Solid Organ Transplantation



Solid Organ Transplantation


Mary L. Turgeon and Kyle P. Miller




The first organ transplantation, using a kidney from an identical twin, was performed in 1954 by Dr. Joseph Murray at Peter Bent Brigham Hospital in Boston. The recipient survived for 9 years. Dr. Murray was ultimately recognized for his work by receiving the Nobel Prize in Medicine in 1990.


At present, a variety of tissues and organs are transplanted in human beings, including bone marrow, peripheral stem cells, bone matrix, skin, kidneys, liver, cardiac valves, heart, pancreas, corneas, and lungs. Transplantation is one of the areas, in addition to hypersensitivity (Chapter 26) and autoimmunity (Chapter 28), in which the immune system functions in a detrimental way.


Early in the history of transplantation, tissue antigens were recognized as important to successful grafting. If significantly different foreign antigens were introduced into an immunocompetent host, the transplanted tissue or organ would undoubtedly fail. Currently, tissue (histocompatibility) matching with concomitant immunosuppression of the host in many cases is used to enhance the probability of success in organ and tissue transplantation.


Transplantation presents the following two basic problems.




Histocompatibility Antigens


The major histocompatibility complex (MHC) is a cluster of genes found on the short arm of chromosome 6 at band 21 (6p21; see Fig 2-1). These genes code for proteins that have a role in immune recognition.


The MHC encodes the human leukocyte antigens (HLAs), which are the molecular basis for T cell discrimination of self from nonself. The HLA complex contains over 200 genes, more than 40 of which encode leukocyte antigens, with the rest an assortment of genes not directly related to the HLA genes. Many genes in this complex have no role in immunity.


Transplanted tissue may trigger a destructive mechanism, rejection, if the recipient’s cells recognize the MHC protein products on the surface of the transplanted tissue as foreign, or if immunocompetent cells transplanted on the donor tissue target the foreign cells of the recipient for elimination.



Nomenclature of Human Leukocyte Antigen Alleles


Each HLA allele has a unique four-, six-, or eight-letter or digit name (Table 31-1). The length of the allele designation depends on the sequence of the allele and that of its nearest relative. All alleles receive a four-letter or digit name; six- and eight-digit names are only assigned when necessary.



Table 31-1


HLA Naming System











































Nomenclature Indicates
HLA Human leukocyte antigen (HLA) region and prefix for an HLA gene
HLA-DRB1 Particular HLA locus (e.g., DRB1)
HLA-DRB113 Group of alleles that encode the DR13 antigen
HLA-DRB11301 Specific HLA allele
HLA-DRB11301N Null allele
HLA-DRB1130102 Allele that differs by a synonymous mutation
HLA-DRB113010102 Allele that contains a mutation outside the coding region
HLA-A2409N Null allele
HLA-A3014L Allele encoding a protein with significantly reduced or low cell surface expression
HLA-A24020102L Allele encoding a protein with significantly reduced or low cell surface expression, where the mutation is found outside the coding region
HLA-B44020102S Allele encoding a protein expressed as a secreted molecule only
HLA-A3211Q Allele that has a mutation previously shown to have a significant effect on cell surface expression, but where this has not been confirmed and its expression remains questionable

As of June 2007, no alleles have been named with the “C” or “A” suffixes.


The first two digits describe the type, which often corresponds to the serologic antigen carried by an allotype. The third and fourth digits are used to list the subtypes, with numbers assigned in the order in which DNA sequences have been determined.


Alleles whose numbers differ in the first four digits must differ in one or more nucleotide substitutions that change the amino acid sequence of the encoded protein. Alleles that differ only by synonymous nucleotide substitutions (also called silent or noncoding substitutions) within the coding sequence are distinguished by the use of fifth and sixth digits. Alleles that only differ by sequence polymorphisms in the introns or in the 5′ or 3′ untranslated regions that flank the exons and introns are distinguished by the use of seventh and eighth digits.


In addition to the unique allele designation, optional suffixes may be added to an allele to indicate its expression status. Alleles shown not to be expressed, termed null alleles, have been given the suffix N. Alleles shown to be alternatively expressed may have the suffix L, S, C, A, or Q.


