Transplantation immunology
The transplantation of organs from unrelated donors into
recipient to replace damaged or diseased organs is now widely
established. Cadaveric renal transplantation has become a routine
treatment for end stage renal failure and transplantation of the
heart, liver and pancreas, are being carried out with increasing
frequency. Unless suitable and reliable artificial support systems
become available or great advances in the prevention or early
treatment of the original disease occur, neither of which seem
likely in the near future, then the practice of organ transplantation
will continue to increase. The major barrier to effective organ
transplantation is rejection of the transplant by the recipient's
immune system. Before discussing the immunological aspects in
detail it is important to outline the terminology involved.
Autograft: A graft taken from one part of an individual and
transplanted to another site in the same individual, e.g. a skin graft
in a burned patient.
Isograft: a graft between genetically identical individuals
e.g. renal transplantation between identical twins.
Allograft: A graft between genetically non-identical
individuals of the same species (syn. Homograft), e.g. a cadaveric
renal transplant from an unrelated donor.
Xenograft: A graft between individuals of different species,
e.g. a monkey kidney into a human.
Orthotopic transplant: A transplant grafted into its normal
site.
Heterotopic transplant: A transplant placed into an abnormal
site, e.g. kidney transplantation into the iliac fossa and not the loin.
Immunological basis of allograft rejection
Tissue and organs transplanted between genetically disparate
individuals are, in the absence of immunosuppressive therapy,
rejected within a few days of grafting. The rejected graft becomes
heavily infiltrated by lymphocytes and monocytes, suggesting that
cell mediated responses rather than antibodies are involved in graft
rejection. The T lymphocytes plays an essential role in graft
rejection is clearly shown by the inability of congenitally athymic
or neonatally thymectomized animals to reject an allograft.
Restoring such animals with normal T cells readily restores their
ability to reject a graft.
Three main categories of graft rejection are recognized:
Hyperacute rejection:
This occurs almost immediately after transplantation. It is due to
the presence in the recipient of preformed cytotoxic antibodies
because of previous sensitization to graft antigens by blood
transfusion or an earlier graft. The complement fixing antibodies
rapidly damage the graft vasculature with platelet aggregation,
thrombosis and infarction. Fortunately careful pre-transplant
screening has almost eliminated this type of rejection.
Acute rejection:
This is due to classical cellular immune mechanisms and occurs to
some extent during the first few weeks of most transplants. The
graft becomes heavily infiltrated with T cells, B cells,
macrophages and NK cells. Both cytotoxic T cell lysis and delayed
type hypersensitivity responses appear to be capable of causing
graft damage, but in the majority of cases any such rejection
episode can be reversed by the immunosuppressive therapy.
Chronic rejection:
If a graft is not acutely rejected within the first few weeks, the
chances of long term survival are much improved. Nevertheless, a
small proportion of grafts steadily succumb to chronic rejection
many months after transplantation. Humeral responses appear to be
involved in late chronic rejection. The glomerular capillaries
become coated with antibodies and gradual fibrosis and loss of
renal function occur.
Tissue typing
Close matching of donor and recipient with respect to
transplantation antigens is an essential requirement for bone
marrow transplantation to avoid rapid graft rejection or graftversus-host disease. By contrast, matching for corneal graft is
unnecessary since the cornea appears to be an immunologically
privileged site. In renal transplantation (and possibly for other
transplanted organs) attempts to match the tissue types of donor
and recipient produce a definite increase in graft survival.
The first requirement in organ matching is to match ABO
blood group antigens, since these antigens are expressed not only
on red blood cells but on most cells of the body, and if mismatched
will elicit a strong immune response in the recipient due to the
presence of naturally occurring antibodies to the ABO antigens.
Tissue typing is employed to identify an individual's leukocyte
antigens or HLA type. These transplantation antigens are
expressed on most cells of the body and they are coded for by a
cluster of genes---- the major histocompatibility complex (MHC)--- situated on chromosome 6. The MHC codes for the alleles of the
HLA-A, HLA-B and HLA-C loci (class I MHC antigens), and for
the alleles of the HLA-DR, -DP and –DQ loci (class II MHC
antigens). HLA-A and -B loci and in particular the DR loci are
considered to code for the most important transplant antigens. To
date, there are 20 well defined A locus alleles, 33 B locus alleles
and 10 DR alleles. An individual possesses 2 alleles for each locus
(one inherited from each parent), but may be homozygous at one or
more loci. Logistically, for the majority of renal transplants,
perfect matching of donor and recipient at all loci is not possible.
