Download Antigen Presentation and Dendritic Cells

Antigen Presentation and Dendritic Cells
Outline:
1. Review of MHC Class I and MHC Class II Presentation
2. Overview of Dendritic Cells
3. Dendritic Cell Paradoxes
4. Cross Presentation
5. Problems in Cross Presentation
6. Pathogens Exploit DCs as an Entry Route for Infection
7. Pathogens Block Antigen Presentation
8. Additional Reading
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Antigen Presentation and Dendritic Cells
1. Review of MHC Class I and MHC Class II Presentation:
A. How T cells are primed to
recognize microbial
antigens: MHC I vs. MHC II
presentation
Intracellular antigens
(typically viral) presented
via MHC I on antigen
presenting cells (APC) to
CD8+ (cytotoxic) T cells.
Note: MHC class I expressed
on all cell types.
Intravesicular antigens
(typically bacterial or
parasitic) presented via
MHC II on APC to CD4+ T
cells
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From Janeway, Immunobiology, 5th edition
J. R. Lingappa, Pabio 552
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Antigen Presentation and Dendritic Cells
1. Review of MHC Class I and MHC Class II Presentation, cont.:
A. MHC class I presentation:
Viral antigens from cytosol are presented by MHC class I
From Janeway, Immunobiology, 5th edition
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Antigen Presentation and Dendritic Cells
1. Review of MHC Class I and MHC Class II Presentation, cont.:
A. MHC class I presentation, cont.:
MHC class I chains are retained in the ER until peptide binds
From Janeway, Immunobiology, 5th edition
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Antigen Presentation and Dendritic Cells
1. Review of MHC Class I and MHC Class II Presentation, cont.:
B. MHC class I presentation results in binding of T cell
receptor and activation of CD8+ (cytotoxic) T cells:
From Janeway, Immunobiology, 5th edition
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Antigen Presentation and Dendritic Cells
1. Review of MHC Class I and MHC Class II Presentation, cont.:
C. MHC class II presentation:
Endocytosed bacterial antigens presented by MHC class II
From Janeway, Immunobiology, 5th edition
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J. R. Lingappa, Pabio 552
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Antigen Presentation and Dendritic Cells
1. Review of MHC Class I and MHC Class II Presentation, cont.:
C. MHC class II presentation, cont.:
Invariant chain prevents other proteins in the ER from binding to MHC class II
From Janeway, Immunobiology, 5th edition
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Antigen Presentation and Dendritic Cells
1. Review of MHC Class I and MHC Class II Presentation, cont.:
D. MHC Class II presentation on APC’s activates CD4+ T cells:
From Janeway, Immunobiology, 5th edition
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Antigen Presentation and Dendritic Cells
1. Review of MHC Class I and MHC Class II Presentation, cont.:
E. Activation of naïve T cells requires two signals (“co-stimulation”).
1. Signal 1: recognition of peptide-MHC complexes by T cell receptor.
2. Signal 2: interactions between B7(CD80 or CD86) on the APC cell surface and
CD28 receptors on the T cell. B7:CD28 binding is promoted and sustained by CD40
binding to CD40 ligand.
ONLY PROFESSIONAL APCS CAN CO-STIMULATE AND ACTIVATE NAÏVE T CELLS.
PROFESSIONAL APCS ONLY EXPRESS B7 WHEN ACTIVATED BY INFLAMMATION.
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Antigen Presentation and Dendritic Cells
3. Overview of Dendritic Cells:
A. Basic Concepts:
1. DCs are the most effective professional APCs
-Only professional APCs are capable activation of naïve T cells
-Amateur APCs lack the ability to elicit signal 2 and therefore only recognize
previously activated T cells. Most cells that become infected are amateur
APCs.
-DCs: most effective because of costimulation efficiency, wide distribution, location
at critical sentinel sites, and ability to migrate
-Activated T cells in turn activate B cells, T cells, and macrophages
-Thus, DCs initiate immune responses that are conducted by other cell types.
