Page 1950 - Hematology_ Basic Principles and Practice ( PDFDrive )

1730 Part XI Transfusion Medicine

preexistence of alloantibodies restricts the identification of compatible THE HUMAN LEUKOCYTE ANTIGEN MOLECULES
donors even further. Highly sensitized recipients with PRA activity
exceeding 85% of tested specificities (generally between 30% and AS ANTIGENS AND HLA ALLOIMMUNIZATION
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60%) represent a particularly challenging group. An alternative
approach to the exclusion of alloreactive determinants is the inclusion By no means are HLA molecules antigenic in physiologic condition
of acceptable antigen mismatches expressed in a panel of cells that (with the exception of maternofetal alloimmunization). However,
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give negative reactions with the recipient sera, which extends the because of their high density on the surface of cells, they can become
repertoire of possible donors. All these are fundamental tools for the highly immunogenic in the nonphysiologic event in which cells
identification of nonrelated, not fully matched donor-recipient pairs. from different individuals are exposed to another person’s immune
Unfortunately, even these compromises often fail to identify a suit- system. The mechanism or mechanisms leading to allosensitization
able match. are believed to follow two pathways. The first pathway mimics the
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Duquesnoy described a molecularly based algorithm to identify one followed during most immune reactions in which antigen is
histocompatible pairs called HLAMatchmaker. This method focuses up-taken by antigen-presenting cells and presented to autologous
on the structural basis of HLA class I polymorphism so that compat- lymphocytes (indirect pathway). In this case the donor’s HLA mol-
ible HLA mismatches can be identified without extensive serum ecules are processed into peptides through the exogenous pathway
screening. This algorithm is based on the principle that short amino of antigen presentation and presented to autologous T cells as linear
acid sequences (triplets) characterizing polymorphic sites of the HLA peptides. 154,155 This pathway is believed to be responsible for the
molecules are critical components of allosensitizing epitopes. Such development of alloantibodies as well as T-helper cell responses, but
amino acids reside in the α-helices and β-loops of the heavy chain. its role in the development of cytotoxic T-cell responses remains
Because each HLA molecule expresses a characteristic string of these unclear. Because this pathway depends on the presentation of donor
determinants, it is possible to characterize each molecule according HLA molecules by recipient HLA alleles, it may explain why the
to the linear sequence of amino acid triplets present on its surface. humoral response to HLA class I allodeterminants correlates with
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Based on the reasonable assumption that none of the triplets present the HLA phenotype of the responder. The indirect pathway of
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in the HLA repertoire of the recipient is self-immunogenic, it is HLA allorecognition has been associated with allograft rejection.
possible through a process of electronic recombination to identify Because the function of HLA molecules is to present antigenic
donors with HLA alleles different from the recipient’s but containing determinants to T cells, it could be easily envisioned how minor
exclusively shared triplets. These HLA alleles will be compatible, changes in their structure could be misinterpreted as antigenic
because they do not contain any epitope absent in the recipient. epitopes. Intact HLA molecules residing on the surface of donor
In theory, a large number of triplets could occur if polymorphisms cells are a perfect target for T cell-mediated allorecognition (direct
were randomly distributed. However, most HLA molecules span pathway) either through the direct cytotoxic effect of T cells against
conserved domains, and only a total of 142 different polymorphic target cells or by the activation of helper T cells through HLA class
triplets designate serologically defined HLA-A, HLA-B, and HLA-C II engagement, leading to stimulation of antibody-mediated immune
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antigens. Triplet polymorphism can occur in 30 locations on responses. 158
HLA-A, 27 in HLA-B, and 19 in HLA-C chains. Because the Humoral responses mediated most frequently by IgM are pre-
HLAMatchmaker algorithm includes interlocus comparison, it is dominant in the sensitization to infrequent allogeneic exposure
possible to accumulate the information into a single database. Among because they require smaller amounts of antigenic material. T-cell
the 142 polymorphic triplets, 29 are polymorphic for one class I locus responses become more manifest in the context of transplantation in
but monomorphic for another class I locus. Such polymorphic triplets which the persistence of the allogeneic stimulus allows the expansion
cannot be immunogenic because they are always present on the and sustenance of alloreactive cytotoxic T cells. Antibodies and TCR
patient’s own HLA antigens, whereas the remaining 113 triplets have have different requirements for their engagement, which means
immunogenic potential. With this algorithm, it is possible to signifi- epitopes recognized by T cells and antibodies are different. Antibodies
cantly broaden the number of molecularly matched HLA alleles and require interaction with a small structure, including a limited number
significantly increase the chances of identifying a compatible donor, of amino acids; thus any sequence combination on the surface of an
particularly in those cases in which the recipient has a rare HLA HLA allele not present in the individual exposed to the alloreaction
phenotype. In addition, HLAMatchmaker considers triplets that are may represent an epitope. T cells have much lower binding affinity
present in the panel cells that give negative reactions with the recipi- for their ligand and require a complete interaction with the peptide
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ent’s serum. These negative panel cells can be expected to share as well as the α- and β-helices of the HLA class I heavy chain.
antigens with the patient’s, but other HLA antigens may be present Although several B-cell epitopes recognized by antibodies can be
and contain mismatched triplets apparently not immunogenic for identified in a given HLA molecule, generally the whole HLA mol-
that patient. Such triplets are therefore acceptable and can be added ecule is necessary for T cell-dependent allorecognition. Definition
to the algorithm for the identification of possible donors. Thus and topographic mapping of epitopes defined by serologic or cellular
HLAMatchmaker assesses HLA compatibility at a molecular level by methods has revealed distinct regions of hypervariability in the α-1
determining whether or not a triplet in a given position of a mis- and α-2 domains of the class I heavy chains and in the α-1 and β-1
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matched HLA antigen is also found in the same position in any of domains of class II molecules. Two types of antibody-defined
the recipient’s own HLA-A, HLA-B, and HLA-C molecules. A shared epitopes can be identified according to their frequency among HLA
triplet in the same position on a mismatched HLA antigen cannot alleles. Private epitopes are almost, but not totally unique for a single
elicit a specific antibody response in that patient. Preliminary verifica- serologically defined HLA antigen and are used for typing. These
tion of the algorithm in a series of high-PRA renal patients suggested epitopes are generally shared by all molecular alleles present in that
that this is a proper strategy at least in highly sensitized renal trans- given family, and fine differences among alleles within a general
plantation candidates waiting for unrelated donors. 150,151 HLAMatch- family cannot be distinguished by antibodies. Public epitopes are
maker is also effective at selecting an optimal HLA-typed platelet more widely distributed and cluster distinct serologic families into
component for alloimmunized thrombocytopenic patients. 152 groups. These epitopes bear an immunodominant character. Immune
A new version of HLAMatchmaker considers so-called eplets and sera that identify public epitopes have been considered predictive of
is based on the structural definitions of functional epitopes on well- major CREGs with the idea that alloreactivity among patients
characterized protein antigens that have been complexed with an belonging to the same CREG may be less likely (Table 113.3). The
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antibody. Eplets represent amino acid residue configurations within predictive value of CREGs in transplant outcome or platelet transfu-
a 3- to 3.5-Å radius of each polymorphic residue on the HLA sion results, however, remains to be demonstrated.
molecular surface. Many eplets correspond to triplets, but eplets Not all patients who have been exposed to alloantigens develop
represents a more complete repertoire of structurally defined alloantibodies, and in fact, exposure to low doses of donor-specific
epitopes. HLA antigens through donor transfusion may have a beneficial effect