Lecture 9 - Transplantation Immunology

transplant

grafting of living cells, tissues or organs from one individual to another

what do we transplant?

kidney (best survival rate), liver, heart, pancreas, lung, intestine, cornea and HSCs, all except corneal ones need long-term immunosuppression to avoid rejection

donor liver

able to regenerate to normal size and mass after 8-12 weeks

types of organ donors

person after death, living donor related to recipient, living donor non-related to recipient and person after brain death

isograft

from a genetically identical sibling or clone

allograft

donor is a genetically different member of the same species, majority of transplants

xenograft

transplant between two different species, highest rejection risk, clotting factors would cause an acute phase response

autograft

when donor and recipient are the same person, from one site to another

allograft rejection

adaptive immune responses to the grafter tissues are the major impediment to successful transplantation, defined as the response to non-self antigens on transplanted organs

graft rejection

when tissues containing nucleated cells are transplanted, T-cell responses to the highly polymorphic MHC molecules almost always trigger a response against the graft organ,

if nude mice lack a thymus what will happen?

they will not reject grafts since they lack T cells, immunodeficient, but the ability to reject cells can be restored in these mice via the adoptive transfer of normal T cells

skin graft rejection is the result of a T cell mediated response

if the transplant occurs between a syngeneic recipient (identical) the graft is tolerated, if its allogenic so there is an MHC mismatch the graft is rejected rapidly and if they try again the second time rejection is accelerated due to immune memory, second set rejection can also be transferred to new recipients by T cells from the initial recipient proving its caused by a memory type response, transfer accelerated rejection from a sensitized donor to a naive recipient

role of MHC in transplantation

grafting between different strains of mice results in distinct rates of graft rejection, this analysis defined one major locus that resulted in rapid graft rejection, MHC, in humans its HLA but in mice its H-2

first vs second set rejection

the grafted epidermis results in revascularization then there is cellular infiltration via active T cell response which results in thrombosis and necrosis but for second set it skips revascularization cause of immune memory

minor histocompatibility antigens

even with perfect match on all 6 MHC loci we still get slow rejection if there is no immunosuppression, these antigens are due to normal proteins that are in themselves polymorphic in a given population, amino acid differences in minor proteins can cause the grafter tissue to be slowly and sometimes chronically rejected,

H-Y antigens (male specific)

mothers of boys can be immunizes against male specific minor histocompatibility antigens during pregnancy and H-Y immunity can cause graft vs host disease after stem cell transplantations, female patients who received male kidney allografts are more likely than other gender combinations to develop H-Y antibodies and reject their allografts

tissue typing

the closer the tissue match the weaker the rejection response

negative crossmatch

this is when no donor-specific HLA antibodies are in the recipient serum so no antibody binds meaning no binding of fluorescein-labelled antibody

positive crossmatch

this is when donor-specific HLA antibodies are in the recipient serum so the antibody binds so you get binding of fluorescein labelled antibody

direct allorecognition

initiated by CD4+ and CD8+ T cells recognizing either allo-MHC molecules (MHC1 and 2 respectively) on donor APCs

indirect allorecognition

initiated by CD4+ and CD8+ T cells recognizing alloantigens bound to self-MHC molecules on recipient APCs, mismatched antigens that recognize their own APCs (presenting antigen in context of recipient MHC)

main difference between indirect and direct allorecognition

comes from donor/recipient origin of macrophages (type of APC), if its direct the DCs are donor derived and if its indirect the APCs are recipient derived

acute kidney rejection through direct allorecognition

kidney graft has DCs which then migrate to lymph nodes and spleen via blood, where they activate effector T cells, they migrate to the graft via the blood, destroying the graft, the APCS that are activating T cells are donor derived

hyperacute rejection

occurs within minutes to hours due to pre-existing antibodies to donor ABO or HLA class I antigens, its antibody-mediated so if you removed them it could be avoided, avoid by ABO-matching as well as cross matching, if you have a healthy kidney grafter into a patient with kidney failure and those pre-existing antigens are present against donor blood group antigens they will bind to the vascular endothelium of the graft, initiating an inflammatory response that occludes blood vessels causing clotting so the graft becomes engorged and purple due to hemorrhage

cross-matching

determining whether the recipient has antibodies that react with the WBCs of the donor

acute rejection

occurs within 10 to 13 days, its T cell mediated, reduced by tissue typing and treated with immunosuppressive agents, its a process of vascular and parenchymal injury mediated by T cells and antibodies that usually begins after the first week of transplantation

chronic rejection

occurs within months to years, the etiology is unclear but its associated with frequent acute rejection episodes due to the involvement of many immune cells, there is no effective treatment but aggressive immunosuppression and immunotherapy might approve outcome

how does chronic rejection work?

