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HLA
Human Leukocyte Antigen (HLA)
Human leukocyte antigen system (HLA) is the name of the human
major histocompatibility complex (MHC). This group of genes resides on
chromosome 6, and encodes cell-surface antigen-presenting proteins and
many other genes. There are a total of 128 genes in the HLA complex
(not including the extended HLA class I complex) of which 40% are
assumed to have an immune function. C2 and C4 are complement
genes,TNF is the gene encoding tumor necrosis factor. The provisional
borders of the extended HLA complex are the HSET gene and the
hemochromatosis gene HFE (Shennan et al., 2006).
(Donadi et al., 2000)
Figure (8): The HLA complex and some of its genes .Only a small
number of genes in this gene complex is shown. The genes encoding
class I and II molecules are depicted in red, other genes in yellow.
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HLA
HLA antigens are proteins found in the membranes (outer coating)
of nearly every cell in the body (all nucleated cells). They are termed so
as they were initially discovered on human lymphocytes (Donadi et al.,
2000).
These antigens are in especially high concentration on the surface
of white blood cells (leukocytes), which is controlled by a series of highly
polymorphic genes located on the short arm of chromosome six in
humans in a cluster called major histocompatibility complex (MHC).
The MHC has genes, which are integral to normal function of the
immune response. Because some HLA antigens are recognized on all
tissues of the body, the identification of HLA antigens is described as
tissue typing (Nathalang, 2006).
HLA antigens are the major determinants used by the body's
immune system for recognition and differentiation of self from non-self.
There are many different major histocompatibility (HLA) proteins, and
each person possesses only a small, relatively unique set that is inherited
from their parents (Sheldon and Poulton, 2006).
Nomenclature
There are two parallel systems of nomenclature that are applied to
HLA. The, first, and oldest system is based on serological (antibody
based) recognition. In this system antigens were eventually assigned
letters and numbers (e.g. HLA-B27 or, shortened, B27). A parallel system
was developed that allowed more refined definition of alleles, in this
system a "HLA" is used in conjunction with a letter * and four or more
digit number (e.g. HLA-B*0801, A*68011, A*240201N N=Null) to
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HLA
designate a specific allele at a given HLA locus. HLA loci can be further
classified into MHC class I and MHC class II (or rarely, D locus). Every
two years a nomenclature is put forth to aid researchers in interpreting
serotypes to alleles (Marsh et al., 2005).
Modern HLA alleles are typically noted with a variety of levels of
detail. Most designations begin with HLA- and the locus name, then *
and some (even) number of digits specifying the allele. The first two
digits specify a group of alleles. Older typing methodologies often could
not completely distinguish alleles and so stopped at this level. The third
through fourth digits specify a synonymous allele (Shennan et al., 2006).
HLA structurally consists of:
I) Major histocompatibility antigens: Which can be divided in to three
major classes?Class 1 molecules: almost all nucleated cells.Class 2
molecules Antigen presenting cells (APCs), endothelium of renal arteries
and glomeruli.Class 3 molecules: Proteins in serum and other body fluids
(e.g. C4, C2, factor B, TNF).
II) Minor histocompatibility antigens; are polymorphic, endogenously
synthesized products that can be recognized by allo reactive T cells
(Duqesnoy, 2002).
Major histocompatibility antigens
Class I MHC:
The class I gene complex contains three major loci, A, B, C and
other undefined minor loci. Each major locus codes for a polypeptide the
alpha chain that contains antigenic determinants, is polymorphic (has
many alleles). It associates with beta-2 micro globulin (beta chain),
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HLA
encoded by agene outside the MHC complex, and expressed on the cell
surface. With out the beta-micro globulin, the class I antigen will not be
expressed on the cell surface (Abdul Ghaffar, 2006).
Figure (9): MHC class I structure (Donadi et al., 2000).
Classification of MHC class I
Form a functional receptor on most nucleated cells of the body.
There are 3 major: (HLA-A HLA-B HLA-C) and 3 minor MHC class I
(HLA-E, HLA-F and HLA-G) genes in HLA.Β2-microglobulin binds
with major and minor gene subunits to produce a heterodimer (Apanius
et al., 1997).
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HLA
Figure (10): Schematic representation of MHC class I (Donadi et
al., 2000).
