Immunology & Disease - Transplantation
Learning Objectives
Explain how MHC restriction and HLA types affect the success of transplantation.
Describe the immune response to transplanted cells or organs.
Explain how we use HLA matching and immunosuppression approaches to try to prevent rejection.
Transplant Fundamentals
Organs and tissues that are transplanted include bone marrow, stem cells, cornea, and sclera.
Unless the donor and recipient are genetically identical, the donated tissue is viewed as non-self.
Without drug-induced suppression of the recipient’s immune system, the donated tissue is destroyed in an immune response called graft rejection.
Transplant Terminology
Donor: Person donating the organ or tissue.
Recipient: Person receiving the organ or tissue.
Graft or Transplant: The organ or tissue being transplanted.
Syngeneic graft or Isograft: Transfer of cells or tissues between genetically identical individuals.
Autologous graft: Donor and recipient are the same patient (skin grafts or HSCT).
Xenograft: Transfer of tissues between individuals from different species.
Allograft: Transfer of cells or tissues between genetically disparate individuals of the same species.
Prefix allo- can be combined with other terms: allorecognition, alloresponse, alloreactive, alloantigen, allogeneic
What Drives Graft Rejection?
The immune system tolerates:
'Self' antigens.
Non-self, not human, and not harmful antigens.
The immune system does not tolerate:
Different, human, and 'non-self' antigens (transplant).
Different, not human and 'non-self', harmful.
Breakdown in tolerance can lead to:
Autoimmune disease (breakdown in tolerance to 'self').
Allergic disease (breakdown in tolerance to harmless food and environmental antigens).
Main Drivers of Graft Rejection
The donor organ or tissue is recognized as non-self:
Allogeneic HLA (MHC) – highly polymorphic.
Other polymorphic proteins.
The donor organ or tissue is perceived as a threat:
Innate immune responses triggered by donor brain death or cardiac death and ischemia-reperfusion injury signal danger to the recipient.
DAMPs or “alarmins” engage Toll-like receptors and other pattern-recognition receptors.
Complement activation.
Human MHC: Human Leukocyte Antigen (HLA)
Major histocompatibility (MHC) molecules are also known as Human leukocyte antigens (HLA).
MHC Class I:
Expressed on all nucleated cells.
Presents peptide antigen to CD8+ T cells.
MHC Class II:
Expressed on antigen-presenting cells.
Presents peptide antigen to CD4+ T cells.
Genes for HLA molecules
MHC Class I alleles: HLA-A, HLA-B, HLA-C (genes encode α chain).
MHC Class II alleles: HLA-DP, HLA-DQ, HLA-DR (genes encode α chain and β chain, paralogues exist).
HLA Expression
Alleles inherited from maternal and paternal chromosomes are co-dominantly expressed.
A person can express six different HLA Class I molecules on their cells.
A person can express six-eight different HLA Class II molecules on their cells (depending on expression of paralogues).
Polymorphism of HLA Genes
Genes encoding HLA molecules are HIGHLY polymorphic.
Many alleles are possible (alternate forms of a gene that code for slightly different varieties of the same protein product).
HLAI: alleles.
HLAII: alleles.
HLA Haplotype
The set of HLA genes are tightly linked and inherited together in Mendelian fashion.
The ‘inherited package’ of genes from a parent is called a: HLA haplotype.
Significance of HLA Polymorphism
Haplotypes are about as unique as a fingerprint.
Good for survival: polymorphisms in peptide-binding cleft mean there should always be individuals who can present peptide from different microbes and mount an effective immune response to a newly encountered or mutated microbe.
Bad for transplantation: it is almost impossible to get an exact match, and different MHC molecules are a strong ‘non-self’ trigger for rejection.
How are Grafts Rejected?
Non-self drivers of graft rejection:
Major histocompatibility complex (MHC) antigens (MHCI and MHCII).
Minor histocompatibility antigens:
Discovered after graft rejection in HLA-identical sibling HSCT.
Non-HLA proteins: proteins encoded by the Y chromosome, by mitochondrial DNA, gene deletion in recipient.
MHC class I related chain A antigens (MICA) (non-conventional MHC class I molecule).
ABO blood group antigens.
Donor-specific antibodies (from previous transplant, transfusion, or pregnancy).
How T Cells Recognize Donor MHC Molecules
Direct pathway:
Donor APCs migrate out of graft.
Recipient T cells recognize donor MHC (+ peptide).
CD8+ cytotoxic T cells can directly recognize allo-MHC on graft and destroy it.
CD4+ helper T cells produce cytokines and help B cells.
Indirect pathway:
Recipient APCs ingest donor Ag (commonly MHC).
CD4+ helper T cells produce cytokines and help B cells to produce donor-specific Ab.
CD8+ cytotoxic T cells stimulated this way CANNOT kill graft.
Semidirect pathway:
Intact donor MHC/peptide fused with the cell membrane of recipient APC.
Generates effector T cells that are donor-MHC restricted – so CD8+ cytotoxic T cells stimulated this way CAN kill graft.
Strength of the Alloresponse
Precursor T cell frequency:
Alloantigen ~ % of T cells.
Conventional Ag ~ % of T cells.
One of the strongest immune responses because the main driver is the different MHC molecule.
T Cell Activation and Graft Rejection
T cells are positively selected in the thymus to survive when their TCR binds to peptide + self-MHC molecules.
Cross-reactivity: some TCRs that bind self-MHC + peptide also recognize non-self-MHC + peptide.
We also have T cells that recognize non-self-MHC as the antigen (i.e., MHC molecules chopped up and presented as peptide) via direct and semi-direct pathways.
