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Dr William Bowers |
BACTERIOLOGY |
IMMUNOLOGY |
MYCOLOGY |
PARASITOLOGY |
VIROLOGY |
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IMMUNOLOGY - CHAPTER TEN
MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) and T-CELL RECEPTORS
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TEACHING OBJECTIVES
The structure and function
of cell surface molecules involved in immune cell interactions:
major histocompatibility complex molecules, the T cell receptor (TCR),
the CD3 complex, and accessory and costimulatory molecules |
MAJOR HISTOCOMPATIBILITY COMPLEX (MHC)
A. History
In transplantation studies, MHC gene products
were identified as responsible for graft rejection
In studies on responses to antigens, MHC gene
products were found to control immune responses, called the immune response (Ir)
genes
It was determined that antigen-specific T
cells recognize portions of protein antigens that are bound non-covalently to MHC gene products
a. Helper T cells recognize peptide bound to
class II MHC gene products
b. Cytolytic T cells recognize peptide bound
to class I MHC gene products
The complete three-dimensional structure for
both class I and class II MHC molecules has been determined by x-ray
crystallography.
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Figure 1 The MHC class 1 molecule has three globular domains alpha 1 (yellow),
alpha 2 (green) and alpha 3 (blue). The alpha 3 domain is closely
associated with the non-MHC -encoded beta 2 microglobulin (pink). The
latter is stabilized by a disulfide bridge (red) and is similar to an
immunoglobulin domain in three-dimensional structure. The alloantigenic
sites which carry determinants specific to each individual are found in
the alpha 1 and 2 domains. The latter also has a carbohydrate chain
(blue, CHO). There is a phosphate in the cytoplasmic domain. Papain
cleaves near the outer surface of the plasma membrane |
B. Structure of MHC Molecules
1. Class I MHC:
Figures 1 shows the
structure of class I MHC.
a. Class I MHC molecules contain two separate polypeptide chains
(1) MHC-encoded alpha (or heavy) chain; 43
kDa
(2) non-MHC-encoded beta chain (beta2
microglobulin) 12 kDa
b. There are four separate regions
(1) peptide-binding region is a groove formed
from the alpha1 and alpha2 regions which interact
to form a "floor" of an 8-stranded, beta-pleated sheet with two opposite
"walls" consisting of parallel strands of an α-helix. (alpha1
and alpha2 each contribute 4 strands of beta-pleated sheet
and one alpha-helix). A peptide 8-10 amino acids long sits in the groove.
The greatest variability in amino acids occurs in the alpha1
and alpha2 sequences that form the
groove that interacts with amino acids in the peptide fragment (figure 2).
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Figure 2 Most variability in amino acids at different positions along the alpha
chain of class I MHC molecules occurs in the alpha 1 and alpha 2
regions. The greatest polymorphism is found for amino acids that line
the wall and floor of the groove that binds the peptides |
Thus, the
polymorphism among class I MHC gene products creates variation in the
chemical surface of the peptide-binding groove. For any given MHC
molecule, binding of a peptide usually requires the peptide to have one or
more specific amino acids at a fixed position, frequently the terminal or
penultimate amino acid of the peptide. Binding of the specific amino acid
in the groove of the MHC molecule occurs in what is termed the anchor
site(s). The other amino acids can be variable so that each MHC molecule
can bind many different peptides. Other polymorphic residues of the MHC
molecule are those in contact with the T cell receptor (TCR), which
interacts with both peptide and the MHC molecule itself.
(2) immunoglobulin-like region is composed of
an alpha3 segment that is highly conserved and is homologous
to Ig constant domains and non-covalently bound beta2
microglobulin, an invariant molecule, also homologous to Ig constant
domains. These two interact with alpha1 and alpha2 to
maintain their proper conformation.
The importance of the highly conserved region
of alpha3 is that CD8, a molecule expressed on cytolytic T
cells that recognize class I MHC molecules, binds to this region.
(3) transmembrane region is a stretch of ~25
hydrophobic amino acids.
(4) cytoplasmic region is the carboxy
terminal 30 amino acids. Contains phosphorylation sites and provides
binding sites for cytoskeletal elements.
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CHIME
Chime presentation showing the
regions of variability of MHC I molecules and the interaction of the
alpha chain with other subunits of the MHC I complex and the bound
peptide (requires
Chime plug-in. Get Chime here) |
Figure
3 MHC class II molecules comprise two non-identical peptides (alpha and
beta) which are non-covalently associated and traverse the plasma
membrane with the N terminus to the outside of the cell. The
domains closest to the membrane in each chain are structurally
related to immunoglobulins. With the exception of the alpha 1
domain, all domains are stabilized by disulfide bridges (red).
