PRINCIPALES OF ORGAN TRANSPLANTATION

The field of organ transplantation has made remarkable progress in a short period of time. Transplantation has evolved to become the treatment of choice for end-stage organ failure resulting from almost any of a wide variety of causes. Transplantation of the kidney, liver, pancreas, intestine, heart, and lungs has now become commonplace in all parts of the

world .

Definition:

Transplantation is the act of transferring an organ, tissue, or cell from one place to

another . .

Types:

Broadly speaking, transplants are divided into three categories based on the similarity between the donor and the recipient:

1-Autotransplants

2-Allotransplants 3-Xenotransplants

.

1-Autotransplants involve the transfer of tissue or organs from one part of an individual to another part of the same individual. They are the most common type of transplants and include skin grafts and vein grafts for bypasses.

NO immunosuppression is required

2-Allotransplants involve transfer from one individual to a different individual of the same species—the most common scenario for most solid organ transplants performed

today .

Immunosuppression is required for allograft recipients to prevent rejection .

3 -Xenotransplants involve transfer across species barriers. Currently, xenotransplants are largely relegated to the laboratory, given the complex, potent immunologic barriers to success.

TRANSPLANT IMMUNOBIOLOGY:

The success of transplants today is due in large part to control of the rejection process, thanks to an ever-deepening understanding of the immune process triggered by a transplant.

Transplant Antigens:

The main antigens involved in triggering rejection are coded for by a group of genes known as the major histocompatibility complex (MHC). In humans, the MHC complex is known as the human leukocyte antigen (HLA) system. It comprises a series of genes located on chromosome 6.

HLA molecules can initiate rejection and graft damage, via humoral or cellular mechanisms:

Humoral rejection mediated by recepient's AB. (e.g. blood transfusion, previous transplant, or pregnancy)Cellular rejection is the more common type of rejection after organ transplants. Mediated by T lymphocytes, it results from their activation and proliferation after exposure to donor MHC molecules.

Complications Of Organ Transplantation

1 -Rejection

2-Malignancy

1-Clinical Rejection:

Graft rejection is a complex process involving several components, including T lymphocytes

,B lymphocytes, macrophages, and cytokines, with resultant local inflammatory injury and graft

damage .

Rejection can be classified into the following types based on timing and pathogenesis: hyperacute,

acute, and chronic.

A-Hyperacute rejection:

This type of rejection, which usually occurs within min after the transplanted organ is reperfused, is because of the presence of preformed antibodies in the recipient, antibodies that are specific to the donor. These bind to the vascular endothelium in the graft and activate the complement cascade, leading to platelet activation and to diffuse intravascular coagulation. The result is a swollen, darkened graft, which undergoes ischemic necrosis.

B-Acute rejection:

This used to be the most common type of rejection, but with modern immunosuppression it is becoming less and less common. Acute rejection is usually seen within days to a few months posttransplant. It is predominantly a cell-mediated process, with lymphocytes being the main cells involved. With current immunosuppressive drugs, most acute rejection episodes are generally asymptomatic. They usually manifest with abnormal laboratory values (e.g., elevated creatinine in kidney transplant recipients, and elevated transaminase levels in liver transplant recipients) .

C-Chronic rejection:

This form of rejection occurs months to years posttransplant. Now that shortterm graft survival rates have improved so markedly, chronic rejection is an increasingly common problem. Histologically, the process is characterized by atrophy, fibrosis, and arteriosclerosis. Both immune and nonimmune mechanisms are likely involved. Clinically, graft function slowly deteriorates over months to years

CLINICAL IMMUNOSUPPRESSION:

The success of modern transplantation is in large part because of the successful development of effective immunosuppressive agents.

Two types of immunosuppression are used in transplantation: Induction and

Maintenance immunosuppresion.

1-Induction immunosuppression

refers to the drugs administered immediately posttransplant to induce

immunosuppression .

