Transcript MHC

Chapter 8
The Major Histocomptibility Complex
(MHC) and Ag Presentation
Chapter 9
T-cell Receptor
Chapter 10
T-cell Maturation, Activation, and
Differentiation
Chapter 8
The Major Histocomptibility Complex (MHC)
and Ag Presentation
Nov 21, 2006
你需要瞭解的課題：
1. MHC 是如何發現的？
2. MHC 的生理功能是什麼？
3. MHC 包含哪些基因及分子？
4. MHC polymorphism 有什麼生物意義？
5. Self-MHC restriction of T cells 是什麼意思？
6. 抗原是如何被呈現 (presented) 到 MHC 分子上的？
7. 與 MHC 結合的胜肽有什麼性質？
TCR and MHC-peptide
TCR (on a T cell)
 peptide (embedded in an
MHC molecule)
MHC (on an Ag-presenting
cell or on a virusinfected cell)
Gorer (1930s):
1. Rejection of foreign tissue is the result of an immune
response to cell-surface molecules.
2. Identification of I, II, III and IV blood-cell Ags and
genes in mice.
Gorer and Snell (1940s & 1950s):
1. Antigens encoded by the genes in the group II took part
in the rejection of transplanted tumors and other tissues.
2. Snell called these genes “histocompatibility (組織相容)
genes” (or H-2 genes)
3. Snell was awarded the Nobel Prize in 1980.
Human MHC: human leukocyte antigen (HLA)
- the major reason for transplantation rejection
Mouse MHC: H-2
The MHC Encodes 3 Major Classes of Molecules
Class I MHC:
- Expressed on the surface of nearly all nucleated cells; the major
function of the class I gene products is presentation of peptide Ags
to CD8+ T cells.
Class II MHC:
- Expressed primarily on Ag-presenting cells (macrophages, dendritic
cells, and B cells), where they present processed antigenic peptides
to CD4+ T cells
Class III MHC: - no function in Ag presentation
- Generally encode various secreted proteins that have immune
functions, including components of the complement system and
molecules involved in inflammation, e.g., C2, C4, factor B,
21-hydroxylase enzymes, TNFα, TNFβ, heat shock proteins (HSP)
D as class I
D as class II
- The MHC loci are highly polymorphic - many alternative
forms of the gene, or alleles, exist at each locus among
the population.
- The MHC genes lie close together - the recombination
frequency within the H-2 complex is only 0.5%.
Thus, most individuals inherit the closely linked MHC
genes as two sets, one from each parent. Each set of genes
is referred to as a haplotype.
- The MHC genes are codominantly expressed; that is,
both maternal and paternal gene products are expressed
in the same cells.
(H-2k)
Inheritance of MHC Haplotypes
inbred
homozygous
Acceptance or Rejection of Skin Grafts is
Controlled by the MHC Type of the Inbred Mice
Inheritance of HLA Haplotype in a
Hypothetical Human Family
outbred
heterzygous
Congenic MHC Mouse Strain
- Inbred mouse strains are syngeneic or identical
at all genetic loci.
- Two strains are congenic if they are genetically
identical except at a single locus or region.
- Congenic strains can be produced by a series of
crosses, backcrosses, and selections.
Production of Congenic Mouse Strain
Strain A.B
Genetically identical to
strain A except for the
MHC locus or loci
contributed by strain B.
if a/a = C57BL/10 (B10)
b/b = A
A.B = B10.A
Examples of Recombinant Congenic Mouse Strains
MHC Molecules and Genes
Class I and Class II Molecules
45kDa
33 kDa
12 kDa
Both class I and II molecules are glycoproteins.
28 kDa
Organization of Class I MHC Genes
b2M gene
Organization of Class II MHC Genes
= IAb
IAb
= IAa
IAa
3-D Structure of the External Domains
of a Human Class I HLA Molecule
peptide-binding cleft of MHC class I molecule:
- 25Å x 10Å x 11Å
- can bind a peptide of 8-10 a.a.
Superimposition of the peptide-binding cleft
of class I and class II MHC molecules
red: HLA-A2 (Class I)
blue: HLA-DR1 (Class II)
Peptide Binding by MHC Molecules
- Several hundred different allelic variants of class
I and II MHC molecules have been identified in
humans.
