Graft rejection - Biomed PDF

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Clinical stages of graft rejectionand the futureClinical phases of graft rejectionBasic classification of clinical phases for graft rejection Post transplantation is based on histopathologic feature of the graft and also the time course of the rejection (hyper-acute rejections occur in first 24h, a...

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Clinical stages of graft rejection and the future Clinical phases of graft rejection Basic classification of clinical phases for graft rejection 

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Post transplantation is based on histopathologic feature of the graft and also the time course of the rejection (hyper-acute rejections occur in first 24h, acute rejections occur in the first few weeks and chronic rejections can occur from a few months to years after transplantation.) It is not based on the molecular basis of graft recognition and immune effector mechanisms

Different clinical phases of rejection Hyper-acute rejection 

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Rejection of organ graft within 24 hours of reperfusion = most severe response. directly comparable to type III hypersensitivity reactions

Acute rejection  Acute rejection is a process of

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Organs transplanted between disparate species can also cause this. (Pig Sugar galactose-α-1, 3galactose (αGal) has been implicated as a major factor in hyper-acute rejection in xenotransplantation.) Pre-existing antibodies binding to xenograft antigens on endothelium and prompting activation of alternative pathway owing presumably to speciesspecific function of factor H, Graft endothelial cell activation and destruction, activation of the coagulation cascade, and graft rejection within minutes or hours. Promotes thrombosis Leads to endothelial cell injury therefore exposing the subendothelial basement membrane proteins which activate platelets. The endothelial cells are stimulated to secrete high– molecular-weight forms of von Willebrand factor, which causes platelet adhesion and aggregation. Both endothelial cells and platelets undergo membrane vesiculation, leading to shedding of lipid particles that promote coagulation. Endothelial cells lose the cell surface heparan sulphate

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injury to the graft parenchyma and blood vessels mediated by alloreactive T cells and antibodies Before immunosuppressant acute rejection would take few days to few weeks after transplantation. The time of onset of acute rejection reflects the time needed to generate alloreactive effector T cells and antibodies in response to the graft. It’s a type IV cell mediated delayed hypersensitivity reaction Occurs if HLA incompatibility ➝ recipient T cells react to donor MHC molecules or donor peptides presented by MHC Alloantigens specific Tcells: Infiltrates of lymphocytes and macrophages. In kidney allografts, the infiltrates may involve the tubules (called tubulitis), with associated tubular necrosis, and blood vessels (called endotheliitis), with necrosis of the walls of capillaries and small arteries. The cellular infiltrates present in grafts undergoing acute cellular rejection include both CD4+ helper T cells and CD8+ CTLs specific for graft alloantigens, and both types of

Chronic rejection 

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Chronic rejection develops insidiously during months or years Chronic rejection of different transplanted organs is associated with distinct pathologic changes. In the kidney and heart, chronic rejection results in vascular occlusion and interstitial fibrosis. A dominant lesion of chronic rejection in vascularized grafts is arterial occlusion as a result of the proliferation of intimal smooth muscle cells, and the grafts eventually fail mainly because of the resulting ischemic damage Likely mechanisms underlying the occlusive vascular lesions of chronic rejection are activation of alloreactive T cells and secretion of IFN-γ and other cytokines that stimulate proliferation of vascular smooth muscle cells. As the arterial lesions of graft arteriosclerosis progress, blood flow to the graft parenchyma is compromised, and the parenchyma is slowly replaced by non-functioning fibrous tissue. Chronic rejection leads to congestive heart failure or arrhythmias in cardiac

proteoglycans that normally interact with antithrombin III to inhibit coagulation. These processes contribute to thrombosis and vascular occlusion, and the grafted organ suffers irreversible ischemic necrosis.

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T cells may cause parenchymal cell and endothelial injury. The helper T cells include IFN-γ– and tumour necrosis factor (TNF)-secreting Th1 cells and interleukin-17 (IL-17)–secreting Th17 cells, both of which contribute to macrophage and endothelial activation and inflammatory damage to the organ. Alloreactive antibody: binding of the alloantibodies to the endothelial cell surface triggers local complement activation, which causes lysis of the cells, recruitment and activation of neutrophils, and thrombus formation. Alloantibodies may also engage Fc receptors on neutrophils and NK cells, which then kill the endothelial cells. In addition, alloantibody binding to the endothelial surface may directly alter endothelial function by inducing intracellular signals that enhance surface expression of pro-inflammatory and procoagulant molecules.

transplant patients or loss of glomerular and tubular function and renal failure in kidney transplant patients.

Major Tissues and Mechanisms of rejections Cornea High success rate of initial corneal transplants is attributed to a process called immune privilege.  After the first corneal transplant is accepted, T regulatory cells prevent other types of immune cells from attacking and rejecting the transplant.  However severing corneal nerves, which occurs during the first transplantation, releases high levels of the neuropeptide Substance P. The resulting high Substance P levels disable the T regulatory cells needed for acceptance of subsequent corneal transplants. Heart Hyper acute Ab-mediated rejection (AMR) = swollen myocardium - complement & Ig in myocardial capillaries - inflammation reactions, intravascular macrophages, endothelial cell swelling and tissue oedema  Acute cell-mediated rejection leads to lymphocytic infiltrate begins in perivenular and progresses to cardiac interstitium amount which is the basis of grading rejection Bone marrow (Haematopoietic Cell Transplantation) major complication = graft vs host disease Rejection caused by recipient T-cells, NK-cells or antibodies which target CD34+/VEGFR-2+ cells (generate both haematopoietic and endothelial cells)

Skin Allogeneic skin grafts are invariably rejected in an acute fashion o Skin dendritic cells (DCs) migrate out of the graft through lymphatic vessels and infiltrate the recipient’s draining lymph nodes where they present donor antigens via two mechanisms: the direct pathway and indirect pathway. Activation of T cells via direct or indirect allorecognition is sufficient to trigger acute rejection of allogeneic skin grafts Kidney  Hyper acute rejection - mechanism begins is antibodies mediated and there is no treatment for it so kidney must be removed  Acute rejection in 10-30% - early decline in renal function  Chronic allograft nephropathy = immunological (acute rejection due to a degree of Human Leukocyte Antigen [HLA] mismatch) and nonimmunological is driven by chronic Ab-mediated rejection or other factors e.g. calcineurin inhibitor Blood Acute transplant rejection induced by blood transfusion reaction to the Kidd blood group system.  Due to antibody production against donor antigens usually present on contaminating white cells.

