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Chapter
11
Antigen Processing and
Presentation
T cells do not recognise native antigens
Y
Y
Y
Y
Y
Y
Y
Cross-linking of
surface membrane Ig
YYY Y
Y Y Y Y
B
B B
B B BB
B B
Proliferation and
antibody production
T
T
No proliferation
No cytokine release
Y
Y
Antigens must be processed in order
to be recognised by T cells
T
Y
Soluble
native Ag
Cell surface
native Ag
Soluble
peptides
of Ag
APC
No T cell
response
No T cell
response
Cell surface peptides of
Ag presented by cells that
express MHC molecules
Cell surface
peptides
of Ag
ANTIGEN
PROCESSING
No T cell
response
No T cell
response
T cell
response
Chapter
11
Antigen Processing and
Presentation
Contents
 PartⅠ Introduction--concepts
 PartⅡ Characteristics of APCs
 PartⅢ Ag Processing and presentation
PartⅠ Introduction-concepts
Endogenous Ags: antigens synthesized within cells,
including self and unself protein----class Ⅰ MHC molecules.
Exogenous Ags:
antigens comes outside the cells,
including self and unself protein----class Ⅱ MHC molecules.
Antigen processing:
the conversion of native
proteins to peptides which can combine with MHC molecules.
Antigen presentation:
the course of formation and
display of peptide-MHC complexes on the surface of APCs
and the course of peptide-MHC complexes recognition by T
cells.
Ag capturing----Endocytosis
(internalization)
Phagocytosis, Pinocytosis, Receptor-mediated endocytosis
Production of endogenous Ags and exogenous Ags
The site of pathogen replication or mechanism of antigen
uptake determines the antigen processing pathway used
Y
EXTRACELLULAR OR
ENDOSOMAL REPLICATION
Vesicular Compartment
Contiguous with extracellular fluid
Exogenous processing
(Streptococcal, tumor antigens)
INTRACELLULAR REPLICATION
Cytosolic compartment
Endogenous processing
(Viral, tumor antigens )
Antigens must be processed in order
to be recognised by T cells
T
Y
Soluble
native Ag
Cell surface
native Ag
Soluble
peptides
of Ag
Cell surface peptides of
Ag presented by cells that
express MHC antigens
Cell surface
peptides
of Ag
APC
No T cell
response
No T cell
response
No T cell
response
No T cell
response
T cell
response
Antigen-Presenting Cells (APC)
APC
(Accessory cells) : A group of cells play
important roles in the immune response which
can uptake, process antigens and present
peptide-MHC complexes to T cells.
 Professional APC: express classⅡMHC molecules
Dendritic cell
Macrophage
B lymphocyte
 Facultative APC: endothelial cells, epithelial cells,
fibroblast, etc
APC
•
•
•
Express classⅠ, Ⅱ MHC molecules
and co-stimulatory molecules
Uptake, process endogenous/exogenous
antigens and present peptide-MHC to T
cells
Including dendritic cells,
macrophages and B cells
PartⅡ Characteristics of APCs
 Dendritic cell (DC)
 Macrophage
 B lymphocyte
1. Dendritic cell (DC)
• History: DCs were first found by Steinman in
1973，named for their special spinelike
projections. DCs were cultured successfully in
vitro in 1993 by Inaba.
• Characteristic: The most efficient APC, can
present antigens to naive T cells to elicit primary
immune response.
Fig2-4 Mature DC suspended in media by colony（ ×400）
Fig2-5 scattered
mature DC（ ×400）
Scanning electron micrograph
1. Dendritic cell (DC)
(1) Identification of DC:
Typical morphology—spinelike projection
MLR—stimulate naïve T cells activation
Surface markers : CD1a, CD11c, CD83(human)
high expression of classⅡMHC
co-stimulatory molecules-CD80,CD86
others—CKs, CAMs, R
(2) Source of DC: pluripotent hematopoietic stem cells
myeloid DCGM-CSF, IL-4 myeloid progenitor
progenitor
lymphoid DC
lymphoid
1. Dendritic cell (DC)
(3) Classification of DC :
 DC in lymphoid tissue:
Interdigitating DC (IDC) ,
Folicular DC (FDC)
 DC in non lymphoid tissue:
Langerhans cell (LC)
 DC in body fluid: Veiled cell, Blood
DC
Interdigitating DC( IDC ）
Express high level of classⅠ, Ⅱ MHC molecules and B7，lack of
FcR and CR, can stimulate T cells.
Folicular DC（FDC）
Lie in follicle of LN,
no expression of
class Ⅱ MHC, high
level of FcR and C3bR.