- Peptide binding by class I and class II molecules
does not exhibit the fine specificity characteristic
of Ag binding by Ab and TCR.
- A given MHC molecule can bind numerous
different peptides, and some peptides can bind to
several different MHC molecules.
Binding Affinity of MHC to Peptides
- The association of peptide-MHC molecule is
very stable under physiological conditions.
- The Kd values range from ~10-6 to 10-10.
- Most of the MHC molecules expressed on the
membrane of a cell are associated with a peptide
of self or nonself origin.
Peptide-binding Cleft Is Blocked
at Both Ends in Class I Molecules
β2M
HLA-A2 with
HIV RT
309-317 a.a.
MHC class I molecules bind to 8 ~ 10 a.a.,
most commonly 9.
Anchor Residues in Nonameric (9) Peptides Eluted
from Two Class I MHC Molecules
amino-terminal anchor:
2nd or 2nd & 3rd positions
carboxylterminal
anchor
Anchor residues
tend to be hydrophobic a.a.
In general, any peptide of correct length that contains the same or similar
anchor residues can bind to the same class I MHC molecules.
Conformational Difference in Bound
Peptides of Different Lengths
class I
An Influenza Virus Peptide and an Endogenous
Peptide Bound to a Class I MHC Molecule
influenza virus
endogenous
Peptide-binding Cleft is Open at
Both Ends in Class II Molecules
HLA-DR1
DRα
Influenza virus
hemagglutinin
306-318 a.a.
DRβ
MHC class II molecules can bind to 13 ~ 18 a. a. A central core
of 13 a.a. determines the ability of a peptide to bind class II.
Interacting T cells
presented to CD8+ T cells
presented to CD4+ T cells
Source of peptides
endogenous
exogenous
Ag processing pathway
cytosolic pathway
endocytic pathway
Polymorphism of class I and class II molecules
- The diversity of the MHC within a species stems from
polymorphism, the presence of multiple alleles at a given
genetic locus within the species.
- The MHC possesses an extraordinarily large number of
different alleles at each locus and is one of the most
polymorphic genetic complexes known in higher vertebrates.
HLA-A : 370 alleles, HLA-B : 660 alleles, HLA-C : 190 alleles
- The theoretical diversity possible for humans :
class I : 370 (A) x 660 (B) x 190 (C) = 4.6 x 107
class II : ~ 8 x 1011
Total : 4.6 x 107 x 8 x 1011 ~ 4 x 1019
實際上你認為人類的 MHC polymorphism 比
4 x 1019 多？還是比 4 x 1019 少？
Antibody diversity
MHC polymorphism
Variability in the amino acid sequence
of allelic class I MHC molecules
Location of Polymorphic Amino Acid Residues
Most of the
residues with
significant
polymorphism
are located in the
peptide-binding
cleft
Genomic Map of MHC Genes
Mouse H-2 is on the chromosome 17
Class I
Nonclassical
Class II
Class III
Class I
Nonclassical
Human HLA is on the chromosome 6
class II nonclassical
class II
class III
class I
nonclassical
Cellular Expression of
MHC Molecules
Cellular Distribution of MHC Class I Molecules
- In general, the classical MHC class I molecules are
expressed on most nucleated cells.
- The highest level of class I molecules are expressed on
lymphocytes: 1 % of the total plasma membrane proteins
or 5 x 105 molecules / cell.
- Fibroblasts, muscle cells, hepatocytes and neural cells
express very low levels of class I molecules.
- A few cell types (e.g., neurons and sperm cells at certain
stages of differentiation) appear to lack class I MHC
molecules altogether.
Cellular distribution of MHC class II molecules
- Class II molecules are expressed constitutively only by
Ag-presenting cells (APC), e.g., macrophages, mature
dendritic cells, and B cells.
- Thymic epithelial cells and some other cell types can be
induced to express class II molecules and to function as
APC under certain conditions and under cytokine
stimilation.
- In some cases, class II expression depends on the cell’s
differentiation stage.
Various MHC Molecules Expressed on
APC of a Heterozygous H-2k/d Mouse
Nonclassical MHC Class I and Class II Molecules
- Structurally similar to class I or class II molecules
- Less polymorphic
- Expressed at lower level
- Tissue distribution is more limited
- Functions are being studied
Function of HLA-G (nonclassical class I)
The expression of HLA-G molecules on
cytotrophoblasts (滋養層細胞) at the fetalmaternal interface has been implicated in
protection of the fetus from being recognized
as foreign and from being rejected by maternal
TC cells.