2-Maintenance immunosuppression

refers to the drugs administered to maintain immunosuppression once recipients have recovered from the operative procedure. Individual drugs can be categorized as either biologic or nonbiologic agents. Biologic agents (monoclonal and polyclonal antibodies) consist of antibody preparations directed at various cells or receptors involved in the rejection process; they are generally used in induction (rather than maintenance)

protocols .Nonbiologic agents (e.g. corticosteroids,azathioprine and cyclosporines)form the mainstay of maintenance protocols.

2-Malignancy:

Transplant recipients are at increased risk for developing certain types of de novo malignancies, including nonmelanomatous skin cancers (3–7-fold increased risk), lymphoproliferative disease (2–3-fold increased risk), gynecologic and urologic cancers, and Kaposi sarcoma. The risk ranges from 1 percent among renal allograft recipients to approximately 5–6 percent among recipients of small bowel and multivisceral transplants.

The most common malignancies in transplant recipients are skin cancers. They tend to be located on sun-exposed areas and are usually squamous or basal cell carcinomas. Often they are multiple and have an increased predilection to metastasize. Diagnosis and treatment are the same as for the general population.

Patients are encouraged to use sunscreen liberally and avoid significant sun exposure.

Sources of organs for transpalntation: The current Main Sources of organs for transpalntation are:

1-Deceased (cadaver) donor (however the recipient has to wait till this cadaver becomes available)

2-Living donor transplantation (has medical, ethical, financial, and psychosocial problems).

The biggest problem facing transplant centers today is the shortage of organ donors. Mechanisms that might increase the number of available organs include:

( 1 )optimizing the current donor pool (e.g., the use of multiple organ donors or marginal donors) ;

(2 )increasing the number of living-donor transplants (e.g., the use of living unrelated

donors) ;(3 )using unconventional and controversial donor

sources (e.g., using deceased donors without cardiac activity or anencephalic donors);

(4 )performing xenotransplants.

New directions for organTransplantation:

STEM CELLS ,

CELL THERAPY

AND

TISSUE ENGINEEERING

Cell therapy can be defined as «The use of living cells to restore, maintain or enhance

the function of tissues and organs .»The use of isolated, viable cells has emerged as an experimental therapeutic tool in the past decade, due to progress in cell biology and particularly in techniques for the isolation and culture of cells derived from several organs and tissues

Cell-based therapy is one of the more recent approaches in regenerative medicine that aims at replacing or repairing organs and tissues. Different cell types have been used, such as skeletal myocytes, which have been injected into infarcted cardiac scar tissue, or neuronal cells inoculated into the brains of patients with nervous disorders. Alternative approaches include extracorporeal organ replacement for kidney and liver failure, the potential transplantation of xenogenic organs and cells and stem cell therapy.

Forms (types) of cell therapy:

1-Extracorporeal bioartificial organs used as assistance devices.

2-Injections, implantations or transplantation of cells.

1-Bioartificial Organs (Assistance

Devices): Extracorporeal support systems most frequently

use a hollow fiber cartridge containing immobilized cells with mass exchange requiring either direct contact with perfused blood or through a semi permeable membrane separating cells from blood.

Howevr, although the bioartificial organs are an attractive technology with therapeutic potential, the limited availability of normal human cells has prevented the technology from being utilized in clinical settings

2-Injections, implantations or transplantation of cells

Strategies (Methods) of Transplantation:

1-Transplantation into blood stream:The reported problems with this method are :emboli,

cells carried to inappropriate sites, difficulties for engraftment, and cells not in ideal environment.

2-Transplantation by grafting (Tissue engineering):

It is ideal for cells from solid organs with less complication than blood infusion. It requires implanting aggregated cells or, ideally, cells on scaffolding [e.g., polylactide meshes.

Cell sourcing remains among the most critical difficulties in the development of cell therapies, whether for bioartificial organs

or for cell transplantation.

