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Immunologic Basis of Graft Rejection
The degree of immune response to a graft varies with the type of graft. The following
terms are used to denote different types of transplants:
Auto graft: is self-tissue transferred from one body site to another in the
same individual. Transferring healthy skin to a burned area in burn patients and
use of healthy blood vessels to replace blocked coronary arteries are examples
of frequently used auto grafts.
Iso graft: is tissue transferred between genetically identical individuals. In
inbred strains of mice, an iso graft can be performed from one mouse to
another syngeneic mouse. In humans, an iso graft can be performed between
genetically identical (monozygotic) twins.
Allograft: is tissue transferred between genetically different members of the
same species. In mice, an allograft is performed by transferring tissue or an
organ from one strain to another. In humans, organ grafts from one individual
to another are allografts unless the donor and recipient are identical twins.
Xeno graft: is tissue transferred between different species (e.g., the graft of a
baboon heart into a human). Because of significant shortages in donated
organs, raising animals for the specific purpose of serving as organ donors for
humans is under serious consideration.
Immunologic Basis of Graft Rejection
FULL ACCEPTANCE
FULL REJECTION
Schematic diagrams of the
process of graft acceptance
and rejection. (a)
Acceptance of an autograft
is completed within 12–14
days. (b) First-set rejection
of an allograft begins 7–10
days after grafting, with
full rejection occurring by
10–14 days. (c) Second-set
rejection of an allograft
begins within
3–4 days, with full
rejection by 5–6 days. The
cellular infiltrate that
invades an allograft (b, c)
contains lymphocytes,
phagocytes, and
other inflammatory cells.
Transplant Rejection
• Graft rejection depends on host recognition the
grafted tissue as foreign.
• The antigens responsible for such rejection are those
of the major histocompatibility antigen (HLA)
system.
• Rejection is a complex process in which both cellmediated immunity and circulating antibodies play a
role.
• The relative contributions of these two mechanisms
to rejection vary among grafts and are often
reflected in the histologic features of the rejected
organs.
T Cells Play a Key Role in Allograft Rejection
For example, nude mice, which lack a thymus and consequently lack
functionalT cells, were found to be incapable of allograft rejection; indeed,
these mice even accept xenografts
T Cells Play a Key Role in Allograft Rejection
Experimental demonstration that T cells can transfer allograft rejection. When T cells
derived from an allograft-primed mouse are transferred to an unprimed syngeneic
mouse, the recipient mounts a second-set rejection to an initial allograft from the
original allogeneic strain.
T Cells Play a Key Role in Allograft Rejection
The role of CD4+ and CD8+ T cells in allograft rejection is demonstrated by the curves
showing survival times of skin grafts between mice mismatched at the MHC. Animals
in which the CD8+ T cells were removed by treatment with an anti-CD8 monoclonal
antibody (red) showed little difference from untreated control mice (black). Treatment
with monoclonal anti-CD4 (blue) improved graft survival significantly, and treatment
with both anti-CD4 and anti-CD8 antibody prolonged graft survival most dramatically
(green). [Adapted from S. P. Cobbold et al., 1986, Nature 323:165.]
Similar Antigenic Profiles Foster Allograft Acceptance
the major histocompatibility complex (MHC) in the mouse and human. The MHC is referred to as
the H-2 complex in mice and as the HLA complex in humans. In both species the MHC is
organized into a number of regions encoding class I (pink), class II (blue), and class III (green)
gene products. The class I and class II gene products shown in this figure are considered to be
the classical MHC molecules. The class III gene products include complement (C) proteins and
the tumor necrosis factors (TNFα and TNFβ).
Similar Antigenic Profiles Foster Allograft Acceptance
Illustration of inheritance of MHC haplotypes in inbred mouse strains.
Similar Antigenic Profiles Foster Allograft Acceptance
Graft donors and recipients are typed for RBC and MHC antigens
(a) White blood cells from potential
donors and the recipient are added
to separate wells of a microtiter
plate.
The example depicts the reaction of
donor and recipient cells with a
single antibody directed against an
HLA-A antigen. The reaction
sequence shows that if the antigen
is present on the lymphocytes,
addition of complement will cause
them to become porous and unable
to exclude the added dye
Graft Donors and Recipients Are Typed for RBC and MHC Antigens
Typing procedures for HLA antigens. (a) HLA typing by microcytotoxicity.
