Chapter 21 Transplantation and Rejection

• Transplantation is the only form of treatment for most end-stage organ failure.

• The barrier to transplantation is the genetic disparity between donor and recipient.

• The immune response in transplantation depends on a variety of factors. Host versus graft responses cause transplant rejection. Histocompatibility antigens are the targets for rejection. Minor antigens can be targets of rejection even when donor and recipient MHC are identical. Graft versus host reactions result when donor lymphocytes attack the graft recipient.

• Rejection results from a variety of different immune effector mechanisms. Hyperacute rejection is immediate and caused by antibody. Acute rejection occurs days to weeks after transplantation. Chronic rejection is seen months or years after transplantation.

• HLA matching is one of two major methods for preventing rejection of allografts. The better the HLA matching of donor and recipient, the less the strength of rejection.

• Successful organ transplantation depends on the use of immunosuppressive drugs. 6-MP, azathioprine, and MPA are antiproliferative drugs. Ciclosporin, tacrolimus, and sirolimus are inhibitors of T cell activation. Corticosteroids are anti-inflammatory drugs used for transplant immunosuppression. Antibodies to the IL2 receptor, or to leukocytes, are widely used.

• The ultimate goal in transplantation is to induce donor-specific tolerance. There is evidence for the induction of tolerance in humans and novel methods for inducing tolerance are being developed. Alloreactive cells can be made anergic. Immune privilege can be a property of the tissue or site of transplant.

• Shortage of donor organs and chronic rejection limit the success of transplantation. Living donation is one way to overcome the shortage of donor organs. Alternative approaches are being investigated. The favored animal for xenotransplantation is the pig.

Transplantation is the only form of treatment for most end-stage organ failure, and it is a central topic for immunologists for two reasons:

• the first is that transplantation is an important clinical procedure;

• the second is that transplantation has proved an important tool for understanding immunological mechanisms – for example, the major histocompatibility complex (MHC; see Chapter 5) was first described in the context of transplantation, and transplantation models continue to be widely used as tools in basic as well as applied immunology.

As a clinical procedure, transplantation is used to replace tissues or organs that have failed. The first successful transplants were those of the cornea, first described in 1906.

Q. Why in retrospect was corneal transplantation more likely to be successful than transplantation of other tissues?

A. The cornea is an immunologically privileged site (see Figs 12.w1–12.w3).

World War II provided an important impetus, with the problems of skin grafting airmen who had extensive burns motivating a number of scientists, most notably Peter Medawar, to investigate the immunological basis of graft rejection.

The subsequent demonstration by the Medawar group that it was possible to manipulate a recipient animal so that it accepted grafts from an unrelated donor animal encouraged the subsequent clinical development of transplantation. The discovery (by Calne and others) of immunosuppressive drugs and agents then allowed the widespread practice of transplantation in the last three to four decades of the 20th century.

In modern practice many transplants are performed routinely (Fig. 21.1). In addition to routine transplantation of the cornea, kidney, heart, lungs, and liver there is increasing interest in transplanting other organs, such as whole pancreas or islet cells for diabetes mellitus and also small bowel.

Fig. 21.1 Clinical transplantation

Organs and tissues shown in blue are routinely transplanted to treat various conditions. Transplantation of other organs (shown in yellow) is being developed, but has yet to be routinely applied in most centers.

In general most transplants use organs from dead donors (cadaveric transplants), though there is an increasing number of living donors (usually related to the recipients) for kidney transplantation (see below).

Hematopoietic stem cell transplantation

Hematopoietic stem cell transplants are performed for two main reasons. One is to treat children who have inherited immune deficiencies. These children are very prone to infection, and will normally die young as a consequence. However, if they are given stem cells from a healthy donor, the infused stem cells can replace the defective bone marrow stem cells. The stem cells can then mature and into fully effective immune cells, thus giving the child a functioning immune system.

The second major application is for patients with leukemia. It is possible to eradicate the patient’s leukemic cells with chemotherapy and radiotherapy. However, this also results in destruction of the patient’s stem cells in the bone marrow and circulation. The patient therefore becomes immunodeficient and will die of infection. Stem cell transplantation can ‘rescue’ the patient by providing a fresh source of stem cells. In some cases the stem cells are autologous (in which they are harvested before chemotherapy, stored, and then infused back into patients after the therapy is over). In these settings there is no risk of graft versus host disease (see below). However, there is a risk that leukemic cells will be present in the stored stem cells, and will then grow in the patient. In other cases the stem cells come from a well matched donor. This removes the chance of carry over of leukemic cells, but does run the risk of graft verus host disease. In some forms of leukemia it has been shown that there is a graft versus leukemic effect, in which the allogeneic T cells mount a response against any leukemic cells remaining in the patient and prevent them from growing.

