2. Discovery of the Major
Histocompatibility Complex
Transplantation experiments in Mice
Serologic Studies in Humans
Structure and Function of MHC
Molecules
Class I and Class II
Expression and Regulation of MHC
Molecules
Parungao-Balolong 2011
3. INTRO...
• MAJOR HISTOCOMPATIBILITY COMPLEX
(MHC)
• a region of highly polymorphic gene whose
products are expressed on the surfaces of a
variety of cells
• discovered in the 1940s via an artificial
transplantation experiments
• principal determinants of graft rejection
• THUS: individuals who express the same MHC
molecules accept tissue grafts from one another,
and, individuals who differ at their MHC loci
vigorously rejects such grafts
Parungao-Balolong 2011
4. INTRO...
MHC: ROLE IN IMMUNE RESPONSE???
1960s: Benacerraf et al
demonstrated that different inbred strains of guinea pigs and mice did
or did not produce antibodies in response to immunization with
simple polypeptide antigen
this immune responsiveness was an autosomal dominant trait mapped
to the MHC region
genes that controlled such immune response = Ir genes or immune
response genes
controlled the activation of helper T lymphocytes
necessary for antibody response to protein antigens
Parungao-Balolong 2011
5. INTRO...
• MHC: ROLE IN IMMUNE RESPONSE???
• 1970s : central role of MHC genes in immune
response to protein antigens was explained
• demonstrated that antigen specific T
lymphocytes do not recognize antigens in
free or soluble form but recognize portions
of protein antigens that are non-covalently
bound to MHC gene products
Parungao-Balolong 2011
6. INTRO...
MHC: TYPES OF GENE PRODUCTS
MHC Class I molecules
MHC Class II molecules
any given T cell recognizes foreign antigen bound to only
one specific class (I or II)
THUS : MHC molecules are integral components of the
ligands that T cell recognize
Parungao-Balolong 2011
7. IMPORTANCE
IMPORTANCE: specificity of T-lymphocytes for self MHC associated antigens?
1. MHC molecules are membrane - associated and not secreted : T-
lymphocytes can recognize foreign antigens only when bound to surfaces of
other cells
This limits T-cell activation such that T cells interact most effectively
with other cells that bear MHC-associated antigens and not with
soluble antigens (i.e antigen presentation)
The recognition of antigen on a cell surface also serves to localize the
effector functions of the activated T cell to the anatomic site of
antigen presentation
NOTE: In contrast, antibodies can function in the circulation by binding
to and neutralizing soluble antigens
Parungao-Balolong 2011
8. IMPORTANCE
IMPORTANCE: specificity of T-lymphocytes for self MHC
associated antigens?
2. the patterns of antigen association with class I or II
MHC molecules determine the kinds of T cells that are
stimulated by different forms of antigens
peptide fragments derived from extracellular proteins =
binds to class II
endogenously synthesized peptides = associates with
class I
Parungao-Balolong 2011
9. IMPORTANCE
IMPORTANCE: specificity of T-lymphocytes for self MHC
associated antigens?
3. the immune response to a foreign protein is determined by
the presence or absence of MHC molecules that can
bind and present fragments of that proteins to T cells
since MHC genes are polymorphic, many different alleles exist
within a population and these alleles differ in their ability to
bind and present different antigenic determinants of proteins
this is how MHC genes control immune responses to protein
antigens
Parungao-Balolong 2011
10. IMPORTANCE
IMPORTANCE: specificity of T-lymphocytes for self MHC associated
antigens?
4. Mature T cells in any individual recognize and respond to foreign
antigens but are responsive to self proteins
this antigen recognition is shaped by the selection of foreign antigen-
specific T cells from developing lymphocytes based on their
recognition of self MHC molecules with or without bound
peptide antigens
THUS, a second means by which MHC can influence immune responses
to particular antigens is through the role of MHC molecules in
shaping the repertoire of mature T cells
Parungao-Balolong 2011
11. DISCOVERY!
MURINE MHC
George Snell and colleagues
used classical genetic techniques to analyze
rejection of transplanted tumors and other
tissues (grafts)
examined the outcome of skin grafts between
individual animals using inbred strains of
laboratory mice
Parungao-Balolong 2011
12. DISCOVERY!
THE EXPERIMENT
principle: RECALL....
