Download The MHC Genes

Transplantation Immunology
Major Histocompatibility Complex (MHC)
The MHC represent a set of genes that control the
types of cellular antigens known as MHC antigens or
Human Leucocytes Antigens (HLA). These antigens
differ from one individual to the other. The immune
system differentiate foreign from self antigen via the
recognition of the HLA antigens. These antigens
represent the crucial factor in acceptance or rejection
of transplanted / grafted tissues or organs.
In early seventies, it was discovered that sera from
multiparous women could agglutinate leucocytes of
different individuals. Later, it became clear that these
sera were anti-HLA antibodies fromed due to the
differences between the women HLA and their carried
emberyos (had also paternal HLA antigens).
The MHC Genes:
In humans, the MHC or HLA genes occupy a portion on
the short arm of the 6th chromosome which control
the MHC (HLA) antigens. These genes are found in
regions ABC and D (Fig.). The genes in each region
are clustered and located in one or more locus (
locus = location of a gene). There are two groups of
MHC genes controlling the proper MHC antigens. A
third group is also present which mainly related to
the complement system, and it does not belong to
the MHC antigens:
1. Genes of class I MHC : A, B, C.
2. Genes of Class II MHC : DP, DQ, DR (D related)
3. Genes of Class III MHC: C2, C4A, C4B, BF, 21A, 21B
TNF- alpha & beta (complement or steroid related)
Heat Shock Protein…..cont..
Cont/... MHC genes
Each individual inherits one chromosome from each
parent. So, there are MHC genes of maternal and
paternal origin. Each half is called haplotype. The
genes of both haplotypes are co-dominant. Each
individual have a formula for the alleles in both
haplotypes.
In each locus there alleles of the genes; there are about
767 alleles for A, 1178 for B, 439 for C, and more
than 900 for the D-related (DP, DQ & DR) genes. By
combination of the alternative, there will be a
possibility of 1/ 2,000,000 for a person to have
another identical individual.
The genes of haplotypes are inherited according to
Mandelian Law of heredity; 25% of brothers/ sisters
are identical, 50% are partially identical and 25% are
non-identical.
Nomenclature of HLA antigens
1. Serologic: Give number, e.g. HLAB27, HLA-DR3, HLA-DR4.
2. Genetic: Give 4-8 digit number, e.g.
HLA- B*01:02, HLA-B* 01:04: 78: 23
‘or HLA-B*01027823). Some times
add letters as L, N, Q, S (e.g N= Null)
First 2: group of alleles, Second 2:
Synonymous alleles, Third 2:
mutation in genes, Fourth 2: mutation
outside genes.
The MHC (HLA) Antigens:
Class I MHC antigens:
These are membrane glycoproteins on the surface of
all nucleated cells and platelets. They are not
present on trophoblast cells , sperms and RBC.
These antigens are composed of:
1. Heavy polypeptide chain (alpha): This has M.W. of 43
KD and three extracellular domains alpha 1, 2, & 3).
This chain has also transmembranous portion and a
small cytoplasmic tail.
2. Beta- 2 microglobulin: The heavy chain is noncovalently bound to Beta- 2 microglobulin which has
M.W. of 12 KD and one extracellular domain only.
This chain is encoded by chromosome 15.
All domains of class I antigen has –S-S- bond except
alpha-1 (Figure).
Class II MHC Antigens:
They are glycoproteins found chiefly on the
surface of APC, B cells activated T cells and
some actively dividing cells as basal layer of
epidermis and some testicular cells. They
consist of two polypeptide chains:
1. Alpha- chain: has M.W. of 29 KD, two extracellular domains (alpha 1 and 2),
tansmembranous portion and a small
cytoplasmic tail.
2. Beta- chain: has M.W. Of 33 KD, two
extracellular domains (beta 1 & 2),
transmembranous portion and a small
cytoplasmic tail, Figure.
All domains have –S-S- bonds except alpha-1.
Non – classical HLA Loci:
There are loci recently discovered which are nonclassical loci:
1. For class I ; there are 2 loci which are G and E. The G
is related to Beta-2 microglobulin and is resposible
for the foetal- mother immunological control (no
response to foetal antigens). The E is responsible for
making complexes with antigens to prevent NK cells
from attaking normal body cells or tissues.
2. For class II ; there are DO and DM loci. The DM is
responsible for promoting the loading of peptides
intracellularly. The DO modulates the function of
DM.
Also, there are areas called Tapacin Binding Proteins
“TBP” (TAP1 and TAP2), and Low Molecular Protein
Areas (LTLT). Both the TBP and LTLT are
responsible for controlling the peptides binding
inside the cells.
Specificity of HLA Antigens:
The heavy chain of class I has hypervariable regions in
its N- terminal (alpha-1 domain) which contains the
antigen binding groove and constant domains. They
can bind to 8 – 10 amino acid derived from
endogenous antigens.
The alpha chain of class II has variable (V), Joining (J)
and constant (C) regions, while beta chain has VDJC
regions. The specificity of these chains is in alpha-1
and beta-2 domains in particular the latter one. The
class II antigen has also a single peptide binding
groove which binds to 13 – 18 amino acids derived
from exogenous antigens.