The suffix L is used to indicate an allele shown to have low cell surface expression compared with normal levels. The S suffix is used to denote an allele specifying a protein that is expressed as a soluble secreted molecule but that is not present on the cell surface. A C suffix indicates an allele product that is present in the cytoplasm but not on the cell surface. An A suffix indicates aberrant expression, where there is some doubt as to whether a protein is expressed. A Q suffix is used when the expression of an allele is questionable, given that the mutation seen in the allele has previously been shown to affect normal expression levels.



Major Histocompatibility Complex Regions


The MHC is divided into four major regions (Table 31-2)—D, B, C, and A. The A, B, and C regions are the classic or class Ia genes that code for class I molecules. The D region codes for class II molecules. Class I includes HLA-A, B, and C. The three principal loci (A, B, and C) and their respective antigens are numbered 1, 2, 3, and so on. The class II gene region antigens are encoded in the HLA-D region and can be subdivided into three families, HLA-DR, HLA-DC (DQ), and HLA-SB (DP).




Classes of Human Leukocyte Antigen Molecules


Structurally, there are two classes of HLA molecules, class I and class II (Table 31-3). Both classes are cell surface heterodimeric structures. Class I HLA molecules consist of an alpha chain, a highly polymorphic glycoprotein, encoded within the MHC on chromosome 6. This alpha chain noncovalently associates with beta-2 microglobulin, a nonpolymorphic glycoprotein, encoded by a non-HLA gene on chromosome 15. Class II HLA molecules are composed of alpha chains and beta chains encoded within the MHC. The conformation of class I and class II HLA molecules provides each with a groove in which linear peptides, consisting of 8 to 25 peptides, are displayed for recognition by the cell surface expression on lymphocytes of a transmembrane heterodimeric receptor. All nucleated cells of the body display transmembrane class I HLA molecules in association with the non–transmembrane beta-2 microglobulin molecule.



Class I and class II antigens can be found on body cells and in body fluids. Class I and class II molecules are surface membrane proteins. Class I molecules are transmembrane glycoproteins, but the class II dimer molecule differs from class I in that both dimers span the cell membrane. Class I and class II gene products are biochemically distinct, although they appear to be distantly related through evolution. Class III gene products such as C2, C4A, C4B, and Bf complement components are incomplete but these structures are defined by genes lying between or very near the HLA-B and HLA-DR loci.


Multiple alleles occur at each locus. Genes of class I, II, and III antigens at each locus are inherited as codominant alleles. Inheritance within families closely follows simple mendelian dominant characteristics. Conservation of entire haplotypes through generation after generation is the general rule. Very strong linkage disequilibrium is displayed between several HLA loci, creating super or extended haplotypes that may differ from race to race. For example, the most frequent Caucasoid superextended haplotype, AL, Xw7, BB, BfS, C2-1, C4AQOB1, DR3, is almost absent in Asians.



Role of Major Histocompatibility Complex and Human Leukocyte Antigens


The histocompatibility complex that encodes cell surface antigens was first discovered in graft rejection experiments with mice. When the antigens were matched between donor and recipient, the ability of a graft to survive was remarkably improved. A comparable genetic system of alloantigens was subsequently identified in human beings.


The presence of HLA was first recognized when multiply transfused patients experienced transfusion reactions despite proper crossmatching. It was discovered that these reactions were caused by leukocyte antibodies rather than by antibodies directed against erythrocyte antigens. These same antibodies were subsequently discovered in the sera of multiparous women.


The MHC gene products have an important role in clinical immunology. For example, transplants are rejected if performed against MHC barriers; thus, immunosuppressive therapy is required. These antigens are of primary importance and are second only to the ABO antigens in influencing the genetic basis of survival or rejection of transplanted organs.


Although HLA was originally identified by its role in transplant rejection, it is now recognized that the products of HLA genes play a crucial role in our immune system. T cells do not recognize antigens directly but do so when the antigen is presented on the surface of an antigen-presenting cell (APC), the macrophage. In addition to presentation of the antigen, the macrophage must present another molecule for this response to occur. This molecule is a cell surface glycoprotein coded in each species by the MHC. T cells are able to interact with the histocompatibility molecules only if they are genetically identical (MHC restriction).