Organ donation

The most limiting factor for organ transplantation is the shortage of donors. There are many more people who would benefit from an organ than there are organs available. Indeed, given the good survival of most forms of transplantation, there are some conditions where it would be beneficial to receive a transplant but they are never considered and treated in other ways because there will always be higher priority cases.

There are several solutions to this issue. One is to increase the number of donors, using advertising and donor recruitment campaigns. This also involves raising the awareness in the public about organ donation, and this can be a particular issue in countries where there are cultural or religious barriers to donation. One possibility in this area is legislation; some countries operate a ‘presumed consent’ policy whereby everyone is assumed to have given consent for their organs to be used, unless they have indicated otherwise. Other countries operate an ‘opt in’ approach where individuals (or their relatives) have to give permission for organs to be used. It is also important to improve the donation rate in ethnic minority groups, who often have a low donation rate. The second is to improve the process by which relatives are approached about organ donation, and to ensure that there are enough facilities to allow all the organs to be harvested (remembering that one donor can provide many organs for transplantation). Transplant coordinators (as they are termed in the UK, but equivalent positions are found in other countries) are key to this process, as are the surgical teams that obtain the organs. The third is to relax the donor selection criteria, to use organs that would not have been used previously. Thus the use of non heart beating donors is increasing, and there is increased use of ‘marginal donors,’ for example from older donors or those that have diseases that would have previously excluded them.

The final ways to increase donation is the use of living donor or using animals as donors (xenotransplantation), both of which are discussed below.

Ethical issues are an important factor in living donation

The shortage of organs from cadaveric donors has increased the use of living donors. Clearly this is only possible in situations where donation of an organ poses little risk to the living donor. Kidney transplantation is routinely performed from living donors, as it is quite possible to live with just one healthy kidney. It is also possible to donate a lobe of a liver, as it can grow back again. Donation of a lung lobe is also possible, though this is a much riskier procedure. Hematopoietic stem cell transplants also involve living donors, though the issues with this form of transplant are somewhat different than for solid organ transplants as the stem cells are rapidly replaced.

Before any donation from a living donor is carried out, it is important to ensure that the potential donor is healthy, that the donation is compatible (of the appropriate blood group), and that the donor is aware of the risks and has not been coerced into giving an organ. It would be unethical for a donor to be put under extreme pressure to donate an organ, and the transplant teams must take great care to ensure that this is not happening. In the majority of cases donation is carried out between close relatives (including spouses) or sometimes friends. However, there are a few ‘altruistic donors’ that give their organs to be used by whoever needs them. In most countries it is not allowed to sell organs for transplantation, and the majority of those involved in transplantation would see that as unethical. However, there is a real problem with ‘transplant tourism’ where people from rich, developed, countries travel to buy organs from poor, marginalized, donors.

Organ exchange programs have been set up to maximize the number of transplants performed. In the simplest cases these are paired donations. Jane Smith may need a kidney and have a relative, Steve Smith, who is willing to donate it. However, if Steve Smith’s kidney is incompatible with Jane then he cannot donate it to her. However, if there is another couple in a similar predicament, so that Jim Jones needs a kidney and Stella Jones is willing to donate one but they are incompatible, then it might be possible for Stella Jones to donate her kidney to Jane Smith and for Steve Smith to donate to Jim Jones. More complex arrangements are also possible, with three or more pairs donating in such a way as to maximize the number of transplants. Indeed it is possible to use an altruistic donor to initiate a chain of donation, such that they donate to recipient A, recipient A’s potential donor gives to recipient B, their donor gives to C and so on, until the final recipient is an individual who does not have a suitable donor.

In addition to transplantation of organs, there is a large program of transplanting hematopoietic stem cells (cells capable of regenerating blood cells), for example in patients with leukemia or with primary immune deficiencies. These stem cells were previously normally harvested from the bone marrow of (living) donors, though increasingly peripheral stem cells obtained from the blood are used – stem cell transplantation has its own particular problems.

Clinical trials of stem cell therapy, using mesenchymal stem cells or embryonic stem cells are now underway. The aim of these is that the stem cells can differentiate and repair damaged organs. In some cases the stem cells are taken from the patient, in which case there is no immunological issues. However, if they are taken from another individual then rejection is possible.

Genetic barriers to transplantation

The main immunological problem with transplantation is that the grafted organ or tissue is seen by the immune system as ‘foreign’ and is recognized and attacked – leading to rejection of the organ.