In practice, it is common to attempt to match both or at least one of
the DR loci (since there are fewer DR types than HLA-A and-B
types) and there after, where possible the HLA-A and-B loci.
Retrospective studies after renal transplantation show that with an
increasing number of HLA-DR, -A and-B mismatches there is a
progressive decrease in the graft survival rate.
An individual's tissue type is determined in the laboratory
from a preparation of peripheral blood leucocytes; these are tested
serologically, using typing anti-sera. In addition to tissue typing, as
part of the organ matching procedure it is essential to perform a
cross match prior to transplantation, using recipient serum and
donor lymphoid cells, in order to exclude the presence in the
recipient of pre-formed cytotoxic antibodies if these antibodies are
present it could result in hyperacute rejection of the graft. It should
be emphasized that despite a complete match of the HLA-A, -B
and –DR locus antigens, rejection episodes may still occur in
response to minor histocompatability antigen differences, but these
tend to be amenable to immunosuppressive therapy.
Methods of preventing rejection
Although tissue typing reduces the chances of graft rejection,
it is essential to give immunosuppressive drugs after organ grafting
and these have to be continued indefinitely to prevent rejection.
Until recently the most widely used immunosuppressive regimen
was a combination of azathiobrine and prednezolone which act by
non-specifically ablating the immune response. Recently,
cyclosporin A, a newer drug, has been used increasingly. This
important drug is a metabolite of a fungus and its action is more
specifically directed at lymphoid cells than conventional
immunosuppressive drugs. cyclosporin A exerts its powerful
immunosuppressive effect mainly through its action on T cell
activation, preventing the development of cytotoxic T cells.
Unfortunately, its side effects include nephrotoxicity and
hepatotoxicity.
If signs of rejection occur, despite the presence of the above
immunosuppressive drugs, then additional immunosuppressive
measures may be introduced. These include the use of antilymphocyte serum (ALS). This serum is raised by injecting an
animal (commonly a rabbit or horse) with human lymphocytes.
When the globulin fraction of the resulting antiserum is injected
into the transplant recipient, it selectively destroys lymphocytes. A
drawback of this treatment is that it may cause hypersensitivity
reactions to the injected animal proteins. Recently the
administration of monoclonal antibodies directed at T lymphocytes
has also been tried with some success (OKT3). After injection, the
monoclonal antibody binds to T cells which are rapidly depleted
(probably by opsonisation) from the body. Both of the above
agents may be successful in reversing a graft rejection episode.
Tow other supplementary immunosuppressive treatments
which have been tried are thoracic duct drainage and total
lymphoid irradiation. Although both methods effectively deplete
the body of circulating lymphocytes, they are serious undertaking
and neither has been widely adopted in clinical practice.
Complications of immunosuppression
All immunosuppressive regimens used in organ
transplantation increase the risk of infection and malignancy.
Transplant recipients using immunosuppressive therapy are at high
risk from opportunistic infection, especially by viruses.
Opportunistic infection is a potential problem in all transplant
recipients, but those receiving aggressive immunosuppressive
therapy after liver, heart, lung and small intestinal transplantation
are most at risk. Chemoprophylaxis is important in high-risk
recipient.
The risk of bacterial infection is higher during the first month
after transplantation. It is a common practice to use a prophylactic
antibiotic in such patients as any other patient going to have a
major surgery to cover the per operative period. This is especially
applicable in patients who are ill before surgery.
The risk of viral infection is higher during the first 6 months
after transplantation and the major common problem is CMV
infection that may present as pneumonia, gastrointestinal disease,
hepatitis, retinitis or encephalitis. Now, it is common practice to
use a prophylaxis in the form of passive immunization
(hyperimmune immunoglobulin) or administration of antiviral
agents in the form of aciclovir.
Protozoal infection is serious and commonly affects the
recipient within the first 6 months after transplantation, the most
common organism is pneumocystis carinii and the best prophylaxis
is high doses of co-trimoxazole for 6 months after transplantation.
Fungal infections not common after renal transplantation but
more common after other solid organ transplantation. It occurs
within the first 3 months after transplantation and the most
common organisms are Candida or Aspergillus, if not early
diagnosed and aggressively treated it might be fatal.
Malignancies especially those viral related are common with
immunosuppressive therapy after transplantation and it might arise
up to 20 years after transplantation. The most commonly recorded
malignancy with immunosuppressive therapy are skin malignancy
like squamous cell carcinoma and basal cell carcinoma, also nonhodgikin's lymphoma is common whether affecting the lymph
nodes or extralymphatic.