Due to the similarity between the structures of Ig &
HLA antigens, the latter are called Histoglobulins.
HLA Typing:
Indications:
1. Transplantation; inparticular for kidney and
bone marrow transplatations.
2. Disease association; there are certain
diseases associated with specific HLA types
as SLE with HLA-DR3, rheumatoid arthritis
with HLA-DR4, myasthenia gravis with HLAB8 or ankylosing spondylitis with HLA-B27.
This association could be due to: (a) HLA
antigens are similar to (molecular mimicry)
or receptors for the causative agents. (b)
HLA antigens are markers for immunosuppression or immuno-stimulation.
3. Paternity testing; it is accepted in courts.
4. Anthropological studies.
Methods of HLA typing:
1. Microcytoxicity: For HLA class I and HLADQ. Principle; HLA antigen + HLA antibody +
complement leads to lysis in microtiter plate.
2. Mixed Lymphocytes Reaction (MLR): For
HLA- DR. Principle; different lymphocytes
reacts against each other and become blast
cells.
3. Primed Lymphocytes typing (PLT): For HLADP. Principle is similar to MLR.
4. Polymerase Chain Reaction (PCR): For any
type, but inparticular the D- related
antigens.
Preparations for transplatations:
1. ABO & Rh compatibility test.
2. Screening for reaction between leucocytes
of recipient and serum of donor.
3. HLA typing; 2 loci compatibility as A & B
preferably with DR.
4. Vaccination against common bacterial and
viral diseases.
5. Antibiotics umbrella as penicillins or
cephalosporins with aminoglycosides
specially the new ones.
6. Splenectomy is preferable.
7. Use of immunosuppressive agents
Immunosuppressive Agents:
Immunosuppressive therapy is used to prevent or treat graft rejection by
non-specifically interfering with the induction or expression of the
immune response. The following agents or measures are in use:
1. Immunosuppressive drugs:
A. Cyclosporine A is an antibiotic produced by a fungus. It prevents T
cells activation and blocks the accompanying cytokine production.
B. FK – 506 (Tacrolimus) , rapamycin (Sirolimus) or Everolimus are
relatively new immunosuppressive drugs with action similar to
cyclosporine.
C. Corticosteroids are given in big doses; they act on blocking IL-1 & 2
release and cytokines receptor expression and suppressing
macrophages.
D. Anti-mitotic drugs as Azathioprine and Methotrexate which inhibit DNA
synthesis and block the growth of T cells.
All used in prevention, but Cortisones is also used in treatment.
2. Anti-lymphocyte or anti-thymocyte globulin or anti- CD3 MCA. These
agents lyses T cells and can block their functions. Used in prevention
and treatment.
3. Antibody to block co-stimulatory molecules.
4. Total lymphoid irradiation while shielding the marrow, lungs and other
vital organs before engraftment.
5. Antigen specific immunosuppression by induction of tolerance to the
graft antigen.
Types of grafts:
1. Autograft is the transfer of an individual own
tissues from place to place, e.g., skin.
Accepted.
2. Isograft (syngeneic)is the transfer of tissues
between genetically identical individuals,
e.g., identical twins. Accepted.
3. Allografts (homograft) is a graft between
genetically different members of the same
species as humans, e.g., Kidney. Rejection
may occur.
4. Xenograft (heterograft) is the transfer of
tissues between different species. Always
rejected.
5. Split graft: e.g. part of liver.
6. Domino graft: Combined lung/Heart.
Types of graft rejection:
1. Hyperacute: This occurs minutes or
hours after transplantation. Graft
should be removed.
2. Acute rejection: This occurs 10 – 30
days after transplantation. It is mainly
cell mediated.
3. Chronic or late rejection: This occurs
slowly over a period of months or
years. It may be cell mediated or
antibody mediated or both.
Graft Versus Host Disease (GvHD):
This condition occurs when there are:
1. Histoincompatibility between donor and recipient tissues.
2. Immuno-compromized recipient (host).
3. Immunocompetent donor tissues.
The grafted cells survive and react against the host cells or
tissues (i.e. Instead of reaction of host against the graft, the
reverse occurs).
There are three types of GvHD:
1. Hyperacute: maculopapular skin rash. Dangerous.
2. Acute: Extensive skin rash with hepato-splenomegaly,
lymphadenopathy, cardiac irritability, and confusion. The
patient is very ill.
3. Chromic GvHD: Hepato-splenomegaly and lymphadenopathy
with some skin rash. The transplanted organ is functionally
suboptimal.
Treatment:
The GvHD is very serious condition. It has mortality up to 90%.
The best treatment is combination of Methotrexate +
corticosteroids + Anti-sera. So, the prevention is better than
treatment.
Prevention of GvHD:
1. Anti- CD3 MCA to remove T cells from
grafted tissue as the bone marrow.
2. The blood or blood products should be
irradiated prior giving to the recipient
using about 3000 Rad.
3. Take all the measures necessary
before transplantation (precautions) to
prevent GvHD rather than treating it.