Transcript M261 MHC class I antigen presentation April 17, 2000

M261
ANTIGEN PRESENTATION/MHC CLASS I
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Pete Sieling
Phone: 825-6964
email: [email protected]
Office: 52-127 CHS
• Reading for Wednesday (4-20) and Friday (4-22)
lectures: Chapter 5 (pp169-198), Janeway et al.
6th edition.
ANTIGEN PRESENTATION
MHC CLASS I
• How do T cells “see” antigen and
what does MHC have to do with it?
• MHC class I antigen processing and
presentation pathway.
• Mechanisms of immune evasion.
MOLECULES OF LYMPHOCYTE RECOGNITION
Although B and T cell receptor genes are rearranged similarly
to generate a high degree of specificity for antigen, the
receptors “see” antigen in entirely different ways.
T-CELL
B-CELL
Iga
CD3
a b
Igb
TCR
Surface Ig
Complete
molecule
(conformational
epitope)
MHC
b1
a1
b2
a2
Processed
peptide
(linear
epitope)
ANTIGEN PRESENTING CELL
MOLECULES OF T LYMPHOCYTE RECOGNITION
Major histocompatibility complex (MHC); human=Human Leukocyte Antigen (HLA); mouse=H-2
Gorer and Snell identified a genetic basis for graft rejection and Snell named it histocompatibility 2
(H-2). Nobel prize awarded to Snell.
Highly polymorphic genes organized in a complex on chromosome 6 (human) and 17 (mouse).
Glycoproteins expressed on the surface of cells. MHC class I is composed of one polypeptide, noncovalently associated with b2microglobulin. MHC class II is composed of two polypeptides, referred
to as a and b.
CD8 T-CELL
CD4 T-CELL
a b
TCR ab
CD4
b1
MHC
CLASS II
b
CD3
b2
a1
a
CD8
15 aa
peptide
a2
ANTIGEN PRESENTING CELL
a b
TCR ab
a2
MHC
CLASS I
CD3
a3
a1
9 aa
peptide
b 2m
ANTIGEN PRESENTING CELL
RELATIONSHIP BETWEEN PATHOGEN AND
MHC COMPARTMENTS: A TWO-PRONGED
APPROACH TO ANTIGEN PRESENTATION
Text Figure 5.2
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Two places for pathogens to be found within eukaryotic cells,
cytosol/endogenous and vesicles/exogenous.
Two pathways for antigen presentation for peptides (MHC class I and II).
One place for antigens to end up in order for the TCR to recognize
foreign antigen-on the surface of the cell in the context of MHC.
TISSUE DISTRIBUTION OF MHC EXPRESSION
SUPPORTS ANTIGEN PRESENTATION PATHWAYS
•
The expression of MHC
molecules differs between
tissues. MHC class I molecules
are expressed on all nucleated
cells, although they are most
highly expressed in
hematopoietic cells. MHC
class II molecules are normally
expressed only by a subset of
hematopoietic cells and by
thymic stromal cells, although
they may be expressed by
other cell types on exposure to
the inflammatory cytokine
interferon-g.
FUNCTION OF MHC CLASS I MOLECULES
• Provide genetic basis for T
cell recognition of foreign
molecules, e.g. virus
infection. Nobel prize
% 51Cr release
70
CBA/H (H2k)
60
C57Bl (H2b)
50
awarded 1996.
CBA/HxC57Bl (H2k/b)
40
nu/nu
30
20
10
C3H (H2k)
0
Infected
51Cr
Uninfected
Zinkernagel and Doherty,
Nature 248:701, 1974
FUNCTION OF MHC CLASS I MOLECULES
• T cell receptor recognition
requires correct MHC as
well as correct peptide.
•
The antigen-specific T-cell receptor (TCR)
recognizes a complex of antigenic peptide
and MHC. One consequence of this is that a
T cell specific for peptide x and a particular
MHC allele, MHCa (left panel), will not
recognize the complex of peptide x with a
different MHC allele, MHCb (center panel), or
the complex of peptide y with MHCa (right
panel). The co-recognition of peptide and
MHC molecule is known as MHC restriction
because the MHC molecule is said to restrict
the ability of the T cell to recognize antigen.
