Antigen processing and presentation Monika Raulf Lecture 25.04.2018
What is Antigen presentation? 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). Monika Raulf - Antigen processing and presentation 2
Which tools are necessary for antigen presentation? What is antigen processing? Monika Raulf - Antigen processing and presentation 3
- APC - T cells Monika Raulf - Antigen processing and presentation 4
Antigen presenting cells DC Macrophages B-cell Monika Raulf - Antigen processing and presentation 5
Properties of different antigen presenting cells Antigen exposure +++ macro pino cytosis and phagocytosis by dendritic tissue cells; virus infection phagocytosis +++ antigen specific receptor (Ig) ++++ MHC-expression on dendritic tissue cells low; on dendritic cells in lymphatic tissue significantly expressed via bacteria and cytokines inducible; from to +++ constitutive increase by activation; from +++ to ++++ Emission of costimulating signals constitutive via mellow, not phagocytic, lymphatic dendritic cells++++ inducible; from to +++ inducible; from to +++ Presented antigens peptides, viral antigens, allergens particular antigens, intra- and extra cellular pathogens soluble antigens, toxins, viruses Localisation lymphatic tissue, conjunctive tissue, epithelia lymphatic tissue, conjunctive tissue, body cavities lymphatic tissue, blood in the periphery Monika Raulf - Antigen processing and presentation 6
Pathogens and their products are either in the cytoplasm or in the vesicles of the cell Pathogens in cytosol Intra vesicular pathogens 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 from: Immunologie, Janeway et al. Activation for killing intra vesicular bacteria and parasites Activation of B-cells, Ig to secrete and extra cellular bacteria or to eliminate toxins Monika Raulf - Antigen processing and presentation 7
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 Monika Raulf - Antigen processing and presentation 8
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. Monika Raulf - Antigen processing and presentation 9
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. Monika Raulf - Antigen processing and presentation 10
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) Monika Raulf - Antigen processing and presentation 11
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 Monika Raulf - Antigen processing and presentation 12
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 Monika Raulf - Antigen processing and presentation 13
1. Step: Antigen exposure/-processing Antigen APC Proteases T-cell-peptide MHC-molecule T-cell Monika Raulf - Antigen processing and presentation 14
2. Step: Stimulation of the T-cells by an antigen presenting cell Antigen 1. signal T-lymphocyte TCR MHC+ peptide APC CD28/ CTLA-4 CD 80/86 2. signal Monika Raulf - Antigen processing and presentation 15
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 T c -lymphocytes binds on CD8 express MHC II presents peptides for helper T H -lymphocytes binds on CD4 express Monika Raulf - Antigen processing and presentation 16
Different antigen degeneration and processing ways, which lead to endogenous peptides associated with MHC I exogenous peptides associated with MHC II Monika Raulf - Antigen processing and presentation 17
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 Presentation CD8-cells 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 CD4-cells Monika Raulf - Antigen processing and presentation 18
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 MHC II-peptide 13 18 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) MHC II: α-ue β-ue β 2 microglobulin (β 2 m), very conserved MHC-gene coded αβ-dimer tends to associate with (αβ) 2 Monika Raulf - Antigen processing and presentation 19
MHC binds peptides Class I Class II Monika Raulf - Antigen processing and presentation 20
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 Monika Raulf - Antigen processing and presentation 21
MHC class I- and class IImolecules extra cellular domain *2 Structure of the MHC class I-molecule Constituted of α-chain + β2- microglobulin *1 Structure of the MHC class II-molecule *1 12 kd; not polymorph *2 belongs to the immunoglobulin superfamily Monika Raulf - Antigen processing and presentation 22
Genetic organisation of the main (major) histocompatibility complex in humans HLA-locus (human leucocyte antigen) In humans on chromosome 6 e.g. TAP 1, 2 MHC-molecules are polymorphic transmembrane glycoproteins β2-microglobulin (is part of the MHC-class I-molecules, but not located on chromosome 6) Monika Raulf - Antigen processing and presentation 23
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 Monika Raulf - Antigen processing and presentation 24
in ER: MHC I-α-chains store themselves together with calnexin (membrane-bound) Binding of the MHC I-α/calnexin-complex to β 2 -microglobin (β 2 m) New MHC I-αβ 2 m-dimer breaks away from calnexin Complex in correlation with tapasin (is TAP-associated) Chaperon molecules calreticulin + Erp57 contribute to the stability of the complex 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 Golgiapparatus to the cell surface MHC I-molecules exist in a cell abundantly, therefore in case of a viral infection the cell is able to react very fast Monika Raulf - Antigen processing and presentation 25
Peptides, that are presented of MHC II-molecules, arise in acidified endocytotic vesicles Extra-cellular proteins or pathogens, which reproduce intra-cellular (parasites, mycobacteria) Monika Raulf - Antigen processing and presentation 26 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 *e.g. Cysteine proteases as Cathepsin B, D and S as well as L are conducted by the invariant chain(ii) to the acidified vesicles
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 Monika Raulf - Antigen processing and presentation 27
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 Monika Raulf - Antigen processing and presentation 28
The MHC-II way of presentation Antigen MHC-II proteolytic enzymes Invariant chain with CLIP-fragment Phagocytosis Endosome Receptor mediated EE Antigen presenting cell IE ER MIIC Golgi EE = early endosomes IE = intermediate endosomes ER = endoplasmatic reticulum MIIC = MHC-loaded compartment Monika Raulf - Antigen processing and presentation 29
Different features of the MHC-molecules Expression MHC class I upon all nucleated cells of the body MHC class II 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 Loading max. 8-10 amino acids long in the endoplasmic reticulum at least 13 amino acids long in the intra-cellular vesicle Monika Raulf - Antigen processing and presentation 30
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: Cytomegalic virus synthesizes a protein, that can retain MHC I-molecules in the ER precipitates the retrograde transport (HCMV): of MHC I-molecules back in the cytosol, where they are degraded Monika Raulf - Antigen processing and presentation 31
The role of T-effector cells in the cellular and humoral immune response against different disease agents Cell-mediated immunity Humoral immunity 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 T H 1-cell T H 2/T H 1-cell Antigen recognition Peptide: MHC-class-I on infected cell Peptide: MHC-class-II on infected macrophages Peptide: MHC-class-II on antigen specific B-cell Mode of action of the effector Killing of the infected cell from: Immunologie, Janeway et al. Activation of infected macrophages Activation of specific B- cells to synthesise antibodies Monika Raulf - Antigen processing and presentation 32
Antigens, which are recognized by T-cells, have two different Interaction areas Agretope interacts with the MHC-molecule Epitope interacts with the TCR Class II-MHC Agretope TCR Epitope Monika Raulf - Antigen processing and presentation 33