Both class I and class II antigens function as targets of T lymphocytes that regulate the immune response. Class I molecules regulate interactions between cytolytic T cells and target cells and class II molecules restrict the activity of regulatory T cells (helper, suppressor, and amplifier subsets). Thus, class II molecules regulate the interaction between helper T cells and APCs. Cytotoxic T cells directed against class I antigens are inhibited by CD8 cells; cytotoxic T cells directed against class II antigens are inhibited by CD4 cells. Many genes in both class I and class II gene families have no known functions.


The class I and class II molecules can also bind to self antigens produced in the normal process of cellular protein degradation. Usually, these are not recognized by the T cell receptor (TCR; tolerance). In transplant patients, most immune responses are generated not from bacterial antigens, viral antigens, or self antigens, but from the presentation of alloepitopes derived from the transplanted tissue to circulating T lymphocytes. Two types of alloepitopes are present on transplanted tissue, private and public. Cross-reactive groups have been defined that categorize the cross-reactive alleles of HLA-A and HLA-B.


Class III molecules bear no clear relationship to class I and II molecules aside from their genetic linkage (presence of the gene in or near the MHC complex). Class III molecules are involved in immunologic phenomenon because they represent components of the complement pathways.



Human Leukocyte Antigen Applications


HLA matching is of value in organ transplantation, as well as in the transplantation of bone marrow. The most important HLA antigens are HLA-A, and HLA-B. Everyone has two types of each of these major HLA antigens; there are many different subtypes of HLA-A and of the others. The best possible match is 6/6; the worst possible match is 0/6.


In kidney allografts, the method of organ preservation, the time elapsed between harvesting and transplanting, the number of pretransplantation blood transfusions, the recipient’s age, and the primary cause for kidney failure are all important determinants of early transplantation success or failure. HLA compatibility, however, exerts the strongest influence on long-term kidney survival. The 1-year survival for kidneys transplanted from an HLA-identical sibling approaches 95%. Approximately 50% to 65% of cadaveric kidneys mismatched for all four HLA-A and -B antigens function for 6 months but deteriorate thereafter with time. Only 15% to 25% of these mismatched cadaveric kidneys remain functioning 4 years after transplantation.


It is obligatory to select HLA-identical donors for bone marrow transplantation to reduce the frequency of graft-versus-host disease (GVHD; see later). A method, that depletes donor marrow T cells capable of recognizing foreign host antigens has greatly reduced the incidence of GVHD.


HLA-matched platelets are useful for patients who are refractory to treatment with random donor platelets. In paternity testing, HLA typing is used, along with the determination of ABO, Rh, MNS, Kell, Duffy, and Kidd erythrocyte antigen. In the past, most laboratories involved in testing individuals in disputed parentage cases used only the ABO, Rh, and MNS systems. The chances of identifying a falsely accused man with these tests were 58%. Additional testing for Kell, Duffy, and Kidd erythrocyte antigens and for HLA typing has an exclusion rate estimated at 92%.


HLA typing is also useful in forensic medicine, anthropology, and basic research in immunology. In studies of racial ancestry and migration, some antigens are almost excluded or confined to a race (e.g., A1 and B8 are rarely detected in peoples indigenous to central and eastern Asia, and Bw57 is uncommon in whites and African Americans). These distinctions allow for precise conclusions to be drawn regarding origin and ancestry.


HLA testing has increasingly been used as a diagnostic and genetic counseling tool. Knowledge of HLA antigens and their linkage has become important because of the recognized association of certain antigens (Box 31-1) with distinct immunologic-mediated reactions, autoimmune diseases, some neoplasms, and other disorders; these disorders, although nonimmunologic, are influenced by non-HLA genes also located within the major MHC region.



The estimated relative risks or chances of developing a disease if a given antigen is present may be elevated in individuals bearing certain HLA antigens compared to individuals who lack the antigen (Table 31-4). The HLA-B27 antigen is the only HLA antigen with a disease association strong enough to be useful in differential diagnosis. Although the degree of association between HLA antigens and other diseases may be statistically significant, it is not strong enough to be of diagnostic or prognostic value.



