Transplant, also called graft or organ transplant, in medicine, a section of tissue or a complete organ that is removed from its original natural site and transferred to a new position in the same person or in a separate individual. The term, like the synonym graft, was borrowed from horticulture. Both words imply that success will result in a healthy and flourishing graft or transplant, which will gain its nourishment from its new environment.
Transplants of animal tissue have figured prominently in mythology since the legend of the creation of Eve from one of Adam’s ribs. Historical accounts of surgical tissue grafting as part of the cure of patients date back to the early Hindu surgeons who, about the beginning of the 6th century BCE, developed techniques for reconstructing noses from skinflaps taken from the patient’s arm. This method was introduced into Western medicine by Italian surgeon Gaspare Tagliacozzi in the 16th century. The flap was left attached to the arm for two to three weeks until new blood vessels had grown into it from the nose remnant. The flap was then severed and the arm freed from the reconstructed nose.
It was found that extremely thin pieces of skin could be cut free and would obtain enough nourishment from the serum in the graft bed to stay alive while new blood vessels were being formed. This free grafting of skin, together with the flap techniques already mentioned, constituted the main therapeutic devices of the plastic surgeon in the correction of various types of defects. Skilled manipulations of such grafts produced surprising improvements in the appearance of those born with malformed faces and in the disfigurements resulting from severe burns. Cornea, which structurally is a modified form of transparent skin, can also be free grafted, and corneal grafts have restored sight to countless blind eyes.
Blood transfusion can be regarded as a form of tissue graft. The blood-forming tissues—bone marrow cells—can also be transplanted. If these cells are injected into the bloodstream, they home to the marrow cavities and can become established as a vital lifesaving graft in patients suffering from defective marrow.
The chief distinguishing feature of organ and limb grafts is that the tissues of the organ or limb can survive only if blood vessels are rapidly joined (anastomosed) to blood vessels of the recipient. This provides the graft with a blood supply before it dies from lack of oxygen and nourishment and from the accumulation of poisonous waste products.
As can be seen from the examples cited, living-tissue grafts may be performed for a variety of reasons. Skin grafts can save life in severe burns, can improve function by correcting deformity, or can improve appearances in a cosmetic sense, with valuable psychological benefits. Organ grafts can supply a missing function and save life in cases of fatal disease of vital organs, such as the kidney.
A tissue removed from one part of the body and transplanted to another site in the same individual is called an autograft. Autografts cannot be rejected. Similarly, grafts between identical twins or highly inbred animals—isografts—are accepted by the recipients indefinitely. Grafts from a donor to a recipient of the same species—allografts or homografts—are usually rejected unless special efforts are made to prevent this. Grafts between individuals of different species—xenografts or heterografts—are usually destroyed very quickly by the recipient. (The methods used to prevent rejection are discussed in full below.)
Tissue or organ grafts may be transplanted to their normal situation in the recipient and are then known as orthotopic—for example, skin to the surface of the body. Alternatively, they may be transplanted to an abnormal situation and are then called heterotopic—for example, kidneys are usually grafted into the lower part of the abdomen instead of into the loin (the back between the ribs and the pelvis), as this is more convenient. If an extra organ is grafted, it is called auxiliary or accessory—for example, a heterotopic liver graft may be inserted without removal of the recipient’s own liver.
Grafts are usually performed for long-term effects. Occasionally, the limited acceptance of a skin allograft may be lifesaving by preventing loss of fluid and protein from extensive burned surface in severely ill patients. The graft also provides a bacteria-proof covering so that infection cannot occur. When the allograft is removed or rejected, the patient may be sufficiently recovered to receive permanent autografts.
Certain tissues, including bone, cartilage, tendons, fascia, arteries, and heart valves, can be implanted even if their cells are dead at the time of implantation or will be rejected shortly thereafter. These are structural implants rather than true grafts or transplants. They are more akin to the stick to which a rose is attached for support—although their support is essential, their function does not depend on biological processes. In fact, xenografts or inert manufactured devices may often be equally suitable substitutes.
Most skin grafting is with autografts; the special indication for skin allografts in severely burned patients has been mentioned. Skin allografts seem to be rejected more aggressively than any other tissue, and there are many experimental situations in which skin grafts between two inbred strains of animal fail, although kidney grafts between the same strains survive indefinitely. With autografts, the donor skin is limited to what the patient has available. If allografts were not rejected, skin from cadavers could be used for coverage of burned areas without the need for subsequent autografting.
Flaps as used by Tagliacozzi are particularly valuable if adipose (fat) tissue as well as skin has been lost. This is because flaps are thicker than skin grafts; they consist of the full thickness of skin, with fascia and fat, or may also contain muscle. The procedure of raising a flap and keeping the donor site adjacent to the recipient bed can be complicated and uncomfortable for the patient. The cosmetic results are good, and the fat and other tissues under the skin contained in the flap can be used to cover exposed bone or to allow movement in a contracted joint or to fashion a new nose.