Screening methods to reduce immunogenicity of allografts The ABO blood group system is the most important blood type system (or blood group system) in human blood transfusion. The associated anti-A and anti-B antibodies are usually IgM antibodies, which are usually produced in the first years of life by sensitization to environmental substances such as food, bacteria, and viruses. Grafts will not survive if there are ABO incompatibilities between the donor and recipient. The ABO blood group antigens of the graft donor are selected to be compatible with the recipient. Blood group testing is particularly important in renal and cardiac transplantation. ABO blood group antigens of the graft donor are selected to be compatible with the recipient. Blood group testing important in renal and cardiac transplantation. HLA phenotyping Initially defined using serological micro-lympho-cytotoxicity techniques that used a battery of carefully selected antisera to recognize distinct HLAs on the surface of lymphocytes.

MHC/HLA contain non-polymorphic regions (conserved regions) so it is useful to design primers that bind to non-polymorphic sequences of exons encoding MHC class I & II. PCR can be performed to amplify the gene/ gene fragments (primers bind conserved regions & polymorphic regions are then amplified amplified). This method helps to provide precise information on molecular tissue typing o So the larger number of MHC alleles matched between donor and recipient the better the graft survival will be. o High resolution HLA typing, which is achieved by SBT, allows for differentiation of unique epitopes to which a recipient may make specific antibodies. High resolution HLA typing, which is achieved by Sequencing Based Typing, allows for differentiation of unique epitopes to which a recipient may make specific antibodies. One major limitation of SBT is that it often results in ambiguous typing due to the sharing of nucleotide sequences across the specific exons interrogated and additional testing is required to report the allele level typing results HLA matching in renal transplantation is possible because donor kidneys can be stored for up to 72 hours. In the case of heart and liver, organ preservation is more difficult and potential recipients are often in critical condition. Screening for preformed antibodies Patients in need of allografts are also tested for the presence of preformed antibodies against donor MHC molecules or other cell surface antigens. This can identify the risk for hyper-acute or acute rejections. Complement-dependent cytotoxicity (CDC) test consists of incubating patient serum with potential donor lymphocytes to establish if the recipient has donor-specific HLA antibodies (HLA-DSA). Rabbit serum as a source of complement is added and if HLA-DSA are present lysis of the cells occurs. This lysis can be detected by the original method of dye exclusion or by later developments which included fluorescence. o It lacks the sensitivity of the other assays described, and second, the assignment of positive and negative reactions can be compromised by viability of the cells used. Flow cytometry - The principle of the flow cytometry cross-match (FCXM) involves incubating donor cells with recipient serum and then adding a fluorescein-labelled second anti-human immunoglobulin antibody that binds to patient antibody bound to the donor cells. The test is read on a flow cytometer, and the degree of positivity is expressed as a channel shift. The main advantage of the FCXM is its sensitivity for antibody detection over the conventional CDC cross-match.

Xenografting Involves the transplantation of nonhuman tissues or organs into human recipients. The concept was pioneered a century ago, when transplanting human organs was considered ethically controversial. In light of the lack of supply of human organs for transplantation, several alternatives have been investigated and debated and an interest in the use of animal organs has increased. Transplantation with pig organs is currently being considered. However the problem with this is hyper-acute rejection--mediated by natural antibodies in

humans to pig antigens, complement fixation to endothelial cells, and the rapid onset of intravascular coagulation.  The major target of the natural IgM and IgG antibodies is the terminal carbohydrate epitope Gal α-(1, 3) Gal, formed by the α-1,3galactosyl transferase, which places a terminal galactose residue in a α-linkage to another galactose. The α-1,3galactosyl transferase in the pig gives rise to very high endothelial cell expression of Gal α-(1, 3) Gal, a ready explanation for the hyper-acute rejection of vascularized organs. In addition the parenchuma of liver and kidneys have high levels of Gal α(1, 3) Gal.  These tissues will all fail in a pig-to-human transplant in what can now be precisely defined in terms of antigen and antibody. We have already made some suggestions for removal of anti-Gal α (1, 3) Gal antibodies and if the procedure were technically feasible xenotransplantation could be attempted now, especially in patients doomed to a certain death because of the absence of a donor (especially for liver where ex vivo perfusion could be performed). Bacteria have Gal--1, 3-gal sugar structures on their surfaces this means that there will be a reaction to gut flora in the first six months after birth and also repeated infections with bacteria lead to formation of antibodies against Gal--1, 3-gal and these cross react with xenografts. To prevent hyper-acute rejection, it is possible to change the swine genome by a human gene modifying the set of donor’s cell surface proteins. The gene construct pGal-GFPBsd containing the human gene encoding α-galactosidase enzyme under the promoter of EF-1α elongation factor ensuring systemic expression can be introduced by microinjection into a male pro-nucleus of the fertilised porcine oocyte. This will then be injected back into the male pig which has the transgene mapping to chromosome 11p12. This will make the pigs organs be more viable for transplantation....