The transplantation era began in 1944 when Peter Medawar demonstrated that human skin grafts between unrelated donors were rejected, but the rejection rate decreased when a graft was obtained from a related donor. He suggested that the immune system recognized the skin from unrelated donors as foreign and mounted a vigorous immune response. In 1958, Jean Dausset described the first human leukocyte-specific antigen (HLA-A2) and demonstrated that these antigens were important in the recognition of foreign tissue. Between 1962 and 1980, immunologists and geneticists identified different HLA loci and determined that HLAs on donated organs were the targets of an immune response to transplanted tissue or organs. Matching donor and recipient HLAs prior to transplantation greatly increased graft survival. Introduction of anti-rejection drugs such as azathioprine, cyclosporine, and tacrolimus has significantly lessened the risk of organ rejection over time.

Between 28,000 and 36,000 organ transplantations are performed in the United States yearly. The most commonly transplanted organs are kidney, heart, and liver. But other organs, including cornea, lung, liver, pancreas, intestine, bone, skin, and bone marrow, can be transplanted. Organ transplantation has been hampered by a scarcity of available organs and bone marrow donors. Currently, over 85,000 individuals in the United States are awaiting organ transplantation.

Types of Transplantations

The four types of transplantations are as follows: (1)When the same individual serves as the donor and the recipient of the transplanted tissue, the graft is termed an autologous graft, or an autograft. (2) Tissue transplanted between genetically identical twins is called an isograft, or a syngeneic transplant. (3) A graft between unrelated or mismatched individuals is an allogeneic transplant. (4) When organs are transplanted across species barriers from animals to humans, the grafts are termed xenografts (Figure 17-1).

Figure 17-1 The genetic relationship between the donor and the recipient determines whether or not rejection will occur. Autografts or isografts are usually accepted, whereas allografts and xenografts are not. (From Roitt I, Brostoff J, Male D, et al: Immunology, ed 7, Philadelphia, 2006, Mosby.)

Xenografts

Xenografts merit more discussion because they solve the problem of organ shortages and long waiting times for transplants. Pigs are useful for transplantation purposes because of the similarity of organ size, weight, and physiology between pigs and humans. However, xenotransplantation has proved impractical because humans have pre-existing immunoglobulin M (IgM) antibodies directed at porcine carbohydrate antigens containing α-galactosyl residues that are expressed on porcine cells. When porcine organs are transplanted into humans, hyperacute rejection occurs within hours.

To prevent xenograft rejection, recombinant deoxyribonucleic acid (DNA) technology has been used to create transgenic pigs that can be used as a source of tissue and organs. Cells from these pigs do not express the carbohydrate antigens that elicit the IgM response. Moreover, cells express molecules that downregulate natural killer (NK) cells (ecto-5´-nucleotidase) or prevent complement activation (CD55 and CD59).

Before xenotransplantation can be implemented in clinical practice, questions concerning the risk of microbial and viral transmission from pigs to humans must be addressed and resolved. Although pigs and humans have been in close contact for centuries and xenografts are unlikely to harbor new or novel infectious agents, little is known about exogenous and endogenous retroviruses in pigs. In vitro, endogenous porcine retroviruses can infect human cell lines.

Human Leukocyte Antigen Markers and Transplantation

Major histocompatibility complex (MHC) antigens in animals and HLAs are considered “self-molecules” and determine acceptance or rejection of transplanted tissue (see Chapter 4). The three class I loci (HLA-A, HLA-B, and HLA-C) and the three class II loci (HLA-DR, HLA-DQ, and HLA-DP) molecules in humans are important in transplantation.

Each individual has two complete sets of the six HLA molecules. One set is inherited from the father and another set from the mother. Each set of HLA molecules is called a haplotype. Since extensive polymorphism and multiple alleles exist in each locus, individuals may have homozygous or heterozygous alleles at each locus.

Test for Tissue Compatibility in Syngeneic and Allogeneic Grafts

Antibody Cross-Matching

As a consequence of blood or platelet transfusion, a previous organ transplantation, or multiple pregnancies, many individuals have antibodies directed at major (classes I and II) or minor (class III) HLA antigens. For example, approximately 20% of patients on the kidney transplantation list have high levels of anti-HLA antibodies in serum. The presence of pre-existing antibodies limits the population of potential donors and extends the waiting time for a kidney. Individuals with no pre-existing HLA antibodies usually receive a kidney within 500 days from the date their name appears on the transplantation registry. If high levels of pre-existing antibodies are present, the waiting time is 2300 days.

An initial test determines the presence of anti-HLA–specific antibodies in the recipient’s serum. In a lymphocytotoxicity assay, serum from the recipient is made to react with ethidium bromide–stained T and B cells from the donor. In the presence of antibodies and complement, dead cells stain red, and live cells stain green. Cytotoxicity is reported as a percentage of the donor’s cells killed in the assay. This means that the higher the concentration of cells that register as dead, the higher the antibody concentration. The presence of preformed antibodies to the HLA molecules on the donor tissue precludes the possibility of successful transplantation.

The recipient’s serum is also tested for non-HLA–specific antibodies directed at the donor’s endothelial cells. Endothelial cells are the initial contact point between the graft and host tissue, and rejection begins at this interface. The presence of anti–endothelial cell antibodies increases the risk of antibody-mediated organ rejection.

Tissue Cross-Matching

Several DNA-based molecular diagnostic techniques are used to type and cross-match tissue for transplantation. Polymerase chain reactions (PCRs) are used to amplify stretches of genomic DNA encoding HLA molecules. Allelic polymorphism within each locus is identified through hybridization with sequence-specific primers (SSP) or sequence-specific oligonucleotides (SSO) or sequence-based typing (SBT). The choice of assays depends on the clinician’s requirements with regard to resolution, sensitivity, and speed.

The PCR-SSO is a low-cost, high-volume assay that can screen a large number of potential donors with low or intermediate HLA resolution. Each probe is complementary to different motifs within an allelic hypervariable region of HLA molecules. Thus, the technique can identify heterozygous or homozygous combinations.

PCR-SSP has intermediate resolution for HLA-A, HLA-B, HLA-C, DR, and DQ and can be performed rapidly usually within 3 to 4 hours. It is especially useful in typing cadaver organs that are used for transplantation or identifying closely related HLA alleles.

PCR-SBT has the highest resolution for determining allelic polymorphisms. In the assay, the coding region of the entire HLA sequence on chromosome 6 is amplified by PCR. Coding sequences in exons 2 and 3 (HLA-A, HLA-B, and HLA-C) and exon 2 (DR, DQ, and DP) are then sequenced to determine HLA alleles. Employing different formats, SBT can be used to determine heterozygous sequences at each locus, haploid sequences, or a combination of heterozygous and homozygous sequences.