(b) Because cells express numerous HLA antigens, they are tested separately with a
battery of antibodies specific for various HLA-A antigens. Here, donor 1 shares HLA-A
antigens recognized by antisera in wells 1 and 7 with the recipient, whereas donor 2
has none of HLA-A antigens in common with the recipient
Graft donors and recipients are typed for RBC and MHC antigens
Typing procedures for HLA antigens. (b) HLA typing by Mixed lymphocyte
reaction
Mixed lymphocyte reaction to determine identity of class II HLA antigens between a
potential donor and recipient.
Total lymphoid irradiation eliminates lymphocytes
the recipient receives multiple x-ray exposures to the thymus,
spleen, and lymph nodes before the transplant surgery. The
typical protocol is daily x-irradiation treatments of about 200
rads per day for several weeks until a total of 3400 rads has
been administered. The recipient is grafted in this
immunosuppressed state. Because the bone marrow is not xirradiated, lymphoid stem cells proliferate and renew the
population of recirculating lymphocytes. These newly formed
lymphocytes appear to be more tolerant to the antigens of the
graft
Graft donors and recipients are typed for RBC and MHC antigens
The effect of HLA class I and class II
antigen matching on survival of
kidney grafts.
Mismatching of one or two class I
(HLA-A or HLA-B) antigens has little
effect on graft survival. A single
class II difference (line 4) has the
same effect as 3 or 4 differences in
class I antigens (line 3). When both
class I and class II antigens are
mismatched, rejection is
accelerated. [Adapted from T.
Moen et al.,
1980, N. Engl. J. Med. 303:850.]
Cell-mediated graft rejection occurs in two stages
The process of graft rejection can be divided into
two stages:
1- A sensitization phase, in which antigen-reactive
lymphocytes of the recipient proliferate in response
to alloantigens on the graft,
2- An effector stage, in which immune destruction of
the graft takes place.
Transplant rejection Mechanism
1.
T Cell-Mediated Reactions:
–
It involves both delayed type hypersensitivity and T cell
mediated cytotoxicity.
The host recognition of donor HLA by two ways:
–
I.
Indirect recognition:
–
–
II.
Host CD 4+ T cells recognize donor HLA after they are processed and
presented by the host’s APC.
This recognition activates DTH.
Direct recognition:
–
–
–
Host T cells recognize HLA molecules on the surface of APC of the
donor.
Host T cells encounter the donor dendritic cells within the grafted
organ or after these cells migrate to the draining lymph nodes.
Both host CD4+ and the CD8+ T cells are involved in this reaction.
SENSITIZATION STAGE
EFFECTOR STAGE
Effector mechanisms (purple blocks) involved in allograft rejection. The generation or activity
of various effector cells depends directly or indirectly on cytokines (blue) secreted by
activated TH cells. ADCC = antibody-dependent cell-mediated cytotoxicity.
EFFECTOR STAGE
Antibody-dependent cellmediated cytotoxicity
(ADCC).
Nonspecific cytotoxic cells
are directed to specific
target cells by binding
to the Fc region of
antibody bound to surface
antigens on the target
cells. Various substances
(e.g., lytic enzymes, TNF,
perforin, granzymes)
secreted by the
nonspecific cytotoxic cells
then mediate target
Cell destruction.
EFFECTOR STAGE
Generation of effector CTLs.
Upon interaction with
antigen–class I MHC
complexes on appropriate
target cells, CTL-Ps begin
to express IL-2 receptors (IL2R) and lesser amounts of IL2. Proliferation and
differentiation of antigenactivated CTL-Ps generally
require additional IL-2
secreted by TH1 cells
resulting from antigen
activation and proliferation of
CD4+ T cells. In the
subsequent effector phase,
CTLs destroy specific target
cells
Mechanism of graft rejection
Both TH and TC are activated
- TC cells destroy graft cells by direct contact
TH cells secrete cytokines that attract and activate macrophages, NK
cells and polymorphs leading to cellular infiltration and destruction
of graft
- B cells recognize foreign antigens on the graft and produce
antibodies which bind to graft cells and
. Activate complement causing cell lysis
. Enhance phagocytosis, i.e. opsonization
. Lead to ADCC by macrophages, NK,PML
- Immune complex deposition on the vessel walls induce platelets
aggregation and microthrombin leading to ischemia and necrosis of
graft
Transplant rejection Mechanism
2.
Antibody-Mediated Reactions.
–
Ab’s produced against donor Ag can also mediate
rejection through two forms:
Hyperacute rejection:
I.