FDC
B cell
Langerhan’s cells(LC)—Birbeck particle
Lie in the epithelia of the skin, gastrointestinal and
respiratory tracts, express FcR and C3bR.
After uptaking antigens, migrating to draining LN and
becoming IDC.
1. Dendritic cell (DC)
(4) Development and Maturation of
DC
Pre-DC phase
Immature DC（iDC） phase
Migration phase
Mature DC（mDC） phase
Blood
Pre-DC
Differentiation
Non-lymphoid tissue
Widely distributed in the body
Possess ability of Ag capture and process
Cytokines and Ag
DC mature and move into lymphoid tissue
Ability of Ag capture and processing decreases
while its ability of Ag presenting increases
ImmatureDC
Distribute
Difference between iDC and
mDC
• Ability of uptaking and processing
antigens decreases.
• Ability of antigen presentation
increases.
• Express high level of MHC, costimulatory molecules(CD80,CD86),
CAMs(ICAM-1).
• Ability to stimulate naïve T cell
activation increases.
1. Dendritic cell (DC)
(5) Antigens capturing:
• Phagocytosis—cell, bacteria
• Pinocytosis—soluble antigen
• Receptor-mediated endocytosis
FcγRⅡ, C3bR, mannose receptor
1. Dendritic cell (DC)
(6) Function of DC :
• Capture, process, present antigens—APC
• Stimulate T or B lymphocytes—mature DC
• Induce immune tolerance—immature DC
2.
Macrophage（
MФ）
Stem from monocytes in blood
•
• Have strong phagocytosis (big phagocyte)
• Can not stimulate naïve T cells
• Capture antigens by phagocytosis,
pinocytosis, receptor-mediated
endocytosis
2. Macrophage（ MФ）
Function :
•
•
Phagocytosis
Presentation of antigens
Unactivated macrophage
Activated macrophage:
Class Ⅱ MHC molecules
and co-stimulatory
molecules
3. B cells
Functions
• Mediate humoral immune response
• Immunological regulation
• Present antigens to T cell
Soluble Ag--pinocytosis
Specific receptor-mediated endocytosis
The three kinds of professional APC
Antigens must be processed in order
to be recognised by T cells
T
Y
Soluble
native Ag
Cell surface
native Ag
Soluble
peptides
of Ag
Cell surface peptides of
Ag presented by cells that
express MHC antigens
Cell surface
peptides
of Ag
APC
No T cell
response
No T cell
response
No T cell
response
No T cell
response
T cell
response
PartⅢ Ag Processing and Presentation
Class Ⅱ MHC pathway ------exogenous antigens
Class Ⅰ MHC pathway ------endogenous antigens
Cross – presentation of antigen
SectionⅠ Class Ⅱ MHC pathway
1. Capture of exogenous Ag
2. Processing of Ag
3. Synthesis and transportation of class Ⅱ MHC
molecules
4. Peptide loading of classⅡ MHC molecules
5. Presenting to CD4+T cells
1. Capture of exogenous Ag
• Endocytosis:
Phagocytosis: particles or granules
Pinocytosis: soluble antigens
Receptor-mediated endocytosis:
• Form endosome
Uptake of exogenous antigens
Membrane Ig
receptor mediated
uptake
Y
Phagocytosis
Complement receptor
mediated phagocytosis
Pinocytosis
Y
Fc receptor mediated phagocytosis
2. Processing of Ag
endosome + lysosome
Cathepsin
Ag
antigen peptides(10-30aa)
Exogenous pathway
Cell surface
Uptake
Protein antigens
In endosome
Endosomes
To lysosomes
Cathepsin B, D and L proteases are activated by the decrease in pH
3. Synthesis and transportation of class Ⅱ
MHC molecules
Synthesis of class Ⅱ MHC molecules in ER
Ii chain --- class Ⅱ MHC molecule (Ii3α3β3 )
①Promote formation of class Ⅱ MHC dimer
②Preventing endogenous peptide from combining with
classⅡMHC molecules within ER
③Leading classⅡMHC molecules into endosome from ER
Endosome (MIIC)
*Ii chain: Ia-associated invariant chain
Invariant chain CLIP peptide
 and b chains of MHC
class II molecules
CLIP
A peptide of the invariant chain blocks the MHC molecule binding site.
