematological malignancies.

Historical Context

• A personal account highlights the emotional impact of organ donation:
  • 24 years ago, a family was informed their 6.75-year-old daughter was brain dead and they opted for organ donation.
  • Decision made after discussing donation as a family.
  • The experience provided hope to other families despite their tragedy.

Types of Organ and Tissue Transplantation

• Variety of Transplant Options:
  • Organs: Heart, kidneys, pancreas, lungs, liver, intestines.
  • Tissues: Bone, corneas, skin, heart valves, veins, cartilage, other connective tissues.
  • Most common form of transplantation is blood transfusion.

Categories of Transplants

1. Autograft:

   • Self-tissue transferred from one site to another within the same individual.

2. Isograft:

   • Tissue transferred between genetically identical individuals.

3. Allograft:

   • Tissue transferred between genetically different individuals of the same species.

4. Xenograft:

   • Tissue transferred between different species.

Blood Transfusion and Compatibility

• Blood types categorized by antigens present:
  • Type A: A antigen present, Anti-B antibodies.
  • Type B: B antigen present, Anti-A antibodies.
  • Type AB: A and B antigens present, no antibodies.
  • Type O: No antigens, Anti-A and Anti-B antibodies present.

Bone Marrow Transplantation

• Used specifically for conditions like:
  • Leukemia
  • Anemia
  • Immunodeficiency (e.g., SCID)
• Procedure:
  • Approx. $10^9$ cells per kg of host body weight injected intravenously.
  • The recipient undergoes immunosuppression before grafting, often using cyclophosphamide and total body irradiation.
  • Risk of graft-versus-host disease (GVHD) due to the introduction of immunocompetent cells.

Immunological Challenges in Transplantation

• Main Immunological Obstacle:
  • Immune response against allogenic MHC molecules.
  • Definition of Allogenic: Genetically different but from the same species.

Five-Year Survival Rates

• Displayed as a function of mismatched HLA alleles:
• Survival rate decreases with increasing number of mismatched alleles:
  • 0 mismatch: ~75% survival
  • 1 mismatch: ~70% survival
  • Increasing downward trend as mismatches increase.

MHC Complex and Transplantation

• MHC (HLA) Complex:
  • Unique molecules present on all nucleated cells, responsible for antigen presentation.
  • Essential for host immune recognition in grafts.

Allorecognition Mechanisms

• DIRECT ALLORECOGNITION:
  • Host T cell receptor recognizes non-self MHC (from donor).
  • Involves both CD8+ and CD4+ T cells:
  • Mechanism: Donor APCs present to recipient TH cells, activating immune response leading to graft rejection.
• Two Phases of Direct Allorecognition:
  1. Sensitization Phase: Allo antigens transported to lymph nodes, leads to activation of T cells.
  2. Rejection Phase: T cells migrate to graft, inducing damage and cytokine release.

Role of Cytokines in Graft Rejection

• Key cytokines involved in graft rejection:
  • IL-2: Promotes T cell proliferation.
  • IFN-γ: Important for DTH (Delayed Type Hypersensitivity) response.
  • TNF-α: Directly cytotoxic to graft tissues.
• Certain cytokines increase expression of MHC Class I and II molecules, contributing to rejection.

Importance of MHC Matching

• MHC matching improves graft survival significantly.
• Advances in MHC typing enhance precision in unrelated donor matching, crucial for bone marrow transplants.

Indirect Allorecognition

• Host APC presents graft antigen to T cells.
• Less common (<0.01%) for T cells but still significant in non-self recognition.

Mechanisms of Graft Rejection

1. Hyperacute Rejection:

   • Occurs within minutes/hours post-transplant.
   • Characterized by thrombotic occlusion due to pre-existing antibodies binding to donor antigens, activating the complement cascade.

2. Acute Rejection:

   • Can begin after the first week without immunosuppression.
   • High incidence (~30%) within the first 90 days post-transplant.
   • Mediated by T cells and antibodies, leading to vascular injury.

3. Chronic Rejection:

   • Common in solid organ transplants (heart, kidney, lung, liver).
   • Characterized by fibrosis, vascular abnormalities, and gradual loss of graft function.

Immunosuppression in Transplantation

• Immunosuppressive Strategies:
  • Systemic immunosuppression is essential for preventing rejection.
  • Initial therapy is often aggressive and can be reduced over time to promote tolerance.
• Common Immunosuppressive Drugs:
  • Cyclosporins: Inhibit T cell activation.
  • Azathioprines: Disrupt DNA/RNA synthesis.
  • Corticosteroids: Suppress inflammation and immune responses.

Side Effects of Immunosuppressants

• Increased risk of infections and malignancies.
• Specific adverse effects: hypertension, hyperlipidemia, diabetes.

Strategies for Inducing Immunological Tolerance

• Research on Tolerance:
  • Focus on achieving T cell tolerance minimizing the need for continuous immunosuppression.
  • Techniques include:
  • Co-stimulatory blockade to induce peripheral tolerance.
  • Pre-treatment with donor antigen to facilitate acceptance.

Case Studies of Induction of Tolerance

• Renal transplant preceded with bone marrow from donor was able to induce tolerance despite HLA mismatches over time, leading to sustained acceptance of the kidney graft.

Conclusion

• Not all allogenic MHC interactions provoke immune responses, allowing for potential therapeutic strategies that leverage regulatory T cells (Tregs) to induce tolerance.

Anecdotes on Organ Donation

• Emotional accounts shared regarding the impact of organ donation:
  • Stories of individuals appreciating the life-saving impact of transplants, ranging from heart to lung donations.