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Antigen processing and presentation
Monika Raulf
Lecture 25.04.2018
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AP is the display of peptide antigens (created via antigen processing) on the cell surface together with either MHC class I or class II molecules, which permit T cells to recognize antigens on the cell surface of an antigen-presenting cell (APC).
What is Antigen presentation ?
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Which tools are necessary for antigen presentation?
What is antigen processing?
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- APC - T cells
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DC Macrophages B-cell Antigen presenting cells
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Properties of different antigen presenting cells
lymphatic tissue, blood in the periphery
lymphatic tissue, conjunctive tissue, body cavities
lymphatic tissue, conjunctive tissue, epithelia
Localisation
soluble antigens, toxins, viruses
particular antigens, intra- and extra cellular
pathogens
peptides, viral antigens, allergens
Presented antigens
inducible; from − to +++
inducible; from − to +++
constitutive via mellow, not phagocytic, lymphatic
dendritic cells++++
Emission of co-stimulating signals
constitutive increase by activation;
from +++ to ++++
via bacteria and cytokines inducible;
from − to +++
on dendritic tissue cells low; on dendritic cells in lymphatic tissue significantly expressed
MHC-expression
antigen specific receptor (Ig) ++++
phagocytosis +++
+++ macro pino cytosis and phagocytosis by dendritic tissue cells; virus infection
Antigen exposure
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from: „Immunologie“, Janeway et al.
Pathogens and their products are either in the cytoplasm or in the vesicles of the cell
Intra vesicular pathogens
Pathogens in cytosol
Extra cellular pathogens and toxins
Cytoplasm acidified vesicle acidified vesicle
MHC-class I MHC-class II MHC-class II
CD8 T-cells CD4 T-cells CD4 T-cells
Cell death
Activation for killing intra vesicular bacteria and
parasites
Activation of B-cells, Ig to secrete and
extra cellular bacteria or to eliminate toxins
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Why is antigen presentation necessary?
Activation of T-cells* Viruses and some bacteria Parasites
Proliferate intracellular and are not accessible for antibodies
*T cells recognize antigens only in the context of self MHC molecules on the surface of accessory cells
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Key player of the adaptive immunity
T-cells (origin is the thymus)
T-cells are essential for the control of intracellular viruses and for the activation of B-
cell-reactions against most antigens.
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Cell-mediated reactions base on direct interactions between T-lymphocytes and cells, which support the T-cells to recognize an antigen.
T-helper cells [CD4⊕]
Cytotoxic cells [CD8⊕]
In both cases T-cells identify their target cells due to the peptide fragments, which originate from foreign proteins.
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Human leukocyte antigen complex (HLA)
or
Major histocompatibility complex (MHC)
Are membrane surface glycoproteins used by T cells to recognize antigens
(structures necessary for the antigen-specific T cell activation)
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MHC-Genes Important for die compatibility of tissue transplants
⇒ As several, closely connected, very polymorphic genes specify the histocompatibility, the concept „Histocompatibility complex“ is used
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T-cells just recognize peptide fragments, which arise from an foreign antigen, if they are bound to MHC-molecules.
There are 2 classes of MHC-molecules:
MHC I-molecules
MHC II-molecules
⇒ Important distinguishing feature: Origin of the peptide, which is bound to the MHC-molecules and used it for the transport to the cell surface and the presentation
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T-cell-peptide
MHC-molecule
T-cell
APC Proteases Antigen
1. Step: Antigen exposure/-processing
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APC T-lymphocyte
TCR
CD28/ CTLA-4
2. signal
CD 80/86
1. signal
MHC+ peptide
2. Step: Stimulation of the T-cells by an antigen presenting cell Antigen
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MHC = Major Histocompatibility Complex ⇒ Primary function: Binding and presentation of antigenic peptides on the cell surface for binding on antigen specific TCR on T cells.