Class I molecules are expressed on almost all nucleated cells. Their
most important function is to present peptide fragments predominantly
derived from endogenous, cytosolic proteins to CD8+ cytotoxic T
lymphocytes. (Undlien et al, 2001)
Class II MHC:
The class II gene complex also contains at least three loci DP, DQ,
DR; each of these loci codes for one alpha and one beta chain polypeptide
which associate together to form the class II antigens. Like the class I
antigen, the class II antigens are also polymorphic. The DR locus may
contain more than one, possibly four functional beta chain genes
(Sheldon and Poulton, 2006).
Classification of MHC class II
There are 3 major and 2 minor MHC class II Proteins encoded by
the HLA. The genes of the class II combine to form heterodimer (αβ)
protein receptors that are typically expressed on the surface of antigen
presenting cells .
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HLA
Figure (11): MHC class II structure (Donadi et al., 2000).
Figure (12): Illustration of an HLA-DQ molecule (magenta and blue)
with a bound ligand (yellow) floating on the plasma membrane of
(Donadi et al., 2000).



HLA-DP
o
α-chain encoded by HLA-DPA1 locus
o
β-chain encoded by HLA-DPB1 locus
HLA-DQ
o
α-chain encoded by HLA-DQA1 locus
o
β-chain encoded by HLA-DQB1 locus
HLA-DR
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HLA
o
α-chain encoded by HLA-DRA locus
o 4 β-chains (only 3 possible per person), encoded by HLADRB1, DRB3, DRB4, DRB5 loci (Valluri et al., 2005).
(Abdul Ghaffar, 2006)
Figure (13): Showing class I and class II MHC.
HLA haplotypes
Particular alleles of the different HLA genes are frequently
inherited as conserved blocks called haplotypes. Such haplotypes are
listed using the individual alleles of the various genes separated by
hyphens. To avoid lengthy designations serological specificities are used
in a much of the literature instead of the full genomic allele designations
even if the alleles are determined genomically. For example, the HLADQB1*0201-DQA1*0501- DRB1*03-B*0801 haplotype is often written
as the HLA-DQ2-DR3-B8 haplotype (Nejentsev, S. et al, 2000).
HLA functions

Disease defense

Reproduction (may be involved in mate selection)

Cancer (may be protective or fail to protect)
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HLA

Human disease: autoimmunity and act as antigens-responsible for
organ transplant rejection.
Under normal circumstances class II molecules are expressed only on
specialized antigen-presenting cells such as dendritic cells, B
lymphocytes, macrophages and thymic epithelial cells. Their main
function is to present peptides to CD4+ helper T lymphocytes (Undlien
et al., 1999), as show in figure (14).
(Undlien et al., 1999)
Figure (14): Function of the classical HLA class I and II molecules.
Aside from the genes encoding the 6 major antigens, there are a
large number of other genes, many involved in immune function located
on the HLA complex. Diversity of HLA in human population is one
aspect of disease defense, and, as a result, the chance of two unrelated
individuals having identical HLA molecules on all loci is very low
(Valluri et al., 2005).
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HLA
The proteins encoded by HLAs are the proteins on the outer part
of body cells that are (effectively) unique to that person. The immune
system uses the HLAs to differentiate self cells and non-self cells. Any
cell displaying that person's HLA type belongs to that person (and
therefore is not an invader). DR protein (DRA: DRB1*0101 gene
products) with bound Staphylococcal enterotoxin ligand (subunit I-C),
view is top down showing all DR amino acid residues within 5
Angstroms of the SEI peptide as shown in figure(15).
(Marsh et al., 2005)
Figure (15): A piece of a poisonous bacterial protein (SEI peptide) bound
within the binding cleft portion of the HLA-DR1 molecule.
In graft rejection: Any cell displaying some other HLA type is
"non-self" and is an invader, resulting in the rejection of the tissue
bearing those cells. Because of the importance of HLA in transplantation,
the HLA loci are among of the most frequently typed by serology or PCR
relative to any other autosomal alleles (Grewal et al., 2003).