Full-thickness free-skin grafts, which include dermis and epidermis, are the maximum thickness that can survive without a blood supply; they are therefore in some danger of failure to survive. These grafts produce good cosmetic appearances and are especially useful on the face. The main defect of a full-thickness free-skin graft is that, unless it is very small, the donor site from which it comes becomes a defect that needs to be closed in its own right and may itself need skin grafting.
Split, or partial-thickness, skin grafts are by far the most commonly used grafts in plastic surgery. Superficial slices of skin the thickness of tissue paper are cut with a hand or mechanical razor. The graft, which contains living cells, is so thin that it usually gains adequate nourishment directly from the raw surface to which it is applied, and the risk of failure to take (that is, to survive in the new location) is therefore much less than with full-thickness grafts. Another major advantage is that the donor site is not badly damaged. It is tender for only two or three weeks, and it resembles a superficial graze both in appearance and in the fact that healing takes place from the deep layer of the skin left behind. Split skin grafts can be taken quickly from large areas to cover big defects. They tend to have an abnormal shiny reddish appearance that is not as satisfactory cosmetically as the other types of skin graft.
There are certain forms of blindness in which the eye is entirely normal apart from opacity of the front window, or cornea. The opacity may be the result of disease or injury, but, if the clouded cornea is removed and replaced by a corneal transplant, normal vision can result. Since cells of the cornea remain viable for some 12 hours after death, a cornea can be grafted if it is removed within that period. Cooling will slow the process of deterioration, although the sooner the section of cornea is transplanted the better. The graft bed to which a cornea is transplanted has no blood supply. Nourishment comes directly by diffusion from the tissues. Because most rejection factors are carried in the bloodstream, the lack of blood vessels permits most corneal allografts to survive indefinitely without rejection. Rejection can occur if, as sometimes happens, blood vessels grow into the graft.
By far the most satisfactory blood-vessel transplant is an autograft, similar in principle to skin autografts. Blood-vessel grafts are frequently used to bypass arteries that have become blocked or dangerously narrowed by fatty deposits, a condition caused by degenerative atherosclerosis (hardening of the arteries). Such atherosclerotic deposits in the coronary and carotid arteries are responsible, respectively, for most heart attacks and strokes. If atherosclerosis affects the main artery of the leg, the result is first pain in the calves and then gangrene of the foot, necessitating amputation of the leg. If dealt with early, the effects of the arterial blockage can often be overcome by removing a nonessential superficial vein from the leg, reversing it so that the valves will not obstruct blood flow, and then joining this graft to the affected artery above and below the block—thus bypassing the obstruction. Grafting for coronary artery bypass has become one of the most common surgical operations in developed countries.
Vein or arterial allografts are far less successful. In time the walls tend to degenerate, and the vessels either dilate, with the danger of bursting, or become obstructed.
Valvular diseases of the heart can be dangerous, since both a blocked valve and a valve that allows blood to leak backward create a strain on the heart that can lead to heart failure. If the valve is seriously damaged, it can be replaced with a xenograft valve or a manufactured mechanical valve. Neither is ideal. Xenograft valves have a normal central blood flow, but after a few years they may become rigid and cease to function. Plastic valves—usually of the ball-valve or trapdoor types—force blood to flow around the surface of the ball or trapdoor flap, and this tends to damage red blood cells and cause anemia. Synthetic heart valves require ongoing anticoagulation therapy.
When fractures fail to unite, autografts of bone can be extremely valuable in helping the bone to heal. Bone allografts can be used for similar purposes, but they are not as satisfactory, since the bone cells are either dead when grafted or are rejected. Thus, the graft is merely a structural scaffold that, although useful as such, cannot partake actively in healing.
Fascia, sheets of strong connective tissue that surround muscle bundles, may be used as autografts to repair hernias. The principle of use is like that for skin.
Nerves outside the brain and spinal cord can regenerate if damaged. If the delicate sheaths containing the nerves are cut, however, as must happen if a nerve is partially or completely severed, regeneration may not be possible. Even if regeneration occurs it is unlikely to be complete, since most nerves are mixed motor and sensory paths and there is no control ensuring that regenerating fibres take the correct path. Thus, there will always be some fibres that end in the wrong destination and are therefore unable to function. Defective nerve regeneration is the main reason why limb grafts usually are unsatisfactory. A mechanical artificial limb is likely to be of more value to the patient.
Blood transfusion has been one of the most important factors in the development of modern surgery. There are many lifesaving surgical procedures that are possible only because the blood loss inevitable in the operation can be made up by transfusion. Blood transfusion is of value in saving life following major injury, bleeding ulcers, childbirth, and many other conditions involving dangerous loss of blood. Purified blood components can be transfused to treat specific defects; for example, platelets are used to correct a low platelet count, and clotting factor VIII is given to counteract the clotting defect in classic hemophilia.