•
•
•
Immediate rejection soon after transplantation.
Occurs when preformed antidonor Ab’s are present in the
circulation of the recipient.
Seen in:
–
–
–
Recipient who has already rejected a kidney transplant.
Multiparous women who develop anti-HLA antibodies against
paternal antigens shed from her fetus.
Recipient of prior blood transfusions from HLA-nonidentical donors,
platelets and white cells are particularly rich in HLA antigens.
Transplant rejection
Mechanism
II.
Anti-HLA humoral Ab’s
•
•
•
It develops concurrently with T-cell mediated rejection.
Seen in recipients not previously sensitized to
transplantation antigens, exposure to the class I and
class II HLA antigens of the donor may evoke
antibodies.
The initial target of these antibodies in rejection
appears to be the graft vasculature.
Transplant rejection
Classification & Morphology
• On the basis of the morphology and the
underlying mechanism, rejection reactions are
classified as:
– Hyperacute.
– Acute.
– chronic.
Transplant rejection
Classification & Morphology
1. Hyperacute Rejection:
– This form of rejection occurs within minutes or hours after
transplantation and can be recognized by the surgeon soon
after the graft vasculature is anastomosed to the
recipient's.
– Hyperacutely rejecting kidney rapidly becomes cyanotic,
mottled, and flaccid and may excrete few drops of bloody
urine.
– This form of rejection is due to the presence of preformed
antidonor Ab’s in the host circulation.
– This form of rejection is rarely seen in today's practice
Pre-existing recipient antibodies mediate hyperacute rejection
Hyperacute Rejection
Steps in the hyperacute rejection of a
kidney graft.
Transplant rejection
Classification & Morphology
2. Acute Rejection:
– This may occur within days of transplantation in the
untreated recipient or may appear suddenly months or
even years later, after immunosuppression has been
employed and terminated.
– Acute graft rejection is a combined process in which both
cellular and humoral tissue injuries play parts.
– Histologically, humoral rejection is associated with
vasculitis, whereas cellular rejection is marked by an
interstitial mononuclear cell infiltrate, edam, and tissue
injury as well as mild interstitial hemorrhage.
Acute rejection is mediated by T-cell responses
Cell-mediated allograft rejection
manifests as an acute rejection
of the graft beginning about 10
days after transplantation
.because of a massive infiltration
of macrophages and lymphocytes
at the site of tissue destruction,
suggestive of TH-cell activation
and proliferation.
Transplant rejection
Classification & Morphology
3. Chronic Rejection:
– Present late after transplantation (months or years).
– Chronic changes are commonly seen in the renal allograft.
– Patients with chronic renal transplant rejection present
clinically with a progressive rise in serum creatinine level
over a period of 4 to 6 months.
– It is characterised by vascular changes (dense intimal
fibrosis), interstitial fibrosis, and loss of renal parenchyma.
Chronic rejection occurs months or years post-transplant
•Chronic rejection reactions develop months or years after acute
rejection reactions have subsided. The mechanisms of chronic
rejection include both humoral and cell-mediated responses by
the recipient . While the use of immunosuppressive drugs and
the application of tissue-typing methods to obtain optimum
match of donor and recipient have dramatically increased
survival of allografts during the first years after engraftment, little
progress has been made in long-term survival.
•Chronic rejection reactions are difficult to manage with
immunosuppressive drugs and may necessitate another
transplantation
Methods of increasing graft survival
1. Minimization of the HLA disparity between the donor
and the recipient by better HLA matching of the
donor and the recipient.
2. Immunosuppressive therapy:
–
drugs such as azathioprine, steroids, cyclosporine,
antilymphocyte globulins, and monoclonal anti-T cell
antibodies (e.g., monoclonal anti-CD3) are used.
General Immunosuppressive Therapy
Most of the immunosuppressive treatments that have been
developed have the disadvantage of being nonspecific;
-its slowing the proliferation of activated lymphocytes. And
any dividing non-immune cells (e.g., epithelial cells of the
gut or bone-marrow hematopoietic stem cells)
-are also affected, serious or even life-threatening
complications. Patients on long-term immunosuppressive
therapy are at increased risk of cancer, hypertension, and
metabolic bone disease.
some Transplantation rejection drugs
Mitotic inhibitor
Azathioprine (Imuran),, is often given just before and
after transplantation to diminish T-cell proliferation in
response to the alloantigens of the graft
methotrexate. Cyclophosphamide is an alkylating
agent that inserts into the DNA helix and becomes
cross-linked, leading to disruption of the DNA chain
Methotrexate acts as a folic-acid antagonist to block
purine biosynthesis
Corticosteroids
such as prednisone and dexamethasone, are potent
anti-inflammatory agents that exert their effects at
many levels of the immune response.