From Janeway, Immunobiology, 6th edition
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Antigen Presentation and Dendritic Cells
2. Overview of Dendritic Cells, cont.:
A. Basic Concepts, cont.:
2. DCs exist in 2 functionally different states (mature and immature).
DCs were initially thought to be good phagocytes and weak APCs.
Change in our understanding of DC: observation that freshly isolated
Langerhans cells were initially weak APCs, but after culture for 1-3 days in presence of
antigen they become capable of presentation (due to maturation).
Immature and mature DC’s function differently.
3. DCs link the innate and adapative immune response.
As immature cells, they act as sensors of the environment.
Upon stimulation, dendritic cells undergo maturation and migration to
secondary lymphoid organs where they stimulate naïve T cells that then go on to
activate T cells, B cells, and macrophages.
DC’s are the link between the innate and adaptive immune response.
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Antigen Presentation and Dendritic Cells
2. Overview of Dendritic Cells, cont.:
B. Immature Dendritic Cells: Sentinels
a. Specialize in endocytosis (antigen
uptake) including recycling of MHC I&II
b. Express low levels of MHC class I and
MHC class II molecules on cell surface
c. Short half life of MHC class II on surface
d. Rich in vesicles with lysosomal markers
that are full of MHC II and HLA-DM called
MHC class II compartments or MIIC
e. Express low levels of co-stimulatory
molecules on cell surface
f.
Antigens taken up are retained for use
as immunogenic peptides days later
g. Peripheral location: blood, lymphoid, &
non-lymphoid tissues (skin &mucosa)
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From Mellman and Steinman, Cell 106: 255 (2001)
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Antigen Presentation and Dendritic Cells
2. Overview of Dendritic Cells, cont.:
C. The Process of Dendritic Cell Maturation:
Step 1: DCs sense pathogens or inflammatory signals.
i. Microbial products
ii. Pro-inflammatory cytokines
Maturation occurs only upon detection of microbial products or cytokines.
Links efficient antigen presentation to a danger signal: favors activation of T cells
ONLY in presence of a pathogen.
Step 2: immature DC’s capture the microbe that has been sensed via:
i. Phagocytosis (for particulate antigens)
ii. Receptor-mediated endocytosis
iii. Macropinocytosis (for soluble antigens)
Step 3: Migration of DC’s from site of infection towards T cell areas of the proximal
LN via afferent lymphatics.
Upregulation of the chemokine receptor CCR7 on DC which drives DC migration to:
1. lymphatic vessels by binding 6Ckine released by lymphatic endothelial cells
2. draining lymph nodes (DNL) by binding MIP 3
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Antigen Presentation and Dendritic Cells
2. Overview of Dendritic Cells, cont.:
C. The Process of Dendritic Cell Maturation:
Step 4: Decreased endocytic & phagocytic capability
Due to down-regulation of macropinocytosis & phagocytosis
Mechanism: reduced activation of Cdc42 & Rac (GTPases that regulate actin).
Clathrin-mediated endocytosis continues
Step 5: Increased surface expression of MHCII bound to peptide
Little or no increase in mRNA and protein synthesis
Almost entirely due to post-translational events, especially intracellular transport
In immature DCs, MHC II accumulates in late endosomes and lysosomes
In mature DCs, MHC II accumulates at cell surface
Step 6: Enhanced expression of co-stimulatory molecules
B7 (CD80 and CD86, CD40) and Fas
Step 7: Release of cytokines and chemokines by DCs
Including IL-12, IL-18, IL-10
Cytokines polarize T cell responses (see Responses to DCs, below)
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Antigen Presentation and Dendritic Cells
2. Overview of Dendritic Cells, cont.:
D. Mature DC’s: Antigen Presenting Cells
a.
b.
c.
d.
e.
f.
g.
h.
i.