Diseases in which the bone marrow is defective, such as aplastic anemia, may be treated by marrow grafting. Some forms of leukemia can be cured by destroying the patient’s bone marrow—the site of the cancerous cells—with drugs and irradiation. Marrow grafting is then necessary to rescue the patient. There is a tendency for the patient to reject the allografted marrow, and there is an additional hazard because immune cells in a marrow graft can react against the patient’s tissues, causing serious and sometimes fatal graft-versus-host disease. To avoid these complications, special immunosuppressive treatment is given. The use of monoclonal antibodies (see below Monoclonal antibodies) to selectively remove harmful lymphocytes from the donor marrow has produced encouraging results in preventing graft-versus-host disease.
Organ transplants are, for a variety of reasons, more difficult to perform successfully than are most other grafts. Despite these difficulties, kidney transplant has become a routine operation in most developed countries. Heart and liver grafting have also become established, and promising results have been obtained with pancreas and combined heart–lung grafts.
The surgery of kidney transplantation is straightforward, and the patient can be kept fit by dialysis with an artificial kidney before and after the operation. The kidney was the first organ to be transplanted successfully in humans, and experience is now considerable. Effective methods of preventing graft rejection have been available since the 1960s.
Fatal kidney disease is relatively common in young people. When there is deterioration of kidney function, eventually, despite all conventional treatment, the patient becomes extremely weak and anemic. Fluid collects in the tissues, producing swelling, known as edema, because the kidneys cannot remove excess water. Fluid in the lungs may cause difficulty in breathing and puts an excessive strain on the heart, which may already be suffering from the effects of high blood pressure as a result of kidney failure.
Waste products that cannot be removed from the body can cause inflammation of the coverings of the heart and the linings of the stomach and colon. As a result, there may be pain in the chest, inflammation of the stomach leading to distressing vomiting, and diarrhea from the colitis. The nerves running to the limbs may be damaged, resulting in paralysis. Treatment with the artificial kidney followed by kidney grafting can eliminate all these symptoms and has a good chance of permitting the dying person to return to a normal existence. Unfortunately, in most countries only a minority of patients receive this treatment because of a shortage of donor kidneys.
Artificial kidney treatment lasting about three to four hours, two to three times a week, removes all the features of kidney failure in one to two months. The patient then is able to leave the hospital and can be assessed as to suitability for a transplant. As has been mentioned, the kidney graft is heterotopic. The diseased kidneys are left in place, unless their continued presence is likely to impair the patient’s health after a successful graft.
The patient may receive a kidney from a live donor or a dead one. Cadaver kidneys may not function immediately after transplantation, and further treatment with the artificial kidney may be required for two to three weeks while damage in the transplanted kidney is repaired. The patient is given drugs that depress immune responses and prevent the graft from being rejected. Immediately after the operation, for the first week or two, every effort is made to keep the patient from contact with bacteria that might cause infection. The patient is usually nursed in a separate room, and doctors and nurses entering the room take care to wear masks and wash their hands before touching the patient. The air of the room is purified by filtration. Close relatives are allowed to visit the patient, but they are required to take the same precautions. When stitches have been removed, the patient is encouraged to get up as much as possible and to be active, but, in the first four months after the operation, careful surveillance is necessary to make sure that the patient is not rejecting the graft or developing an infection. The patient may be discharged from the hospital within a few weeks of the operation, but frequent return visits are necessary for medical examination and biochemical estimations of the blood constituents, to determine the state of function of the graft, and to make sure that the drugs are not causing side effects. Each patient requires a carefully adjusted dose of the immunosuppressive drugs that prevent transplant rejection.
Once the dosage of immunosuppressive drugs is stabilized, patients are encouraged to go back to a normal existence and return to work. The only restrictions are that they must continue to take their drugs and make frequent visits to the outpatient department for surveillance. Patients can return even to heavy work, such as driving a bulldozer, but more often a relatively light job is preferable. Women can bear children after a transplant, and men can become fathers. The course of events is not always so happy, unfortunately. If the patient rejects the kidney or develops a serious infection, it may be necessary to remove the graft and stop administration of the immunosuppressive drugs. The patient must then return to regular maintenance treatment with an artificial kidney but may receive a second or even a third graft.
In kidney grafts involving identical twins, in which case rejection is not a problem, recipients have survived more than 25 years. A number of patients who have received kidneys from unrelated cadaver donors have survived more than 20 years, demonstrating that in some patients rejection can be controlled with standard immunosuppressive drugs. There has been a gradual improvement in the overall results of kidney transplants. The patient mortality has declined to around 10 percent per year, death usually being due to infection associated with immunosuppressive treatment; to complications of dialysis in patients whose kidneys have failed; or to other facets of kidney disease, such as high blood pressure and coronary artery disease. Recipients also face an increased risk of developing malignant growths, particularly lymphomas (malignant diseases of the lymphatic system). One cause of this may be related to the effects of immunosuppressive treatment. Kidney graft survival has improved since the introduction of the immunosuppressive agent cyclosporine (also called cyclosporin A), and many centres have achieved a one-year survival rate of about 96 percent and a five-year rate of about 80 percent for patients with a functioning kidney graft from an unrelated cadaver donor. One-year survival rates of 90 to 95 percent have been attained for kidney grafts from well-matched living donors. Thus, the patient who develops permanent kidney failure has a reasonable chance of successful treatment from a combination of dialysis and kidney transplantation. Those fortunate enough to receive a well-functioning kidney can expect complete rehabilitation.