Fungal Metabolites Cyclosporin A (CsA), FK506 (tacrolimus), and rapamycin
these drugs block activation of resting T cells by inhibiting
the transcription of genes encoding IL-2 and the highaffinity IL-2 receptor (IL-2R), which are essential for cell
activation
Specific Immunosuppressive therapy
Monoclonal Antibodies •Monoclonal antibody to the CD3 molecule of the TCR
complex
•Diphtheria toxin is coupled with the antibody
•Monoclonal antibodies specific for the high-affinity IL2 receptor (anti-TAC).
•Monoclonal-antibody therapy for the cell-surface
adhesion molecules. ICAM-1 and LFA-1
Most of these monoclonal antibodies are mouse
origin. Many recipients develop an antibody response
to the mouse monoclonal antibody, rapidly clearing it
from the body. This limitation has been overcome by
the construction of human monoclonal antibodies and
mouse-human chimeric antibodies
Diphtheria toxin is coupled with the antibody
Blocking co-stimulatory signals can induce anergy
TH-cell activation requires a costimulatory signal
provided by antigen-presenting
cells (APCs). Interaction of B7
family members on APCs with
CD28 delivers the co-stimulatory
signal. Engagement of the
closely related CTLA-4 molecule
with B7 produces an inhibitory
signal. All of these molecules
contain at least one
immunoglobulin- liké domain and
thus belong to the
immunoglobulin superfamily
CTLA-4Ig, a chimeric suppressor of
co-stimulation.
(a) CTLA-4Ig, a genetically
engineered molecule in which the
Fc portion of human IgG is joined to
the B7-binding domain of CTLA-4.
(b) CTLA-4Ig blocks costimulation
by binding to B7 on antigen
presenting cells and preventing the
binding of CD28, a major costimulatory molecule of T cells
Blocking co-stimulatory signals at the time of transplantation can cause anergy instead of
activation of the T cells reactive against the graft. T-cell activation requires both the
interaction of the TCR with its ligand and the reaction of co-stimulatory receptors with
their ligands (a). In (b), contact between one of the co-stimulatory receptors, CD28 on the T
cell, and its ligand, B7 on the APC, is blocked by reaction of B7 with the soluble ligand CTLA4Ig. The CTLA4 is coupled to an Ig H chain, which slows its clearance from the circulation.
This process specifically suppresses graft rejection without inhibiting the immune response
to other antigens
Immune tolerance to allografts
There are two general cases in which an allograft may be
accepted:
1. Is when cells or tissue are grafted to a so-called
privileged site that is sequestered from immunesystem surveillance.
2. Is when a state of tolerance has been induced
biologically, usually by previous exposure to the
antigens of the donor in a manner that causes immune
tolerance rather than sensitization in the recipient.
Privileged sites accept antigenic mismatches
These sites include the anterior chamber of the eye, the
cornea, the uterus, the testes ,and the brain
(blood-brain barrier prevents the entry or exit of many
molecules, including antibodies.) cartilage or heart valves
Transplantation of artificial privileged tissue
(pancreatic islet cells were encapsulated in semi
permeable membranes (fabricated from an acrylic
copolymer) The transplanted cells are produce insulin
were not rejected, because the recipient’s immune cells
could not penetrate the membrane
Early exposure to alloantigens can induce specific tolerance
Experimental support for the notion that tolerance comes
from exposure of the developing organism to alloantigens
came from neonates of mouse strain experiments. If strain
A are injected with cells from strain C they will accept grafts
from C strain as adults.
Immunocompetence of the injected A-strain mice and
specificity of the tolerance is shown by the fact that they
reject grafts from other strains as rapidly as their untreated
littermates.
cattle of dizygotic twin
44
Transplantations routinely used in clinical practice. For the solid organs, the number of
transplants performed in the United States in 2000 is indicated. Estimates are included for
other transplants if available
Clinical Transplantation
Kidney
•The most commonly transplanted organ is the kidney
•Many common diseases, such as diabetes and various
types of nephritis, result in kidney failure that can be
alleviated by transplantation
•Two major problems are faced by patients waiting for a
kidney.