Example of transplantation processes
Kidney transplantation
Renal transplantation is the preferred treatment for many
patients with end-stage renal disease because it provides a better
quality of life for them than dialysis. Transplantation releases
patients from the dietary and fluid restriction of dialysis and the
physical constraints imposed by the need to dialyse it is also more
cost-effective than dialysis.
The transplant kidney is placed in the retroperitoneal
position, leaving the native kidneys in place. After induction of
general anesthesia a central venous line and a urinary catheter are
inserted. It is helpful to distend the bladder with saline containing
methylene blue to allow to be identified with certainty prior to
ureteric implantation. A curved incision is made in the lower
abdomen and after dividing the muscles of the abdominal wall, the
peritoneum is swept upward to expose the ilial vessels. These are
dissected free so that they can be controlled with vascular clamps.
The donor renal vein is then anastomosed end to side to the
external iliac vein. The donor renal artery is anastomosed end to
end to the internal iliac artery. While the vascular anastomosis is
being under taken the kidney is kept cold by application of ice.
Following completion of the venous and arterial anastomosis the
vascular clamps are removed and the kidney is allowed to
reperfuse with blood. The ureter, which is kept reasonably short to
avoid the risk of distal ischemia, is then anastomosed to the
bladder in an antireflux manner. Before closing the transplant
wound it is important to ensure that the kidney is lying in a
satisfactory position without kinking or torsion of the vessels.
Technical complications:
Renal artery thrombosis occurs in 1% of cases and renal vein
thrombosis occurs in 5% of cases. The incidence might be
minimized by using small doses of heparin; the diagnosis is done
by the appetence of sudden onset of pain with swelling at the site
of the transplant and confirmed by Doppler study. The treatment is
nephrectomy.
Renal artery stenosis presents late, even years after
transplantation (about 10% of cases), presented with deterioration
of the kidney function and hypertension, usually treated now with
angioplasty.
Ureteric leak occurs in up to 10% of cases and can be
avoided with stenting and can be treated with reimplantation.
Early or late ureteric obstruction can be presented with
deterioration of the kidney function and diagnosed by
ultrasonography; surgical correction is needed although trials of
balloon dilatation may be of help.
Outcome after transplantation:
The half life of the transplanted kidney of living related
donor is much better than that of living unrelated which is again
much better than the cadaveric one. The range is 7 to 24 years.
Pancreas transplantation
Successful pancreas transplantation restores normal control
of glucose metabolism and obviates the need for insulin therapy in
patients with diabetes mellitus. Improved control of blood glucose
levels in diabetes undoubtedly reduces the risk of secondary
complications such as retinopathy, peripheral vascular disease and
nephropathy. However in considering the indications for pancreas
transplantation, these advantages have to be weighted carefully
against the risks posed both by the transplant procedure itself and
the immunosuppressive therapy required to prevent graft rejection.
Careful patient selection is essential to avoid excessive
mortality and morbidity. The procedure is usually reserved for
those patients with type I diabetes who are relatively young (under
the age of 50 years0 and do not have advanced coronary artery
disease or peripheral vascular disease. Echocardiography and
coronary angiography are mandatory during assessment of
recipient suitability for transplantation.
Most centers now perform transplantation of the whole
pancreas together with a segment from the duodenum. The
pancreas graft is placed intraperitonially in the pelvis and the donor
vessels of the graft are anastomosed to the recipient iliac vessels
and the exocrine secretions are dealt with by anastomosing the
graft duodenum to either the bladder (urinary drainage) or the
small bowel (enteric drainage).although urinary drainage has a
higher incidence of urinary tract infections but it has the advantage
that urinary amylase levels can be used to monitor graft rejection.
The results of pancreas transplantation have improved
significantly over the last decade. The one year patient survival
rate is greater than 90%.
The treatment of diabetes by transplantation of isolated islet
cells injected into the portal vein in the liver is under trial but the
results now is not that good as the number of the cells needed for
this is very high, the rejection rate is very high and the use of
capsules of semipermiable membranes containing the cells is under
trial.
Liver transplantation
The indication for liver transplantation falls into four groups:
 Chronic cirrhosis.
 Acute fulminant liver failure.
 Metabolic liver disease.
 Primary hepatic malignancy.
