Page 1949 - Hematology_ Basic Principles and Practice ( PDFDrive )

Chapter 113 Human Leukocyte Antigen and Human Neutrophil Antigen Systems 1729

important, because antibodies identify structural differences on the alleles result in ambiguous allele combinations that require additional
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surface of HLA molecules, variants caused by nucleotide polymor- testing for resolution. Finally, new methods based on high-density
phism in nonexposed areas such as the peptide-binding groove of the array technology are being developed that may allow extensive typing
HLA heavy chain are not detectable. However, these differences are of known and unknown polymorphisms on microchips. 143,144
of functional significance because they determine the specificity and High-resolution methods yield high-resolution information of an
affinity of peptide binding and T-cell recognition of self and alloge- individual’s HLA type. However, the wealth of information is coun-
neic target cells. 23,131–133 DNA-based typing directly determines the terbalanced by increased difficulty in identifying suitable HLA alleles
134
sequence, and its resolution is limited only by the number of during donor-recipient pairing or accrual into immunization proto-
allele-specific probes used to identify an ever-growing number of cols restricted to specific HLA-epitope combinations. Thus, at
alleles (see www.anthonynolan.com/HIG/index.htm). Various poly- present, clinicians are faced with the daunting task of applying high-
merase chain reaction (PCR)-based methods have been described, resolution typing results of unclear relevance to clinical settings. 145
among which sequence-specific primer and sequence-specific oligo-
nucleotide probe-based methods are the most universally used. 134–137
The rich nature of HLA has led to proportionally increasing complex- TESTING FOR ALLOSENSITIZATION AND
ity of the assays used to cover all possible alleles. As a consequence,
accurate HLA typing for donor and recipient matching in transplan- DETERMINATION OF COMPATIBLE
tation has become increasingly complex and burdensome. In addi- RECIPIENT-DONOR PAIRS
tion, because of the important role that HLA molecules play in
antigen presentation and the stringency of the relationship between Any cell-containing product transfused or transplanted between
epitope and associated HLA allele, high-resolution typing is increas- different individuals should be compatible in an ideal situation. Yet,
ingly requested for appropriate enrollment of patients into immuni- in most cases, histocompatibility is not prospectively sought. Thus
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zation protocols aimed at the enhancement of T-cell responses. patients with multiple exposures to blood products often become
Therefore high-resolution HLA typing is increasingly in demand in reactive to various antigens, including HLA. Transplant candidates
clinical and experimental settings. often develop prior alloreactivity following transfusion of platelet
Although oligonucleotide-based methods could theoretically dis- concentrates contaminated with leukocytes, even though the inci-
criminate any known polymorphic site, they have two major limita- dence of allosensitization is much less because of leukodepletion of
tions. First, they require a specific PCR reaction for each allele blood products. Alloreactivity must be documented before transplan-
investigated. Because each individual has only two alleles for each tation, because alloreactive patients can still undergo transplantation,
locus, a disproportionately large number of PCR reactions must be provided that the donor has no mismatched HLA antigens reacting
performed to cover all possible polymorphisms to identify the two with the patient’s antibodies. Patients who have received repeated
borne by the individual tested. Because both methods are based on platelet transfusions may become allosensitized and consequently
specific interactions with known oligonucleotide sequences unique to refractory to further transfusions unless HLA-compatible platelets are
a particular allele, they cannot identify unknown polymorphisms used. Obviously the best compatibility consists of identical matching.
unless the variation occurs within the region spanned by one of the It is often impossible to identify a perfectly matched unrelated donor,
oligonucleotides used in the assay. Because of these limitations, interest particularly in the case of rare HLA types. Thus other strategies are
is growing for definitive typing methods that yield conclusive informa- adopted to identify the best possible match or compatible mismatch.
tion about the identity of the alleles typed. The most comprehensive Selection of unrelated donor-recipient pairs is carried out through
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method is sequence-based typing. Unfortunately, its use has been typing with serologic, cellular, and molecular methods. The chances
limited by the cost of equipment and reagents and by the high level of identifying compatible donors based on full or partial HLA match-
of expertise and time required for the interpretation of each typing. ing have become increasingly low with the increasing resolution of
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More recently, high-throughput, robotic, sequence-based typing has the typing methods. To broaden compatibility, matching criteria
been developed that allows sequencing of hundreds of genomic frag- of donor-recipient pairs are based on shared public epitopes assigned
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ments each day. 139–141 However, even sequence-based typing has some to cross-reactive groups (CREGs) or shared amino acid polymor-
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technical limitations. Some combinations of HLA class I and class II phisms defined through sequence information (Table 113.3). The
TABLE Population Frequencies of Major Cross Reactive or Determinants Present on HLA-A and HLA-B Gene Products
113.3
Major Cross- Public Approximate Epitope
Reactive Group Epitope Associated Private Epitopes Frequency (%) a
1C 1p A1, 3, 9 (23, 24), 11, 29, 30, 31, 36, 80 79
10p A10 (25, 26, 34, 43, 66), 11, 28 (68, 69), 32, 33, 74
2C 28p A2, 28 (68, 69), 9, 17 70
9p A2, 28 (68, 69), 9 (23, 24)
17p A2, B17 (57, 58)
5C 5p B5 (51, 52), 18, 35, 53, 78 50
21p B5 (51, 52), 15 (62, 63, 75, 76, 77), 17 (57, 58), 21 (49, 50), 35, 53, 70 (71, 72), 73, 74, 78
7C 7p B7, 8, 41, 42, 48, 81 54
22p B7, 22 (54, 55, 56), 27, 42, 46
27p B7, 13, 27, 40 (60, 61), 47
8C 8p B8, 14 (64, 65), 16 (38, 39), 18 38
12C 12p B12 (44, 45), 13, 21 (49, 50), 40 (60, 61), 41 44
Bw4 Bw4 B13, 27, 37, 38, 47, 49, 51, 52, 53, 57, 58, 59, 63, 77, A24, 25, 32 79
Bw6 Bw6 B7, 8, 18, 35, 39, 41, 42, 45, 46, 48, 50, 54, 55, 56, 60, 61, 62, 64, 65, 67, 71, 72, 73, 75, 87
76, 78, 81
a North American white populations of European origin.