non-polymorphic: some genes are represented by only one normal nucleic
acid sequence
variant nucleic acid sequence is an uncommon mutation and may result in a
disease state
polymorphic: genes may vary at relatively high frequency among normal
individuals in the populations polymorphic
any individual animal can have the same allele at a genetic locus on both
chromosome of the pair (homozygous) or two different alleles one on each
chromosome (heterozygous)
Parungao-Balolong 2011
13. DISCOVERY!
THE EXPERIMENT
inbred mouse strains: produced by repetitive matings of siblings (> 20
generations)
every individual animal of a given inbred mouse strain will have identical nucleic
acid sequences at all locations on both members of each pair of chromosomes
completely homozygous at every genetic locus
genetically completely identical to every mouse of the same strain = syngeneic
THUS, when a tissue or organ, such as patch of skin, is grafted from one animal to
another, two possible outcomes may ensue:
grafted skin survives and functions as normal skin or;
immune system destroys the graft (graft rejection)
Parungao-Balolong 2011
14. DISCOVERY!
THE EXPERIMENT: genetic basis of graft rejection among inbred mice
1. grafts of skin from one animal to itself (isogeneic or isografts) or grafts
between animals of the same inbred strain (syngeneic grafts of syngrafts)
are usually NEVER rejected
2. grafts between animals of different inbred strains or between outbred
mice (allogeneic grafts or allografts) are almost ALWAYS rejected
Distinguishes the grafts as FOREIGN: the genes responsible for causing a
grafted tissue to be perceived as similar to one’s own tissue or as foreign
as called histocompatibility genes
differences between foreign and self were attributed to
genetic polymorphisms among diffrent histocompatibility
alleles
Parungao-Balolong 2011
15. DISCOVERY!
THE CONGENEIC MOUSE STRAINS EXPERIMENT
differed only by genes responsible for graft rejection (MHC)
NOTE: although several different genes could contribute to rejection, a single genetic region is
responsible for most rejection phenomena
this gene encodes a polymorphic blood group antigen called antigen II or
histocompatibility-2 (H-2)
JUSTIFICATION
initially, MHC congeneic strains were thought to differ at a single locus
occasional recombination events occurred within the MHC during interbreeding of different
strains, suggesting that the MHC actually contained several different genes, each involved in graft
rejection
H-2 region is now known to be homologous to genes that determine the fate of grafted
tissues in other species (Major Histocompatibility Complex)
Parungao-Balolong 2011
16. DISCOVERY!
GENETICS OF GRAFT REJECTION
indicated that the products of MHC genes are co-
dominantly expressed
alleles on both chromosomes of a pair are
expressed
as a consequence, each parent of a genetic cross
between two different strains can reject a graft
from the offspring by recognizing MHC
alleles inherited from the other parent
Parungao-Balolong 2011
17. DISCOVERY!
SEROLOGIC STUDIES IN HUMANS (Dausett et al)
development of allogeneic blood transfusion and allogeneic organ transplantation in clinical medicine
provided ways to detect and define genes that control rejection in humans
OBSERVATIONS: patients who rejected kidneys or had transfusion reactions to WBC often develops
circulating antibodies reactive with antigens on the WBC of the blood or organ donor
in the presence of complement, the recipient’s serum would lyse lymphocytes obtained from the donor
and also lyse lymphocytes obtained from some but not all third parties (individuals other than the blood
or organ donor or the recipient)
this sera which react against cells of allogeneic individuals are called alloantisera or
allosera
said to contain alloantibodies whose molecular targets are alloantigens (HLA)
HLA or human leukocyte antigens: products of polymorphic genes that distinguish foreign tissues
from self
HLAs = H-2 in mice = MHC
Parungao-Balolong 2011
18. Structure
and Function
Larger chain: alpha 1, 2 and 3 domain
alpha 3-domain is an immunoglobulin fold and is the
attachment point to the membrane
alpha 1 and 2 domain forms an 8-stranded β-sheet that
serves as a platform for peptide binding.