Although only 8% of normal whites carry the HLA-B27 antigen, 90% of patients with ankylosing spondylitis (AS) or spondylitis in association with Reiter’s syndrome are positive for the antigen. An elevated percentage of HLA-B27–positive patients is also observed in juvenile chronic arthritis with spinal involvement. Therefore, the major indication for screening for HLA-B27 test is to rule out AS when back pain develops in relatives of patients with the disease and to help distinguish incomplete Reiter’s syndrome from gonococcal arthritis, or chronic or atypical Reiter’s syndrome from rheumatoid arthritis. A negative test for HLA-B27, however, does not exclude the diagnosis of AS or Reiter’s syndrome.



Laboratory Evaluation of Potential Transplant Recipients and Donors


Systems developed to ascertain compatibility between donor and recipient include HLA typing and screening of the potential recipient’s serum for the presence of antibodies associated with rejection. Graft success is generally correlated with the presence of a compatible crossmatch, although some transplantation teams have proceeded despite a positive (incompatible) crossmatch.



Human Leukocyte Antigen Typing


A potential recipient needs to have HLA typing (Fig. 31-1, A). A family search may be conducted for a suitable donor. If a suitable match is not found, the patient is placed on a waiting list (see Fig. 31-1, B). When an organ becomes available, the donor is HLA-typed and a computerized search is made for a suitable recipient (see Fig. 31-1, C).



A newer method of HLA typing is polymerase chain reaction (PCR) amplification of DNA, followed by probing with sequence-specific oligonucleotide probes (SSOPs) and PCR amplification of alleles at loci using allele-specific primers. A simple computer program has been developed to assign the alleles and genotypes based on the probe hybridization pattern.


The difference between HLA-genotyped and zero mismatches reflects the imperfection of the HLA-typing process. HLA genotype–matched means an HLA-identical sibling when all alleles are truly identical. A zero-mismatched unrelated donor may be mismatched because typing does not distinguish between very closely related alleles. In addition, there may be some effect of non-HLA loci because HLA-identical siblings will share only 50% of their minor histocompatibility loci. The degree of donor-recipient mismatch is somewhat obvious, even in the first year.




Complement-Mediated Cytotoxicity


Class I antigens are determined by several techniques; the popular classic method is the lymphocyte microcytotoxicity method (complement-mediated cytotoxicity). With this technique, a battery of reagent antisera and isolated target cells are incubated with a source of complement under oil to prevent evaporation. If a specific alloantibody and cell membrane antigen combine, complement-mediated damage to the cell wall allows for penetration of a vital dye and the cells are killed. Cell death is determined by staining. A stain such as trypan blue will penetrate dead cells but not living cells. Unaffected cells remain brilliantly refractile when observed microscopically.


This assay can be insensitive and scoring is subjective. Test sensitivity can be enhanced by the addition of anti–human globulin (AHG) antibody. This assay is used for pretransplantation crossmatching and for antibody specificity analysis. For HLA class I typing or anti–class I antibody identification, a purified T cell population is preferred because human T lymphocytes express class I but not class II molecules. Conversely, B lymphocytes are required for class II typing or antibody identification because human B cells express class I and class II HLA molecules.


Class II HLA-DR and HLA-DQ specificities are also recognized by similar serologic methods, except that isolated B cells are the usual target cells because their surface is rich in these molecules, as well as in class I determinants. At present, HLA-Dw and HLA-DP cannot be serologically defined, and their detection relies on the ability of these molecules to stimulate newly synthesized DNA when added to primary mixed lymphocyte (HLA-Dw) or when re-added to secondary primary lymphocyte (HLA-DP) in vitro cultures.


Class III complement specificities are recognized by the availability of diagnostic reagents, but reagents remain scarce.




Flow Cytometry


Single-cell analysis by flow cytometry is the most sensitive method for crossmatching and antibody identification (see Chapter 13). Tagged T or B lymphocytes are incubated with the patient’s serum to allow the formation of antigen-antibody complexes on the cell surface. Unbound proteins are washed away and the bound antibodies are detected with a second antibody, anti–human immunoglobulin G (IgG) labeled with a chromophore. An alternative flow cytometry format uses microparticles coated with HLA antigens of known specificity (obtained through recombinant techniques) instead of lymphocytes.



