ANTIGEN PROCESSING AND PRESENTATION

1. Mechanisms of antigen processing and presentation.

2. How cellular and molecular interactions in antigen processing lead to activation of T cell immunity.

Antigen (HEL)bound to the Fab portion of Anti-HEL Antibody.

Fab: antigenbinding fractionof antibody

T CELLS (IN CONTRAST TO B CELLS) RECOGNIZE AN ANTIGEN ONLY WHEN ITS FRAGMENT IS BOUND WITHIN THE GROOVE OF MAJOR HISTOCOMPATIBILITY COMPLEX (MHC) MOLECULE

T CELLS RECOGNIZE AN ANTIGEN ONLY WHEN ITS FRAGMENT IS BOUND WITHIN MHC GROOVE

Peptide-MHC Class II

complex

TCR

MHC-peptidecomplex

TCR-Ligand Interactions

CDR3 region

1. A central event in generation of both humoral and

cell-mediated immune response is activation of helper T lymphocytes.

2. T lymphocytes interact with specific antigen via their T cell receptors.

3. T cell receptors are unable to recognize a soluble antigen.

Why do antigens need to be processed and presented in order to activate an immune

response ?

4. A prerequisite for recognition by T cell receptors is that an antigen fragment, in the form of peptides, be bound within the groove of an MHC molecule.

5. Thus, in order to induce a humoral or a cell-mediated (T helper or T cytotoxic) immune response, it is required that an antigen or a pathogen be degraded by a series of biochemical events called antigen processing.

6. The antigenic peptides bound within the antigen binding groove of MHC need to be expressed on the APC cell surface to interact with T cell receptors.

Why do antigens need to be processed and presented in order to activate an immune

response ?

Antigen= any molecule or pathogen that stimulates an immune response. e.g., pathogen proteins or carbohydrates foreign to host.

Antigen processing= degradation of pathogens and proteins intopeptides that can bind to MHC molecules for presentation to T cells.

Antigen Presenting cells (APC) =highly specialized cells that can process antigens and display their peptide fragments on the cell surface together with molecules required for T cell activation.

The main APCs for T cells are dendritic cells, macrophages, and B cells.

MHC class I and II= Major Histocompatibility Complex class I and II, a set of highly polymorphic membrane glycoproteins that present peptides to T cells. The recognition of MHC molecules loaded withpathogen-derived peptides by T cells initiates the immune response.

Short Glossary

Steps involved in Antigen Processing and presentation

1. Fragmentation of pathogens into proteins and then into peptides

2. Association of peptides with an MHC molecule

3. Transport to cell surface for expression.

4. Different cellular pathways exist for association of peptides with either MHC Class I or class II molecules.

Antigen Processing

Exogenous and Endogenous Antigens andPathways of Antigen Presentation

1. Pathogens and antigens can be extracellular (Exogenous) or intracellular (Endogenous). Exogenous antigens are usually eliminated by antibody response.

2. Exogenous antigens/pathogens are internalized by APC using phagocytosis, micropinocytosis or receptor-mediated endocytosis and brought into to endosomal vesicles and then displayed on the cell surface for activation of helper T cells.

3. Examples of exogenous antigens are bacteria, allergens, protein antigens.

4. Intracellular infectious agents are endogenous antigens and can exist and replicate in either (a) cytosol or (b) endocytic vesicles.

5. Pathogens/antigens that are either (i) internalized (though were originally extracellular) into the endosomal vesicles and (ii) those that reside within the endocytic vesicles are processed and presented via the Exogenous or Vesicular (sometimes also called endocytic pathway) Pathway of antigen presentation.

Exogenous and Endogenous Antigens andPathways of Antigen Presentation

6. Pathogens/antigens that are either synthesized (e.g. viral proteins) or reside within the cytoplasmare processed and presented via the Endogenous Pathway of antigen presentation.

Exogenous and Endogenous Antigens andPathways of Antigen Presentation

The peptides produced in the Endocytic Pathway are brought to the cell surface by MHC class II molecules and are displayed to CD4 T cells.