In cancer: Some HLA mediated diseases are directly involved in
the promotion of cancer. Gluten sensitive enteropathy is associated with
increased prevalence of Enteropathy-associated T-cell Lymphoma, and
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HLA
DR3-DQ2 homozygotes are within the highest risk group with close to
80% of gluten sensitive EATL cases. More often; however, HLA
molecules play a protective role, recognizing the increase in antigens that
were not tolerated because of low levels in the normal state. Abnormal
cells may be targeted for apoptosis mediating many cancers before
clinical diagnosis. Prevention of cancer may be a portion of heterozygous
selection acting on HLA (Oshima et al., 2002).
In infectious disease: When a foreign pathogen enters the body,
specific antigen-presenting cells (APCs) engulf the pathogen through
phagocytosis process. Proteins from the pathogen are digested into small
pieces (peptides) and loaded onto HLA antigens (specifically MHC class
II). They are then displayed by the antigen presenting cells for certain
cells of the immune system (T cells), which then produce a variety of
effects to eliminate the pathogen (Sheldon and Poulton, 2006).
Through a similar process, proteins (both native and foreign, such
as the proteins of viruses) produced inside most cells are displayed on
HLA antigens (specifically MHC class I) on the cell surface. Infected
cells can be recognized and destroyed by components of the immune
system (specifically CD8+ T cells) (Marsh et al., 2005).
In autoimmunity: HLA types are inherited, and some of them are
connected with autoimmune disorders and other diseases. People with
certain HLA antigens are more likely to develop certain autoimmune
diseases, such as Type I Diabetes, Ankylosing spondylitis, Celiac
Disease, SLE (Lupus erythematosus), Myasthenia Gravis, inclusion body
myositis and Sjögren's syndrome (Shennan et al., 2006).
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HLA
HLA typing has led to some improvement and acceleration in the
diagnosis of Celiac Disease and T1D; however for DQ2 typing to be
useful it requires either high resolution B1*typing (resolving *0201 from
*0202), DQA1*typing, or DR serotyping. Current serotyping can resolve,
in one step, DQ8. HLA typing in autoimmunity is being increasingly used
as a tool in diagnosis. In GSE (glutin sensitive enteropathy) it is the only
effective means of discriminating between 1st degree relatives who are at
risk from those who are not at risk, prior to the appearance of sometimes
irreversible symptoms such anallergies and secondary autoimmune
disease (Mitchell et al., 2007) .
Table (18): HLA and autoimmune diseases
HLA and autoimmune diseases
Diseases with increased
risk
Relative risk
(%)
Ankylosing spondylitis
90-100
Postgonococcal arthritis
14
Acute anterior uveitis
15
Autoimmune hepatitis
14
Primary Sjögren syndrome
10
Diabetes mellitus type 1
5
Rheumatoid arthritis
4
Diabetes mellitus type 1
6
HLA-DR3 and-DR4
combined
Diabetes mellitus type 1
15
HLA-B47
21-hydroxylase deficiency
15
HLA allele
HLA-B27
HLA-DR3
HLA-DR4
(Mitchell et al., 2007)
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HLA
Genetic polymorphisms in susceptibility to Type 1 DM
Genetic factors play an important role in determining the risk for
T1D, since it shows a strong clustering in some families, with a sibling
recurrence rate of 6%, and a monozygotic twin concordance rate of 30–
50% ) (Behrooz et al, 2008).
More direct evidence is seen in the strong association between
several genetic variants and T1D. The first major genetic risk locus was
detected as early as 1974 who detected an association with the HLA
system with allelic forms of HLA class I genes that encode for HLA-B8
and -B15 molecules (Behrooz et al., 2008).
(Von Herrath, 2009)
Figure (16): A Model for the Pathogenesis of Type 1 Diabetes Based on
Genetic Etiological Studies in Humans.
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HLA
Shown genes in figure (16) are (e.g. those that encode MHC HLA
class II and class I antigen-presentation molecules, preproinsulin (INS) in
the thymus, CTLA-4 in Treg cells, cytokines) believed to be causal in the
process of type I diabetes. Also shown are influences of viral infections
and type 1 interferon (IFN) production and effects; autoimmune
repertoire development in the thymus; and the main immune cell types,
CD4+ and CD8+ T cell subsets, T regulatory (Treg) cells, and B cells and
various other antigen-presenting cells, acting together to kill pancreatic b
cells (Von Herrath, 2009).