Transplantation is normally performed between individuals of the same species who are not genetically identical, and the antigenic differences are known as allogeneic differences, and result in an allospecific immune response (Fig. 21.2).

Fig. 21.2 Genetic barriers to transplantation

The genetic relationship between the donor and recipient determines whether or not rejection will occur. Autografts or isografts are usually accepted, whereas allografts and xenografts are not.

However, it is also possible in experimental circumstances (and possibly in the future in the clinical setting) to perform grafting between different species. This is termed xenotransplantation, and the antigenic differences between donor and recipient form the xenogeneic barrier.

Transplantation can also be performed within an individual (e.g. skin grafting), when it is known as an autograft.

Syngeneic or isografts can be performed between genetically identical individuals. This can occur clinically for identical twins, but is more commonly seen in experimental settings with inbred strains of animals.

In the case of autografts and isografts there should be no antigenic differences between donor and recipient, and so no immune response. This can be readily illustrated using transplantation of skin or organs between inbred strains of animals (Fig. 21.3).

Fig. 21.3 Host versus graft reactions

Grafts between genetically identical animals are accepted. Grafts between genetically non-identical animals are rejected with a speed that is dependent on where the genetic differences lie. For example, syngeneic animals that are identical at the MHC locus accept grafts from each other (1). Animals that differ at the MHC locus reject grafts from each other (2). The ability to accept a graft is dependent on the recipient sharing all the donor’s histocompatibility genes; this is illustrated by the difference between grafting from parental to (A × B) F₁ animals (3) and vice versa (4). Animals that differ at loci other than the MHC reject graft from each other, but much more slowly.

Graft rejection

Host versus graft responses cause transplant rejection

Immune recognition of the antigenic differences between the donor organ and the recipient will, unless treated, lead to an immune response in which the host immune system responds to, and attacks, the donor tissue.

Q. What are the main genetic differences that are recognized by the host, and why should this be so?

A. Differences in MHC molecule allotypes are most important. The reason is that all nucleated cells express MHC molecules, and the T cell receptor on host T cells has a basic structure that interacts with and recognizes MHC molecules. Also the MHC class I and class II molecules have an extremely high level of genetic variability (see Chapter 5).

The nature of the host versus graft response is discussed in more detail below.

There is a high frequency of T cells recognizing the graft

One of the main features of the immune response against a transplanted organ is that it is much more vigorous and strong than the response against a pathogen, such as a virus. This is largely reflected by the frequency of T cells that recognize the graft as foreign and react against it.

Thus, in a naive or unimmunized individual fewer than 1/100 000 T cells respond upon exposure to a virus or a protein immunization; however, 1/100–1/1000 T cells respond to allogeneic antigen-presenting cells (APCs). This is reflected in the strong T cell response (proliferation) seen when naive T cells are stimulated with allogeneic dendritic cells (Fig. 21.4)

Fig. 21.4 Measuring the strength of the alloresponse

The strength of the alloresponse can be measured in a mixed lymphocyte reaction. In this assay T cells from an individual were mixed with varying numbers of dendritic cells (DCs) from either the same (autologous DC) or a different (allogeneic DC) donor. The dendritic cells were irradiated to prevent their proliferation. As a control cultures were included that contained just the dendritic cells or just the T cells. Five days later, T cell proliferation was measured by incorporation of ³ H-thymidine, which is incorporated into the DNA of dividing cells. There is strong proliferation of T cells exposed to allogeneic dendritic cells, even though these T cells have not been exposed to the allogeneic cells, i.e. it represents a primary immune response.

Histocompatibility antigens are the targets for rejection

Early experiments showed that the bulk of the allospecific response is against molecules of the MHC. We now know that these molecules are the MHC class I or class II molecules (see Chapter 5), which are responsible for presenting antigen (in the form of peptides) to either:

• CD8 T cells (MHC class I); or

• CD4 cells (MHC class II).

As discussed in Chapter 8, MHC molecules are highly polymorphic, and it is these polymorphic differences that are seen by alloreactive T cells.

Why is there such a high frequency of allospecific T cells?

Several models seek to explain why there is such a high frequency of allospecific cells in the T cell repertoire.

The first model (high determinant density model) (Fig. 21.w1) suggests that allospecific T cells recognize the foreign MHC molecules directly, with a low affinity, in a peptide-independent manner. The affinity of the interaction would normally be too low to activate the T cells; however, because the T cells see the MHC molecules directly, and are not recognizing the peptide, this low affinity is compensated for by the high concentration of MHC molecules on allogeneic cells.