The Endocytic Pathway of antigen presentationis thus responsible for antigen specific activation ofCD4 T helper cells.

Exogenous (or endocytic) Pathway of Antigen Presentation

Endogenous Pathway of Antigen Presentation

The peptides produced in the Endogenous Pathway are brought to the cell surface by MHC class I molecules and are displayed to CD8 T cells.

The Endogenous Pathway of antigen presentationis thus responsible for antigen specific activation ofCD8 T cytotoxic cells.

The antigens in this pathway are usually those that happento be produced in the cytosol or reside in the cytosol.

Figure 5-2

Antigen Presenting Cells (APC)

APC are cells that degrade the foreign antigen into peptides.

After active metabolic processing of antigens, they present the peptides within the antigen binding grooves of either class I or MHC class II molecules.

They provide a co-stimulatory signal to T cells for activation.

Professional antigen presenting cells: Constitutively express high amounts of MHC molecules (especially MHC class II molecules), and co-stimulatory molecules, e.g. dendritic cells.

Target cells: APC expressing MHC class I bound antigenic peptides to cytotoxic CD8 T cells are often called target cells. All cells can be target cells. Professional APC are extremely efficient target cells due to effective co-stimulation.

Figure 8-18 part 1 of 2

Figure 8-18 part 2 of 2

Facultative Antigen Presenting Cells

Phagocytosis Type Location Class II expression

+ astrocytes brain inducible

follicular cells thyroid inducible -/+

endothelium vascular and - to inducible(++)lymphoid tissue

fibroblasts connective tissue - to inducible (++)

These can be induced to present antigens

MHC class II pathway (exogenous pathway)of antigen processing

The Endocytic (Exogenous) Pathway of antigen Presentation

Leads to presentation of MHC II bound peptides to CD4 T cells.

Antigen is processed within the endocytic vesicles.

MHC II traffics through the endocytic pathway and binds antigenic peptides enroute to the cell surface of an APC.

Trafficking of MHC and antigen within the endocytic pathway

Antigen presentation by B cells: the same as other APC exceptB cells take up antigen via Ig receptors and are thus can bemore efficient.

MHC class I pathway(endogenous pathway)of antigen presentation

The Endogenous Pathway of antigen Presentation

Leads to presentation MHC I bound peptides to CD8 T cells.

Antigen is processed within the cytosol (proteasomes).

Processed peptides are transported across the ER membrane.

MHC I-peptide complexes are formed in the ER and traffic through the conventional secretory pathway to eventually be expressed on the cell surface.

MHC class I pathwayPeptides for transport into ER are generated in cytosol by degradation carried out by a large multicatalytic protease complex-PROTEASOME

The structure of proteasome

Peptides that bind to MHC class I molecules are actively transported from the cytosol to the ER.

Transporters associated with Antigen Processing (TAP)TAP1 and TAP2 form a peptide transporter in the ER membrane.Mutations in either gene can prevent Ag presentation by MHC I.

Expression of peptide-MHC class I complex on the surface of a target cells

Non-protein antigens are presented by CD1 molecules

Co-stimulatory and adhesion molecules required for successful T cell-APC interactions.

Lymphocyte Activation

Absolute requirement for activation of T helper cells leading to cytokine production: (i) Specific interaction of TCR with MHC II-peptide complex and (ii) non-specific interaction of CD28 with B7 molecules.

CD4 T helper cells can be separated into 2 different subsets of T helpercells: TH1 and TH2 depending on the set of cytokines they secrete.

CONCLUSIONS

Pathogens and antigens need to be partially degraded into peptides before they bind MHC molecules and are presented on the surface of an APC.

Newly synthesized MHC molecules traffic through either the endocytic pathway (MHC class II) or the conventional secretory pathway (MHC class I) and bind the antigen derived peptide enroute to the cell surface.

At a minimum, activation of T cells requires a specific signal via the TCR-MHC-peptide complex and a non-specific signal via CD28-B7 molecules.