Mechanisms of Graft Rejection
Hyperacute rejection: Occurs within minutes to hours of transplant.
Acute rejection: Occurs within days or weeks of transplant.
Chronic rejection: Occurs over months or years after transplant.
Hyperacute Rejection
Mediated by preformed antibodies:
Antibodies specific for blood group antigens.
Donor-specific antibodies (DSA) (specific for allo-MHC), present due to previous blood transfusion, pregnancy, or prior organ transplantation.
Antibodies immediately bind to Ag on graft vascular endothelium and activate complement and clotting systems.
Leads to injury to endothelium and thrombus formation.
Rare in modern transplantation due to blood type matching and cross-matching to test for DSA.
Acute Rejection
Mediated by:
CD8+ T cells directly destroy graft, and CD4+ T cells secrete cytokines and induce inflammation that destroys graft.
Antibodies specific for alloantigens in endothelium lead to complement activation and vascular damage.
Current immunosuppressive therapy is aimed at blocking alloreactive T cells that cause acute rejection (direct or semi-direct pathway of allorecognition).
Chronic Rejection
Thought to be mediated by:
CD4+ T cells that produce cytokines that activate fibroblasts and vascular smooth muscle cells.
Antibodies may also contribute.
A slow form of graft damage that leads to progressive graft loss.
Manifests as fibrosis and narrowing of graft blood vessels.
Successes in preventing acute rejection haven’t translated to chronic rejection – it is becoming the principal cause of graft failure (indirect pathway of allorecognition).
How to Prevent Graft Rejection - Before a Transplant
Combination approach to prevent graft rejection:
Screen for donor-specific antibodies.
Blood type match.
HLA haplotype match.
At the time of transplant and after transplant: Immunosuppression, including induction therapy, maintenance therapy, and rejection episode therapy.
Detecting Donor-Specific Antibodies (DSAs)
Crucial for preventing hyperacute rejection.
DSAs against allo-MHC present due to blood transfusion, pregnancy, or prior transplant.
When a donor becomes available:
Lymphocyte cross-match determines the presence of antibodies against the specific donor.
On the waiting list, patient sera are frequently screened for HLA sensitization.
Lymphocyte Cross-Match
Does the recipient have antibodies that react with donor HLA (MHC) molecules?
Complement method: If antibodies are present, they bind to donor cells and activate complement, which lyses cells.
Flow cytometry method: If antibodies are present, they bind to donor cells, and fluorochrome-labelled anti-human IgG binds to these antibodies.
HLA Haplotypes: Inheritance
Consequences of Mendelian inheritance of HLA haplotypes:
% chance of being genotypically HLA identical.
% chance of being haploidentical (sharing one haplotype).
% chance of sharing no haplotypes.
HLA Typing (Histocompatibility Testing)
Traditionally done by serological testing, now done with molecular methods (i.e., sequencing of most important alleles).
Most important: HLA-A, -B, -DRβ1.
Importance of HLA Typing
With modern immunosuppression, is HLA matching important?
Depends on the type of transplant.
Some organs/tissues are more immunogenic than others and some have more risk of GVHD
HLA Typing – How Far to Go?
Organ allocation is based on HLA matching plus many other parameters (comorbidities and lifestyle), which all contribute to the estimated likelihood of survival after 5 years.
Very important for kidney transplants, where patients may receive more than one kidney throughout their life and be susceptible to HLA sensitization.
Very important for haematopoietic stem cell transplantation: highly immunogenic and high risk of GVHD.
Graft versus host disease: donor immune cells attack recipient cells.
How to Prevent Graft Rejection - Immunosuppression
Immunosuppression is mainly aimed at T cells.
Induction therapy.
Maintenance therapy (including rejection episode therapy).
Therapies that deplete T cells and reduce T cell activation:
Therapeutic monoclonal antibodies that deplete T cells or block the IL-2 receptor, combined with traditional immunosuppression (like corticosteroids).
Calcineurin inhibitors.
mTOR inhibitors.
Anti-proliferatives.
Steroids.
Transplant Immunosuppression - Targeting B Cells
Treating antibody-mediated rejection:
Monitor the development of donor-specific anti-HLA antibodies.
May need a biopsy.
Mainstays of therapy: Plasma exchange and/or IVIg + steroids.
Therapeutic monoclonal antibodies that deplete B cells are being investigated.
Can Tolerance to Graft Be Achieved?
Immunosuppression is associated with a higher risk of opportunistic infections and cancer.
Achieving ‘operational tolerance’ (also called functional tolerance) is the holy grail of transplantation: Stable graft function for a year after the withdrawal of immunosuppression.
When has it occurred?
Liver transplant: Up to % of liver transplants spontaneously develop tolerance.
For multiple myeloma with end-stage kidney disease: Kidney + Bone Marrow transplant can achieve mixed chimerism (co-existence of donor and recipient cells)
Tolerance Induction: Regulatory T Cell (Treg) Therapy
Many pre-clinical studies support the potential of Tregs to induce long-term graft survival or tolerance.
Treg manufacturing for cellular therapies is in clinical trials for organ transplantation.
Summary
The major drivers of graft rejection are the human Major Histocompatibility molecules (MHC) (human leukocyte antigens – HLA).
HLA genes are highly polymorphic, so there are thousands of alleles in the human population, making it very difficult to find an HLA match for transplantation.
Grafts are rejected after allo-specific T cells recognize alloantigen through direct presentation, indirect presentation, or semi-direct presentation.
Graft rejection is prevented by:
Blood type match.
Screening for donor-specific antibody.
Close HLA haplotype match.
Immunosuppression: induction and maintenance therapy (main focus is reducing T cell activation).