Reduced capacity for antigen uptake
Increased antigen presentation and T cell stimulation (better than macrophages)
Redistribution of MHC II from intracellular compartments to cell surface
MIICs replaced by CIIVs where peptide loading occurs
CIIVs deliver peptide-loaded MHC II to cell surface
Dramatic increase in cell surface expression of co-stimulatory molecules
MHC II loaded with peptide at cell surface has a much longer half life
Surface expression of co-stimulatory molecules
Long dendritic processes (membrane folds) for capture of T cells
j. Chemokine receptor expression for migration & homing to lymph nodes
From Mellman and Steinman, Cell 106: 255 (2001)
MIIC
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Antigen Presentation and Dendritic Cells
3. Dendritic Cell Paradoxes:
1. How are 9 - 16 amino acid peptides (ideal for antigen presentation) produced?
Lysosomes normally degrade proteins to single amino acids.
DC’s regulate lysosomal proteolysis in a developmentally-linked fashion. This may
occur in endosome-like vesicles that are specialized for antigen processing.
Macrophages contain high levels of proteases resulting in rapid and complete
degradation.
In contrast, DC’s are protease poor with limited capacity for proteasomal
degradation.
Dellamare et al., Science 307: 1630, 2005
Limited lysosomal proteolysis favors antigen presentation.
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Antigen Presentation and Dendritic Cells
3. Dendritic Cell Paradoxes:
2. How do DC’s present antigen that was sequestered in lysosomes for days?
Lysosomal enzyme content, pH, and morphology are regulated in DC’s.
A. Regulation of post-translational trafficking of MHC II:
1. Regulation of cathepsin S
Cathepsin S cleaves Ii chain that occupies the MHC II peptide binding groove.
Immature DCs: cystatin C antiprotease slows Ii cleavage - MHC II goes to lysosomes.
Mature DC’s: cystatin C activity decreases, cathepsin S activity increases and more
MHC II reaches the cell surface.
2. Regulation of vacuolar ATPase
Low pH favors peptide loading onto MHC II
DC maturation lowers endosomal pH
Low pH results from recruitment of cytoplasmic
ATPase subunits forming functional ATPase
Trombetta et al. Science 299: 1400, 2003.
(Helicobacter and MTb have ways to
block the reduced endosomal pH)
Trombetta and Mellman, Annual Reviews of Immunology, 23:975, 2005
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Antigen Presentation and Dendritic Cells
3. Dendritic Cell Paradoxes:
2. How do DC’s present antigen that was sequestered in lysosomes for days?
Lysosomal enzyme content, pH, and morphology are regulated in DC’s.
B. Regulation of lysosomal morphology:
Time lapse confocal images reveal that within 30 min after stimulation by LPS,
lysosomal structures form tubules that take MHC II to plasma membrane. Thus,
dendritic cells can rescue components from degradation in lysosomes and transfer
them to the plasma membane. Not blocked by MT inhibitors.
Formation of tubules may be a specialized function of DC lysosomes.
Boes et al., Nature 418: 982, 2002; Chow et al., Nature 418: 988, 2002.
Immature (a),
maturing (b), and
mature (c)
dendritic cells
stained for MHC
class II (green) and
lysosomal marker
Lamp-1 (red).
From Mellman et al., TICB 8: 231 (1998)
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Antigen Presentation and Dendritic Cells
4. Cross-presentation:
Do Class I and Class II Presentation explain antigen presentation adequately?
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Antigen Presentation and Dendritic Cells
4. Cross-presentation:
Do classical MHC class I and class II Presentation explain antigen presentation fully?
Problem 1: Classical MHC class I presentation would require DC’s to get infected and
produce peptides in the DC cytoplasm. However, many viruses do NOT infect dendritic
cells and still activate cytotoxic CD8+ T cells.
There must be a way that dendritic cells can use intracellular peptides produced
in other cells to activate cytotoxic T cells.
