Transplantation Lecture Flashcards

Why is Transplantation Difficult?

• Humans have the same organs and red blood, but immune responses cause complications.
• MHC restriction and HLA haplotypes affect transplantation success.
• The immune system responds to transplanted organs, requiring immunosuppression.

Fundamentals of Transplantation

• Various organs and tissues can be transplanted, including kidneys, heart, and liver.
• Unless the donor and recipient are genetically identical, the transplant will be seen as foreign.
• Without immunosuppression, the immune system will destroy the transplant.

Terminology

• Donor: Person donating the organ or tissue.
• Recipient: Person receiving the transplant.
• Graft/Transplant: The organ or tissue being transplanted.

Types of Transplants

• Autologous Graft: Donor and recipient are the same person (e.g., skin graft for burns).
• Syngeneic Graft/Isograft: Between genetically identical individuals (twins).
• Allograft: Between genetically different individuals (most common).
  • Terms like allorecognition and alloresponse are related.
• Xenograft: Transplanted tissue from another species (e.g., pig xenografts).

Graft Rejection

• Grafts are rejected unless drug-induced immunosuppression is used.

Immune System Tolerance

• Self Cells: Tolerated, even though they differ from immune cells.
• Pathogens: Not tolerated; incite an immunological response.
• Harmless Foreign Antigens (Allergens): Should be tolerated, but intolerance leads to allergic disease.
• Non-Self Human Cells (Transplants): Not tolerated; cause an immunological response.

Drivers of Graft Rejection

• Recognition of non-self, mainly due to MHC molecules (also known as HLA).
• MHC molecules were first identified in transplantation as major histocompatibility molecules.
• Innate immune system activation by danger signals during surgery.
  • Activates innate immune cells and complement.

Human MHC (HLA) Molecules

• MHC and HLA are the same thing.

Class I MHC/HLA

• Present on all nucleated cells.
• Present antigen to CD8 T cells (cytotoxic T cells).

Class II MHC/HLA

• Only on antigen-presenting cells.
• Present antigen to CD4 T cells (helper T cells).

Gene Location

• Genes for MHC/HLA molecules are clustered on human chromosome 6.

Forms of Class I and Class II

• Three different forms of Class I and Class II.
• Class I has one alpha chain paired with beta-2 microglobulin.
• Class II has an alpha and a beta chain.

Class I Genes

• Three genes.

Class II Genes

• Three versions (HLA-DP, HLA-DQ, HLA-DR).
• Genes encode alpha and beta chains.
• Gene duplications occur (paralogs).

Expression of MHC/HLA Molecules

• Inherited alleles from mother and father are co-dominantly expressed.
• Each cell expresses six different types of Class I molecules.
• Antigen-presenting cells express at least six different types of Class II molecules.

Polymorphism of MHC Molecules

• Sequence variability exists across the human population.
• Over 23,000 different types of Class I and almost 9,000 Class II molecules.

HLA Haplotype

• Genes are closely packed together on chromosome 6 (tightly linked).
• Inherited as a package.

Polygenic and Polymorphic HLA Genes

• Multiple versions of each gene (polygenic).
• Massive genetic diversity between individuals (polymorphic).

Importance of HLA Haplotypes

• Haplotypes are unique like a fingerprint.
• Diversity in MHC allows binding to different epitopes, influencing T cell selection in the thymus.
• Variety in MHC is beneficial for species survival against new pathogens.
• Diversity is detrimental to transplantation due to the difficulty of finding exact matches.
• Different MHC molecules strongly trigger rejection.

Other Factors in Graft Rejection

Minor Histocompatibility Antigens

• Not related to antigen presentation.
• Proteins encoded by the Y chromosome (male to female transplants), mitochondrial DNA, gene deletions, or polymorphisms.

Non-Conventional Class I Molecules

• MICA antigens.

Antibodies

• Antibodies against the donor are formed after previous transplants, blood transfusions, or pregnancy.

Blood Group Antigens

• Can drive rejection.

T Cell Recognition of MHC Molecules

• Three different pathways.

Direct Pathway

• Donor antigen-presenting cells (APCs) migrate out of the graft.
• Recipient T cells recognize allo-MHC and peptide on donor APCs.
• CD8 T cells are stimulated and migrate to the graft, killing cells with donor MHC molecules.

Indirect Pathway

• Recipient APCs ingest antigen from the graft and present it to their own T cells.
• Stimulates mostly CD4 T cells, which produce cytokines and help B cells.
• CD8 T cells cannot bind to the blue MHC molecules, they will not recognize it.

Semi-Direct Pathway (Cross Dressing)

• Recipient APCs fuse their membrane with donor cells, acquiring donor MHC molecules.
• Stimulates CD8 T cells, which migrate to the graft and destroy cells with donor MHC molecules.

