3: Antigen Capture and Presentation of Lymphocytes

The capture and display of microbial antigens: dendritic cells

Step 1: Microbes enter through different parts of the body, including the skin, gastrointestinal tract, and respiratory tract.

Step 2: Microbes and their antigens are present near the epithelium and in the connective tissue beneath it.

Step 3: Antigens in the tissue are captured by antigen-presenting cells, like a dendritic cell.

Step 4: Dendritic cell-associated antigens move into lymphatic vessels in the connective tissue.

Step 5: The lymphatic vessels transport the antigens toward a lymph node.

Step 6: Protein antigens are displayed for recognition by T cells

Step 6: The lymph node collects antigens from the tissue

The capture and display of microbial antigens: cell-free antigen

Step 1: Microbes enter through different parts of the body, including the skin, gastrointestinal tract, and respiratory tract.

Step 2: Microbes and their antigens are present near the epithelium and in the connective tissue beneath it.

Step 3: Free-cell antigens are present in the tissue

Step 4: Some antigens enter the bloodstream and travel through a venule.

Step 5: The blood carries these antigens through the circulation toward the spleen.

Step 6: Protein antigens are displayed for recognition by T cells

Step 7: Antigen-presenting cells in the spleen capture the blood-borne antigens.

Where do immature dendritic cells reside?

Epithelia

Where do mature dendritic cells reside?

T-cell-rich areas of the lymph nodes and the spleen

What makes dendritic cells special?

• They can travel!

  • They capture antigens and take them to the training lymph nodes

• They are the bridge between innate and adaptive immunity

What is the major location of classical dendritic cells?

Tissues

What is the major location of plasmacytoid dendritic cells?

Blood and tissues

What are the expression of toll-like receptors in classical dendritic cells?

TLRs 4, 5, 8 high

What are the expression of toll-like receptors in plasmacytoid dendritic cells?

TLRs 7, 9 high

What are the major cytokines produced by classical dendritic cells?

TNF, Il-6, IL-12, and IL-23

What are the major cytokines produced by plasmacytoid dendritic cells?

Type 1 interferons

Assumed major functions of classical dendritic cells?

Induction of T cell response against most antigens

Assumed major functions of plasmacytoid dendritic cells?

ANTIVIRAL INNATE IMMUNITY and induction of T cell responses against viruses

Process: capture and presentation of protein antigens by dendritic cells

1. Immature dendritic cells in the epithelium capture microbial antigens.

2. Macrophages and epithelial cells in tissues encounter microbes.

3. These cells respond by secreting TNF and IL-1 which lead to the DCs to round up and lose their adhesiveness for the epithelium.

4. The dendritic cells migrate to draining lymph nodes, being attracted there by chemokines produced in the nodes.

5. During their migration, and probably in response to the microbe, the dendritic cells mature

6. Once in the lymph nodes, the dendritic cells present antigens to naive T lymphocytes.

What do surface receptors of immature dendritic cells express?

Immature dendritic cells express surface receptors that capture microbial antigens

What do surface receptors of mature dendritic cells express?

Mature dendritic cells express high levels of MHC molecules and costimulators, which function to stimulate T cells.

How does a T- cell receptor (TCR) recognize an antigen?

• A T cell receptor (TCR) recognizes a complex of a peptide antigen displayed by a major histocompatibility (MHC) molecule.

  • MHC molecules are expressed on antigen-presenting cells and function to display peptides derived from protein antigens.

• Peptides bind to the MHC molecules by anchor residues, which attach the peptides to pockets in the MHC molecules.

• The TCR of every T cell recognizes some residues of the peptide and some (polymorphic) residues of the MHC molecule.

What does the TCR recognize?

Some residues of the peptide and some polymorphic residues of the MHC molecule.

What do MHC molecules display?

Peptides derived from protein antigens

How do peptides bind to MHC?

Anchor residues attach the peptide to pockets in the MHC molecule.

Where are MHC molecules expressed?

On antigen-presenting cells.

What is the main function of the TCR?

Recognize a peptide antigen displayed by an MHC molecule.

How do MHC and T cell receptors bind?

MHC binds to a T-cell receptor & the T-cell receptor binds to MHC

M.H.C.

Major Histocompatibility Complex

• A molecule on antigen-presenting cells that holds and displays pieces of proteins (peptides) so T cells can recognize them.

Class-I MHC

Expressed on all nucleated cells in man, and also on erythrocytes in mice

Class-II MHC

Expressed primarily on antigen-presenting cells (dendritic cells, macrophages, and B cells)

• These cells also have Class-I MHC because they are nucleated

Principal function of dendritic cells?

Antigen presentation to naive T cells in the initiation of T cell responses to protein antigens (priming)

Principal function of macrophages?

Antigen presentation to CD4+ effector T cells in the effector phase of cell-mediated immune response

Principal function of B lymphocytes?

