Immunologic Aspects of Organ Transplantation

Susan Smith MN, PhD


June 17, 2002


"Histo" means "tissue." Histocompatibility is defined as a measure of how well 2 tissues "get along with one another" when they find themselves in a confined space. The major physiologic barrier in transplantation is the potential for rejection of transplanted organs as a result of normal, protective host immune responses. Put another way, tissue transplanted from 1 individual to another will be rejected if the recipient's immune system recognizes the transplanted organ or tissue as foreign.

Histocompatibility testing is used to minimize graft foreignness and reduce donor-specific immune responses to the transplanted organ. The type of histocompatibility testing performed varies, depending on the organ or tissue transplanted. The reasons for this variability are that the immunogenicity of organs and tissues varies and the cold ischemic times for different organs and tissues vary considerably depending on the expression of class II MHC antigens and the relative number of APCs in the tissues. Therefore, it is desirable to perform more extensive histocompatibility testing for the most immunogenic organs, but the length of cold ischemia time an organ will tolerate limits the extent of testing that can be done. In some cases this is a severely limiting factor. For example, the maximum cold ischemia time for heart transplantation is 4 hours. In this case, there simply is not enough time to perform HLA typing between donor and recipient. Tissues and organs are listed in Table 6 according to their capacity to induce allogeneic reactions. Bone marrow is most immunogenic; the liver is the least immunogenic.

The ABO and HLA systems have been identified as the major transplantation antigens in man. ABO antigens are present in most body tissues as well as on RBCs. Two categories of histocompatibility testing are routinely performed in preparation for organ and some types of tissue transplantation -- typing and matching procedures.

Typing Procedures: Determining the Presence of Potentially Reactive Antigens

Typing procedures identify the exact antigens that would be responsible for incompatibility between the donor and recipient tissue.

ABO typing. Basic ABO compatibility depends on the presence or absence of antigens on donor RBCs and the presence or absence of specific antibodies to these antigens in the recipient's serum. Anti-ABO antibodies are of the IgM classification and cause agglutination, complement fixation, and hemolysis. If an ABO-incompatible graft is transplanted, hyperacute rejection will occur (the possible exception being a liver graft). In kidney transplantation, preformed circulating cytotoxic antibodies in the recipient react with ABO isoagglutinins produced by the graft, and the graft quickly turns dark and soft as a result of diffuse thrombosis of the microvasculature.

Rho (D) antigens. Rho antigens are not expressed on endothelial tissue and therefore play no apparent role in graft rejection or survival. In other words, an organ from a donor with ABO type B positive can be safely transplanted into a recipient with ABO type B negative.

Minor red cell antigens. At least 15 different minor red cell antigen systems have been identified in humans. The most important of these appears to be the Lewis system.[11] Transplant recipients who are highly sensitized to minor red cell antigens as a result of numerous blood transfusions, for example, may experience antibody-mediated rejection responses (hyperacute or chronic rejection). For this reason, the potential recipient's blood is screened for the presence of antibodies to the known minor red cell antigens before transplantation.

Vascular endothelial antigens. Vascular endothelial antigens are known to occur, but are not easily detected and therefore cannot necessarily be avoided. These antigens may stimulate antibody production in the recipient and trigger hyperacute rejection. Sensitization to these antigens occurs from exposure to monocytes through blood transfusions. In the early 1970s, there was some evidence that blood transfusions administered to kidney recipients before transplantation had beneficial effects related to increased graft survival. However, many centers have stopped this practice because of the relatively high incidence of patients who become sensitized to HLA antigens.

HLA typing (microlymphocytotoxicity testing). Microlymphocytotoxicity testing is used to detect class I MHC antigens in order to "match" as many class I antigens between the donor and recipient as possible. When 2 people share the same HLAs, they are said to "match." In other words, their tissues are immunologically compatible with each other.

HLA tissue typing is performed serologically by adding a standard panel of typing antisera, complement, and tryphan blue stain to purified lymphocytes and observing for lymphocytotoxicity. Cell death confirms that the test cells (recipient and donor cells) possess the antigens being tested for, namely HLA-A, HLA-B, and HLA-DR antigens. There are many different specific HLA proteins within each of these groups. For example, there are 59 different HLA-A proteins, 118 different HLA-B, and 124 different HLA-DR.

HLA matching improves kidney, heart, and lung graft survival, but an HLA-based (ABO, HLA, lymphocytotoxicity cross-match) donor organ allocation has been implemented only for kidney transplantation. Many studies have shown a stepwise decrease in graft survival of cadaver kidneys with increasing numbers of HLA mismatch.[12] The superior results with 0 HLA-A, HLA-B, and HLA-DR mismatches have led to a system of mandatory sharing of such donor kidneys. Time constraints regarding the preservation of donor hearts and lungs do not permit prospective HLA matching for these organs. However, cross-matching may be done for cardiothoracic transplantation if the patient is HLA sensitized.