Hunt, J.S., M.G. Petroff, R.H. McIntire & C. Ober. HLA-G and
immune tolerance in pregnancy. FASEB J 19: 681-693, 2005
CTB: cytotrophoblast cell
Regulation of MHC Expression
- Defect in class II MHC transcriptional activator CIITA or
RFX cause one form of bare lymphoyte syndrome (BLS).
- The expression of MHC molecules is regulated by various
cytokines, e.g., interferons (α, β,γ) and tumor necrosis
factors (TNF), etc.
- MHC expression on cell surfaces is decreased by infection
with certain viruses, including human cytomegalovirus
(CMV), hepatitis B virus (HBV), adenovirus 12 (Ad12), etc.
- Decreased expression of class I MHC molecules is likely to
help viruses evade the immune response by reducing the
presentation of viral peptide on virus-infected cells.
MHC and Immune Responsiveness
Important Roles of MHC in the Immune Response
1. MHC molecules act as antigen-presenting structure.
2. MHC molecules expressed by an individual influence the
repertoire of antigens to which that individual’s TH cells
and TC cells can respond.
3. MHC partly determines the response of an individual to
antigens of infectious organisms.
4. MHC has been implicated in the susceptibility to disease
and in the development of autoimmunity.
Self-MHC Restriction
of CD4+ T Cells
APC
measure the 2nd
response against
immunized Ag
T cells
Class II
MHC
Restriction
Self-MHC
Restriction of
CD8+ T Cells
Zinkernagel & Doherty
(1974), 1996 Nobel prize
lymphocytic
choriomeningitis
virus (淋巴性脈絡
叢腦膜炎病毒)
Class I
MHC
Restriction
Self-MHC Restriction:
T cells can recognize Ag only when it is presented
by a self-MHC molecule on the membrane of an APC.
* This observation is actually the only case under
physiological situations.
Role of Ag-Presenting Cells
Since all cells expressing either class I or
class II MHC molecules can present peptides to T
cells, strictly speaking, they all could be designated
as Ag-presenting cells (APC). However,…………
- Ag-presenting cells (APC): Cells that display
peptides associated with class II MHC molecules to
CD4+ (mostly TH) cells are called APC.
- Target cells: Cells that display peptides associated
with class I MHC molecules to CD8+ (mostly TC ) cells
are referred to as target cells.
A Variety of Cells Can Function as APC
can be induced to
express class II
MHC & costimulatory signal.
They function
only for short
periods of time
during a
sustained
inflammatory
response.
TH-cell Activation Requires A Costimulatory Signal
Provided by APC
TCR
MHC
costimulatory
signal
Dendritic cells are the most effective APC.
They constitutively express a high level of class
II MHC molecules and have co-stimulatory
activity, and can activate naïve TH cells.
Macrophages must be activated by
phagocytosis of particular Ags before they
express a high amount of class II MHC
molecules or the co-stimulatory B7 membrane
molecule.
B cells constitutively express class II MHC
molecules but must be activated before they
express the co-stimulatory B7 molecule.
Processing of Ag Is Required for
Recognition by TH cells
(paraformaldehyde,
三聚甲醛)
(glutaraldehyde,
戊乙醛)
Ag processing is a
metabolic process
that digests
proteins into
peptides, which
can then be
displayed on the
cell membrane
together with a
class II MHC
molecules.
Processing of Ag Is Also Required for Recognition by
TC cells
- In influenza virus-infected cells, not only viral
envelop proteins (more exposed), but also
polymerase and nucleoproteins (internal proteins
of the virus), can be recognized by TC cells.
- TC cells recognize short linear peptide sequences
of viruses.