Both the alpha and beta chains are glycosylated. The beta chain is
shorter than the alpha chain (beta mol. wt = 28,000) and contains
the alloantigenic sites. There is some polymorphism in the alpha
chain of some MHC II molecules |
2. Class II MHC
Figure 3 shows the structure of
class II MHC.
a. The protein contains two non-covalently associated
polypeptide chains, both MHC-encoded and polymorphic
(1) alpha chain; 34 kDa
(2) beta chain; 28 kDa
b. MHC proteins have four separate regions
(1) peptide-binding region is formed by
interaction of the alpha1 and beta1 segments.
There is a groove having a "floor" of 8 beta-pleated strands and two
"walls" with alpha-helices; alpha1 and
beta1
make equal contributions to this structure. As in the case for class I MHC,
the greatest polymorphic variability in the amino acids is in those facing
the groove (figure 4).
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Figure 4 The
greatest polymorphism for the beta chain of class II MHC molecules is
found for those amino acids in the beta I region that line the wall
and floor of the groove that binds the peptide |
Thus, as for class I MHC, the genetic polymorphism determines
the chemical structure of the groove and influences the specificity and
affinity of peptide binding and T cell recognition. Peptides associated
with class II MHC are 13-25 amino acids long; the longer peptides project
from the ends of the groove. As with class I MHC, anchor sites for one or
more amino acids also exist in the groove of the class II MHC molecule,
but these occur at more variable locations.
(2) immunoglobulin-like region formed by alpha2 and
beta 2 is folded into Ig-like domains.
These are largely non-polymorphic. The correlation of CD4 expression on
helper T cells with a specific TCR for class II MHC molecules is due to
binding of the CD4 molecules to the Ig-like non-polymorphic beta2 domain
of the class II MHC molecules.
(3) transmembrane region has proposed functions similar to
its counterpart in class I MHC.
4) cytoplasmic region also has proposed function similar to
itscounterparts in class I MHC.
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C. Similarities between class I and class II MHC
molecules
Despite differences in the two-chain composition
of class I and class II MHC molecules, they are quite similar structurally.
D. Important aspects of MHC
1. Because MHC molecules are membrane-associated
and not soluble, T cells must make cell to cell contact with cells expressing
MHC molecules.
2. In general, peptide fragments of proteins in
the cytosol associate with class I MHC; those of vesicular proteins associate
with class II MHC. Each is recognized by functionally distinct T cell
populations. Tc recognize class I MHC-peptide; Th recognize class II MHC-peptide.
3. There are many different MHC gene products for
class I and class II in the human population (polymorphism), only some of
which are found in an individual. The maximum number of class I MHC gene
products expressed in an individual is six; that for class II MHC products can
exceed six but is also limited. Whether or not a peptide fragment can associate with a given MHC
product determines whether there will be an immune response. This is one level
of control of immune responses.
4. Mature T cells respond to foreign antigens,
but not self protein. The repertoire of antigen recognition is based on
selection processes involving MHC molecules that occur mainly in the thymus.
This is another level of control of immune responses.
5. Only a single binding site exists on a class I
or class II MHC molecule; all peptides must bind to the same site.
6. The MHC polymorphism is determined only in the
germline. There is no somatic DNA recombination that occurs for antibodies and
for the TCR, so the MHC genes lack recombinational mechanisms for generating
diversity. As a result, the affinity and selectivity of MHC molecules for
foreign proteins are considerably lower than those of antibodies and T cell
antigens.
7. Because each MHC molecule can bind many
different peptides, the binding is said to be degenerate. Compare with the
restricted binding of a hormone to a receptor, for example, where a difference
in even one amino acid may impair binding.
8. Cytokines, especially interferon gamma (IFN-gamma),
increase the level of expression of class I and class II MHC molecules.
9. Alleles for MHC genes are co-dominant, i.e.
each gene product is expressed on the cell surface. (Contrast this with
allelic exclusion.)
10. Why the high polymorphism of MHC molecules?
Although an individual may not possess MHC molecules capable of binding
certain antigenic peptides, say for example against a virulent organism, the
likelihood is great that other MHC in the species can do so. Overall this
affords an evolutionary protection for the continuation of that species.
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Figure 5 The T cell receptor heterodimer comprises two transmembrane
glycoproteins, the alpha and beta chains. There are two domains in the
external part of each chain and these resemble immunoglobulin variable
and constant regions. There are sugar chains on each domain.
There is a short sequence similar to the immunoglobulin hinge region
that connects the immunoglobulin-like domains to the transmembrane
sequence. This contains cysteines that form a disulfide bridge. The
hydrophobic transmembrane helical structures are unusual in that they
contain positively charged amino acids (basic amino acids). The alpha
chain has two positively charged residues while the beta chain has
one.
Structure of A6-T cell receptor bound to MHC
class I molecule complexed with an altered Htlv-1 Tax Peptide Y8a. The HIV peptide is shown in gray. MHC class I molecule is in dark blue,
the associated beta 2 microglobulin in light blue. T cell receptor is
in green and yellow. Y. H.Ding, B. M.Baker, D.
