Chapter 15- Transplantation of Tissues and Organs Flashcards
Which solid organs are the most commonly transplanted?
Kidneys (58%) followed by liver (23%), heart (9%), lung (7%). Other transplants include kidney-pancreas and corneal transplants
The wait for organ transplantation
Every 10 minutes, a person is added to the waiting list. The average wait time is 2-3 years. Although over 138 million Americans are registered organ donors, the demand is greater than the supply. Around 8000 people die yearly waiting for an organ. This has spawned an international trade for human organs and tissues (especially kidneys)
Reasons why someone would need an organ transplant (6)
- Heart disease
- Diabetes
- Hepatitis
- Cirrhosis
- Injury
- Birth defects
Complications of organ transplants (4)
- Nosocomial infections
- Surgical complications
- Rejection
- Immunosuppressive drugs make people more prone to opportunistic infections
Blood transfusions
Blood is the most common transplanted tissue. Every 2 seconds someone in the US needs blood, and 21 million blood components are transfused each year in the US. The need increases during natural disasters and accidents
Blood matching
RBCs do not express MHC class 1 or class 2. However, donors and recipients must be matched for ABO and Rhesus D antigens
ABO blood antigens
These antigens are the carbohydrate component of the glycolipids on RBCs. There are O, A, and B antigens that all have different structures. Individuals generate antibodies against the antigen that they don’t express
O, A, and B antigen structures
The A antigen has the same basic structure as the O antigen, but contains a Gal-Nac molecule at the end farthest from the cell surface. The B antigen also has the basic structure of the O antigen, but contains a galactose residue at the end farthest from the cell surface
Alloantigens
Antigens that differ between genetically unrelated members of a species, from one person to the next. Blood types are one example, where one person might have an A blood type, one might have B, etc
Alloantibodies
Antibodies against alloantigens
Which antibodies do specific blood types generate?
Type O- generates antibodies against A and B
Type A- antibodies against B
Type B- antibodies against A
Type AB- neither antibody
What happens if a person is transfused with a blood type they have generated antibodies against?
The antibodies they have produced against the antigen fix complement and cause rapid clearance of the RBCs- this is a type 2 hypersensitivity reaction
Rhesus (Rh) antigens
+/- blood type indicates Rh status. An Rh - blood type will produce antibodies against the Rh antigen
Universal donor
O- blood type- lacks A and B, as well as Rh, antigens for other individuals to produce antibodies against
Universal recipient blood type
AB+ blood type- the recipient lacks antibodies against A, B, and Rh antigens, as these are all antigens they express themselves
Types of MHC molecules that everyone has (6)
- MHC class 1- HLA-A, B, and C
- MHC class 2- HLA-DP, DQ, and DR
Genetics of MHC molecules
MHC= major histocompatibility complex, can indicate how alike or different the molecules are from person to person. Polymorphisms can cause these molecules to change from one organism to the next. Each person expresses MHC haplotypes from their parents and can express antigens of paternal and maternal origin. All of these factors can contribute to a high amount of variability
Polygenic
Each person expresses 3 different types of MHC class 1 and MHC class 2
MHC matching
There must be sufficient matching between donor MHC molecules and recipient MHC molecules. If there aren’t enough matches, the donor organ will be seen as foreign by the recipient’s immune system
Alloresponse
Any immune response against antigens expressed on a transplanted issue or organ. This relies heavily on immunogenetics (HLA molecules)
Rejection
The recipient’s immune system reacts against a donor tissue or organ. May require another transplant
Graft vs host disease
In stem cell transplants- when donor T cells attack recipient tissues and organs
Hyperacute rejection
A type 2 hypersensitivity reaction- blood type must be matched, or antibodies specific for ABO and/or HLA antigens will bind to the blood vessels of the graft. This causes complement fixation, inflammation, hemorrhage, thrombosis, and the lack of oxygenated blood. The response occurs in minutes to hours, meaning that the organ must be removed
How do anti-HLA antibodies arise? (3)
- Multiple blood transfusions- different donors mean exposure to a variety of different HLA allotypes, which patients could develop antibodies to
- Prior organ transplants
- Pregnancy
How do anti-HLA antibodies arise during pregnancy?
