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Transplantation
Transplantation os the transfer (engraftment) of human cells, tissues or organs from a donor
to a recipient with the aim of restoring function(s) in the body
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Transfusion is the transfer of blood resulting in antibody
mediated reactions, whereas transplantation is the transfer of tissues or organs and results
in T cell mediated reactions
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Tissues that share sufficient antigenic
similarity, allow transfer without immunologic rejection, and are referred to as
histocomapatible
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Tissues that display significant antigenic
differences are histoincompatible and typically induce an immune response leading to tissue
rejection
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The
loci responsible for the most vigorous allograft rejection reactions are located within the
MHC
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Xenografts exhibit the greatest genetic
and antigenic disparity, engendering a rapid and vigorous graft rejection response
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When mice from two different inbred strains are mated, the
F1 progeny each inherit one maternal and one paternal haplotype
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However, neither of
the parental strains from the F1 offspring because each parent lacks one of the F1
haplotypes and will therefore reject these MHC antigens
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in matings between members
of an outbred population, there is only a 25% chance that two offspring will inherit identical
MHC haplotypes unless the parents share a haplotype
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Acceptance of an autograft is completed within 12-14 days; with extraversion occurring
between days 3-7, and healing occurring between days 7-10, and finally resolution occurring

between days 12-14
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Second set rejection (based on a secondary response) of an allograft begins
within 3-4 days, with full rejection by 5-6 days; cellular infiltration of leukocytes, phagocytes,
and other inflammatory cells occurs between days 3-4, and thrombosis and necrosis occurs
between days 5-6
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MHC matching
The function of MHC molecules is to bind to peptide fragments derived from pathogens and
display them on the cell surface for recognition by the appropriate T cells
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The MHC is polyallelic: contains multiple alleles at its genetic loci
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MHC molecules are codominantly expressed
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MHC polymorphism imparts an evolutionary survival advantage against mortality
from infectious disease
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Given
immunosuppressive drug regimens and immune tolerance induction protocols, solid organ
transplantations between individuals with significant or even total mismatch can be
successful, and this is still the course of action with tissues like the heart or lungs that do not
survive for more than a few hours outside the body, as this window of time is too short to
transport MHC-matched organs any significant distance
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Either serologic or molecular tests can be used to
determine the HLA compatibility - tissue typing
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Molecular assays provide greater
specificity and higher resolution than assays that characterize MHC molecules serologically,
using antigen-antibody interactions alone
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The transplant is rejected so quickly, the grafted tissue never becomes vascularised
...
Hyperacute rejection is an example of a type II
hypersensitivity reaction
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The most common targets
are ABO blood group antigens or MHC alloantigens
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This results in

endothelial damage that obstructs capillaries, preventing vascularisation of the graft
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Several mechanisms can account for the presence of antibodies specific for
MHC alloantigens, including; past blood transfusions that induced antibodies to MHC
antigens expressed on allogeneic white blood cells in the blood; past pregnancies, in which
women develop antibodies against paternal alloantigens of the fetus; exposure to infectious
agents, which can elicit MHC cross-reactive antibodies; or a previous transplant, which
results in high levels if antibodies to the allogeneic MHC antigens present in that graft
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Hyperacute rejection is irreversible and untreatable,
requiring removal of the transplanted organ
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Acute rejection occurs in
the absence of pre-existing immunity
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During the sensitisation phase, CD4+ and CD8+ T cells
recognize alloantigens expressed on cells of the foregin graft and proliferate in response
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The response to minor
histocompatibility antigens is weak, although the combined response to several minor
differences can be rigorous
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Activation of naive T cells requires presentation by an antigen presenting cell
that expresses the appropriate antigenic ligand/MHC molecule and provides the requisite
costimulatory signal
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Antigen presenting cells of host origin can also migrate
into a graft and endocytose the forgein alloantigens (both major and minor histocompatibility
molecules), where they become activated and present these antigens indirectly as
processed peptides bound to self MHC molecules
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The
cross-presentation ability of dendritic cells also allows them to present endocytic antigens in
the context of MHC class I molecules to CD8+ T cells, which can then participate in allograft
rejection
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T cells that respond to antigen via the T cell receptor in the absence of
costimulation or danger signals can become tolerant
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This suggests that exposure to
donor cells in this non-inflammatory context encourages tolerance to donor alloantigens
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Hallmark of the effector phase is a large influx of leukocytes, especially CD4+ T
cells and macrophages
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Th1 cells secrete IL-2 and IFN-gamma mediate graft rejection by promoting T cells
proliferation, delayed type hypersensitivity reactions, and the synthesis of IgG by B cells,
with resulting complement activation
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Acute rejection is cellular mediated rejection
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Alloreactive T cells
Alloreactivity refers to the ability of T cells to recognize peptide-allogeneic-MHC complexes
that were not encountered during thymic development, and manifests itself clinically as
transplant rejection and graft-versus-host-disease
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High determinant density: all graft’s MHCmolecules can
act as ligands for the alloreactive T cell receptor, thus there are thousands more available
ligands to the T cell receptor
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Numerous different T cell clones are activated by the allogeneic MHC/peptide
complexes
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The mechanisms include
both humoral and cell-mediated responses
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Chronic rejection is an example of a type III
hypersensitivity reaction
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The recruitment of inflammatory cells
causes; damage to graft vascular endothelium, smooth muscle proliferation, and migration of
effector T cells, in the tissue causing increasing damage and leading to vascular occlusion
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Chronic rejection is the
result of a chronic inflammatory state which results in diffuse scar tissue and stenosis of
vasculature of the transplanted organ
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Chronic rejection is resistant to reversal by standard immunosuppression, necessitating
another transplant
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The major clinical applications of bone marrow transplant include; providing a functional
immune system to individuals with a genetically determined immune deficiency such as