Facts About Solid Organ Transplantation


Organ transplantation is widely viewed as the preferred treatment for end-stage organ failure because of the quality of life that the treatment offers for patients and because of the long-term cost benefits. The increased demand for organ transplantation is fueled by the transplantation success rate. Worldwide, the demand for transplantation procedures is increasing by about 15%/year, but the number of donated organs has remained static.


The waiting time for allergenic organ transplantation varies widely for many reasons. Each patient’s situation is different. Some patients are more ill than others when they are put on the transplant waiting list. Some patients become sick more quickly than others or respond differently to treatments. Patients may have medical conditions that make finding a good match more difficult.


How long a patient waits for a transplant depends on the following factors:



Depending on the type of organ needed, some factors are more important than others.


In 1984, the U.S. Congress passed the National Organ Transplant Act. The goal of this legislation was to match a low supply of organs with the most critically ill patients, regardless of where they reside. About 79 patients receive an organ transplant every day in the United States. In 2009, more than 28,000 patients received an organ transplant.


On March 22, 2012, the United Network for Organ Sharing patient waiting list contained 113,612 names. The list continues to grow because of the scarcity of organs (Box 31-2). Most of these registrants are waiting for a kidney transplant, followed by those waiting for a liver transplant and heart transplant. Other transplant registrants are waiting for lung, kidney and pancreas, pancreas, pancreatic islet cell, heart and lung, and intestine. Approximately 25% of patients waiting for a liver transplant are children younger than 10 years.



The most common reasons for needing a transplant vary by the type of organ. Kidney recipients usually have diabetes, glomerulonephritis, hypertensive nephrosclerosis, or polycystic kidneys. Liver recipient patients typically have noncholestatic cirrhosis, cholestatic liver disease, biliary atresia, acute hepatic necrosis, or hepatitis C infection. Patients with cardiomyopathy, congenital heart disease, valvular heart disease, or coronary artery disease are the most frequent heart transplant recipients.


The number of patients living with a function graft has generally increased over the last decade. Graft survival time depends on many factors, including the type of organ transplanted (Fig. 31-2 and Table 31-5).





Transplantation Terminology


The transplanting or grafting of an organ or tissue ranges from self-transplantation, such as skin grafts from one part of the body to another to correct burn injuries, or hair transplants from one area of the scalp to another to correct pattern baldness, to the grafting of a body component from one species to another, such as transplanting a pig’s heart valve to a human. Table 31-6 defines the most recent terms used in transplantation.




Types of Transplants


Eleven different organs or human body parts can be transplanted—blood vessels, bone, bone marrow or stem cells (see Chapter 32), cornea, heart, kidneys, liver, lung, middle ear, pancreas, and skin. Successful organ transplants have increased since the advent of the immunosuppressive drug cyclosporine (cyclosporin A).


Living donor transplants have attracted significant media attention. According to the United Network for Organ Sharing and the Health Resources and Services Administration of the U.S. Department of Health and Human Services, a living donor may donate a single kidney, segment of the liver, portion of the pancreas, or the lobe of a lung.



Bone


Bone matrix autografts or allografts are common. Transplantation of bone matrix is used after certain limb-sparing tumor resections and to correct congenital bone abnormalities. The major criteria for bone donation are a lack of infection, no history of IV drug use, and no history of prolonged steroid therapy or human growth hormone treatment. Bone can be easily harvested and frozen. Freezing not only preserves the bone but offers the additional benefit of concomitant diminution of histocompatibility antigens.


The major technical requirement for allograft transplantation is maintaining the periosteal sheath of the recipient bone to strip the donor bone completely of all periosteal elements. Transplantation of bone is an easy procedure. Processed bone lacks significant quantities of immunogenic substances; therefore, the need for immunosuppression is almost completely eliminated.


Jun 12, 2016 | Posted by in IMMUNOLOGY | Comments Off on Solid Organ Transplantation

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