The heart is a pump with a built-in power supply; it has a delicate regulatory mechanism that permits it to perform efficiently under a wide range of demands. During moments of fear or intense exercise, the heart rate increases greatly, and the contractions become more forceful. Cessation of the heartbeat has been, throughout the ages, the cardinal sign of death. Thus, it is perhaps not so surprising that there was an intense public interest when the first attempts were made at transplanting a human heart. The objectives of heart transplantation, nevertheless, are the same as those of other organ grafts.
One of the most important advances in surgery after World War II was in direct operations on the heart. Heart valves are repaired or replaced with artificial valves, and techniques have been developed so that the heart can be stopped and its function taken over temporarily by an electrical pump. If, however, the muscle of the heart is destroyed, as occurs in certain diseases, the only operation that can cure the patient is to replace the heart with a graft or possibly an artificial heart. Blockage of the coronary arteries and certain other heart muscle diseases can kill the patient because the muscle of the heart cannot contract properly. A patient with one of these diseases who is close to dying is, therefore, a possible recipient for a heart transplant.
A group of American investigators perfected the technique of heart transplantation in the late 1950s. They showed that a transplanted dog’s heart could provide the animal with a normal circulation until the heart was rejected. The features of rejection of the heart are similar to those of the kidney. The cells that produce immune reactions, the lymphocytes, migrate into the muscle cells of the heart, damage it, and also block the coronary arteries, depriving the heart of its own circulation. Some of the lymphocytes (i.e., B cells) also secrete antibodies that are toxic. In most experiments it was more difficult to prevent rejection of the heart than of the kidney. Despite this, rejection was prevented for long periods in animals. Based on this experimental work, the next step was to transplant a human heart into a patient dying of incurable heart disease. This step was taken in 1967 by a surgical team in Cape Town, South Africa.
In the years immediately following the first transplant, numerous heart allografts were performed at medical centres throughout the world. Unfortunately, many recipients succumbed to rejection of the transplanted organ. Furthermore, the heart is more sensitive to lack of blood than the kidneys are; it must be removed from the donor more quickly and can be preserved without damage for only a short period of time. Because of these difficulties—particularly the problem of rejection—the number of heart transplants performed worldwide dropped considerably after the initial excitement abated. Steady advances in detecting and treating rejection were made throughout the 1970s, however, and the introduction of the immunosuppressant cyclosporine in the 1980s brought even further improvements in the long-term survival rates for heart graft recipients. Interest in the procedure revived, and numerous heart transplants were performed. A number of patients have lived five or more years after the operation, and heart grafting has become an accepted therapy for otherwise incurable heart disease. Experimental artificial hearts have also been implanted, but these require an external power supply and long-term survival rates are not known.
The liver is a complicated organ that produces clotting factors and many other vital substances in the blood and that removes many wastes and toxic by-products from the circulation. It is, in effect, a chemical factory. The two categories of fatal liver disease that may be treated by liver grafting are nonmalignant destructive diseases of the liver cells—for example, cirrhosis—and primary liver cancer affecting either the main liver cells or the bileducts. The liver is extremely sensitive to lack of blood supply and must be cooled within 15 minutes of the death of the donor. The operation can be difficult, since the liver is rather large and of complex structure. Both its removal from the cadaver and its grafting into the recipient are major surgical operations. The operation is more difficult in humans than in animals—particularly, the removal of the diseased liver from the recipient. This may be much enlarged and adherent to surrounding structures so that its removal may result in serious bleeding. Once transplanted, the liver must function immediately or the patient will die. There is no treatment available that is comparable to the use of the artificial kidney for kidney disease. If the liver functions well immediately after transplantation, the rest of the management is similar to that followed in kidney operations, and the same drugs are given. Many early liver transplantation operations failed, but an increasing number have successfully restored dying patients to normal existence. Children do especially well following liver transplantation. The commonest fatal liver disease in childhood is a congenital deficiency of bile ducts called biliary atresia. Several centres have obtained an 80 to 90 percent one-year survival in children after liver grafting, although up to 25 percent of these patients may require retransplantation because of failure of the first graft.