This peptide is called the CLass Ⅱ associated Invariant chain Peptide
(CLIP)
MHC class II maturation and invariant chain
In the endoplasmic reticulum
Ii chain
CLIP
Invariant chain stabilises MHC class
Need to prevent newly
II by non- covalently binding to the
synthesised, unfolded
self proteins from binding immature MHC class II molecule and
forming a nonomeric complex
to immature MHC
4. Peptide loading of class Ⅱ MHC molecules
Ii - class Ⅱ MHC molecules
protease
Ii chain cleaving
CLIP - class Ⅱ MHC molecules
HLA-DM
CLIP releasing
Antigen peptide - class Ⅱ MHC complexes
Class II associated invariant chain peptide (CLIP)
Cell surface
Uptake
(binv)3 complexes
directed towards
endosomes by
invariant chain
Endosomes
Cathepsin L degrades
Invariant chain
CLIP blocks groove in MHC
molecule
MHC Class II
containing vesicles
fuse with antigen
containing vesicles
Removal of CLIP
?
How can the peptide stably bind to a floppy binding site?
Competition between large number of peptides
HLA-DM catalyses the removal of CLIP
HLA-DM
Replaces CLIP with a
peptide antigen using a
catalytic mechanism (i.e.
efficient at substoichiometric levels)
Discovered using mutant
cell lines that failed to
present antigen
HLA-DR
HLA-DM
MIIC compartment
HLA-DO may also play a
role in peptide exchange
Sequence in cytoplasmic
tail retains HLA-DM in
endosomes
5.Presenting to CD4+T cells
Antigen peptide-class Ⅱ MHC molecuels
presented on cell membrane by
exocytosis
Surface expression of class II MHC peptide complexes
Exported to the cell surface (t1/2 = 50hr)
Sent to lysosomes for degradation
MIIC compartment sorts peptide-MHC complexes for surface expression or
lysosomal degradation
CD4+T cells
sectionⅡ class Ⅰ MHC
pathway
1. Processing of endogenous Ag
2. Transporting of antigen peptide into ER
3. Peptide loading of class Ⅰ MHC molecules
4. Presenting to CD8+T cells
1. Processing of endogenous Ag
• Proteosome : 20S, 26S
• Low molecular weight polypeptide (LMP) :
LMP2, LMP7，LMP10
• Ag
antigen peptides (6-30aa)
Degradation in the proteasome
Cytoplasmic cellular proteins, including non-self proteins
are degraded continuously by a multicatalytic protease
Binding ubiquitin
The components of the proteasome include LMP2, LMP7, MECL-1（LMP10）
*MECL-1：Multicatalytic endopeptidase complex subunit
2. Transporting of antigen peptide
into ER
TAP(transporter associated with antigen
processing):
Consisting of TAP1 and TAP2
ATP dependent transporter
Selective transporting (8-15aa)
Peptide antigens produced in the cytoplasm are
physically separated from newly formed class I MHC
ENDOPLASMIC RETICULUM
Newly synthesised
class I MHC molecules
CYTOSOL
Peptides need
access to the ER in
order to be loaded onto
class I MHC molecules
Transporters associated with
antigen processing (TAP1 & 2)
Hydrophobic
transmembrane
domain
Lumen of ER
Peptide
ER membrane
Cytosol
Peptide
Peptide
Peptide antigens
from proteasome
ATP-binding cassette
(ABC) domain
Transporter has preference for >8 amino acid peptides
with hydrophobic C termini.
3. Peptide loading of class Ⅰ MHC molecules
ER: antigen peptide—class Ⅰ MHC complexes
Maturation and loading of class I MHC
Peptide
Peptide
Peptide
Endoplasmic reticulum
B2-M
Calnexin binds
binds and
to nascent
stabilises
class I chain
floppy
until b2-M binds
MHC
Tapasin, calreticulin, TAP
1 & 2 form a complex with
the floppy MHC
Cytoplasmic peptides
are loaded onto the
MHC molecule and the
structure becomes
compact
4. Presenting to CD8+T cells
Antigen peptide-class Ⅰ MHC molecuels
presented on cell membrane by exocytosis
Fate of class I MHC
Exported to the cell surface
Sent to lysosomes for degradation
Ag(cytosolic protein)
Proteasome
proteolytic degradation
Ag peptide
TAP complex
transporting into ER
antigen peptide-class Ⅰ MHC complexes
Golgi complex exocyotsis
Presenting to CD8+T cells
CD8+T cells
CD8+T cells
CD4+T cells
SectionⅢ Cross-presentation of antigens
Cross-priming:
• Class Ⅰ MHC molecules also present
exogenous antigens to CD8+T cells
• Class Ⅱ MHC molecules also present
endogenous antigens to CD4+T cells
CD8+T cell(Tc)
CD4+T cell(Th)
T cell
Receptor
T cell
Receptor
Peptide
CD4
MHC
Class II
Peptide
CD8
MHC
Class I
Antigen Presenting
Cell
Target cell