Rapid graft rejection can occur even when MHC-matched tissues are transplanted due to minor histocompatibility (mH) antigens, which are peptides from allelically polymorphic host proteins other than MHC molecules, presented in the groove of MHC class I and II molecules.[13] The mH antigens explain the need (in some cases) for systemic immunosuppressive therapy to recipients of HLA-identical organ grafts and GVHD after HLA-identical stem cell transplantation.

Mixed leukocyte culture (reaction). The mixed leukocyte culture detects class II antigens and measures donor-recipient compatibility between HLA-D loci. HLA-D loci disparity can occur even when HLA-A and HLA-B loci are identical. Because this test takes several days to complete, it is used only in preparation for living-related donor kidney transplantation.

During recent years, alternative strategies for HLA matching considered in kidney transplantation include cross-reactive group (CREG) matching, "public" epitope matching (the conventional HLA antigens are called "private" epitopes), and residue matching (determined from amino acid residue sequence information of HLA antigens).[12] All 3 strategies are based on the concept that HLA molecules contain multiple antigenic determinants and that some are more important for matching than others. CREG-matching strategies are now being implemented in kidney transplantation.

Histocompatibility testing for liver transplantation remains somewhat of an enigma. HLA compatibility does not seem to benefit the overall group of liver transplant recipients.[12] In fact, several studies have shown lower survival rates for HLA-DR-matched livers. HLA matching seems to have a dualistic effect on liver transplant outcome. First, it reduces graft rejection. Second, it promotes other immune mechanisms of graft injury related to viral infection (eg, cytomegalovirus and hepatitis viruses) and recurrent disease. Moreover, a liver allograft has a distinguished feature of promoting a hematolymphoid chimeric state associated with transplant tolerance, but liver graft-derived immunocompetent cells may also induce GVHD.

Matching Procedures: Detecting Preformed Circulating Antibodies

Matching procedures provide an opportunity for donor and recipient antigens to interact and predict the degree of compatibility between donor and potential recipient. Pretransplantation cross-matching involves mixing the recipient's serum with potential donor lymphocytes to identify preformed antibodies in the recipient. Cross-matching can be done between the recipient and a specific potential donor or between the recipient and a panel of random potential donors.

White cell cross-match. The white cell cross-match is done to identify in the potential recipient the presence of preformed circulating cytotoxic antibodies to antigens on the lymphocytes of a specific donor. The recipient's serum is incubated with a specific donor's lymphocytes. A negative cross-match means that the recipient does not have cytotoxic antibodies against the donor's lymphocytes. A positive cross-match means that the recipient has cytotoxic antibodies in their serum against the donor's lymphocytes. A positive cross-match is a contraindication for organ transplantation because of the risk for hyperacute rejection and the higher incidence of vascular rejection during the early posttransplant period. This applies particularly for kidney and heart transplants, whereas the liver allograft is more resistant to antibody-mediated injury.

In kidney transplantation, several modifications of the cross-match assay have been used to increase its sensitivity, including antihuman globulin augmentation, flow cytometry, enzyme-linked immunoassays (ELISA), and B-cell cross-matches.[12] Recently, additional serologic methods have been developed that do not utilize lymphocytotoxicity as an end point.[12] One is based on flow cytometry analysis of alloantibodies binding to panel donor lymphocytes with different HLA types. Serum screening is also being done with an ELISA assay.

Mixed lymphocyte cross-match. The mixed lymphocyte cross-match is also done to identify preformed circulating cytotoxic antibodies in the recipient. Patients can have HLA antibody as a result of transfusions, prior transplants, and/or pregnancies. The potential recipient's serum is mixed with a randomly selected panel of 60 donor lymphocyte samples to measure their extent of reactivity against the panel. This procedure is also referred to as percentage panel reactive antibody (PRA). For example, if the patient's serum reacts with 30 out of 60 samples, then the patient's PRA is 50%. The PRA can vary from 0% (nonsensitized) to 80% to 100%, indicating a high degree of sensitization. A high PRA suggests a low probability of finding a cross-match-negative donor. Patients with a high PRA must wait much longer for a transplant than patients with a low PRA, and some may never receive a kidney.

In addition to determining the actual PRA, it is important to know the antibody specificity. Some patients have as few as 1 or 2 antibody specificities, while others have numerous specificities. Because the development of antibodies may change over time, the potential renal transplant candidate is usually screened on a regular basis (ie, monthly).

Donor-specific cross-matching has limited relevance to liver transplantation because the liver allograft is relatively resistant to humoral rejection. In some sensitized patients, a liver allograft may even protect a subsequent kidney transplant from hyperacute rejection.[12]