Transplant Immunobiology
Michelle J. Hickey
Phuong-Chi T. Pham
Phuong-Thu T. Pham
BACKGROUND
ABO blood group is the most important tissue barrier to successful kidney transplantation, followed by major histocompatibility complex (MHC) antigens. Non-MHC molecules, referred to as minor histocompatibility antigens or non-human leukocyte antigens (non-HLAs), can also mediate rejection (discussed below and summarize in Table 8.1).
ABO BLOOD GROUP ANTIGENS
ABO blood group antigens are expressed on the surface of red blood cells as well as in the kidneys, gastrointestinal, respiratory, and other organ systems.
ABO-incompatible kidney transplantation results in hyperacute rejection and graft loss.
A number of variant A antigens are known, with the A1 antigen providing more potent antigenicity than the antigen A2. Successful transplantation can be performed using A2 kidneys into O recipients and A2 and A2B kidneys into B recipients.
Various desensitization protocols have allowed successful ABO-incompatible kidney transplantation. Discussion is beyond the scope of this chapter.
MHC, HLA MOLECULES
The major histocompatibility (MHC) genes are located on the short arm of chromosome 6 and represent the most polymorphic genes in human genome.
HLAs are glycoproteins encoded by the MHC genes (Fig. 8.1). In human, the MHC molecule was first discovered in leukocytes; therefore, it is also called the human leukocyte antigen (HLA).
The primary role of Class I and II HLA is to present foreign antigen to the immune system.
Table 8.1 Transplantation antigens and their role in kidney allograft rejection
Transplantation Antigens
(listed in order of importance in allograft rejection)
Comments
ABO blood group
ABO blood group antigens are strong transplantation antigen.
ABO-incompatible transplantation results in hyperacute rejection and graft loss.
Major histocompatibility antigens (also known as human leukocyte antigens [HLA])
HLAs are classified into class I (HLA-A, HLA-B, HLA-C) and class II (HLA-DR, HLA-DQ, HLA-DQA, HLA-DP, HLA-DPA).
Fewer HLA mismatches between donor and recipient correlate better with graft survival.
1-y graft survival is more related to HLA class II mismatching than to class I mismatching.
Repeat HLAs mismatch in the setting of a re-allograft transplantation may trigger reactivation of memory cells and production of donor-specific antibodies.
Minor histocompatibility antigens
MHC-related chain A (MICA) and MHC-related chain B (MICB) are examples of minor antigen expressed on endothelial cells.
Unlike the classic HLA classes I and II, they do not bind peptides and do not engage T-cell receptor. Antibodies against MICA have been shown to be associated with antibody-mediated rejection.
Non-HLAs
Examples of non-HLAs: MICA and MICB (discussed above), AT1R, anti-endothelin-1 type A receptor (ETAR), vimentin, cardiac myosin, collagen V, and agrin
Antibodies to non-HLAs have been shown to be associated with graft rejection.
Currently, clinical tests are available to test for antibodies to MICA, AT1R, and reactivity to antigens expressed on donor endothelial cells.
Abbreviation: AT1R, angiotensin II type 1 receptor.
In kidney transplantation, HLAs are the predominant antigens that form the targets for the immune response.
Over 25,000 distinct HLA alleles have been defined through DNA sequencing. Despite significant diversity at the level of DNA, the majority of polymorphisms that stimulate alloreactivity of the recipient’s immune system are located in the α1 and α2 domains of the α chain of HLA class I, and the α1 and β1 domains of the α and β chains of HLA class II, respectively (discussed below).
The classic HLA class I antigens (HLA-A, HLA-B, and HLA-C) are heterodimers composed of a polymorphic, membrane-spanning α or heavy chain of 44 kDa with three external domains (α1, α2, and α3), noncovalently bound and
stabilized by a nonpolymorphic light-chain, β2-microglobulin (β2-m) of 12 kDa. β2-Microglobulin is encoded by a non-MHC gene located on chromosome 15.
FIGURE 8.1 Major histocompatibility complex, human leukocyte antigen molecules. Class I HLA antigens (HLA-A, HLA-B, and HLA-C) heterodimers consisting of a polymorphic α or heavy chain (encoded by HLA-A, HLA-B, and HLA-C genes) that is noncovalently bound to a nonpolymorphic light-chain, β2-microglobulin. Class II HLA antigens (HLA-DP, HLA-DQ, and HLA-DR) consist of two noncovalently bound glycoproteins: an α chain (encoded by DPA1, DQA1, or DRA1) and a β chain (encoded by DPB1, DQB1, or DRB1, DRB3/4/5). Note: All HLA-DR types have the DRB1 gene, and some contain an additional functional gene, DRB3, DRB4, or DRB5. The majority of polymorphisms that stimulate alloactivation of the recipient’s immune system are located in the α1 and α2 domains of HLA class I and α1 and β1 domains of HLA class II. Abbreviations: β2-m, β2-microglobulin; MICA, MHC class I-related chain A; MICB, MHC class I-related chain B.