Edges of the peptide binding site are defined by long a -
helices, one from a 1 and one from a 2
alpha 3 is paired with β2 microglobulin, which also has a
typical Ig fold
β2 microglobulin is essential for stability and peptide
binding
CD8 on TC cells binds to the alpha domain
Parungao-Balolong 2011
19. FUNCTION and PRODUCTION Parungao-Balolong 2011
PRODUCTION
The peptides are mainly generated in the cytosol by the
proteasome
proteasome degrades intracellular proteins into small
peptides that are then released into the cytosol
The peptides have to be translocated from the cytosol
into the (ER) to meet the MHC class I molecule, whose
peptide-binding site is in the lumen of the ER
FUNCTION
display fragments of proteins from within the cell to T cells
healthy cells will be ignored while cells containing foreign proteins will be attacked by the
immune system
Because MHC class I molecules present peptides derived from cytosolic proteins, the
pathway of MHC class I presentation is often called the cytosolic or endogenous pathway
20. The peptide translocation from the
cytosol into the lumen of the ER is
accomplished by the transporter TRANSLOCATION
associated with antigen processing (TAP)
= TAP 1 and 2
The two subunits form a peptide binding
site and two ATP binding sites that face
the lumen of the cytosol
TAP binds peptides on the cytoplasmic
site and translocates them under ATP
consumption into the lumen of the ER
The MHC class I molecule is then in turn
loaded with peptides in the lumen of the
ER
The peptide-loading process involves
several other molecules that form a large
multimeric complex consisting of TAP,
tapasin, calreticulin, calnexin, and ERP57
Parungao-Balolong 2011
21. Once the peptide is loaded onto
the MHC class I molecule, it TRANSLOCATION
leaves the ER through the
secretory pathway to reach the
cell surface
The transport of the MHC class I
molecules through the secretory
pathway involves several post-
translational modifications of the
MHC molecule
example: change to the N-glycan
regions of the protein, followed
by extensive changes to the N-
glycans in the Golgi apparatus
Parungao-Balolong 2011
22. GENES AND
ISOTYPES
Very Less
polymorphic polymorphic
HLA-A (HLA-A) HLA-E (HLA-E)
HLA-B (HLA-B) HLA-F (HLA-F)
HLA-C (HLA-C) HLA-G (HLA-
G)
HLA-K
HLA-L
Parungao-Balolong 2011
23. Structure
and Function
Almost identically sized a and b chains
Each chain is divided into two segments, e.g. alpha 1
and 2; beta 1 and 2
alpha 2 and beta 2 are immunoglobulin domains that
pair with each other
alpha 2 and beta 2 are the point of membrane
attachment
alpha 1 and beta 1 form the peptide binding domain,
conformation quite similar to Class I MHC, except,
the ends are open allowing the binding of longer
peptides
CD4 on TH cells binds to beta 2 domain
Parungao-Balolong 2011
24. FUNCTION
found only on a few specialized cell types, including
macrophages, dendritic cells and B cells, all
of which are professional antigen-
presenting cells (APCs)
The peptides presented by class II molecules are
derived from extracellular proteins (not cytosolic as
in class I)
MHC class II-dependent pathway of antigen
presentation is called the endocytic or
exogenous pathway
Loading of class II molecules: extracellular proteins
are endocytosed, digested in lysosomes, and bound
by the class II MHC molecule prior to the molecule's
migration to the plasma membrane
Parungao-Balolong 2011
25. SYNTHESIS
result of dimerization of α and β chains, with the assistance of an invariant chain =
a special polypeptide involved in the formation and deliverance of MHC class II
protein
The nascent MHC class II protein in the rough ER has its peptide-binding cleft
blocked by the invariant chain (Ii; a trimer) to prevent it from binding cellular
peptides or peptides from the endogenous pathway
The invariant chain also facilitates MHC class II's export from the ER in a vesicle
which fuses with a late endosome containing the endocytosed, degraded proteins
It is then broken down in stages, leaving only a small fragment called CLIP which
still blocks the peptide binding cleft
An MHC class II-like structure, HLA-DM, removes CLIP and replaces it with a
peptide from the endosome
The stable MHC class-II is then presented on the cell surface