Strength of Immune Response to Non-Self MHC

• Strongest immune response is against other people's MHC.
• Many T cells respond to alloantigen.
• The precursor frequency is higher vs pathogens, so it doesn't take as long to reach the threshold for an active immune response.
• T cells are selected in the thymus to bind to peptide and self-MHC (MHC restricted).

Cross-Reactivity

• MHC molecules are different but can cross-react.
• T cells recognize an MHC molecule that's been chopped up as a peptide and bound when it's held by their own MHC.

Kinetics of Graft Rejection

• Hyperacute rejection.
• Acute rejection.
• Chronic rejection.

Hyperacute Rejection

• Occurs within minutes to hours.
• Mediated by pre-existing antibodies against blood group antigens or donor-specific antibodies.
• Antibodies bind and activate complement, leading to tissue injury and destruction.
• Rare due to blood group matching and crossmatching.

Acute Rejection

• Occurs within days to months.
• Mediated by CD8 T cells destroying the graft and CD4 T cells helping them.
• Passenger APCs migrate out of the graft and stimulate recipient T cells.
• Antibodies bind to the endothelium.
• Target of current immunosuppressive therapies.

Chronic Rejection

• Occurs over months and years.
• Mediated by CD4 T cells and antibodies (indirect pathway).
• Fibrosis and narrowing of blood vessels.
• Most common form of rejection.

Prevention of Graft Rejection

• Combination of pre-transplant and post-transplant strategies.

Before Transplant

• Blood type matching.
• Screening for donor-specific antibodies.
• HLA haplotype matching.

At and After Transplant

• Immunosuppression.
  • Induction therapy.
  • Maintenance immunotherapy.
• Management of rejection episodes.

Pre-Transplant Procedures

Blood Typing

• Match the blood type.

Screening for Donor-Specific Antibodies

• Prevent hyperacute rejection.
• Screen for HLA sensitization.

Lymphocyte Crossmatch

• Two common techniques:
  • Complement-dependent cytotoxicity.
  • Flow cytometry.

Complement-Dependent Cytotoxicity

• Recipient serum is mixed with donor lymphocytes and complement proteins.
• If the recipient has antibodies, they will bind to the donor's cells, activating complement and causing cell death.
• More than 20% cell lysis indicates a positive crossmatch.

Flow Cytometry

• Recipient serum is mixed with donor lymphocytes, and anti-human antibodies are added.
• Antibodies bind to the cells, and FACS (FITC) conjugated antibodies will bind to that antibody and give a positive signal with flow cytometry.
• If get a positive lymphocyte cross match, you cannot use that donor with that recipient.
• A positive signal indicates the recipient has antibodies against the donor.

HLA Typing

Inheritance

• Rare for two people to have the same HLA haplotype.
• Three core HLA genes sequenced:
  • HLA-A (Class I).
  • HLA-B (Class I).
  • HLA-DR (Class II).

Siblings

• Siblings have a one in four chance of being HLA identical.

Haploidentical

• Share one haplotype (50% chance).

Methods

• Sequencing (preferred over serological testing).

Importance of Typing

• Matching HLA types is still very important.
• Fewer mismatches result in more successful transplants.

Immunogenicity of Transplants

• Bone marrow, skin, and kidney are highly immunogenic.
• Cornea and liver are more tolerant.

Graft Versus Host Disease (GVHD)

• Occurs in bone marrow/hematopoietic stem cell transplantation.
• The donor's immune system attacks the recipient's cells.

Organ Allocation

• Depends on HLA typing, donor-specific antibodies, blood type, and other factors.
• HLA typing is vital as some patients may need more than one kidney.
• Important for hematopoietic stem cell transplantation due to high immunogenicity and GVHD risk.

Immunosuppression

• Targeting T cells.

Induction Therapy

• T cell depletion.
• Blocking the IL-2 receptor (IL-2 is important for T cell proliferation).
• Corticosteroids.

Maintenance Therapy

• Traditional immunosuppression drugs.

B Cell Involvement

• Monitored for anti-HLA antibodies.
• Biopsy may be required.
• Plasma exchange, IVIG, and steroids are treatments.
• Therapeutic monoclonal antibodies that deplete B cells.

Tolerance to a Graft

• Lifelong immunosuppression is not ideal (risk of opportunistic infections and cancer).

Operational Tolerance

• Stable graft function for a year after withdrawing immunosuppression.
• Spontaneous tolerance in liver transplants (up to 20%).

Mixed Chimerism

• Donor and recipient immune cells coexist.

Regulatory T Cell (Treg) Therapy

• Boosting Tregs to induce tolerance.

Lecture Summary

• Transplantation is difficult due to graft rejection.
• Human MHC/HLA molecules drive graft rejection.
• High polymorphism makes it hard to find an exact match.
• Allo-specific T cells are the main mechanisms of rejection (direct, indirect, and semi-direct presentation).
• Prevention includes blood type matching, donor-specific antibody screening, HLA matching, and post-transplant immunosuppression.