A transverse abdominal incision with a midline extension is
made and the diseased liver mobilized. Because of portal
hypertension the recipient hepatectomy is often the most difficult
part of the operation, especially if there has been previous surgery
in the region. The common bile duct is divided as are the right and
left hepatic arteries. The inferior vena cava is clamped and divided
allowing the recipient liver to be removed. Occlusion of the vena
cava and portal vein results in a reduction in cardiac output and
may necessitate the use of veno-venous bypass. The bypass
delivers blood from the vena cava and portal vein to the heart via a
canula inserted in the axillary vein or internal jugular vein. After
putting the donor liver in position, the supra and infra hepatic caval
anastomosis are performed. The portal vein and hepatic arterial
anastomosis are then performed and the graft is reperfused.
Finally, biliary drainage is re-established, usually by a duct to duct
anastomosis (without the use of a T tube), it may be necessary, for
example in recipients with biliary atresia or sclerosing cholangitis,
to reconstruct the biliary drainage by a bile duct to Roux loop
anastomosis.
Technical complications
1-Haemorrhage: correction of coagulopathy and meticulous
haemostasis is mandatory.
2-Heptic artery thrombosis may result spontaneously or as a result
of rejection and diagnosed by Doppler study to evaluate the
deterioration in liver function or biliary leak, this is usually need
retransplantation. Portal vein thrombosis is less frequent and
usually dose not need retransplantation.
3-Biliary leak is now less frequent and usually managed by
endoscopic dilatation or stenting.
The outcome of liver transplantation for a chronic liver
disease is much better than the outcome after transplant for acute
liver failure. The outcome after primary liver malignancy is good
but the transplant after a hepatitis C or hepatitis B is usually
followed by recurrence of the viral infection.
Small bowel transplantation
Small bowel transplantation is a treatment option for patients
with intestinal failure as defined by the loss of intestinal function
to the extent that long term parental nutrition is required. Intestinal
failure may be due to:
 Intestinal atresia.
 Necrotizing enterocolitis.
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Volvulus.
Disorders of motility.
Mesenteric infarction.
Crohn's disease.
Trauma.
Desmoid tumors.
The superior mesenteric artery of the graft (it is not advisable
to include the ascending colon in the graft) is anastomosed to the
recipient aorta, and the superior mesenteric vein is anastomosed to
the inferior vena cava or to the side of the portal vein. The
proximal end of the small bowel graft is anastomosed to the
recipient jejunum or duodenum. The distal end of the graft is
anastomosed to the side of the colon.
The one year graft survival rate is about 60% and after three
years the graft survival rate is around 40%.
Heart Transplantation
Transplantation is considered only in patients with end stage
heart disease which has failed to respond to all other conventional
therapy and where predicted survival without transplantation is
only six to twelve months.
A median sternotomy is performed and the patient is given
systemic heparin, placed on cardiopulmonary bypass and cooled to
280C. After cross clamping the aorta, the recipient heart is excised
at the mid-atrial level. The donor heart is then removed from ice
and the left atrium is then opened by making incisions in the
posterior wall between the orifices of the pulmonary veins to create
an atrial cuff. The left and then the right atrial anastomosis are
performed and the aortic and pulmonary arterial anastomosis are
then completed. The patient is then rewarmed and weaned from
cardiopulmonary bypass.
One- and five- year graft survival after heart transplantation
is around 85 and 70% respectively.
Bone marrow transplantation
This offers a chance of cure in an increasing number of
patients with otherwise fatal malignant and non-malignant
conditions. It is most often used in the management of
haematological malignancy. It also has a role in non-malignant
diseases such as sever combined immune deficiency; sever aplastic
anaemia and thalassaemia major. In essence, the patient is given a
very high dose of chemotherapy or total body irradiation to destroy
completely the malignancy and also the patient's own lymphoid
tissue and bone marrow which would otherwise rapidly reject the
graft. Bone marrow from a well matched donor, invariably a
sibling since this offers the best chance of good match, is then
transplanted at the appropriate time and the transplant recolonizes
the recipient's bone marrow. Although the technique is relatively
simple, marrow grafting is a hazardous process. Complications
related to graft rejection, infection and bleeding may be difficult to
control and even when the donor is an HLA identical sibling and in
the presence of immunosuppressive drugs, graft-versus-host
reaction is still common. The risk of graft-versus-host reaction can
be reduced if T cells can be eliminated from the donor marrow
prior to transplantation. A promising way of achieving this
involves incubating the harvested marrow with anti-T-cell
monoclonal antibodies, either with complement or bound to toxin,
in an attempt to destroy any T cells.
Questions:
1- Discuss immunological basis of allograft rejection and
methods of preventing rejection?
2- Discuss indications and technical complications of liver
transplantation?