Chronic fatal disease of the lung is common, but the progress of the disease is usually slow, and the patient may be ill for a long time. When the lung eventually fails, the patient is likely to be unfit for a general anesthetic and an operation. The function of the lung is to allow exchange of gases between the blood and the air. The gas passes through an extremely fine membrane lining the air spaces. This exposure to air makes the lungs susceptible to infection, more so than any other organs that have been grafted. It is consequently not surprising that infection has caused failure of many lung transplants. Even a mild rejection reaction can severely damage the gas-exchange membrane, and the patient may die before the rejection is reversed. The actual ventilation of the lungs by rhythmic breathing is a complicated movement controlled by nerves connecting the brainto the lungs and to the muscles that produce the breathing. Cutting the nerves can interfere with the rhythmicity of breathing, and this may be an important cause of the difficulties of successfully transplanting both lungs. Nevertheless, these difficulties have been overcome. If only one lung is transplanted, however, the patient’s own diseased lung may interfere with the function of the graft by robbing it of air and directing too much blood into the graft. Further progress may depend on a safer, more perfect control of rejection.
The technique of transplanting the heart and both lungs as a functioning unit was developed in animal experiments at Stanford Medical Center in California. Despite the technical feasibility of the operation, rejection could not be controlled by conventional immunosuppression. With the availability of cyclosporine, researchers were able to obtain long-term survivors with combined heart–lung transplants in primate species. Applications to human patients have been remarkably successful. Approximately two-thirds of the patients who initially received transplants at Stanford survived. Other centres subsequently adopted this form of treatment for patients with severe lung fibrosis and failure of the right side of the heart, which pumps blood into the lungs. Unfortunately, many organ donors have been maintained on ventilators, a process that frequently leads to lung infections; as a consequence, the availability of donor heart–lung units is quite limited. Furthermore, the lungs are vulnerable to damage from lack of blood, and so transplantation must be performed expeditiously.
The pancreas consists of two kinds of tissues: endocrine and exocrine. The latter produces pancreatic juice, a combination of digestive enzymes that empty via a duct into the small intestine. The endocrine tissue of the pancreas—the islets of Langerhans—secrete the hormones insulin and glucagon into the bloodstream. These hormones are vital to the regulation of carbohydrate metabolism and exert wide-ranging effects on the growth and maintenance of body tissues. Insufficient insulin production results in type 1 diabetes mellitus, a disease that is fatal without daily injections of insulin. Even with insulin therapy, many diabetics suffer kidney failure and blindness due to the disease’s effects on the small blood vessels. A pancreas transplant can potentially prevent the progression of these complications.
Much effort has been devoted to removing the islets of Langerhans from the pancreas with a view to grafting the separated islets or even the isolated insulin-producing beta cells. Unfortunately, it is very difficult to obtain sufficient islets from the fibrotic human pancreas, and it appears that isolated islets are highly susceptible to rejection. A number of clinical attempts at islet grafting have been made.
Transplanting the vascularized pancreas has been more encouraging. It is customary to graft the body and tail of the pancreas; that is, half the pancreas is transplanted, using the splenic artery and vein for vascular anastomosis. One of the difficulties with this procedure has been dealing with the digestive juice produced by the transplanted pancreas. A further complicating factor has been the fact that corticosteroids—frequently used for immunosuppression in transplant patients—aggravate diabetes. The availability of cyclosporine permitted the avoidance of corticosteroids. Later, triple-drug immunosuppression, using cyclosporine, tacrolimus, and mycophenolate mofetil, was introduced.
Pancreas transplantation is particularly attractive when a patient with diabetic kidney failure can receive a kidney and pancreas graft from the same donor. A technique with encouraging early results has been to insert the pancreas graft very close to the patient’s own pancreas in the so-called paratopic position. This allows drainage of insulin directly into the liver, while the pancreatic juice is diverted into the stomach, where the digestive enzymes are inhibited by stomach acid. It is certainly most gratifying to patients who have been undergoing regular dialysis and taking insulin to be free from both these onerous treatments and to be permitted to eat and drink without restriction. The five-year functional survival rate for pancreas transplant is about 90 percent. It is of interest that the vascularized pancreas probably is less susceptible to rejection than the kidney.
Women with a diseased or malformed uterus or an absent uterus are unable to bear children. Mayer-Rokitansky-Küster-Hauser syndrome (MRKH; also called Müllerian agenesis), characterized by underdevelopment or absence of the vagina and uterus, occurs in about 1 in 4,500 girls at birth. Women with MRKH cannot carry a pregnancy, though those who have functioning ovaries may choose in vitro fertilization (IVF) with use of a surrogate mother for gestation. However, researchers have explored techniques for uterus transplant as a means of enabling pregnancy in MRKH patients with functional ovaries. The first successful birth of an infant to a uterus transplant recipient, using a uterus from a living donor, occurred in 2014. Four years later the first live birth from a deceased donor uterus was reported.
In uterus transplantation with intended pregnancy, the recipient must first undergo IVF to produce embryos, which are then subjected to cryopreservation. The embryos are later transferred to the mother’s uterus once successful transplantation has been established.