donor specific alloantibodies bind HLA molecules expressed on arterial endothelium of transplanted organ and recruit inflammatory cells, then endothelial injury enables immune effectors to enter the wall of the artery and inflict increasing damage, you could technically delete T cells to avoid this but this is hard to due once fibrotic scarring occurs, its characterized by fibrosis and vascular abnormalities with loss of graft function occurring during a prolonged period-no effective treatment

what cells are involved in graft rejection

CD4+/CD8+ T cells, B cells and antibodies, phagocytic cells (macrophages DCs), natural killer cells, unconventional T cells and vascular endothelial cells

how do antigens induce hyperacute rejection?

through fixation of complement, resulting in tissue injury and coagulation, complement activation also recruits macrophages and neutrophils, causing additional endothelial injury, complement causes inflammation via cytokines activating the membrane attack complex leading to damage

acute rejection treatment with immunosuppression

it has some adverse effects like drug toxicity, malignancy and opportunistic infection risk

non specific immunosuppression

cyclosporin, FK506, mycophenolate, anti-CD3 antibodies, prednisone (corticosteroid)

specific immunosuppression

manipulation of T cell activation via TCR antagonists (partial), costimulator blockade, tolerizing DCs, cytokine manipulation, Treg transfer or activating regulatory cascades (PD-1 and CTLA-4)

belatacept

immunosuppressor by interfering with co-stimulation

IL-2

we want to increase this to drive towards regulatory T cells and prevent graft rejection via immunosuppression

corticosteroid therapy

leads to net anti-inflammatory effect,it will reduce inflammation caused by cytokines via reduction of IL-1, TNF-a, GM-CSF, IL-3, IL-4, IL-5 and CXCL8, it reduces NO via reduction of NOS, it reduces prostaglandins and leukotrienes by reducing phospholipase A2, cyclooxygenase type 2 and annexin-1, reduces emigration of leukocytes from vessels by reducing adhesion molecules and induces apoptosis in lymphocytes and eosinophils by increasing endonucleases

graft vs host disease

mostly occurs in bone marrow transplantation cause cells are irradiated causing very weak immune system, donor T cells in the graft (marrow or blood) view the recipient’s body as foreign and the donated cells/bone marrow attack the body, donor CD8+ and/or CD4+ T cells are activated by recipient class 1+/2+ cells, marrow recipients are immunologically compromised and cannot initiate a counter attack, results in skin sloughing, diarrhea, inflammation of the lungs, liver and kidneys resulting from chronic inflammation and altered tissue permeability

T regs

prevents graft vs host disease, you want as many conventional T cells as possible cause patients are dying due to opportunistic infection so T regs can act as a buffer to eliminate inappropriate immune responses

how does graft vs host disease work?

basically its the donor T cells that will attack recipient tissues, the allogeneic HSCs have mature and memory T cells, they will circulate in blood to secondary lymphoid tissues and the alloreactive cells interact with DCs and proliferate, effector CD4 and CD8 T cells enter host tissues and cause injury

mixed lymphocyte reaction

this detects histoincompatibility and determines severity of graft vs host disease, the cells from the person who serves as the stimulator are first irradiated to prevent their proliferation, then they are mixed with cells from the other person who serves as the responder and cultured for 5 days, basically mix MHCa T cells with irradiated MHCb non-T cells as APCs and measure T cell proliferation + T cell cytotoxicity

measuring alloreactivity

early studies on T cell responses to allogeneic MHC molecules used the mixed lymphocyte reaction in which T cells from one person are mixed with lymphocytes from a second person, if the T cells of this person recognize the other individual’s MHC molecules are foreign, the T cells will divide and proliferate and will be cytotoxic, the lymphocytes from the second person are usually prevented from dividing by irradiation or treatment with the cytostatic drug mitomycin C, these studies show 1-10% of all T cells in an individual will respond to stimulation by cells from another, unrelated, member of the same species

graft vs leukemia

donor T cells in the allogeneic preparations of HScs recognize minor histocompatibility antigens expressed by the leukemia cells and kill the leukemic cells, depletion of T cells from HSCs abrogates graft vs host disease and graft vs leukemia effects need to balance the positive of graft vs leukemia with the negatives of graft vs host disease

what are the stages of transplant injury?

