Immunology and Organ Transplantation

Overview of Solid Organ Transplantation

• Definition and Purpose

  • Solid organ transplantation is primarily used for end-stage organ disease or acute organ failure.

  • This treatment is considered a last resort but can significantly extend life.

Challenges in Transplantation

• Immune System as a Barrier

  • The immune system is a critical barrier to organ acceptance and survival.

  • Transplanted organs do not last indefinitely despite medical advances.

• Five-Year Survival Rates

  • Long-term graft survival rates are below 100%.

  • Immunosuppressive drugs have improved these rates but come with complications.

Immunosuppressive Therapy

• Need for Immunosuppression

  • Organ transplant recipients must take multiple immunosuppressive medications to prevent organ rejection.

  • The requirement for these medications can lead to a significant change in quality of life.

• Anecdotal Evidence

  • Example: Amy Silverstein lost her transplanted heart due to rejection and later died from metastatic cancer, which was exacerbated by her over-suppressed immune system.

• Life-Changing Side Effects

  • Patients may experience severe side effects from immunosuppressive drugs, affecting their daily lives.

Immune Responses in Transplant Rejection

• Types of Immune Recognition

  • Direct Allorecognition

  • Patient T cells directly recognize foreign HLA (human leukocyte antigen) molecules and peptides from the donor organ.

  • This response occurs rapidly and is often mitigated by immunosuppressive drugs.

  • Indirect Allorecognition

  • Donor-derived peptides are presented by recipient antigen-presenting cells (APCs) to the recipient's T cells.

  • This process can take years and often results in subclinical inflammation, eventually leading to antibody-mediated rejection.

Balancing Immune Suppression

• The Challenge

  • The key challenge in transplant medicine is to balance the degree of immunosuppression: too little leads to rejection, while too much increases the risk of infections and cancers.

Research Directions in Immunology

• The Pursuit of Immune Tolerance

  • Researchers have long aimed to achieve immune tolerance in transplantation, making it the norm rather than the exception.

• Combination of Kidney and Bone Marrow Transplants

  • Current best-studied method: kidney transplant alongside a bone marrow transplant from the same donor.

  • Goal: Create a mixed chimerism where the patient's immune system incorporates donor immune cells, thus reducing the likelihood of rejection.

Protocols for Inducing Chimerism

• Milder Conditioning Regimens

  • The variant procedure from Stanford Medical School incorporates a nonmyeloablative but lymphocyte-depleting regimen, avoiding the extensive toxicity of total body irradiation.

• Development of Mixed Chimerism

  • Mixed chimerism is achieved with a combination of donor and recipient immune cells.

  • Regenerative capacity of myeloid cells is preserved, allowing for recovery from potential damage during the transplantation process.

Mechanisms of Tolerance

• Central vs. Peripheral Tolerance

  • Central Tolerance involves the hematopoietic stem cells (HSCs) establishing a tolerant immune environment in the recipient's body.

  • Donor HSCs will seed into the recipient’s bone marrow alongside recipient HSCs, enabling a diverse immune cell population.

  • Peripheral Tolerance involves regulatory T cells (Tregs) suppressing donor-reactive T cells to prevent immune responses against the transplanted organ.

Current Practices and Findings at UCLA

• Ongoing Transplantation Studies

  • UCLA's current practices include kidney transplants followed by stem cell infusions from living donors for achieving immune tolerance.

• Success Stories

  • Example of the first success story with HLA identical siblings having transplants at UCLA without ongoing immunosuppression, thus enhancing patient quality of life.

Limitations of Current Procedures

• Need for Living Donors

  • This protocol is restricted to living donors, and not all patients have access to such donors.

• Waiting List Discrepancy

  • A significant number of patients remain on transplant waiting lists with insufficient donor organs, emphasizing the disparity in available organ transplants versus the need.

Research on HLA Mismatched Deceased Donors

• Hematopoietic Stem Cells from Deceased Donors

  • Ongoing research investigates whether stem cells from deceased donors can be utilized without harming other potential transplants.

• Challenges in Collection Methods

  • Initial collection methods for HSCs from deceased donors involved extensive disfigurement of the donor's body and raised concerns about infection and contamination.

New Approaches in Stem Cell Mobilization

• Mobilization Techniques

  • New techniques for HSC mobilization from bone marrow into peripheral blood aim to minimize risks to other organs during the process.

• Use of Apheresis Machines

  • Mobile apheresis machines are utilized to safely collect the necessary stem cells, proving effective in preliminary studies.

Ensuring Organ Viability

• Controlling Stem Cell Migration

  • Care must be taken to prevent mobilized stem cells from migrating to organs like the liver and lungs, to avoid immunological complications.

• Continuous Screening

  • Post-collection, evidence shows that no significant numbers of migrated NK cells were present, indicating a limited immune response.

Conclusion and Future Directions

• Future Studies

  • Continued research focuses on ensuring safety in the use of mobilized stem cells from deceased donors while preserving organ viability and enhancing transplant success rates.

• Funding and Collaborative Efforts

  • Collaborations with organizations like OneLegacy and CIRM are crucial for advancing research in immunology and transplantation therapy.

• Ethical Considerations

  • Ongoing discussions exist regarding ethical implications of using deceased donors for HSC mobilization and potential impacts on their organ donation viability.