They are expressed on all nucleated cells and platelets, but not on red blood cells.
Class I molecules generally present peptides derived from intracellular proteins (e.g., viral proteins) to cytotoxic CD8+ T cells (Fig. 8.2).
The classic class II antigens (HLA-DP, HLA-DQ, and HLA-DR) are composed of two membrane-spanning, noncovalently bound glycoproteins: an α chain of 35 kDa (encoded by DPA1, DQA1, or DRA1) and a β chain of 31 kDa (encoded by DPB1, DQB1, or DRB1).
The majority of polymorphic sites on class II antigens that stimulate alloactivation of the recipient’s immune system are located in the α1 and β1 domains of HLA class II.
HLA class II antigens are constitutively expressed on professional antigen-presenting cells (APCs), including dendritic cells, macrophages, and B lymphocytes. Their expression may be upregulated on activated T cells and epithelial and vascular cells (e.g., renal tubular cells, glomerular endothelium, and capillaries) after exposure to proinflammatory cytokines.
Class II molecules present larger peptides derived from extracellular proteins (e.g., bacterial proteins) to CD4+ T cells (Fig. 8.2).
FIGURE 8.2 Antigen presentation. The endogenous pathway: Endogenous antigens are digested into peptides and loaded into class I MHC. The MHC-peptide complex is assembled within the cell’s endoplasmic reticulum, transported through the Golgi apparatus and expressed on the cell surface where it is recognized by CD8+ TCR, leading to T-cell activation. Exogenous pathway: Exogenous antigens are degraded within endosomes and loaded into class II MHC. The MHC-peptide complex is ultimately expressed on the cell surface where it is recognized by CD4+ TCR, leading to T-cell activation. Abbreviations: MHC, major histocompatibility complex; TCR, T-cell receptor.
Kidney donors and recipients in the United States are typed for HLA-A, HLA-B, HLA-Bw4/6, HLA-C, HLA-DRB1, HLA-DRB3/4/5, HLA-DQB1, HLA-DQA1, and HLA-DPB1.
For interested readers:
HLA-B antigens are distinguished by either the immunogenic Bw4 or Bw6 epitope encoded on this antigen between amino acids 77 and 83.
The DRB3, DRB4, or DRB5 genes encode HLA-DR52, HLA-DR53, and HLA-DR51, respectively. Some HLA-DRB1 genes are commonly linked to one of these additional DRB345 genes. For example, DR17 is commonly linked to DR52; DR7 is commonly linked to DR53; DR15 is commonly linked to DR51. However, DR8, and members of the DR1 group, are not commonly linked to a DRB345 gene (Fig. 8.3).
Terms used for HLA match (or mismatch) when considering HLA-A, HLA-B, and HLA-DR:
Kidney from a parent or a sibling: Each parental chromosome 6 provides a linked set of MHC genes (called a haplotype) to the offspring in a Mendelian codominance inheritance. Statistically, there is a 25% chance that siblings share the same two haplotypes (two-haplotype match), a 50% chance they share one same haplotype (one-haplotype match), and a 25% chance they do not share any of their parental haplotypes (zero-haplotype match or two-haplotype mismatch). By definition, a child is a haplotype match to each parent unless recombination has occurred (Fig. 8.4).
Example: a kidney from a parent donor (father or mother) to a recipient offspring: one-haplotype match.
Example: a kidney from a sibling donor to sibling recipient: will be either a zero-haplotype match, one-haplotype match, or two-haplotype match (Fig. 8.4).
FIGURE 8.3 HLA-DR genomic region (for interested readers). Most DRB1 genes are associated with DRB3, DRB4, or DRB5 gene. The DRB345 genes encode the DR52, DR53, and DR51 antigens, respectively. The DR51 group includes the DR15 and DR16 antigens (encoded by the DRB1 gene depicted in teal; blue-green) that are commonly associated with the DR51 antigen (encoded by the DRB5 gene). Antigens in the DR1 and DR8 groups (DR1, DR10, and DR8) are not commonly associated with DRB345 gene. The majority of polymorphisms in DR antigens are encoded in the DRB1/345 genes (such as DR15, DR51, or DR17). The protein translated from these genes is noncovalently bound to the protein produced by translation of the DRA1 gene (depicted in yellow).