Parungao-Balolong 2011
27. Characteristic MHC-I pathway MHC-II pathway
Polymorphic chain α and β2
Composition of the stable Polymorphic chains α and β, peptide
microglobulin, peptide bound to α
peptide-MHC complex binds to both
chain
Dendritic cells, mononuclear
Types of antigen presenting
cells (APC) All nucleated cells phagocytes, B lymphocytes, some
endothelial cells, epithelium of thymus
T lymphocytes able to Cytotoxic T lymphocytes Helper T lymphocytes
respond (CD8+) (CD4+)
cytosolic proteins (mostly synthetized Proteins present in endosomes or
Origin of antigenic
by the cell; may also enter from the lysosomes (mostly internalized from
proteins extracellular medium via phagosomes) extracellular medium)
Enzymes responsible for Proteases from endosomes and
peptide generation Cytosolic proteasome lysosomes (for instance, cathepsin)
Location of loading the Specialized vesicular
peptide on the MHC molecule Endoplasmic reticulum
compartment
Molecules implicated in
transporting the peptides and TAP (transporter associated with
loading them on the MHC antigen processing) DM, invariant chain
molecules
28. IMPORTANT ASPECTS
OF THE MHC
• High polymorphism in MHC for • NO recombination mechanisms
a species for creating diversity in MHC
• Alleles for MHC genes are co- • Peptide must bind with
dominant individual’s MHC to induce
immune response
• Each MHC gene product is
expressed on surface of • MHC molecules are membrane-
individual cell bound
• Each MHC has ONE peptide • Recognition by Ts requires cell-
binding site but each MHC can cell contact
bind many different peptide one
at a time (Peptide binding is
“degenerate”)
• Mature Ts must have TCR that
recognizes particular MHC
Parungao-Balolong 2011
29. T-CELL RECEPTOR (TCR)
• role in immune response
• Surface molecule on Ts
• Recognize Ag presented in MHC
context
• Similar to Immunoglobulin
• Two types of TCR
• α β: predominant in lymphoid
tissues
• γ δ: enriched at mucosal surfaces
Parungao-Balolong 2011
30. IMPORTANT ASPECTS
OF TCR
• Each T cell has TCR of only ONE specificity
• Allelic exclusion
• αβ TCR recognizes Ag only in the context of cell-cell
interaction and in correct MHC context
• γδ TCR recognizes Ag in MHC-independent manner
• Response to certain viral and bacterial Ag
Parungao-Balolong 2011
31. GENETIC
BASIS FOR
RECEPTOR
GENERATION
• Accomplished by recombination of V, D and J gene
segments
• TCR β chain genes have V, D, and J
• TCR α chain genes have V and J
Parungao-Balolong 2011
32. TCR AND CD3
• TCR is closely associated
with CD3 complex
• Group of 5 proteins
• Commonly called
“invariant” chains of TCR
• Role of CD3 complex
• CD3 necessary for cell
surface expression of TCR
• transduces signal after Ag
interaction with TCR Parungao-Balolong 2011
34. NICE TO KNOW
(from wikipedia
though...)
• MHC and Sexual Selection
• MHC plays a role in the selection of potential mates, via olfaction
• MHC genes make molecules that enable the immune system to
recognize invaders; in general, the more diverse the MHC genes of
the parents the stronger the immune system of the offspring
• It would be beneficial, therefore, to have evolved systems of
recognizing individuals with different MHC genes and preferentially
selecting them to breed with
Parungao-Balolong 2011
35. NICE TO KNOW
(from wikipedia
though...)
• MHC and Sexual Selection
• Yamazaki et al. (1976) showed this to be the case for male mice, which show a
preference for females of different MHC; similar results have been obtained with fish
• Claus Wedekind (1995) determined MHC-dissimilar mate selection tendencies in
humans
• group of female college students smelled t-shirts that had been worn by male
students for two nights, without deodorant, cologne, or scented soaps
• An overwhelming number of women preferred the odors of men with dissimilar
MHCs to their own
• preference was reversed if they were taking oral contraceptives
• Rates of early pregnancy loss are lower in couples with dissimilar MHC genes
Parungao-Balolong 2011
36. • Describe the immune ASSIGNMENT
response to:
: WORK IN
• A bacterial infection in your PAIRS
arm
• A bacterial infection from
your intestinal tract
• A viral infection
• A cancerous cell in your
body
• Which MHC type is most
likely to be involved?
Parungao-Balolong 2011