In countries with established transplant programs, organ transplantation is highly regulated. Of particular concern is organ donation, with legal, medical, and social issues surrounding the procurement of organs, without compensation, for transplantation. Many of those issues are overcome by organ registries, in which individuals choose to become organ donors. Through such registries, donors can indicate which organs they are willing to donate upon death. Whether a person is a registered organ donor can then be indicated on a personal identification card (e.g., a driver’s license), authorizing organ procurement once the individual is deceased. In the absence of legal consent via registration as an organ donor, organ procurement representatives are required to consult with next of kin for authorization to obtain organs from the deceased person.
Transplantation raises important ethical considerations concerning the diagnosis of deathof potential donors, and, particularly, how far resuscitation should be continued. Every effort must be made to restore the heartbeat to someone who has experienced sudden cardiac arrest or to restore breathing to someone who cannot breathe. Artificial respiration and massage of the heart, the standard methods of resuscitation, are continued until it is clear that the brain is dead. Most physicians consider that beyond this point efforts at resuscitation are useless.
In many countries, the question of how to diagnose brain death—that is, irreversible destruction of the brain—has been debated by neurologists and other medical specialists. Most of these experts agree that when the brainstem is destroyed, there can be no recovery. The brainstem controls the vital function of breathing and the reflexes of the eyesand ears, and it transmits all information between the brain and the rest of the body. Most countries have established strict guidelines for how brainstem death is to be diagnosed and what cases are to be excluded—for example, patients who have been poisoned, have been given drugs, or have developed hypothermia. The neurological signs of brainstem death must be elicited by a trained clinician who is not concerned directly with the transplant operation. These signs are reverified after an interval, and, if there is the slightest doubt, further reverifications are made until the criteria are unequivocally met. The guidelines are not seriously disputed, and there has never been a recovery in a case that fulfilled the criteria of brainstem death.
Another area of ethical concern is the dilemma posed by the shortage of donor organs. Advances in immunosuppressive therapy have put increasing pressure on the supply of donor organs, and medical personnel sometimes find themselves having to determine who among the potential recipients should receive a lifesaving graft. Furthermore, there is a danger of commercial interests becoming involved with people willing to sell their organs for personal gain, and there is definite risk of illegal organ trafficking, in which organs are procured from unwilling donors and then sold to facilities that offer transplant services.
Humans possess complex defense mechanisms against bacteria, viruses, and other foreign materials that enter the body. These mechanisms, which collectively make up the immune system, cannot, unfortunately, differentiate between disease-causing microorganisms and the cells of a lifesaving transplant. Both are perceived as foreign, and both are subject to attack by the immune system. This immune reaction leads to rejection, the greatest problem in successful tissue and organ grafting.
In order to understand why rejection occurs and how it may be prevented, it is necessary to know something of the operations of the immune system. The key cells of the immune system are the white blood cells known as lymphocytes. These are of two basic types: T lymphocytes (T cells) and B lymphocytes (B cells). These cells have the capacity to distinguish “self” substances from such “nonself” substances as microorganisms and foreign tissue cells. Substances that provoke an immune reaction are recognized by the presence of certain molecules, called antigens, on their surface.
T lymphocytes are responsible for cell-mediated immunity, so named because the T cells themselves latch onto the antigens of the invader and then initiate reactions that lead to the destruction of the nonself matter. B lymphocytes, on the other hand, do not directly attack invaders. Rather, they produce antibodies, proteins that are capable of initiating reactions that weaken or destroy the foreign substance. The overall immune reaction is exceedingly complex, with T lymphocytes, B lymphocytes, macrophages (scavenger cells), and various circulating chemicals waging a coordinated assault on the invader.
Transplant rejection is generally caused by cell-mediated responses. The process usually occurs over days or months, as the T lymphocytes stimulate the infiltration and destruction of the graft. The transplant may be saved if the cell-mediated reactions can be suppressed. Antibody attack of transplanted tissues is most apparent when the recipient has preexisting antibodies against the antigens of the donor. This situation can arise if the recipient has been previously exposed to foreign antigens as the result of pregnancy(during which the mother is exposed to fetal antigens contributed by the father), blood transfusions, or prior transplants. Unlike a cell-mediated reaction, antibody-mediated rejection is rapid, occurring within minutes or hours, and cannot be reversed.
The factors that provoke graft rejection are called transplantation, or histocompatibility, antigens. If donor and recipient have the same antigens, as do identical twins, there can be no rejection. All cells in the body have transplantation antigens except the red blood cells, which carry their own system of blood-group (ABO) antigens. The main human transplantation antigens—called the major histocompatibility complex, or the HLA (human leukocyte antigens) system—are governed by genes on the sixth chromosome. HLA antigens are divided into two groups: class I antigens, which are the target of an effector rejection response; and class II antigens, which are the initiators of the rejection reaction. Class II antigens are not found in all tissues, although class I antigens are. Certain macrophagelike tissue cells—called dendritic cells because of their finger-like processes—have a high expression of class II antigens. There has been much interest in trying to remove such cells from an organ graft, so that the rejection reaction will not be initiated.