Evidence for Different Ag-processing
and Presentation Pathways
Processing and Presentation of Exogenous
and Endogenous Antigens
interacting
with CD4+
T cell
endocytic or
exogenous
pathway
- new protein synthesis is not required
- inhibited by chloroquine &
protease inhibitors
interacting
with CD8+
T cell
cytosolic or
endogenous
pathway
- requires synthesis of viral protein
- inhibited by protein synthesis inhibitor,
emetine
Endogenous Ags: the cytosolic proteolytic system for
degradation of intracellular proteins
misfolded or
defective proteins:
intact proteins:
Peptides are Transported from the Cytosol to
the Rough Endoplasmic Reticulum (RER)
- by Transporters Associated with Ag Presentation (TAP)
a heterodimer
Both TAP1 and TAP2 belong to the family of ATP-binding cassette (ABC)
proteins found in the membranes of many cells, including bacteria. These
proteins mediate ATP-dependent transport of amino acids, sugars, ions, and
peptides.
TAP Genes in Mouse H-2
Class I
TAP:
Nonclassical
Class II
Class III
Class I
Nonclassical
1. has affinity for peptides containing 8 to 16 amino acids.
2. favors peptides with hydrophobic or basic C-terminal a.a., the preferred
anchor residues for class I MHC molecules.
3. Thus, TAP is optimized to transport peptides that will interact with class I
MHC molecules.
4. different allelic forms of TAP genes exit within the population.
Peptides Assemble with Class I MHC
Aided by Chaperone Molecules
ERAP1 and ERAP2:
ER aminopeptidase
molecular chaperones: facilitate the folding of polypeptides
- calnexin: a resident membrane protein of the ER, promoting the folding of class I α chain.
- calreticulin: a chaperone
- tapasin (TAP-associated protein) : brings the TAP transporter into proximity with
class I MHC and allows it to acquire a peptide.
- ERp57: allows for the release of class I α/β2m after acquisition of peptide.
Exogenous Ags: the endocytic system for degradation
of internalized molecules
- APC can internalize Ag by phagocytosis or endocytosis
(pinocytosis or receptor-mediated endocytosis).
- The endocytic pathway involves 3 increasingly acidic
compartments: early endosome (pH 6.0 - 6.5),
late endosomes (endolysosomes) (pH 5.0 - 6.0),
lysosomes (pH 4.5-5.0).
- Lysosomes contain > 40 acid-dependent hydrolases,
including proteases, nucleases, glycosidases, lipases,
phospholipases and phosphatases.
- Ag is degraded into oligopeptides of about 13 – 18 a.a.,
which bind to class II MHC molecules and are thus,
protected from further proteolysis.
B Cells Can Act as APC through Internalization
of Ag by Receptor-mediated Endocytosis
- using mAb as receptors
The Invariant Chain Guides Transport of Class II
MHC Molecules to Endocytic Vesicles
 catalyzing the exchange
of CLIP with antigenic
peptides into class II
 a negative regulator, binding to
HLA-DM and lessening the
efficiency of the exchange
reaction facilitated by HLA-DM
CLIP : class II-associated invariant chain peptide
Functions of Invariant (Ii, CD74) chain
1. Prevent the binding of endogenous peptides to the
class II molecules.
2. Proper folding of the class II a and b chains
3. Exit of class II from the RER
4. Direct the transport of the class II MHC complex
from the trans-Golgi network to the endocytic
compartment.
Cross-Presentation
of Exogenous Ags
APC may present
exogenous Ag to TC
cells in the context of
class I MHC molecules.
Presentation of
Nonpeptide Ags
Non-MHC, Nonclassical Class I molecules
– CD1 family of genes
(group 2)
(group 1)
- CD1 genes are not located within the MHC.
- The CD1 family of molecules associates with
b2m and has general structural similarity to
class I MHC molecules.
- The Ag-binding groove of the CD1 molecules
is deeper and larger than that of classical
class I molecules.
Ags Presented by CD1 Molecules
Mycolic acid of the Mycobacterium tuberculosis cell wall
(結核桿菌)
Lipoarabinomannan (a glycolipid) of Mycobacterium leprae
(痲瘋桿菌)
CD1 molecules present glycolipids and phospholipids.
- CD1 Ag presentation is another pathway for the
processing of Ags.
- TAP is not required.
- CD1 molecules locate and traffic differently from
class I or class II MHC molecules.
- The distribution of CD1 molecules is different
from the classical class I molecules.
- The mouse CD1d1 is found on T cells, B cells,
dendritic cells, hepatocytes, and some epithelial cells.
- CD1 genes can be induced by GM-CSF or IL-3.
You should be able to explain how these happen now!