MHC class I and class II have different structural features regarding the activation of variable T-lymphocytes
MHC I presents peptides for → cytotoxic Tc-lymphocytes
binds on
CD8 express
MHC II presents peptides for → helper TH-lymphocytes
binds on
CD4 express
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Different antigen degeneration and processing ways, which lead to
⇒ „endogenous“ peptides associated with MHC I
⇒ „exogenous“ peptides associated with MHC II
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MHC I: - endogenous synthesised antigens are proteolytic fragmented - small peptide fragments are transported to ER and bind with the arising (labile) MHC I-molecule - MHC I-peptide complex moves through the Golgi apparatus and reaches the cell surface
MHC II: - exogenous peptides, which are internalised from APC, are proteolytic fragmented - Peptide fragments are compartmentalised in the endosome and - associate to the MHC II-molecule; complex reaches the cell surface
⇒ Presentation → CD8-cells
⇒ Presentation → CD4-cells
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The size of the antigen peptides, which reside in the „binding bag“ of MHC class I or class II, is different:
„MHC I-peptide“ → 8 – 10 AS
Different structures, as varying α- and β-polypeptide subunits exist, which maturate to αβ-heterodimer:
→ MHC I: α-UE (is coded of polymorphic MHC-genes) β-UE (not MHC-gene coded)
β2 microglobulin (β2m), very conserved → MHC II: α-UE
β-UE MHC-gene coded
αβ-dimer → tends to associate with (αβ)2
„MHC II-peptide“ → 13 – 18 AS
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MHC binds peptides
Class I Class II
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Differentiated cell specific expression of class I- and class II-molecules correlates with the specialisation of the immune cells → MHC I-expression is wide-spread on almost every somatic cell
cytotoxic T-cells have a protective function
→ MHC II-expression is exclusively on APC
local activation of helper T-cells
Important feature: Multi-peptide-binding
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MHC class I- and class II- molecules
Structure of the MHC class I-molecule
Structure of the MHC class II-molecule
extra cellular domain*2
Constituted of α-chain + β2-microglobulin*1
*1 12 kD; not polymorph *2 belongs to the immunoglobulin superfamily
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Genetic organisation of the main (major) histocompatibility complex in humans
HLA-locus (human leucocyte antigen) In humans on chromosome 6
MHC-molecules are polymorphic transmembrane glycoproteins
e.g. TAP 1, 2
β2-microglobulin (is part of the MHC-class I-molecules, but not located on chromosome 6)
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Antigen processing and antigen presentation
MHC-molecules are instable without bound peptides; For MHC I-presentation important proteins/structures: • TAP1 + TAP2 (Transporters associated with antigen processing 1/2); TAP1:TAP2-complex is an ATP-dependent peptide transporter • Proteasome (multi catalytic protease complex) (Subunits of the proteasome (LMP2 + LMP7) are coded near the TAP1- and TAP2-genes in MHC; also their expression is inducible by IFN (IFN is virus-inducible)) • Proteins with chaperon-like functions (Calreticulin) („cellular quality management“) MHC I-class I-molecules leave the ER only, if they have bound peptides
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Binding of the peptide (break up of proteins in the proteasome and transport via TAP into ER) to the complex induces the complete folding and the peptide-MHC complex leaves the ER and will be transported via Golgi-
apparatus to the cell surface
in ER: MHC I-α-chains store themselves together with calnexin (membrane-bound)
Binding of the MHC I-α/calnexin-complex to β2-microglobin (β2m)
New MHC I-αβ2m-dimer breaks away from calnexin
⇓
Complex in correlation with tapasin (is TAP-associated)
Chaperon molecules calreticulin + Erp57 contribute to the stability of the complex
MHC I-molecules exist in a cell abundantly, therefore in case of a viral infection the cell is able to react very fast
⇓
⇓
⇓
⇓