Tissue typing involves the identification of an individual’s HLA antigens. Lymphocytes are used for typing. It is important also that the red blood cells be grouped, since red-cell-group antigens are present in other tissues and can cause graft rejection. Although transplantation antigens are numerous and complicated, the principles of tissue typing are the same as for red-cell grouping. The lymphocytes being typed are mixed with a typing reagent, a serum that contains antibodies to certain HLA antigens. If the lymphocytes carry HLA antigens for which the reagent has antibodies, the lymphocytes agglutinate (clump together) or die. Typing serums are obtained from the blood of persons who have rejected grafts or have had multiple blood transfusions or multiple pregnancies; as previously stated, such persons may develop antibodies to transplantation antigens.
If the lymphocytes of both the recipient and the potential donor are killed by a given serum, then, as far as that typing serum is concerned, the individuals have antigens in common. If neither donor nor recipient lymphocytes are affected, then donor and recipient lack antigens in common. If the donor lymphocytes are killed but not those of the recipient, then an antigen is present in the donor and is missing from the recipient. Thus, by testing their lymphocytes against a spectrum of typing sera, it is possible to determine how closely the recipient and donor match in HLA antigens. As a final precaution before grafting, a direct crossmatch is performed between the recipient’s serum and donor lymphocytes. A positive crossmatch usually contraindicates the donor–recipient transplant under consideration.
There is now considerable knowledge concerning the inheritance of transplantation antigens, but, even so, tissue typing is not sufficiently advanced to give an accurate prediction of the outcome of a graft in an individual case, particularly when the donor and recipient are not related to one another. In accordance with Mendelian laws of inheritance, a person obtains one of a pair of chromosomes from each parent. Therefore, a parent-to-child transplant will always be half-matched for transplantation antigens. Siblings have a one-in-four chance of a complete match of the HLA antigens, a one-in-four chance of no match, and a one-in-two chance of a half-match.
Following a blood transfusion, some patients become sensitized to the transplantation antigens of the donor, so it was expected that prior blood transfusion could only harm the recipient’s prospects for a successful organ graft. Careful analysis of results, however, showed the contrary. Specifically, the results of kidney grafting in patients who had received previous blood transfusions without regard to HLA matching were much better than in patients who had never received a blood transfusion. Although a great deal of effort has been expended to determine the mechanisms involved, researchers still do not know how the immune system is modified by prior blood transfusions. Most centres now give blood transfusions before transplantation, though some patients do develop HLA antibodies against a wide spectrum of the population and therefore become very difficult to transplant. This pool of highly sensitized patients is getting larger throughout the world, not only from blood transfusions but also from patients who have rejected kidney grafts and are back on dialysis and from women who have had multiple pregnancies.
A special application of the blood transfusion effect involves repeated small blood transfusions from a potential donor who is a close relative of the patient. If sensitization does not occur, subsequent kidney graft results are excellent. Some patients, however, develop a positive crossmatch to donor lymphocytes and cannot receive a graft from that donor. This, combined with the increasing number of patients who readily develop HLA antibodies, has resulted in a decline in the use of donor-specific blood transfusion.
The aim of transplantation research is to allow the recipient to accept the graft permanently with no unpleasant side effects. With current drugs that are used for this purpose, after some months the dosage can often be reduced and sometimes even stopped without the graft’s being rejected. In such a case, the patient is no longer as susceptible to infections. There would appear to be adaptation of the recipient toward the graft and the graft toward the recipient. The adaptation is probably akin to desensitization, a process used sometimes to cure patients suffering from certain types of allergies by giving them repeated exposure to small doses of the allergen to which they are sensitive.
Azathioprine is one of the most widely used immunosuppressive agents; it also has been used to treat leukemia. It can be given by mouth, but the dose must be carefully adjusted so that the blood-cell-forming tissues in the bone marrow are not damaged, which could lead to infections and bleeding. The white blood cell and platelet counts need to be determined frequently to make sure that azathioprine is not being given in too large a dose. It is an extremely valuable drug and has been the basis of most immunosuppressive regimens in patients with organ grafts. At first, high doses are given, but eventually the doses may be reduced. Even years after transplantation, small doses of azathioprine may still be needed to maintain coexistence between graft and host.
Cortisone and its relatives, prednisone and prednisolone, are very useful in patients with organ grafts. They can be given by mouth, but, although not damaging to the blood-forming cells, they do predispose the body to infection, cause stunted growth in children, and have other injurious effects. Persons receiving these substances may develop complexion problems with swollen faces and may tend to gain weight and become diabetic, and their bones may become brittle. Few recipients of organ transplants, however, can do without corticosteroids, particularly during an active rejection crisis.
If rabbits receive repeated injections of mouse lymphocytes, they become immunized and develop antibodies against the mouse cells. The serum from the rabbits’ blood can be injected into mice and will often prevent them from rejecting grafts, both from other mice and even, sometimes, from other species. Such antilymphocyte serums can be produced between a variety of species, but in higher mammals, particularly humans, it has been difficult to obtain a powerful immunosuppressive serum without side effects of toxicity.