A fetus can be considered an allograft in this situation because it generally expresses different HLA molecules. During birth, antigens can enter the mother’s bloodstream and she can make antibodies against any paternal HLA molecules
Acute rejection
A type 4 hypersensitivity reaction, mediated by T cells, that can take several days to occur. Dendritic cells can enter a transplanted organ and begin to collect antigens that are present. They then travel to secondary lymphoid tissues and present alloantigens to recipient T cells. Any alloantigen-specific T cells can go on to activate B cells, which make alloantigen-specific antibodies. Effector CD8 T cells can then damage and kill cells of the donor organ. Additionally, a TH1 response can occur, where resident macrophages are activated to cause more inflammation
Cross-match
A serological test for the presence of anti-HLA antibodies in the recipient. These assays include complement and flow cytometric cross-match assays
Complement cross match
Donor lymphocytes are mixed with recipient serum (which contains the recipient’s antibodies). If there are any antibodies against the HLA molecules from the donor cells, they will bind to the surface of the donor cell. If complement is added to the system, the complement can become activated, lysing the donor cells
Flow-cytometric crossmatch
A high throughput assay that can test multiple HLA types. Uses donor cells and recipient serum. Again, antibodies against any donor HLAs will bind to the donor cells. We add a second antibody with a fluorescent tag, which detects the interaction between the recipient antibodies and the donor cells. Causes a fluorescent reaction if the interaction does occur
Mixed lymphocyte reactions
Blood lymphocytes, monocytes, and dendritic cells are isolated from the recipient and cultured together with irradiated donor cells for 3-5 days.
Chronic rejection
A type 3 hypersensitivity reaction that occurs months or years post transplant. It is characterized by narrowing of the blood vessels (compromising the function of the donor organ) and chronic inflammation
Chronic rejection immune response
IgG antibodies are produced against alloantigens in the donor organ. The antibodies and the antigens form immune complexes which are deposited in the vessels supplying blood to the donor organ. Over time, the complexes induce inflammation due to activation of the complement system and innate cells. Other cells are recruited and infiltrate the area, such as CD40 expressing B cells and CD40L expressing helper T cells. This chronic rejection response is mostly inevitable- 50% of all kidney and heart transplants fail within 10 years
Pathways of allorecognition
Dendritic cells from the donor organ stimulates allorecognition through one of 2 pathways- direct and indirect
Direct pathway
Donor dendritic cells leave the graft and travel to a draining lymph node (secondary lymphoid tissue) to directly stimulate alloreactive CD4 and CD8 T cells
Indirect pathway
Recipient dendritic cells endocytose dead or dying cells from the donor organ. They present the antigen through MHC molecules on the tissue surface. Alloreactive CD4 or CD8 T cells are activated from there
How was the need for HLA matching determined?
Transplantation was pioneered with kidneys. It was found that outcomes improved when recipients had received an organ from a family member. Outcomes were worse when the organ was received from an unrelated donor, indicating that some genetic component must be involved
HLA role in transplant success
HLA matching significantly improves the success rate of clinical organ transplantation but does not prevent rejection. HLA typing may be imprecise, and there are minor histocompatibility antigens that the recipient may generate an immune response to. Unless the donor and recipient are identical twins, all graft recipients must be given immunosuppressive drugs to prevent rejection
Xenotransplantation
Using other species as a source for organs- solves the issue of an organ donor shortage. Some other mammalian species have similar organ physiology with a similar size to human organs. Although apes and monkeys have a high genetic similarity, they are protected and have low availability. Their gestation time is also longer and they produce less progeny. Pigs are a better alternative- they produce more progeny and have a shorter gestation time, and they are more abundant
Transgenic animals
Animal organs are still foreign, which may cause rejection. Transgenic animals would produce organs that are more similar to human organs. This is difficult to do in apes but has been performed in monkeys and pigs. We can also clone pigs, making them a better source for organs
Concerns about xenotransplantation
Pigs naturally express different antigens, such as alpha-1,3-galactose. Human antibodies then trigger a hyper-acute rejection by binding to the endothelial cells of the graft and initiating the complement and clotting cascades. Unlike in humans. CD59, DAF (CD55) and MCP (CD46) do not regulate the complement system to protect the cells from being attacked by complement. We need to make these molecules more human-like to generate a protective effect. Therefore, transgenic pigs expressing human DAF have been developed and are being studied. Pigs may also be infected with unique pathogens (porcine herpesvirus) that could infect the donor organ
Pregnancy and tolerance
Pregnancy can be viewed as a transplant in that the fetus can be considered an “allograft” that is repeatedly tolerated. This is because the fetus carries paternal MHC molecules and minor antigens that differ from those of the mother. No comprehensive explanation has yet emerged as to why the fetus is tolerated.
