**2.3. Recent changes to HLA nomenclature**

The overall size of the MHC is approximately 3.5 million base pairs. Within this is the HLA Class I genes and the Class II genes each spread over approximately one third of this length. The remaining section, sometimes known as Class III, contains loci responsible for comple‐ ment, hormones, intracellular peptide processing and other development characteristics [1]. Thus the Class III region is not actually a part of the HLA complex, but is located within the HLA region, because its components are either related to the functions of HLA antigens or are

The cell surface glycopeptide antigens of the HLA A, B and C series are called HLA Class I anti‐ gens [2]. HLA Class I antigens are expressed on all nucleated cells of the body. Additionally, they are found in soluble form in plasma and adsorbed onto the surface of platelets. Erythrocytes also adsorb HLA Class I antigens to varying degrees depending on the specificity (e.g. HLA-B7, A28 and B57 are recognizable on erythrocytes as so called "Bg" antigens). Immunological studies in‐ dicate that HLA-B (which is also the most polymorphic) is the most significant HLA Class I locus, followed by HLA-A and then HLA-C. There are other HLA Class I loci (e.g. HLA E, F, G, H, J, K

The HLA Class I antigens comprise a 45 Kilodalton (Kd) glycopeptide heavy chain with three domains, which is non-covalently associated with β2-microglobulin, which plays an important role in the structural support of the heavy chain [3]. The HLA Class I molecule is assembled inside the cell and ultimately sits on the cell surface with a section inserted into the lipid bilayer

The general structure of HLA Class I, HLA Class II and IgM molecules show such similarity of subunits, that a common link between HLA and immunoglobulins, back to some primordial cell surface receptor is likely. The full 3-dimensional structure of HLA-A Class I molecules has been determined from X-ray crystallography [4]. This has demonstrated that the molecule has a cleft on its outermost surface, which holds a peptide. Consequently, if a cell becomes infected with a virus, the virally induced proteins within the cell are broken down into small peptides which are then inserted into this cleft during the synthesis of HLA Class I molecules. The HLA Class I molecules then translocate these virally (or self) induced peptides to the cell surface leading to activation of cytotoxic (CD8) T cells [5]. This role of HLA Class I, in identifying cells, which are changed (e.g. virally infected), is the basis for their expression on all cells [4]. Epitopes on certain expressed HLA Class I molecules also act as ligands for killer inhibitory receptors expressed on natural killer [6] cells, thereby influencing NK cell function [7].

The cell surface glycopeptide antigens of the HLA DR, DP and DQ loci are termed HLA Class II [1]. The tissue distribution of HLA Class II antigens is confined to the "immune competent" cells, including B-lymphocytes, macrophages, and endothelial cells and activated T-lympho‐ cytes. The expression of HLA Class II, on cells, which would not normally express them, is stimulated by cytokines like interferon-γ and is associated with acute graft rejection in the

and L), but most of these may not be important as loci for "peptide presenters".

under similar control mechanisms to the HLA antigens.

372 Current Issues and Future Direction in Kidney Transplantation

of the cell membrane and has a short cytoplasmic tail.

**2.1. HLA Class I antigens**

**2.2. HLA class II antigens**

A new HLA nomenclature was introduced in April 2010, replacing a system which had been in use since the 1990's. The main drive for the change was that the old system could no longer accommodate the increasing number of HLA alleles that were being described. There are currently over 5,700 alleles described across all the classical and non classical HLA loci.

The old system was based on assigning significance to pairs of digits in the allele nomenclature (Fig 1). For example in the allele HLA-A\*02010102L, the designation 'HLA' identifies the allele as a HLA allele. The dash (-) separates the HLA designation from the gene, in this case the 'A' gene. The '\*' is a separator. Of the actual allele name, the first two digits (02010102L) represents the allele group and in most instances, was synonymous with the Serological type (A2 in this case). The third and fourth digits (02010102L) identified the specific allele. All alleles whose no‐ menclature differed in these first four positions (02010102L) must code for proteins with differ‐ ent sequences. Alleles whose nomenclature differed in the fifth and sixth position (02010102L) code for proteins with silent mutations within the coding sequences. A sequence which dif‐ fered by mutations in the introns or in the untranslated regions flanking the 3' and 5' ends of the exons were identified by different digits in the seventh and eighth positions (02010102L). In ad‐ dition, a number of suffixes were used to identify sequences that were null, i.e. not expressed (N), those that had low expression (L), those that were secreted (S), those found only in the cy‐ toplasm (C), those with questionable expression and those with aberrant expression (A).

**Figure 1.** Old HLA nomenclature

A key limitation of this old system was that it only allowed for up to 99 alleles which differ in any of the pairs of positions. The HLA-A\*02 and B\*15 allele groups were the first to run into this problem when more than 99 alleles were detected. At that time, the WHO Nomenclature Committee for the HLA system decided to adopt the rollover sequences A\*92 and B\*95 respectively for A\*02 and B\*15. When A\*0299 was identified, the next A\*02 allele described was named A\*9201. Similarly when B\*1599 was identified the next B\*15 allele described was named B\*9501. Recently however, a number of other HLA types started to fast approach 99 alleles. These include A\*03, B\*40, B\*44 and DRB1\*11. Adopting rollover sequences for all of these was impractical. A rollover system of sorts had already been adopted for HLA-DPB1. When HLA-DPB1\*9901 was identified, the next HLA-DPB1 allele was named 'within the existing sequences' as HLA-DPB1\*0102.