This proplem could be alleviated by use of stem cells (this is called stem cell therapy), especially probably in combination with grafting methods, because the progenitor cells can be cryopreserved, have dramatic expansion potential, and have low or negligible immunogenic antigens that can possibly be managed with minimal need for immunosuppressive drugs.

Why stem cell?

The following stem cell characterisics make them good candidate for cell based therapies:

1-potential to be harvested from patients.

2-High capacity of proliferation in culture.

3-Ease of manipulation to replace existing non functioning genes via gene transfer methods.

4-Ability to migrate to host’s target tissues.

5-Ability to integrate into host tissues.

Stem cells have 4 main properties :

1-Unspecialized.

2- Self renewal.

3-Potency :Stem cells are either:

Totipotent (e.g. fertilized ova).

Pleuripotent(e.g. ES cells, EC cells and EG cells , the last two are less desirable for research).

Multipotent (e.g. tissue stem cells).

Unipotent (e.g. hepatocytes, skin and corneal stem cells).

4-Robust repopulation (functional, long term tissue reconstitution).

And moreover the flexibility in expressing these characteristics and serial transplantability should be feasible

.Cells that fulfill all these criteria are called "actual stem cells." The cells that possess these capabilities but do not express them are named "potential stem cells." (Potten and Loeffler, 1990 and Dabeva et al., 2003).

Scientists took about 20 years to learn how to grow human embryonic stem cells in the laboratory following the development of conditions for growing mouse stem cells.

Scientists primarily work with two kinds of stem cells from animals (mouse) and humans: which are

embryonic stem cells and adult stem cells.

• means treatment in which stem cells are induced to differentiate into the specific cell type required to repair damaged tissues.

• Right now, only few diseases are treatable with stem cell therapies because scientists can only regenerate few types of tissues.

Stem cell therapy

However, the success of the most established stem cell-based therapies (blood and skin transplants) gives hope that someday stem cells will allow scientists to develop therapies for a variety of diseases previously thought to be incurable.

• Only non-ESCs have been used clinically so far. Bone marrow cells were first used successfully 4 decades ago, and cord blood stem cells in the past 10–15 years. These cells have been of benefit for blood disorders such as leukemia, multiple myeloma and lymphoma; and disorders with defective genes such as severe combined immune deficiency.

Stem cell therapy

• As yet, ESC has not been used clinically.

There are no current approved treatments or human trials using embryonic stem cells.

ES cells, being totipotent cells, require specific signals for correct differentiation - if injected directly into

the body; ES cells will differentiate into many different types of cells, causing a teratoma.

There are in fact only few and modest published successes using animal models of disease.

Various potential therapeutic applications of human embryonic stem cells )hES( (Habibullah, 2007).

Much of the work with stem cells is preclinical, relying on results obtained from mice or rats. In the following cases (neurological disorders and cardiovascular disease) phase I clinical trials are still several years into the future (Panno, 2005).

Obstacles to stem cell therapy

There are many ways in which human stem cells can be used in basic research and in clinical research. However, there are many technical hurdles and obstacles between the promise of

stem cells and the realization of these uses, which will only be overcome by continued

intensive stem cell research.

Obstacles to stem cell therapy

A- For ESCs: There are three major problems:1-Ethical proplem (ethical issues): There are many

ethical dilemmas in stem cell and cloning research, and in their use in therapy, concerning

- the isolation of cells,

- consent and donation,

- the destruction of potential life forms for the treatment of others.

These hurdles are:

It must be demonstrated that to alleviate human suffering does not necessarily justify the use of any means to achieve it.

Obstacles to stem cell therapy

A- For ESCs: There are three major problems:2- Immunological rejection problems (rejection).

3- Biological proplems: e.g. teratomas , chromosomal abnormalities and possible contamination of the stem cells with retoviruses and other animal pathogens

These hurdles are:

It must be demonstrated that to alleviate human suffering does not necessarily justify the use of any means to achieve it.

Obstacles to stem cell therapy

B- For Non–ESCs: There have been many technical challenges that have been overcome in adult stem cell research.