What are some reasons why the fetus may be allowed to survive inside the mother?
1/ Although mothers may possess antibodies against the father’s MHC molecules, the placenta does not express the classical MHC class 1 and 2 proteins.
2. The placenta is also known to possess proteins that inhibit NK activation, preventing an innate response against the fetus
3. Additionally, there is the production of inhibitory cytokines like IL-10 and TGF-β at the maternal-fetal interface (the placenta). Suppresses innate and adaptive responses
Allogenic stem cell transplant
Uses tissues or cells from genetically dissimilar individuals (an unrelated donor) of the same species
Autologous stem cell transplant
Uses tissues or cells obtained from the same individual. The cells were collected in the past and cryopreserved
What does the bone marrow transplant process involve?
To make space in the bone marrow, the patient undergoes chemotherapy and/or radiation to kill the existing cells in their bone marrow. The new bone marrow can then be infused into the recipient to repopulate their bone marrow
What are some factors to consider with stem cell transplantation?
Again, HLA matching is critical to prevent rejection and to reconstitute immune function. While we are irradiating a patient’s bone marrow, this does not impact the cells that already reside in the thymus. Therefore, even if the bone marrow is repopulated, the donor MHC molecules may not be able to engage the recipient T cells in the thymus.
What happens if a patient receives bone marrow of a different HLA type?
The bone marrow is repopulated with TCRs that cannot engage MHC molecules expressed in the recipient thymocytes. This is because donor TCRs were educated using different MHC molecules. Antigen presenting cells are derived from the donor (through hematopoiesis) and present antigens on donor MHC molecules. Therefore, recipient T cells do not recognize MHC and cannot recognize antigens. No T cell immune responses will occur
What happens if a patient receives bone marrow that matches their HLA type?
The recipient HLA molecules expressed in the thymus properly educate the donor T cells. When the T cells enter circulation, the MHC they have been educated with is similar to the MHC present on APCs. The T cells and innate cells can interact for an effective adaptive response
Graft vs host disease (GVHD)
A type 4 hypersensitivity reaction. It is a type of “reverse” rejection, where the bone marrow attacks the host rather than the host immune system attacking the organ. The donor bone marrow may contain mature and memory T cells, which will then circulate in the donor blood and enter secondary lymphoid tissues. The T cells can be activated and proliferate in response to the antigen in the recipient. The T cells become effector T cells that leave the secondary lymphoid tissue and attack the recipient’s healthy tissues and organs, causing additional tissue damage from the BMT conditioning regimen
Success of hematopoietic stem cell transplant correlates with
The extent of the HLA match. 30-70% of HSC transplant recipients develop acute GVHD, but the fewer the mismatches, the better the survival and health of the transplanted patient
Acute GVHD symptoms (4)
Symptoms are systemic
1. Rash (skin)
2. Nausea, vomiting, diarrhea (GI tract)
3. Jaundice (liver)
4. Dryness (eyes)
Chronic GVHD
As inflammation continues, a greater number of T cells are able to react to self antigens. May impact a single organ or multiple organs. Symptoms include a scaly rash, mouth ulcers, brittle nails, muscle aches, and breathing issues
Prevention of GVHD (3)
- HLA typing
- Immunosuppressive drugs
- T cell depletion of the donor bone marrow. We need to keep some T cells in order to help attack the recipient’s cancer cells
Immunosuppressive therapy
Utilized to prevent alloreactive T cells from causing organ rejection. These drugs can be given both before and after transplantation. Target the ability of alloreactive T cells to be activated (CD3 signaling), impact the secretion of cytokines like IL-2, impact the proliferation of cells, or block the IL-2 receptor
Adverse effects of immunosuppressive therapy
Immediate treatment with these drugs uses the highest doses and gives patients an increased susceptibility to infections. Long term maintenance doses result in an increased incidence of malignancies
Using antibodies for immunosuppression
Antibodies can be administered before and after transplantation to deplete alloreactive immune cells. They bind to the surface of cells in the recipient and targets them for destruction using complement activation and phagocytosis. One risk of this treatment is the risk of infection if there is over-suppression of the immune response
Rabbit anti-thymocyte globulin (rATG)
A mixture of antibodies that bind T cells, B cells, NK cells, and dendritic cells and remove them prior to transplantation
Alemtuzumab
An anti-CD52 monoclonal antibody. CD52 is expressed on the surface of mature lymphocytes, but not on the stem cells, so it can be used to remove alloreactive T cells in the recipient prior to transplant. Used in kidney transplantation since 1998
Corticosteroids
Widely used to treat acute and chronic inflammation and to reduce alloreactive responses. It works by binding to steroid receptors in the cytosol, which form complexes with heat shock protein 90. The complex of all 3 molecules undergoes a conformational change, and the steroid travels into the nucleus to act as a transcription factor. Generally, corticosteroid inhibit the activity of crucial transcriptional regulators of pro-inflammatory genes, including NF-κB and AP-1
Prednisone
A corticosteroid that targets the NF-κB pathway. Prednisone exists as a pro-drug, when taken up by the cells it assumes its active form (prednisolone). Prednisolone increases the production of IkBa, which traps NFkB in the cytoplasm. Therefore, NFkB is blocked from accessing the nucleus and turning on cytokine genes. Prednisone reduces inflammation and prevent inflammatory cell migration to sites of inflammation through multiple mechanisms
Cyclosporin A
Disrupts the ability of the cell to produce and act in response to IL-2 by disrupting signal transduction from the TCR. Binds to cyclophilin in the cytosol
Tacrolimus
Inhibits both T-lymphocyte signal transduction and IL-2 transcription. Binds to FK-binding proteins
How do cyclosporin and tacrolimus inhibit T-cell activation?