alloantigen independent injury, alloantigen dependent injury or both

allantigen independent injury

can think of the organ like a hotplate where the hotter it gets, the higher the chance of rejection is, if you can cool down the plate you will likely prevent this type of injury, it can look like ischemia/reperfusion injury (sudden rush of blood following o2 depletion in the organ) or injury mediated by hypoxia causing massive inflammation, this can lead to chronic rejection if its accompanied by dependent injury

alloantigen dependent injury

this is when tissue matching immunosuppression leads to acute allograft rejection

hepatic ischemic injury

this leads to leukocyte infiltration and hepatocyte death, in the kupffer cell the ischemia/reperfusion will cause recruitment of chemokines and leukocytes + TNFa which leads to neutrophil infiltration, apoptosis and necrossi in the hepatocyte

T reg cells role in transplant tolerance

they are CD4+CD25+FOXP3+ natural T regs will inhibit the proliferation of CD25- cells, CD4+CD25+ Tregs express cytotoxic T lymphocyte antigen 4 (CTLA-4) which interacts with CD80 and/or CD86 on the surface of APCs like DCs and this interaction delivers a negative signal for T cell activation, they are secreted or cell-surface transforming growth factor B (TGF-B) or IL-10 might have a role in suppression mediated by regulatory T cells

novel methods to test immunotherapies for rejection

ex vivo organ perfusion (can isolate organ pre-transplantation so all therapies can be used before implanting into recipient

the liver

functions in clotting, metabolism, clearing bacteria and processing fats, has regenerative capacity so you can lose up to 80% of it and it will grow back, diseases impair this regenerative capacity, 80% of liver cells are hepatocytes

cell barcoding

each cell is associated with a bead and each bead has a cell barcode then each cDNA fragment also has a barcode that links fragment to original cell so you can see all the genes making up cell transcriptional profile

chemokine receptors

they are involved in T reg recruitment and dampening of immune response, infuse with Tregs to promote graft tolerance, CXCR3 migrates into inflammed liver via CXCL9, 10 and 11, CCR4 allows recruitment to sites of inflammation via CCL17 and CCL22 from liver DC, CCR6 allows recruitment to the inflammed bile ducts via CCL20

pro-inflammatory phenotype

T cell expressing CD3, CD4, CTLA4, RORC and IKZF2 is most consistent with reg T cell with a Th17 like program, often referred to as a RORyt+ reg cell or Th17 like Treg

fetus as an allograft

its an allograft that doesn’t get rejected, the mother and father usually differ in HLA class I and II, during gestation multiple mechanisms establish immune tolerance at the maternal-fetal interface to prevent maternal immunity to the fetus, one of the minor histocompatibility antigens is male-female transplantation post birth due to mismatch, microenvironment in liver transplant rejection is similar to placental microenvironment

why does the fetus not get rejected?

cause it has no MHC class I or II in the trophoblast (it gets downregulated in layers outside the placenta), has HLA-G, IDO, progesterone and FasL, the outer layer of the placenta, the interface between fetal and maternal tissues is the trophoblast, this does not express classical MHC class I and I proteins making it resistant to recognition and attack by maternal T cells, tissues lacking MHC class I expression are vulnerable to attack by NK cells

how is the trophoblast protected from attack by NK cells?

via the expression of nonclassical and minimally polymorphic HLA clas molecules HLA-G, this protein is shown to bind to KIR1 and KIR2 (major inhibitory receptors) inhibiting NK killing

IDO

expressed at a high level by cells at the maternal-fetal interface, it catabolizes and thereby depletes the essential amino acid tryptophan at this site and T cells starved of tryptophan show reduced responsiveness, inhibiting it in pregnant mice using inhibitor 1-methyltryptophan causes rapid rejection of allogeneic but not syngeneic fetuses

progesterone

hormone of pregnancy because in prp the endometrium for embryo implantation and facilitating endometrial development, its critical to the very survival of a pregnancy, it inhibits T cell mediated rejection of tissue through shifting the immune response toward the TH2 pattern (IL-4, IL-5, IL-6)

FasL

appropriately positioned during pregnancy, first in the uterus and then in the placenta to deter trafficking of activated Fas positive immune cells