N.Garboczi, W. E.Biddison & D. C.Wiley
MMDB Id: 11766 PDB Id: 1QSF Image prepared using RasMol
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T CELL RECEPTOR
(TCR)
Figure 5 (top) shows a schematic diagram of the
structure of the T cell receptor (TCR). The three-dimensional structure of the
TCR has been determined by x-ray crystallography. It is a heterodimer of two
polypeptide chains, alpha (40-50kDa) and beta (35-47 kDa), covalently
linked by a disulfide bond. Both alpha and
beta chains are similar to immunoglobulin, each consisting of a variable (V) and constant (C) region. The
variable region also has a joining (J) segment, and for the beta chain only,
a diversity (D) segment as well. Both chains contribute to the receptor formed
by the V regions. There are hypervariable regions in the V regions; these
contribute to the diversity of the TCR. The cytoplasmic tail for each is short
and not involved in signal transduction. The TCR is present on both helper and
cytolytic T cells. A given T cell expresses a TCR of only one specificity. The
TCR recognizes portions of both the MHC molecule and the peptide bound in the
groove.
There is also a small population of T cells that
expresses a TCR comprised of completely different chains, the gamma chain and
the delta chain. These T cells are termed gamma-delta T cells to distinguish
them from the more prevalent alpha-beta T cells. Gamma-deltaT cells do not express CD4,
characteristic of helper T cells, or CD8, characteristic of cytolytic T cells.
The diversity of TCRs is estimated to be as high
as 1016! This is due to the many possibilities for combining V, D,
and J regions, N-region diversification, joining-site variation and multiple D
regions. This number is considerably higher than the number of T cells leaving
the thymus, as is true for the number of possible immunoglobulin specificities
relative to the number of B cells produced.
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TABLE 1
COMPARISON OF
THE MAJOR PROPERTIES OF IMMUNOGLOBULIN (Ig) AND T-CELL
RECEPTOR (TCR) GENES AND PROTEINS |
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GENES |
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Properties |
Ig |
TCR |
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Many VDJs, Few C's |
Yes |
Yes |
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VDJ Rearrangement |
Yes |
Yes |
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V pairs form
antigen-recognition site |
Yes |
Yes |
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Somatic hypermutation |
Yes |
No |
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PROTEINS |
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Transmembrane forms |
Yes |
Yes |
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Secreted forms |
Yes |
No |
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Isotypes with distinct
functions |
Yes |
No |
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Valency |
2 |
1 |
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Adapted from Janeway and
Travers, Immunobiology |
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Structure of a crystal structure of a complex of a human T cell
receptor, influenza Ha Antigen Peptide and an MHC Class II Molecule.
The alpha and beta chains of the MHC II molecules are in dark and
light blue. The T cell receptor is in yellow and green. The influenza
peptide is in gray. Hennecke, J., Carfi, A., Wiley, D.
C. MMDB Id: 14648 PDB Id: 1FYT. Image prepared using RasMol |
CHIME
Click on the image above to view rotatable structure and identify protein chains
of MHC I and TCR interacting with HTLV tax peptide (requires
Chime plug-in. Get Chime here) |
CHIME
Click on the image above to view a
rotatable structure and identify protein chains of MHC II and TCR
interacting with an influenza HA peptide (requires Chime plug-in. Get Chime
here) |
Figure 6 The receptor for antigens on the T cell surface comprises eight
proteins.
(a) Two disulfide-bonded chains of the T cell receptor which form a
heterodimer. These recognize peptides associated with MHC molecules.
(b) Four chains, collectively termed CD3, that associate with the T
cell receptor dimer and participate in its transport to the surface of
the cell. The CD3 complex together with the zeta chains, which form a
homodimer, transduce the signal after antigen has bound |
CD3 COMPLEX
As shown in Figure 6, CD3 complex
is a group of five proteins that are physically associated with the TCR. It
consists of a gamma chain, delta chain, 2 epsilon chains, and 2 zeta chains.
The gamma, delta, and epsilon chains are produced by three closely linked genes and are highly homologous.
The zeta chain is the product of a single gene and is involved in signaling. All
four are invariant and do not
contribute to the specificity of binding of the TCR. Do not confuse the gamma-delta chains of the T-cell receptor with the
gamma chain and delta chain of the CD3 complex; they are distinctly different molecules.
The functions of the CD3 complex
are:
1. To ensure the cell surface
expression of the TCR. The synthesis of the TCR and the CD3 complex, their
assembly, and surface expression are tightly coordinated.
2. To transduce activating signals
to the interior of the T cell when the associated TCR binds antigen. The CD3
complex thus performs a signaling function similar to that of the Ig-alpha
and Ig-beta molecules associated with the immunoglobulin receptor on B
cells.