Many studies have investigated the risk of HLA-DRB1– DQB1
haplotypes in different populations and showed that several haplotypes
are associated with a spectrum of different disease risks, ranging from
strong susceptibility to almost complete protection It is now clear that the
main susceptibility for disease is associated with the following
haplotypes, in contrast to the almost complete protection conferred by
other haplotypes.
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HLA
(Vang et al 2001)
Figure (17): DR4 subtypes in T1D. The risk encoded by DQ8 haplotypes
(DQA1*03–DQB1*0302) is influenced by which DRB1*04 subtype is
carried. For example, DQ8 haplotypes carrying the DRB1*0401 or 0405
alleles are susceptible, whereas those carrying DRB1*0403 or 0406 are
protective.
Many other candidate genes have been studied in T1D in the past
decades, but few have been found to be associated to T1D.Many of the
initially significant associations failed to be confirmed by independent
studies, some have been inconsistently confirmed as risk factors for T1D
are the Variable Number of Tandem Repeats (VNTR) near the Insulin
gene (INS), the Cytotoxic T-Lymphocyte associated Antigen (CTLA4)
and the Protein Tyrosine phosphatasis Pyrosine
22(PTPN22) (Haller et al., 2005).
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Nonreceptor-type
HLA
INS (Insulin gene)
The INS gene was one of the obvious candidate genes for T1D,
Moreover, the region surrounding INS on chromosome 11p15 has been
consistently linked to T1D for more than two decades (the IDDM2 locus).
The main association was found to center around the INS-VNTR
polymorphism in a 4.1 kb region encompassing the INS gene as well as
the Tyrosine Hydroxylase gene (TH), and the Insulin-like Growth Factor
II gene (IGF2). Strong consistent association was found by several
independent groups showing that INS class I alleles are associated with
increased risk, whereas class III alleles are associated with dominant
protection for T1D (Bennett et al,. 1996).
CTLA4 (the Cytotoxic T-Lymphocyte associated Antigen)
CTLA4 is expressed by CD4+ and CD8+ T cells and (similar to
CD28) binds B7-1 and B7-2 ligands during presentation of Antigens
bound to MHC by antigen-presenting cells (Fig. 15).Upon binding of B7,
CTLA4 down-regulates T-cell proliferation and cytokine production,
whereas binding of B7 to CD28 results in a co-stimulatory response. It is
thought that the CTLA4 function is critical for regulating peripheral self
tolerance and prevention of autoimmunity and it has therefore long been
considered as a candidate gene for T1D Indeed, CTLA4 maps on to
chromosome 2q33, which is linked to T1D (IDDM1)( Maurer, et al
2002 )
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HLA
PTPN22 (the Protein Tyrosine phosphatasis Pyrosine Nonreceptortype 22
Little controversy has surrounded the first report of the association
of the C1858T variant of PTPN22, which was discovered and shown to
be consistently associated to several autoimmune diseases, including
T1D. PTPN22 codes for lymphoid-specific phosphatase (Lyp), and
belongs to the family of protein tyrosine phosphatases, which are
regulators of the immune response. Lyp is an intracellular protein that
interacts with Csk kinase and this protein complex inhibits TCR
signaling, leading to reduced T-cell activation (Behrooz et al, 2008).
The C1858T variant results in an amino acid substitution R620W in
Lyp, and was shown to result in a Lyp protein with enhanced binding
properties with Csk (Vang et al, 2005). This is predicted to result in more
efficient suppression of TCR signaling. It is postulated that this gain-offunction may predispose to T1D through an increased survival of
autoreactiveT cells during thymic selection. Alternatively, the variant Lyp
may also affect regulatory T-cell populations, making them less effective
in suppressing an autoimmune response (Matesanz et al., 2005).
MICA (MHC class I chain-related) genes as a Marker for Newborn
Screening
The Diabetes Autoimmunity Study in the Young (DAISY) is a
newborn screening and follow-up study in the state of Colorado, United
States, Where children are followed up from birth to define the incidence
of beta cell autoimmunity by age, race/ethnicity, HLA genotype, family
history of T1DM, and environmental factors. This suggests that MICA
may be, important in early events of autoimmunity that is, events before
acute onset of the disease (Carani and Sanjeevi, 2006).
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HLA
Re
(Carani and Sanjeevi, 2006)
Figure (19): The Environmental Determinants of Diabetes in the Young
(TEDDY) study.
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