Both complexes bind to calcineurin, which prevents its activation by calcium and then blocks NFAT activation. NFAT cannot travel into the nucleus and activate T cells. The drugs inhibit cytokine secretion, proliferation, and cytotoxicity
Anti-CD3 antibody
Internalizes the TCR and prevents T cells from becoming activated, as they can’t engage with antigens on MHC. This is a mouse monoclonal antibody, so serum sickness is a potential risk
T cell co-stimulation
Even alloreactive T cells require 2 signals- from the TCR and the co-receptor. Alloantigens can be recognized using both CD4 and CD8 T cells. CD28 and B7 help to provide this co-stimulatory signal
CTLA-4
An inhibitory molecule that binds to B7 and sends a negative signal to the T cell, causing T cell deactivation. CTLA-4 is found in high levels in regulatory T cells, and is one of the ways that regulatory T cells are able to downregulate immune responses. CTLA-4 binds to B7 much more strongly than CD28
Belatacept
An inhibitor of T cell co-stimulation, FDA approved in 2011. Contains CTLA4, which has a high affinity for B7 and blocks T cell co-stimulation. It also contains an Fc region that allows the molecule to travel deeper into tissues and have a longer half life inside the body. Belatacept binds tightly to B7 and prevents the B7 and CD28 signal from occurring. Therefore, even if a donor antigen is presented to an alloreactive T cell, it will only get signal 1, not signal 2. The lack of the second signal leads to anergy (inactivation of the T cell). Can be given with cyclosporin to prevent IL-2 proliferative effects
Basiliximab
An inhibitor of cytokine signaling that was FDA approved in 1998. A chimeric mouse-human monoclonal antibody, which binds to the CD25 chain of the high affinity IL-2 receptor. Therefore, even if IL-2 is produced, it can’t bind to its receptor. The alloreactive T cell then can’t get a full signal from the cytokine, negatively impacting the activation of the alloreactive T cell
Daclizumab
An inhibitor of cytokine signaling. It is a humanized monoclonal antibody (90% human). Similar mechanism to Basiliximab
Inhibitors of T cell proliferation
An older class of immunosuppressives that kills alloreactive T cells by interfering with their replication and proliferation- therefore, they are considered cytotoxic drugs. Typically administered after transplantation. They have no selectivity and can kill healthy cells and tissues, so this class of drugs has the most severe side effects. Impacts the bone marrow and hair follicles, and can lead to anemia or thrombocytopenia
Cytotoxic drugs (4)
- Azathioprine
- Mycophenolic acid
- Cyclophosphamide
- Methotrexate
Azathioprine
Cytotoxic, inhibits purine biosynthesis
Mycophenolic acid
Cytotoxic, inhibits guanine synthesis, blocks cell division
Cyclophosphamide
Cytotoxic, cross-links DNA, blocks DNA replication
Methotrexate
Cytotoxic, inhibits thymidine synthesis, blocks DNA replication
Infectious disease and transplants
Infections are a concern with transplants, mainly due to immunosuppression. Nosocomial (hospital acquired) infections can occur as well. With long term immunosuppression, we are concerned with the reactivation of latent viruses such as CMV, EBV, HSV, and tuberculosis. Individuals may not be as responsive to vaccines, and could be more susceptible to community infections like influenza