Problem 2: Phagocytosed pathogens such as Salmonella, Brucella, and Leischmania
can elicit MHC class I-dependent cytotoxic CD8+ T cell proliferation.
To elicit Class I responses, pathogens in phagosomes must transfer antigens
into the cytosol.
Problem 3: Vaccine antigens are extracellular and yet result in cytotoxic CD8+ T cell
responses.
Extracellular antigens must be capable of transfer into the cytosol to elicit class
I responses.
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Antigen Presentation and Dendritic Cells
4. Cross-presentation:
Typically MHC II antigens are extracellular and MHC I antigens are cytosolic.
However, this is no longer absolutely true - examples that violate both directions exist.
Thus, ANY protein can potentially be presented by MHC I or MHC II.
Cross presentation = MHC I presentation of exogenous antigens.
Sources of antigen for MHC I presentation:
Sources of antigen for MHC II presentation:
Trombetta and Mellman, Annual Reviews of Immunology, 23:975, 2005
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Antigen Presentation and Dendritic Cells
5. Problems in Cross-presentation:
A. Another topological barrier is crossed if extracellular proteins access the proteasome
B. What channel is used when extracellular peptides cross the phagosome membrane?
C. What is the source of the phagosome membrane?
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Antigen Presentation and Dendritic Cells
5. Problems in Cross-presentation:
2. Models for Cross-Presentation:
Exogenous antigens are taken
up into a phagosomal
compartment composed
mainly of PM/endosomes
OR
Exogenous antigens are moved
from phagosome to ER
In some organelle
(?Phagosome, ? ERphagosome), retrograde
transport to the proteasome
in the cytosol occurs.
Peptides produced by this
phagosome-associated
proteasome are transported
back into the phagosome
via a TAP transporter and
loaded onto MHC class I.
From Ackerman and Cresswell, Nature Immunology 2004 Jul;5(7):678-84.
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Antigen Presentation and Dendritic Cells
6. Pathogens exploit dendritic cells as an entry route for infection:
While dendritic cells use pathogen recognition and uptake to activate the adaptive
immune system, pathogens conversely use exploit dendritic cells as a route of entry
for infection.
A. DC-SIGN captures and transfer HIV-1
DC-SIGN: Adhesion receptor on DCs
Binds ICAM-2 promoting trans-endothelial
migration of DCs
Initiates interaction of DCs with naïve T cells
through ICAM-3 binding
Binds HIV-1 envelope glycoprotein (gp120)
Rather than facilitating infection of DC with
HIV, DC-SIGN protects HIV by sequestering it
into a protease resistant compartment.
DC-SIGN then transfers HIV-1 to CD4+ cells
causing infection of these cells.
DC-SIGN enhances infection of T cells: at
low virus titers T cells not infected unless DCs
are present (expressing DC-SIGN in trans).
DC-SIGN may transmit other viruses such as
Ebola, CMV, and HCV.
From Kyook et al. Trends in Mol. Medicine 9:163 (2003)
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Antigen Presentation and Dendritic Cells
6. Pathogens exploit dendritic cells as an entry route for infection, cont.:
A. DC-SIGN captures and transfer HIV-1, cont. HIV co-opts antigen-capture function of
immature DC’s to recruit virus into infectious synapses that contact the T cell
Fig. 1: Virus captured at surface. HIV-green; HLA - red
Fig. 2: Recruitment to cell contact sites. virusgreen; mitoch-red. Seen even after protease
treatment suggesting internalization
Fig. 4: Localization of HIV at antigen-indep. contacts
McDonald et al., Science 300:1295, 2003
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Antigen Presentation and Dendritic Cells
7. Pathogens block antigen presentation:
Impairment of MHC class I trafficking by viral gene products:
1. Escape of viral proteins from
proteasomal cleavage. Gly-Ala repeat
in EBNA-1 inhibits proteasomal
proteolysis.
2. Blockade of TAP-transporter.
CMV US-6 inhibits TAP by blocking ATP
binding to TAP without affecting peptide
binding to TAP.