Kidney from a deceased donor
Deceased donors are HLA typed at HLA-A, HLA-B, HLA-C, HLA-DRB1/3/4/5, HLA-DQB1, HLA-DQA1, and HLA-DPB1.
The United Network for Organ Sharing (UNOS) uses HLA-A, HLA-B, and HLA-DRB1 matching as part of the donor allocation algorithm. In the current
UNOS allocation system, points are given to patients without HLA-DR mismatch: 2 points if there are no HLA-DR mismatches with the donor and 1 point if there is one HLA-DR mismatch with the donor. Allocation of “zero mismatch” deceased donor kidneys is based on HLA-A, HLA-B, and HLA-DR matching between patient and donor.
FIGURE 8.4 Inheritance of haplotypes and human leukocyte antigen profile in four theoretical siblings. Sibling 1 is a one-haplotype match to sibling 2, a two-haplotype match to sibling 3, and a zero-haplotype match to sibling 4 (or a two-haplotype mismatch to sibling 4).
Terms used for HLA match (or mismatch) when considering HLA-A, HLA-B, and HLA-DR:
0 of 6 HLA-A, HLA-B, HLA-DR mismatch (or a 6 of 6 HLA match)
1 of 6 HLA-A, HLA-B, HLA-DR mismatch (or five HLA match)
Example: Consider the HLA phenotype of the following recipient/donor pair:
Terms used for HLA match (or mismatch) when considering HLA-A, HLA-B, HLA-DR, HLA-DQ, and HLA-DP:
The degree of HLA mismatch between donor and recipient plays an important role in rejection risk and graft loss. In the setting of kidney transplant, fewer HLA mismatches correlate better with graft survival. One-year graft survival is more related to HLA class II mismatching than to class I mismatching. Repeat HLA mismatch in the setting of reallograft transplantation (second or third transplant) may trigger reactivation of memory cells and production of donor-specific antibody (DSA). DSAs are antibodies that are specific for donor antigens and can be formed prior to transplant due to pregnancy, blood transfusion, or prior transplant (referred to as preformed) or developed posttransplant (referred to as de novo). Further discussion on identification of anti-HLA antibodies is presented in a later section.
HLA-TYPING TECHNIQUES
Serotyping
HLA serotyping by serologic methods was previously performed using the complement-dependent microlymphocytotoxicity test; however, more accurate
and higher resolution molecular typing of DNA (described below) has replaced these methods.
The test is performed in a microtiter plate with multiple small wells.
Each well is loaded with a selected antiserum, and lymphocytes from the individual to be typed are added. The antiserum is well characterized and contains strong HLA antibodies with known antigen specificity.
After an incubation period, complement is added.
If anti-HLA antibody from the antiserum binds to its specific HLA target antigen on the cell surface, the complement cascade is activated, leading to cytotoxic injury. A vital dye is added to permit visualization of the proportion of dead cells in each well when the tray is examined under phase-contrast microscopy.
The HLA-typing antiserum does not recognize all antigens and is considered low resolution.
DNA Typing
Generally, DNA isolated from blood anticoagulated with acid citrate dextrose (ACD) is preferred for DNA-based HLA-typing methods; however, any source of cells can serve as a sample for molecular-based tissue typing, including samples isolated from biopsy.
DNA-based tissue typing uses standardized probes, primers, or sequencing to determine an individual’s HLA tissue type.
DNA probes hybridize to the complementary DNA nucleotide sequence that is unique to an HLA locus, allele, or groups of alleles. DNA hybridization probe techniques allow identification at the “antigen level” with varying levels of resolution based on the method used (low-to-intermediate resolution), whereas sequencing provides high-resolution “allele-level” HLA typing.
HLA Nomenclature
The level of resolution provided by various molecular HLA-typing methods leads to complex HLA nomenclature. Methods that produce low-resolution typing results distinguish an antigen such as “A2,” whereas high-resolution probes make it possible to distinguish alleles of that antigen such as “A*02:01:01:02” (Fig. 8.5). Intermediate-resolution HLA-typing results may include a “string” of alleles that cannot be ruled out by the method such as A*02:01/03/09/212 (for explanation, see Fig. 8.5).
MINOR HISTOCOMPATIBILITY ANTIGENS
Minor histocompatibility antigens are small endogenous peptides that occupy the antigen-binding site of the donor MHC molecules. They are encoded by genes located on different chromosomes.Related posts:
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