Topic 5
Antigen Processing
Adaptive Immune Response requires Time
Responses to infection -- T cell component
Innate immunity Recognition by Removal of (0-4 hours) Infection pre-formed, non- infectious agent specific effectors
Early induced Recognition and Recruitment of activation of Removal of response Infection infectious agent (4-96 hours) effector cells effector cells
Late adaptive Transport of Recognition Clonal expansion and differentiation Removal of response Infection antigen to by naïve infectious agent >96 hours) lymphoid organs B and T cells to effector cells
Protective Recognition by Removal of immunity Infection pre-formed, Ab infectious agent and T cells
Recognition by Rapid expansion Immunological and differentiation Removal of memory Infection memory B cells infectious agent and T cells to effector cells
The adaptive immune response involving antigen-specific T cells and B cells is only one part of the immune response and is required to protect against pathogens. A pathogen is by definition an organism that can cause disease. In other words, a pathogen is an organism that can bypass innate immunity and requires an adaptive immune response for clearance.
Page ‹#› Adaptive Immune Response
What you should know by the end of this lecture
• That T and B cells recognise antigen differently • Antigen processing generates antigenic peptides • That antigen processing can take place in lysosomes • That there is an endogenous mechanism of antigen processing • That there is an exogenous mechanism of antigen processing • Antigen processing includes uptake, degradation, complex formation and presentation • The role of invariant chain HLA-DM and CLIP in antigen processing • The role of the proteasome and transporters in antigen processing • Other items covered in summary slides
T Cells DO NOT Recognize Native Antigens
B B
Y
B Y
Y B Y
B Y B Y B B Y B Y Y Y Y Y Y Y Y Y
Cross-linking of Proliferation and surface membrane antibody production Ig
T T No proliferation Y Y No cytokine release
Page ‹#› Antigens Must Be Processed in Order to be
Recognized by T Cells T Y
Cell surface peptides of Ag presented by cells that express MHC antigens Soluble Soluble native Ag peptides Cell surface of Ag Cell surface native Ag peptides of Ag
ANTIGEN PROCESSING
No T cell No T cell No T cell No T cell T cell response response response response response
The recognition of antigens by T cells and B cells is fundamentally different. Because B cells bind antigen that is free in solution, the epitopes they recognize tend to be highly accessible sites on the exposed surface of the immunogen. T-cell epitopes are peptides combined with MHC molecules. Thus, there is no requirement for solution accessibility such as B-cell epitope.
Differing Properties of Epitopes
Page ‹#› Mammalian Toll-like Receptors
PPR - Pattern recognition receptor Sense the presence of general PAMP - pathogen associated molecular pattern characteristics of invader TLR4 - lipopolysaccharide TLR7 - ssRNA
Antigen Presenting Cells
Cells that display peptides associated with class II MHC + molecules to CD4 TH cells are called antigen presenting cells (APCs) Cells that display peptides associated with class I MHC to CD8+ TC cells are called target cells There are both professional (cells that express MHC II and costimulatory molecules) and non-professional APCs
Interplay of Innate & Adaptive Immune Repsonse
Dendritic cells are the “bridge”
Page ‹#› Professional Antigen-Presenting Cells
Why is Antigen Presentation Necessary?
Focus attention of killer T cells on infected cells, rather than extracellular antigens Without MHC I display, pathogen infected cells would go undetected since most proteins made by pathogens remain inside the cell and never make it to the cell surface
Both APC and TH cells must “agree” that there is an ensuing battle before the helper cell can be activated, ensuring that the decision to deploy adaptive response is not made by a single cell.
The T cell Antigen Receptor
Antigen Resembles an Ig Fab fragment combining site V VH H V VL L Fab CH CH CL CL C C Vα Vβ H H Fc CH CH Domain structure: Ig gene superfamily Carbohydrates Monovalent Cα Cβ No alternative constant regions Hinge Never secreted Heterodimeric, chains are disuphide- + bonded + + Very short intracytoplasmic tail Positively charged amino acids in the TM Transmembrane region region Cytoplasmic tail Antigen combining site made of juxtaposed Vα and Vβ regions 30,000 identical specificity TcR per cell
Page ‹#› MHC structures
Peptide binding cleft Peptide binding cleft
α2 α1 α1 β1
α3 β2-microglobin α2 β2
Class I MHC Class II MHC
• MHC classes I and II have an almost identical 3-D structure. • Both classes of MHC are polygenic (each cell has many MHC genes) and polymorphic (there are many alleles for each locus), but the MHC genes do not undergo recombination. Note: Human MHC are called HLA (human leukocyte antigen).
T cell co-receptor molecules
Lck PTK TcR Lck PTKTcR
CD8 α3 CD4 β2 β α
MHC Class I MHC Class II CD4 and CD8 can increase the sensitivity of T cells to peptide antigen MHC complexes by ~100 fold
IgG Superfamily
Page ‹#› IgG Superfamily
Processing Pathways
(Endogenous)
Non-lysosomal
(Exogenous)
Proteasome Degradation
Page ‹#› Changes in Proteasome
Endogenous Pathway
Endogenous Antigen Processing
UPTAKE Antigens/pathogens already present in cell DEGRADATION Antigens synthesized in the cytoplasm undergo limited proteolytic degradation in the cytoplasm ANTIGEN-MHC COMPLEX FORMATION Loading of peptide antigens onto MHC class I molecules is different to the loading of MHC class II molecules PRESENTATION Transport and expression of antigen-MHC complexes on the surface of cells for recognition by T cells
Page ‹#› Exogenous Pathway
MCH II processing
Summary of Exogenous Antigen Processing
• T cells can not recognize native antigens
• Antigens must be processed for recognition by T cells
• Antigens catabolism occurs inside cells
• Catabolism reduces antigens to peptides
• Only metabolically active cells can process antigen (active process)
• Antigen processing involves the lysosomal system
• Because extracellular antigens are dealt with by the lysosomal system, lysosomal antigen processing is part of the EXOGENOUS antigen processing pathway
Page ‹#› Comparison of Presentation Pathways
Stages of Endogenous and Exogenous Antigen Processing
UPTAKE Access of native antigens and pathogens to intracellular pathways of degradation
DEGRADATION Limited proteolysis of antigens to peptides
ANTIGEN-MHC COMPLEX FORMATION Loading of peptides onto MHC molecules
ANTIGEN PRESENTATION Transport and expression of peptide-MHC complexes on the surface of cells for recognition by T cells
Crossover Pathway
• Refers to the presentation of exogenous antigens on MHC class I molecules. • Exact mechanism is WAY unclear; two models have been proposed: recycling and ER loading.
Page ‹#› Summary
• T and B cells recognize antigen differently • Antigen must be catabolized before T cells can recognize it • Antigen processing generates antigenic peptides • Exogenous antigen processing takes place in lysosomes • Endogenous processing is non-lysosomal • The mechanism of antigen processing depends upon the compartment in which the pathogen replicates • Endogenous and exogenous antigen processing both involve uptake, degradation, complex formation and presentation • Exogenous antigen processing uses invariant chain and HLA-DM • Endogenous antigen processing uses proteasomes and peptide transporters in antigen processing • Pathogens can evade immunity by disrupting antigen processing
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