Some of the barriers include: • the rare occurrence of adult stem cells among other differentiated

cells,

• difficulties in isolating and identifying the cells

• difficulties in growing adult stem cells in tissue culture

These hurdles are:

Obstacles to stem cell therapy

Tissue stem cells have been shown by the published evidence to be a more promising alternative for patient treatments, with a vast biomedical potential.

Tissue stem cells have proven success in the laboratory dish, in animal models of disease, and in current clinical treatments.

Tissue stem cells also avoid problems with tumor formation, transplant rejection, and provide realistic excitement for patient treatments.

However

The relative lack of success of embryonic stem cells should be compared with the real success of tissue(adult) stem cells. A wealth of scientific papers published over the last few years document that tissue stem cells are a

much more promising source of stem cells for regenerative medicine. Adult (tissue)stem cells actually do show pluripotent capacity in generation of tissues,

meaning that they can generate most, if not all, tissues of the body.

Tissue engineering

Tissue engineering is the process of creating living, physiological 3D tissues and organs. The process starts with a source of cells derived from a patient or from a donor. The cells may be immature cells, in the stem cell stage, or cells that are already capable of carrying out tissue functions; often, a mixture of different cell types (e.g., liver cells and blood vessel cells) and cell maturity levels is needed. Many therapeutic applications of tissue engineering involve disease processes that might be prevented or treated if better drugs were available or if the

processes could be better understood.

Tissue engineering-based therapies may provide a possible solution to alleviate the current shortage of organ donors. In tissue engineering, biological and engineering principles are combined to produce cell-based substitutes with or without the use of materials. One of the major obstacles in engineering tissue constructs for clinical use is the limit in available human cells. Stem cells isolated from adults or developing embryos are a current

source for cells for tissue engineering .

In general, there are three main approaches to tissue engineering:

(1 )To use isolated cells or cell substitutes as cellular replacement parts ;

(2 )To use acellular materials capable of inducing tissue regeneration; and

(3 )To use a combination of cells and materials (typically in the form of scaffolds and this approach be categorized into two categories:

Open and closed systems. These systems are distinguished based on the exposure of the cells to the immune system

upon implantation

The materials used for tissue engineering are either synthetic biodegradable materials (such as polylactic acid (PLA), polyglycolic acid (PGA), poly lactic-glycolic acid (PLGA), polypropylene fumarate, poly ethylene glycol (PEG) and polyarylates) or natural materials such as collagen, hydroxyapatite, calcium carbonate, and alginate. Natural materials are typically more favorable to cell adherence, whereas the properties of synthetic materials such as degradation rate, mechanical properties, structure, and porosity can be better controlled

Open tissue engineering systems have been successfully used to create a number of biological substitutes such as bone, cartilage, blood vessels, cardiac, smooth muscle, pancreatic, liver, tooth, retina, and skin tissues. Several tissue-engineered products are under clinical trials for FDA approval. Engineered skin or wound dressing and cartilage are two of the most advanced areas with regards to clinical potential. For example, a skin substitute that consists of living human dermis cells in a natural scaffold consisting of type I collagen already received FDA approval to be used for a diabetic foot ulcer. In addition, various cartilage and bone are also currently in clinical stages, and bladder and urologic tissue are being tested in various stages of research (Levenberg et al., 2006).

•Closed tissue engineering systems have been used particularly for the treatment of diabetes,

liver failure, and Parkinson’s disease. This system may prove to be especially useful in conjunction

with ES cells since the immobilization of ES cells within closed systems may overcome the

immunological barrier that faces ES cell-based therapies (Strauer and Kornowski, 2003).

Current approaches for tissue engineering using tissue (postnatal) stem cells:

(A )Expansion of a population ex vivo prior to transplantation into the host ,

(B )Ex vivo recreation of a tissue or organ for transplantation, and

(C )Design of substances and/or devices for in vivo activation of stem cells, either local or distant, to induce appropriate tissue repair