• is the short supply of available organs,
• is the increasing number of sensitized recipients
Transplantation of Hematopoietic Cells
(BM transplant)
• Use of hematopoietic cell transplants for
hematologic malignancies, certain nonhematologic
cancers, aplastic anemias, and certain
immunodeficiency states.
• Hematopoietic stem cells are usually obtained from
the donor bone marrow but may also be harvested
from peripheral blood after they are mobilized from
the bone marrow by administration of hematopoietic
growth factors.
Transplantation of Hematopoietic Cells
(BM transplant)
• In most of the conditions in which bone marrow
transplantation is indicated, the recipient is
irradiated with lethal doses either to destroy the
malignant cells (e.g., leukemias) or to create a graft
bed (aplastic anemias).
Transplantation of Hematopoietic Cells
(BM transplant)
• Two major problems arise in allogeneic bone marrow
transplantation:
– Graft-Versus-Host (GVH) disease
– Transplant rejection.
Transplantation of Hematopoietic Cells (BM transplant)
GVH disease
– Occurs in any situation in which immunologically
competent cells or their precursors are transplanted into
immunologically crippled recipients.
– GVH disease occurs most commonly in the setting of
allogeneic bone marrow transplantation but may also
follow transplantation of solid organs rich in lymphoid
cells (e.g., the liver) or following transfusion of unirradiated blood.
Graft Versus Host (GVH) Reaction
An immunologically competent graft is transplanted into an
immunologically suppressed recipient (host)
* The grafted cells survive and react against the host cells
i.e instead of reaction of host against the graft,
the reverse occurs
* GVH reaction is characterized by fever, pancytopenia,
weight loss, rash , diarrhea, hepatsplenomegaly and death
Transplantation of Hematopoietic Cells
(BM transplant)
– the immunocompetent T cells derived from the
donor marrow recognize the recipient's HLA
antigens as foreign and react against them. With
sensitization, antirecipient CD4+ and CD8+ T cells
are generated.
– It can be acute or chronic.
Heart Transplantation
• The patient must be kept alive by wholly artificial means until
the transplanted heart is in place and beating.
• Heart-lung machines are available to circulate and aerate the
patient’s blood after the heart is removed. The donor’s heart
must be maintained in such a manner that it will begin beating
when it is placed in the recipient
• the one-year survival rate for transplantation of the heart has
become greater than 80%.
• Accident victims who are declared brain dead but have an intact
circulatory system and a functioning heart are the normal
source of these organs
Lung Transplants
• Either by itself or in conjunction with heart transplantation,
• Has been used to treat diseases such as cystic fibrosis and
emphysema or acut damage to the lungs such as that caused
by smoke inhalation.
• First-year survival rate for lung transplants is reported at
about 60%.
Liver Transplants
• Liver malfunction can be caused by damage to the organ from
viral diseases such as hepatitis or by exposure to harmful
chemicals, as in chronic alcoholism.
• Damage to the liver may correct itself and the damaged tissue
can regenerate after the causative injurious agent is cleared
• One-year survival rate has risen approximately 65%.
• A liver from a single donor may be split and given to two
recipients
• GVHD have occurred in liver transplants even when donor and
recipient are blood-group compatible
Pancreas Transplantation
• diabetes mellitus. This disease is caused by malfunction of
insulin-producing islet cells in the pancreas.
• one-year success rates for pancreas transplantation of about
55%.
• Transplantation of the complete pancreas is not necessary
transplantation of the islet cells alone could restore function.
• Kidney failure is a frequent complication of advanced diabetes
occurring in about 30% of diabetics, therefore kidney and
pancreas transplants are indicated
Skin Grafts
• Most skin transplantation in humans is done with autologous
tissue
• Rejection must be prevented by the use of
immunosuppressive therapy
• But burn victims is the high risk of infection, with
immunosuppressive taking
Xenotransplantation may be the answer to the shortage of
donor organs
• The insufficient supply of available organs means that a large
percentage of patients die while waiting for a transplant
• The larger nonhuman primates (chimpanzees and baboons)
have served as the main transplant donors
• No attempt has met with great success
• With xenotransplants is that immune rejection is often quite
vigorous, even when recipients are treated with potent
immunosuppressive drugs
• Xenotransplantation has the potential of spreading pathogens
from the donor to the recipient(HIV-2)
• The possibility of introducing new viruses into humans may be
not greater for transplants from more distantly related
species, such as pigs, because viruses are less likely to
replicate in cells from unrelated species