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Extra-cellular proteins or pathogens, which reproduce intra-cellular (parasites, mycobacteria)
⇓ Affiliation in intra-cellular vesicle (endosomes)
⇓ Decline of the pH-value in the endosomes = Acidification effects the
activation of the proteases (acid proteases*) ⇓
Fusion of the endosomes during the migration into the cell interior with the lysosomes
⇓ Division of the protein antigens + binding on MHC II-molecules of the ER
are conducted by the invariant chain(Ii) to the acidified vesicles
Peptides, that are presented of MHC II-molecules, arise in acidified endocytotic vesicles
*e.g. Cysteine proteases as Cathepsin B, D and S as well as L
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Antigen presentation via MHC II-molecules Important components: Invariant chain (Ii)
a) binds to the MHC II αβ-heterodimer; verifies the binding line for peptides
⇓ prevents the configuration by peptides in ER; in ER are the single components associated to calnexin ⇓
complete complex breaks away from calnexin and will be transported from the ER ⇒ Peptides of ER cannot be bound ⇓
b) provides for the transport of the complex to an endosomal compartment (probably MIIC = MHC II-compartment)
⇓ here Ii will be cut by proteases ⇓
it remains a short Ii-piece = CLIP
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CLIP = class II-associated invariant chain peptide
Still eliminates the loading with the peptide
HLA-DM - binding and stabilisation of empty MHC II-molecules
- catalyses formation of the CLIP-fragment
- analogy to TAP-molecules at MHC I-molecules
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The MHC-II way of presentation
Invariant chain with CLIP-fragment
Antigen
MHC-II
EE IE
Endosome
ER Antigen presenting cell
Golgi MIIC
Receptor mediated
Phagocytosis
proteolytic enzymes
EE = early endosomes IE = intermediate endosomes
ER = endoplasmatic reticulum MIIC = MHC-loaded compartment
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Different features of the MHC-molecules MHC class I MHC class II
Expression upon all nucleated cells of the body
upon professional antigen presenting cells
Activation of CD8+ T-cells of CD4+ T-cells
Genloci HLA-A, HLA-B, HLA-C HLA-DR, HLA-DP, HLA-DQ
Structure Transmembrane constant α-chain associated with
β2-microglobulin
Transmembrane constant heterodimer of the α- and β-
chain
Peptides max. 8-10 amino acids long
at least 13 amino acids long
Loading in the endoplasmic reticulum in the intra-cellular vesicle
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Peptides bound to MHC I-class stimulate CD8-cells
⇒ Strategies of viruses to prevent the mechanism of the antigen presentation: • Herpes simplex-virus: prevents the transport of viral peptide in the ER by the production of a protein that binds on the TAP-transporter and blockades it
• Adenovirus: synthesizes a protein, that can retain MHC I-molecules in the ER
• Cytomegalic virus precipitates the retrograde transport (HCMV): of MHC I-molecules back in the cytosol, where they are degraded
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The role of T-effector cells in the cellular and humoral immune response against different disease agents
Cell-mediated immunity Humoral immunity
from: „Immunologie“, Janeway et al.
Typical pathogens
Vaccinia virus, Influenza virus, Rabies virus,
Listeria
Mycobacterium tuberculosis
Mycobacterium leprae Leishmania donovani, Pneumocystis carinii
Clostridium tetani Staphylococcus aureus
Streptococcus pneumoniae Poliovirus
Pneumocystis carinii
Localisation Cytosol Vesicle of macrophages
Extra cellular fluid
T-effector cell Cytotoxic
CD8-T-cell TH1-cell TH2/TH1-cell
Antigen recognition
Mode of action of the effector
Peptide: MHC-class-I on
infected cell
Peptide: MHC-class-II on
infected macrophages
Peptide: MHC-class-II on antigen
specific B-cell
Killing of the infected cell
Activation of infected macrophages
Activation of specific B-cells to synthesise
antibodies
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Antigens, which are recognized by T-cells, have two different „Interaction areas“
Class II-MHC
Agretope
Epitope TCR
→ Agretope interacts with the MHC-molecule
→ Epitope interacts with the TCR