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Figure 7
A. Molecules involved in the interaction between T cells and
antigen-presenting cells. Some cytokines produced by each cell type are
shown
B. Ligands involved in the interaction of cytotoxic T cells and
their target cells |
CELL SURFACE MOLECULES INVOLVED IN
CELL-CELL INTERACTIONS
Engagement of a TCR and the peptide-MHC
complex it recognizes is not sufficient to activate a T cell. Other cell surface
molecules between the two interacting cells also engage. These form two groups
termed accessory molecules and co-stimulatory molecules.
1. Accessory molecules
When the TCR of a T cell recognizes
the peptide-MHC molecule on another cell, the accessory molecules attach
non-covalently to ligands on that cell (figure 7)
Some of the most important are shown
in table 2.
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TABLE 2
IMPORTANT ACCESSORY MOLECULES |
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T cell molecule |
Ligand on
second cell |
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CD4 on helper T cells |
class II
MHC molecules |
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CD8 on cytotoxic T cells |
class
I MHC molecules |
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LFA-2 (CD2) |
LFA-3 |
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LFA-1 |
ICAM-1, ICAM-2 |
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LFA = Leukocyte
Function-associated Antigen |
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ICAM = Intercellular Adhesion
Molecule |
Key points about accessory
molecules:
a. They are non-polymorphic and invariant
b. They increase strength of adhesion
between a T cell and an antigen presenting cell or target cell
c. They may have increased expression
in response of T cell to cytokines
2. Costimulatory molecules
Engagement of the TCR with peptide-MHC
delivers one signal to the T cell that alone is insufficient to activate a T
cell. Costimulatory molecules when bound to their ligand deliver a second
signal required for the T cell to become activated. The most critical
costimulatory molecule on the T cell is CD28, which binds to either B7-1
(CD80) or B7-2 (CD86). All three molecules are non-polymorphic and invariant.
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This page copyright
2007,
The Board of Trustees of the University of South Carolina
This page last changed on
Thursday, April 05, 2007
Page maintained by Richard Hunt
Please report any problems to rhunt@med.sc.edu
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Figure 1 The MHC class 1 molecule has three globular domains alpha 1 (yellow),
alpha 2 (green) and alpha 3 (blue). The alpha 3 domain is closely
associated with the non-MHC -encoded beta 2 microglobulin (pink). The
latter is stabilized by a disulfide bridge (red) and is similar to an
immunoglobulin domain in three-dimensional structure. The alloantigenic
sites which carry determinants specific to each individual are found in
the alpha 1 and 2 domains. The latter also has a carbohydrate chain
(blue, CHO). There is a phosphate in the cytoplasmic domain. Papain
cleaves near the outer surface of the plasma membrane
Figure 2 Most variability in amino acids at different positions along the alpha
chain of class I MHC molecules occurs in the alpha 1 and alpha 2
regions. The greatest polymorphism is found for amino acids that line
the wall and floor of the groove that binds the peptides
Figure
3 MHC class II molecules comprise two non-identical peptides (alpha and
beta) which are non-covalently associated and traverse the plasma
membrane with the N terminus to the outside of the cell. The
domains closest to the membrane in each chain are structurally
related to immunoglobulins. With the exception of the alpha 1
domain, all domains are stabilized by disulfide bridges (red).
Both the alpha and beta chains are glycosylated. The beta chain is
shorter than the alpha chain (beta mol. wt = 28,000) and contains
the alloantigenic sites. There is some polymorphism in the alpha
chain of some MHC II molecules
Figure 4 The
greatest polymorphism for the beta chain of class II MHC molecules is
found for those amino acids in the beta I region that line the wall
and floor of the groove that binds the peptide
Figure 5 The T cell receptor heterodimer comprises two transmembrane
glycoproteins, the alpha and beta chains. There are two domains in the
external part of each chain and these resemble immunoglobulin variable
and constant regions. There are sugar chains on each domain.
There is a short sequence similar to the immunoglobulin hinge region
that connects the immunoglobulin-like domains to the transmembrane
sequence. This contains cysteines that form a disulfide bridge. The
hydrophobic transmembrane helical structures are unusual in that they
contain positively charged amino acids (basic amino acids). The alpha
chain has two positively charged residues while the beta chain has
one.
Structure of a crystal structure of a complex of a human T cell
receptor, influenza Ha Antigen Peptide and an MHC Class II Molecule.
The alpha and beta chains of the MHC II molecules are in dark and
light blue. The T cell receptor is in yellow and green. The influenza
peptide is in gray. Hennecke, J., Carfi, A., Wiley, D.
C. MMDB Id: 14648 PDB Id: 1FYT. Image prepared using RasMol
Figure 6 The receptor for antigens on the T cell surface comprises eight
proteins. 
Figure 7 