The activity of the antilymphocyte serum lies in its gamma globulin, which contains the antibody proteins. Antilymphocyte globulin is used in humans but contains many proteins that are ineffective and may be harmful. It can be added to immunosuppressant and steroid treatment regimens, and it is extremely useful in treating rejection crisis in kidney graft recipients who have not responded to corticosteroids. The horse has usually been used to produce antilymphocyte serum for the treatment of human patients, but some persons are sensitive to horse proteins and become extremely ill when treated with horse serum. Such patients may, however, be successfully treated with rabbit antilymphocyte globulin.
Antithymocyte globulin is a similar antibody treatment but is produced in animals, usually rabbits or horses, through inoculation with human thymocytes. Antithymocyte globulin triggers a significant reduction in levels of circulating T lymphocytes. It commonly is used to induce immune suppression for kidney transplantation, helping prevent graft rejection.
An important development in antibody production followed the discovery that an antibody-forming lymphocyte can be fused with a cancerous bone marrow cell. The resulting hybrid cell produces the antibody specified by its lymphocyte progenitor, while from the cancercell it obtains the characteristic of multiplying indefinitely in laboratory cultures. The culturing of the hybrid yields a clone of cells that produce one specific antibody—a “monoclonal” antibody. Such agents are exclusively specific in action and there is no theoretical limit to the number of antibodies that can be produced by different hybrid cell lines. Monoclonal antibodies can be regarded as highly specific antilymphocyte globulin without many of the unwanted materials that are present in the ordinary polyclonal antilymphocyte serum described above. Some monoclonal antibodies have been produced that are effective as immunosuppressive agents in humans.
Cyclosporine was found as a natural product of an earth fungus by researchers at the pharmaceutical company Sandoz Laboratories. It is a stable cyclic peptide with powerful immunosuppressive activity affecting especially the T lymphocytes. Cyclosporine was found to prevent organ graft rejection in a number of animal species. When the drug was used in humans, the expected immunosuppressive effect was again observed. It has been used in recipients of all types of organ grafts with improved immunosuppressive results. Cyclosporine can be toxic to the human kidney, however, and may cause permanent renal damage. The drug also increases the growth of hair on the face and body. It is a difficult drug to use because, being fat soluble, its absorption is variable and each patient needs to be individually studied to ensure that the dosage is adequate but not excessive.
It is clear that none of the agents so far used to prevent rejection is ideal. No one would use such dangerous agents except as a last resort in a desperate situation. This, unfortunately, is the exact plight of a person in need of a vital organ transplant. Immunosuppression is, however, much more effective and less dangerous than it used to be, and advances with chemical derivatives, in particular monoclonal antibodies and nontoxic analogues of cyclosporine, have brought significant improvements in immunosuppressive therapy.
Without a blood supply, organs deteriorate rapidly. Cooling can slow down the process but cannot stop it. Organs differ in their susceptibility to damage. At body temperature, irreversible destruction of the brain occurs after more than 3 to 5 minutes; of the heart, liver, pancreas, and lung, after 10 to 30 minutes; of the kidney, after 50 to 100 minutes; and of the skin and cornea, after 6 to 12 hours. The shorter the time the organ is deprived of its blood supply, the better. Although the cornea can be removed for grafting at relative leisure, every minute is of vital importance for a liver transplant. When a kidney is removed from a living donor, it is not necessary to use elaborate preservation techniques. The operations on the donor and recipient are performed at the same time, and the recipient is prepared to receive the graft by the time that the donor organ is removed. Cadaver kidneys are removed as soon as possible after the donor’s death, preferably within an hour. Cool solutions are infused into the blood vessels of the kidney, which is then kept at 4 °C (39 °F) in a refrigerator or surrounded by ice in a vacuum flask. At the same time, the recipient is prepared for operation. Kidneys can be conserved in this simple way for 24 to 48 hours with little deterioration, and during this time they can be moved for long distances. For a kidney to be preserved from 48 to 72 hours, a complicated machine is required to provide artificial circulation. Cool oxygenated physiological solutions with the same osmotic pressure as blood are passed through the blood vessels of the kidney. The imperfections of the machinery mean that there is a slow deterioration of the organ that does not occur normally in the body. To keep a kidney undamaged for longer than 72 hours is difficult. Blood cells, spermatozoa, and certain other dissociated tissue cells can be frozen to subzero temperatures and kept alive indefinitely. Special preserving fluids will prevent cell destruction by ice crystals, but these fluids have damaging effects if introduced into whole organs such as the kidney.
For the heart, lung, liver, and pancreas, grafting is performed as quickly as possible, preferably within eight hours for the liver and pancreas, four hours for the heart, and two hours for the combined heart-and-lung graft. Much research will be necessary before it is possible to keep organs banked in the way that blood can be stored.