severe combined immunodeficiency, replacing a defective hematopoietic system with a
functional one to cure patients with life threatening non-malignant genetic disorders in
haematopoiesis such as thalassaemia, and restoring the hematopoietic system of cancer
patients after treatment with doses of chemotherapeutic agents and radiation so high they
destroy the system
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Bone marrow HSCs are needle aspirated from a living donor and implanted into the
recipient by intravenous injection
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Patients are severely
immunocompromised until bone marrow reconstitution, and thus are at high risk of infection
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GVHD occurs; when the graft contains immunologically competent cells, the recipient
expressess tissue antigens that are not present in the transplant donor, and the recipient is
incapable of mounting an effective response to eliminate the transplanted cells
...
GVHD occurs when donor T cells respond to genetically defined proteins on host
cells, the most important proteins are human leukocytes antigens
...
Genetic
differences in minor histocompatibility antigens between host and recipient can also result in
GVHD
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Clinical
features of chronic GVHD include; dyspigmentation, new onset alopecia, poikiloderma,
lichen planus-like eruptions of scleorotic features, nail dystrophy or loss, xerostomia, mouth
ulcers lichen type features, restrictions of mouth opening from sclerosis, dry eyes, sicca
syndrome, cleartical conjunctivitis, fasciitis, myositis, joint stiffness from contractures, vaginal
sclerosis, vaginal ulceration, anorexia, weight loss, esophageal web of strictures, jaundice,
transaminitis, bronchiolitis obliterans, pleural effusions, nephrotic syndrome, pericarditis,
thrombocytopenia, anemia, and neutropenia
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The
development of acute GVHD occurs in three phases; activation of antigen presenting cells;
donor T cell activation, proliferation, differentiation, and migration; and target tissue
destruction
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Damaged host tissues respond by
producing danger signals, including proinflammatory cytokines, chemokines, and increased
expression of adhesion molecules MHC antigens and costimulatory molecules of host
antigen presenting cells
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The danger signals generated in phase I augment this
activation by increasing the expression of costimulatory molecules, CD4+ cells induced

acute GVHD to MHC class II differences, and CD8+ cells induce GVHD to MHC class I
differences, natural killer T cell subsets of both host and donor modulate acute GVHD
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Th1 secrete
the cytokines IFN-gamma, IL-4 and TNF-alpha in large amounts in acute GVHD
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It also increases the sensitivity to monocytes and
macrophages to stimuli such as LPS and accelerates intracellular cascades in response to
these stimuli
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In contrast, IFN-gamma may suppress GVHD by hastening apoptosis of
activated donor T cells
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These soluble and cellular mediators
synergize to amplify local tissue injury and further promote inflammation and target tissue
destruction
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GVHD can also be
prevented through the use of umbilical cord blood as the source of HSCs, as the umbilical
cord is rich in stem cells and lacks alloreactive mature T cells, due to the lower risk of GVHD
with umbilical cord transplanted stem cells, a greater disparity of HLA can be tolerated
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The treatment of acute GVHD involves the use of
steroids, due to their potent anti lymphocytes and antiinflammatory activity
...
During ECP, the
patient's white blood cells are collected by apheresis, incubated with the DNA intercalating
agent, exposed to ultraviolet light, and returned to the patient
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Another strategy to treat GVHD is the blockade of inflammatory cytokine TNF-alpha
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The pathophysiology of chronic GVHD remains poorly understood, and is
treated with a variety of immunosuppressive agents
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Cross-matching is the
most important level of compatibility testing that occurs prior to solid organ transfer
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A positive cross-match means that the recipient has antibodies against
HLA proteins expressed by the donor and that these are likely to lead to rapid hyperacute
rejection
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Lymphocytes are obtained from peripheral blood and separated by centrifugation with a

ficoll-hypaque gradient
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If the lymphocytes
carry molecules recognized by the antibody, the antibody will bind to the cell and the cell will
be lysed
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Reactions are graded based on the percentage of lysis
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Mixed lymphocyte culture
The mixed lymphocyte reaction (MLR) is an in vitro assay in which leukocytes, from two
genetically distinct individuals of the same species, are cultures resulting in cell blast
transformation, DNA synthesis, and proliferation
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MLRs can identify
discrepancies in the HLA class II loci which microcytotoxicity may not detect
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The donor's
lymphocytes are treated so that they will not proliferate in the presence of the recipient's
lymphocytes
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If the recipient
lymphocytes react to the donor lymphocytes they will uptake the DNA and their radioactivity
can be measured and is a measure of the responsiveness of the recipient's lymphocytes to
the donor cells
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DNA techniques for compatibility testing
Restriction fragment length polymorphism (RFLP) is a difference in homologous DNA
sequences that can be detected by the presence of fragments of different lengths after
digestion of the DNA samples with specific restriction endonucleases
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Then the reaction fragments produced during DNA fragmentation
are analyzed using gel electrophoresis, the fragments are negatively charged and can be
easily separated by electrophoresis, which separates molecules based on their size and
charge
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An
RFLP probe is a labelled DNA sequence, frequently a cDNA clone
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RFLP can be used for HLA typing
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The initial step is the
HLA-locus-specific amplification of DNA by polymerase chain reaction
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Fluorochromes are linked with the probes to allow their detection by chemiluminescence
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The higher the number of probes the better the resolution level
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Most of the vast polymorphism of the HLA system results from conversion

events whereby small nucleotide sections of one allele are transferred to another allele
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Therefore probes are used which are sequence specific
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The advantages of molecular typing techniques over serological typing techniques include;
increased accuracy, increased resolution, high throughput, high reproducibility, and
simplified technique

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