Transplantation Lecture Notes

Increasing Role: The significance of transplantation in clinical practice is growing yearly.
Transplant Statistics: As of 2010, the number of transplants has been on the rise, with kidneys being the most frequently transplanted organs, followed by livers, hearts, and lungs.
Focus of Lecture: Understanding the mechanisms behind transplantation rejection and methods to prevent it through effective matching.

Organ Acquisition:
- Example scenario: a traffic accident leading to the donation of a kidney.
- Organ Harvesting: Organs are harvested from deceased donors if medically viable.
- Transport to Transplant Center: The organ is transported quickly to the transplant center.
- Recipient Preparation: The recipient is alerted and prepares for surgery, often in a stressed or inflamed state.
Surgical Operation: A swift operation is critical to transplant the organ and enhance the likelihood of successful integration.

Inflammatory Conditions: Both donor organs and recipients might experience stress and inflammation due to trauma and underlying disease.
Goal of Transplantation: Reduce inflammation to enable the transplanted organ to function effectively.

Key Considerations: Matching is primarily based on the following factors:
1. Major Histocompatibility Complex (MHC)
- Ideal is to achieve compatibility at all MHC loci, specifically:
  - All three MHC Class I loci
  - All three MHC Class II loci (considering both alpha and beta chains).
1. Blood Type: Mismatched blood types initiate an immediate rejection response.

Alloantigen: Antigens from one individual that trigger an immune response in another individual of the same species.
Alloreaction: Immune reaction against alloantigens.
Allorecognition: Identification of alloantigens by the immune system.
Xenotransplantation: Transplantation of organs or tissues from non-human species, e.g., pigs.

Immunosuppressive Drugs: Utilized to prevent rejection by suppressing the recipient's immune system broadly (unlike vaccines that enhance immune responses).

Blood Transfusion:
- Details: Requires blood type matching (ABO and Rh factors).
- Components of blood may be transfused depending on the match.
Solid Organ and Bone Marrow Transplantation: Requires careful MHC matching.

Solid Organ Rejection:
- Recipient's T cells mediate damage to the grafted organ.
Bone Marrow Transplant Rejection / Graft Versus Host Disease:
- New immune system potentially attacks recipient's tissues.

Characteristics:
- Occurs rapidly.
- Pre-existing antibodies against HLA, MHC, or ABO blood group antigens lead to immediate reactions.
Mechanism:
- Antibodies bind to graft vasculature and trigger an inflammatory response including complement activation and neutrophil recruitment.
Outcome: Rapid graft destruction.

Characteristics:
- Common occurrence days to weeks after transplantation.
Mechanism:
- Mediated by effector T cells recognizing MHC differences.
- Donor-derived dendritic cells prime recipient T cells, leading to inflammation and damage.
Example Effects: Endothelialitis, organ damage due to inadequate oxygen supply.

Characteristics:
- Develops months to years post-transplant.
- Accounts for significant cause of organ failure.
Mechanism:
- Involves indirect recognition of donor MHC or minor histocompatibility antigens by recipient T cells.
- Results in vessel thickening and scarring, impairing function.
Important Notes:
- Minor Histocompatibility Antigens: Polymorphic proteins differentially recognized by the recipient’s immune system, leading to rejection despite MHC matching.
- Example: HY antigens between male donors and female recipients.

MHC Importance: Successful grafts depend on genetic relatedness and matching of MHC.
Example of Mouse Strains:
- Strain A (MHC allele A) vs. Strain B (MHC allele B).
- Graft from strain A to strain A yields no rejection, while strain B to A leads to full rejection.
- Hybrid mice (both A and B alleles) exhibit tolerance towards certain mismatched grafts.

Direct Allorecognition: Donor-derived antigen presenting cells express allo MHC, recognized by recipient T cells.
Indirect Allorecognition: Recipient dendritic cells process and present donor-derived peptides to T cells.

Blood Type Compatibility: Essential for blood transfusions.
- Consider A, B, AB, O blood types with Rh factor.
- Ideal match: O Rh negative donor with an Rh negative recipient.
- Rh positive donors can only donate to Rh positive recipients.

Hyperacute Reaction: Pre-existing antibodies target graft vasculature.
Acute Rejection: Mediated by CD4 and CD8 T cells recognizing donor MHC.
Chronic Rejection: Driven by indirect recognition through recipient-host interactions with donor MHC.

Key Drugs:
1. Cyclosporine: Impairs calcium flux post T cell receptor stimulation, inhibiting T cell activation.
2. Tacrolimus: Similar mechanism to cyclosporine.
3. Methotrexate: Inhibits DNA replication and cell cycle progression.
4. Cyclophosphamide: Also acts on DNA replication.

Indications: Treat immunodeficiencies (e.g., SCID), cancers.
Process: Myeloablative therapy destroys recipient’s immune system, allowing donor marrow to establish.
Graft Versus Host Disease (GVHD): Mature T cells in graft can attack host tissues, often targeting skin, liver, and GI tract.
Avoidance of GVHD:
- Autologous transplants (using patient's own stem cells).
- Removal of mature T cells from allogeneic transplants.

Mechanistic Focus: Different responses depend on organ type, significance of MHC matching, and understanding of rejections.
- Solid Organ Transplants:
- Hyperacute: Pre-existing antibodies reactions.
- Acute: Direct allorecognition mediated by CD4 + CD8 T cells.
- Chronic: Indirect allorecognition leads to long-term graft failure.
- Bone Marrow Transplants:
- Emphasis on myeloablative therapy and prevention of graft vs host disease.
- Importance of MHC matching for transplant success and longevity.