HSV ICP47 blocks peptide-binding
site of TAP.
3. Impaired transport of MHC I.
CMV US3 binds to 2-microglobulin and
causes retention of MHC I-peptide
complex in ER.
From Yewdell and Hill, Nature Immunol. 3: 1019 (2002)
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4. Inhibition of Tapasin. Adenovirus
E3-19k binds to MHC class I and
TAPand inhibits tapasin causing
delayed MHC I maturation.
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Antigen Presentation and Dendritic Cells
7. Pathogens block antigen presentation:
F. Impairment of MHC class I trafficking by viral gene products, cont.:
5. Degradation of MHC I. CMV US 2,
US 11: membrane glycoproteins that
degrade MHC I by promoting ERAD.
HIV-1 Vpu induces degradation of CD4
(which needs to bind to MHC class II for
T cell activation) via ERAD.
6. Diversion to the lysosome. Murine
CMV Gp48 and HHV7 U21 bind to MHC
I in ER and divert it to lysosomes for
degradation.
7. Downregulation of MHC class I on
cell surface. HIV Nef: links MHC I (and
CD4) to clathrin (see PM lecture).
HHV8 K3 protein induces rapid
endocytosis of MHC I.
From Yewdell and Hill, Nature Immunol. 3: 1019 (2002)
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8. Transcriptional effects (not
shown). Adenovirus down-regulates
MHC class I transcription via a
transcriptional repressor.
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Antigen Presentation and Dendritic Cells
8. Additional Reading (Optional):
General Reviews:
1. Mellman, I. And Steinman, R. M. Dendritic cells: specialized and regulated antigen processing machines. Cell 106: 255
(2001)
2. Chow AY, Mellman I. Old lysosomes, new tricks: MHC II dynamics in DCs.Trends Immunol. 26(2):72-8, 2004.
3. Trombetta ES, Mellman I. Cell biology of antigen processing in vitro and in vivo.Annu Rev Immunol. 23:975-1028, 2005.
4. Yewdell J and Hill A. Viral interference with antigen presentation.Nat Immunol. 2002 Nov;3(11):1019-25. Review.
Cross Presentation Reviews:
1. Shen L, Rock KL. Priming of T cells by exogenous antigen cross-presente on MHC class I molecules. Curr Opin
Immunol. 2006 Feb;18(1):85-91.
2. Touret N, Paroutis P, Grinstein S. The nature of the phagosomal membrane: endoplasmic reticulum versus
plasmalemma. J Leukoc Biol. 2005 Jun;77(6):878-85.
3. Gagnon E, Bergeron JJ, Desjardins, M mediated phagocytosis: myth or reality?J Leukoc Biol. 2005 Jun;77(6):843-5.
Primary Articles:
1. McDonald, D. et al., Recruitment of HIV and its receptors to dendritic cell-T cell junctions. Science 300:1295 (2003)
2. Chow, A. et al. Dendritic cell maturation triggers retrograde MHC class II transport from lysosomes to the plasma
membrane. Nature 418: 988 (2002).
3. Houde M et al. Phagosomes are competent organelles for antigen cross-presentation.Nature. 2003 Sep
25;425(6956):402-6.
4. Gagnon E et al., Endoplasmic reticulum-mediated phagocytosis is a mechanism of entry into macrophages.Cell. 2002 Jul
12;110(1):119-31.
5. Trombetta, E et al. Activation of lysosomal function during dendritic cell maturation.Science. 2003 Feb
28;299(5611):1400-3.
6. Delamarre L, Pack M, Chang H, Mellman I, Trombetta ES. Differential lysosomal proteolysis in antigen-presenting cells
determines antigen fate. Science. 2005 Mar 11;307(5715):1630-4.
7. Touret et al., Quantitative and dynamic assessment of the contribution of the ER to phagosome formation.Cell. 2005 Oct
7;123(1):157-70.
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