This restriction may either result from direct
contact between MHC molecule and T-cell
receptor or be an indirect effect of MHC
polymorphism on the peptides that bind or
on their bound conformation.
Text Figure 5.17
H2k with
LCMV pep
H2b with
LCMV pep
H2k with
HSV pep
GENETIC ORGANIZATION OF MHC IN HUMANS
AND MICE
• Human HLA (Chrom. 6)
– Three class I genes, HLAA, HLA-B, HLA-C.
– Three pairs of class II
genes, HLA-DR, HLA-DP,
HLA-DQ.
• Mouse H-2 (Chrom. 17)
– Three class I genes, H2-K,
H2-L, H2-D.
– Two pairs of class II genes,
IE and IA.
Text Figure 5.11
IMMUNOLOGICAL DIVERSITY
GENERATED BY MHC LOCUS
• Expression of MHC gene
products are co-dominant,
meaning that each gene
encoding these proteins
on the parental
chromosome of the diploid
cell, is expressed.
• Polymorphism and
polygeny contribute to the
diversity of MHC
molecules expressed by
any individual.
Upper case=H2 gene
Lower case=H2 allele
H2kxH2d
STRUCTURE OF MHC MOLECULES AND
PEPTIDES BOUND TO THE GROOVE
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Crystal structure (solved in late
1980’s) revealed a binding groove
formed by anti-parallel b-pleated
sheets (bottom of groove) and ahelices (sides of groove).
a1 and a2 of MHC class I and a1
and b1 of MHC class II form “mirror
images of each other to create the
peptide binding groove.
Amino acids along the edges of the
groove interact (through hydrogen
bonds and ionic attractions) with the
amino acids of the peptide to
stabilize peptide binding (Figure).
HOW THE T CELL RECEPTOR
“SEES” PEPTIDE-MHC
View from bystander perspective
View from TCR perspective
STRUCTURE OF MHC MOLECULES AND
PEPTIDES BOUND TO THE GROOVE
• Allelic variation
occurs at specific
sites along the MHC
molecules. These
sites correspond to
amino acids that line
the antigen binding
groove.
• Similar variability is
seen in MHC class II.
Text Figure 5.16
STRUCTURE OF MHC MOLECULES AND
PEPTIDES BOUND TO THE GROOVE
•
The peptides that bind to the
MHC groove were identified by
lysing virus-infected cells with
detergent, (in some cases)
affinity purifying the MHC
molecules, isolating peptides
with HPLC, and identifying the
active peptides using CTL assays
(Rotzschke, et. al., Nature
348:252, 1990).
Uninfected
Infected
CTL
MHC
CLASS I
Anti-HLA Ab
Infected
cell
Lyse
cells
MHC
CLASS I
Mild H+
HPLC
HPLC, CTL, sequence
peptides
PEPTIDES BOUND TO THE GROOVE OF
MOUSE MHC CLASS I
• MHC class I molecules
bind 8-10 amino acid
peptides whereas MHC
class II bind 12 amino acid
or longer peptides.
• Peptides that bind to a
particular MHC protein
share a motif.
Anchor
residue=
H2Kb
H2Kd
PEPTIDES BOUND
TO THE GROOVE
OF HUMAN MHC
CLASS I
Housekeeping
function
Immune function
MHC CLASS I ANTIGEN PROCESSING AND
PRESENTATION PATHWAY
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Peptides presented by MHC class I come from cytosolic
proteins and peptides presented by MHC class II come
from endocytosed proteins. This is an oversimplification.
Many of the components of MHC class I processing are
encoded in the MHC locus.
Cytosolic proteins are degraded in the cytoplasm by a
complex called the proteasome, often after the protein is
ubiquitinylated.
Peptides are transported into the ER via the transporter
associated with antigen processing (TAP), a heterodimeric,
ATP-dependent transporter.
In the meantime, MHC class I heavy chain is being
translated in the ER and folded into the proper
conformation. The MHC protein is stabilized by a protein,
calnexin, a resident ER protein that binds to glycoproteins.
MHC class I then binds to b2microglobulin (b2m) and
calreticulin, a chaperone, that trafficks MHC class I to
tapasin which stabilizes a TAP-tapasin-MHC class I
complex. Peptides are loaded into the binding groove,
stabilizing MHC class I, which then moves to the cell
surface. Peptide deficient MHC class I is unstable and is
rapidly degraded whether or not it reaches the cell surface.