A\*02:100 and B\*15:100 were not used to help make the remapping easier. However other HLA types which exceed 99 alleles will use allele 100. HLA-DPB1 alleles were also remapped.

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A number of other changes were made to the nomenclature. The 'w' was dropped from HLA-Cw alleles but not from Cw antigens. HLA-Cw\*0102 became HLA-C\*01:02. The 'w' was kept in antigen names to avoid confusion with complement factors as well as with KIR ligand groups. For ambiguous allele strings, the codes 'P' and 'G' were introduced. A group of alleles that share the same nucleotide sequences within exons 2 and 3 for HLA class I and exon 2 for HLA class II were named after the first allele in the sequence and given a code of 'G' as a suffix. E.g. HLA-A\*02:01:01 and HLA-A\*02:01:02 could be named HLA-A\*02:01G. A group of alleles that share the same protein sequences in the α2 and α3 domains, irrespective of the nucleotide sequence differences could be named after the first allele in the sequence and given a code of

The most important function of MHC molecule is in the induction and regulation of immune responses. T-lymphocytes recognize foreign antigen in combination with HLA molecules.

In an immune response, foreign antigen is processed by and presented on the surface of a cell (e.g. macrophage). The presentation is made by way of a HLA molecule. The HLA molecule has a section, called its antigen (or peptide) binding cleft, in which it has these antigens inserted. T-lymphocytes interact with the foreign antigen/HLA complex and are activated. Upon activation, the T cells multiply and by the release of cytokines, are able to set up an immune response that will recognize and destroy cells with this same foreign antigen/HLA complex, when next encountered. The exact mode of action of HLA Class I and HLA Class II antigens is different in this process. HLA Class I molecules, by virtue of their presence on all nucleated cells, present antigens that are peptides produced by invading viruses. These are specifically presented to cytotoxic T cells (CD8) which will then act directly to kill the virally infected cell. HLA Class II molecules have an intracellular chaperone network which prevents endogenous peptide from being inserted into its antigen binding cleft. They instead bind antigens (peptides) which are derived from outside of the cell (and have been engulfed). Such peptides would be from a bacterial infection. The HLA Class II molecule presents this "exogenous" peptide to helper T cells (CD4) which then set up a generalized immune response to this bacterial invasion. Thus it is apparent that MHC products are an integral part of immunological health and therefore it is no surprise to see a wide variety of areas of clinical and genetic implications.

HLA typing was applied to kidney transplantation very soon after the first HLA determinants were characterized [10-12]. The importance of reducing mismatched antigens in donor kidneys was immediately apparent with superior survival of grafts from HLA identical siblings compared to one haplotype matches or unrelated donors. It is apparent that the effect of HLA matching is significant, even with the highly efficient immunosuppression used today. The

HLADPB1\*0102 became HLA-DPB1\*100:01.

'P' as a suffix e.g. HLA-A\*02:01:01P.

**2.5. HLA and renal transplants**

**2.4. Clinical relevance of the HLA system**

In 2010, a new nomenclature system was adopted (Fig 2) [8, 9]. This introduced colons ':' as separators between pairs of digits. HLA-A\*02010102L therefore became HLA-A\*02:01:01:02L. The pairs of digits separated by colons are known as Fields. The first and second digits of the old nomenclature form the 1st Field of the new nomenclature. The third and fourth digits of the old nomenclature form the 2nd Field of the new nomenclature. To help reduce confusion in adopting the new nomenclature, the leading '0' in alleles 1-9 of each allele group was kept.

**Figure 2.** New HLA nomenclature

The introduction of the colons means that each Field is no longer restricted to 99 digits but can be expanded limitlessly. Once HLA-A\*03:99 was identified, the next A3 allele could be named HLA-A\*03:100.

With the introduction of colons and therefore the removal of the artificial restriction of 99 digits, there is no more need for rollover sequences. HLA-A\*92 and B\*95 were renamed A\*02 and B\*15 respectively and their associated alleles remapped. A\*9201 became A\*02:101. A\*9202 became A\*02:102 etc. HLA-B\*9501 became B\*15:101. HLA-B\*9502 became B\*15:102 etc. HLA- A\*02:100 and B\*15:100 were not used to help make the remapping easier. However other HLA types which exceed 99 alleles will use allele 100. HLA-DPB1 alleles were also remapped. HLADPB1\*0102 became HLA-DPB1\*100:01.

A number of other changes were made to the nomenclature. The 'w' was dropped from HLA-Cw alleles but not from Cw antigens. HLA-Cw\*0102 became HLA-C\*01:02. The 'w' was kept in antigen names to avoid confusion with complement factors as well as with KIR ligand groups. For ambiguous allele strings, the codes 'P' and 'G' were introduced. A group of alleles that share the same nucleotide sequences within exons 2 and 3 for HLA class I and exon 2 for HLA class II were named after the first allele in the sequence and given a code of 'G' as a suffix. E.g. HLA-A\*02:01:01 and HLA-A\*02:01:02 could be named HLA-A\*02:01G. A group of alleles that share the same protein sequences in the α2 and α3 domains, irrespective of the nucleotide sequence differences could be named after the first allele in the sequence and given a code of 'P' as a suffix e.g. HLA-A\*02:01:01P.