Text Figure 5.6
THE ROLE OF PROTEOLYSIS IN MHC
CLASS I PRESENTATION
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•
The proteasome is a complex of
polypeptides that sequester
proteins signaled for proteolysis.
It degrades all proteins tagged
with ubiquitin, regardless of
whether they will end up in the
ER.
Two subunits, LMP2 and LMP7,
are encoded in the MHC and can
be up-regulated by IFN-g. These Treatment Protein
signaled for
subunits direct the proteasome to
degradation
generate peptides that have
-IFN-g
carboxyl ends that fit MHC
(e.g., uninfected
cell)
molecules well and are easily
transported by TAP. IFN-g
converts proteasome from
+IFN-g
housekeeper to specialized
(e.g., infected
cell)
factory worker.
Proteasome
LMP2
Peptides
Result
Difficult to
transport to ER
and load in MHC
class I
Easy to transport
to ER and load in
MHC class I
LMP7
THE ROLE OF PROTEOLYSIS IN MHC
CLASS I PRESENTATION
-IFN-g
• Peptide profile (HPLC) of
IFN-g -treated cells differs
from untreated cells.
• When peptides are
sequenced, it becomes
apparent that some
proteolytic products are
favored in the cells treated
with IFN-g.
• The favored products are
those that will transport
easily into ER and load on
MHC class I.
Boes, et. al., J.Exp.Med. 179:901, 1994.
Centifugation,
column
purification
+IFN-g
HPLC
Housekeeping
function
Immune function
TRANSPORTER ASSOCIATED WITH
ANTIGEN PROCESSING (TAP)
ER (glycosylation
takes place)
Shepherd,
et. al., Cell,
74:577, 1993
RYWANATRSX
Nature, 367:648, 1994.
TAP-
TAP-/ratTAP
TAP
X
• Two proteins (TAP-1,2) that
form a heterodimer spanning
the ER membrane.
• Facilitates the ATPdependent transport of
peptides across the ER
membrane.
• Selectivity in transporting
peptides.
• Bias towards transporting
peptides that bind well to
MHC class I (amino acids
with hydrophobic or bulky
side chains).
Momburg, et. al.,
ROLE OF TAPASIN IN THE MHC CLASS I
PATHWAY
• Components of the MHC
class I-TAP complex
affinity purified by antiTAP-1 column.
• Tapasin transfection
restores MHC class I
surface expression and
CTL lysis.
Ortmann, et. al.,
Science 277:1306, 1997
CUSTOM PEPTIDES GENERATED FOR
MHC CLASS I BY ERAAP
TAP-deficient fibroblasts
No NH2-terminal amino acids
Seven NH2-terminal amino acids
Serwold et. al. Nature, 419:480-483, 2002
SIGNIFICANCE
OF ERAP
ERAP (endoplasmic
reticulum-associated
aminopeptidase 1) an
IFN-g inducible ER
protease that trims
peptides to fit MHC
class I
MECHANISMS OF IMMUNE EVASION (MHC CLASS I)
gp34 (mCMV)
Nef (HIV)
gp48 (mCMV)
gp40 (mCMV)
E19 (adenovirus)
US3 (hCMV)
E19 (adenovirus)
US2, US11 (hCMV)
Vpu (HIV)
EBNA1 (EBV)
US6 (hCMV)
ICP47 (HSV)
MECHANISMS OF IMMUNE EVASION
(MHC CLASS I)
-HSV
• Herpes simplex virus
encodes for a protein,
ICP47, that prevents TAP
from transporting peptides
into the lumen of the
endoplasmic reticulum.
• Prevention of peptide
transport results in an
inability to load peptide
into nascent MHC class I
proteins.
+HSV
X
Hill, et. al., Nature
375:411, 1995
MECHANISMS OF IMMUNE EVASION
(MHC CLASS I)
Tumor cell
Reduce MHC class I expression
Loss of tumor antigen expression
Secrete anti-inflammatory cytokines
Defective death receptor signaling
Lack of costimulation
T cell
Dendritic cell
HOW DO NAÏVE CD8 T CELLS BECOME
ACTIVATED WHEN THE INFECTED CELL IS
NOT A PROFESSIONAL APC?