Antigen presentation to CD4+ helper T cells in humoral immune response (T cell-B cell interactions)

HLA

Human MHC ((Human Leukocyte Antigen)

What are the Class II MHC locus?

• DP

• DQ

• DR

What are the Class I MHC locus?

• B

• C

• A

What do the MHC loci do?

The MHC locus is a section of DNA that contains the instructions for proteins that help the immune system recognize antigens and fight infections.

ex:

• MHC Class I locus: Contains genes that make MHC Class I molecules.

What is the most polymorphic of the polymorphic loci?

The MHC

Why is it good at the MHC is so polymorphic?

The job of the MHC is to bind to antigens. Therefore, as a whole population, we will always be able to fight a microbe because some will be able to fight it strongly.

Why is the MHC so polymorphic?

The MHC 1 & 2 in everyone are so different because our genes come from our parents

• codominant!

Where does peptide-binding occur in a Class I MHC?

The antigen binds between the alpha 1 and alpha 2

Where does peptide-binding occur in a Class II MHC?

The antigen binds between alpha 1 and beta 1

What is the basic structure of MHC Class I?

One large α (alpha) chain.

One smaller protein called β₂-microglobulin.

What is the basic structure of MHC Class II?

• One α (alpha) chain.

• One β (beta) chain.

Properties of MHC molecules

• Polymorphic genes

• Co-dominant expression

• Class I and Class II

Polymorphic genes of MHC

Many different alleles are present in the population

• Individuals are able to respond to different microbes

Co-dominant expression of MHC

Both parents’ alleles of each MHC gene are expressed

• Increases the number of different MHC molecules that can present peptides

Significance of Class II MHC

CD4+ helper T cells interact with dendritic cells, macrophages, and B cells

Significance of Class I MHC

CD8+ CTLs can kill any type of virus-infected cell

How many peptides do MHC molecules display at a time?

Each MHC molecule displays one peptide at a time

• Each T cell responds to a single peptide bound to an MHC molecule

What do MHC molecules bind to?

MHC molecules bind only to peptides

• MHC-restricted T cells respond mainly to protein antigens

• Lipids, carbohydrates, and nucleic acids cannot bind

Where do Class I and II MHC display their peptides from?

• MHC Class I gets peptides from proteins in the cytosol.

• MHC Class II gets peptides from proteins in the endosome/lysosome.

What does MHC need to be stable on the cells surface?

MHC molecules need a peptide attached to them to stay stable on the cell surface.

• If a peptide is not attached, the MHC molecule is unstable and does not remain properly expressed on the cell surface.

What is the off-rate like on an MHC?

It is very slow.

• Once a peptide binds to an MHC molecule, it stays attached for a long time to be recognized by a T cell

What is antigen processing?

Antigen processing is the process of breaking proteins into smaller pieces called peptides so they can be displayed by MHC molecules.

What is the main difference between the Class I and Class II Ag processing pathways?

They process proteins from different places inside the cell and display the resulting peptides using different MHC molecules.

• Class II: Processes extracellular proteins.

• Class I: Processes cytosolic proteins.

Class I MHC Ag Processing

• The MHC Class I pathway begins with a protein located in the cytosol, which is the fluid inside the cell

• The cytosolic protein is broken down into smaller peptides by a structure called the proteasome

• These peptides are now located in the cytosol.

• The peptides in the cytosol are transported into the ER by a protein called TAP

• Once the peptide enters the ER, it can bind to the MHC Class I molecule.

• The peptide-MHC Class I complex is displayed to a CD8⁺ cytotoxic T cell (CTL).

Class II MHC Ag Processing

• The MHC Class II pathway begins with a protein located outside the cell, called an extracellular protein.

• The cell takes in the extracellular protein through a process called endocytosis.

• The protein is broken down into smaller pieces called peptides located in the endosome/lysosome

• The peptide produced from the extracellular protein binds to the MHC Class II molecule.

• The peptide-MHC Class II complex is displayed to a CD4⁺ T cell.

Invariant chain (Ii)

Invariant chain (Ii) is a protein that temporarily binds to MHC Class II while it is being made in the ER.

• The invariant chain acts like a cover that blocks the peptide-binding groove of MHC Class II until the MHC molecule reaches the right compartment to bind an antigen peptide.

The Class I MHC Pathway of Processing Antigens

1. Proteins enter the cytoplasm of cells either from phagocytosed microbes or from endogenous synthesis by microbes, such as viruses, that reside in the cytoplasm of infected cells.

2. Cytoplasmic proteins are un-folded, ubiquitinated, and degraded in proteasomes.

3. The peptides that are produced are transported by the TAP transporter into the endoplasmic reticulum (ER), where the peptides bind to newly synthesized class I MHC molecules.

4. The peptide-class I MHC complexes are transported to the cell surface and are recognized by CD8+ T cells

The Class II MHC Pathway of Processing Antigens

1. Protein antigens are ingested by APCs into vesicles, where they are degraded into peptides.

2. Class II MHC molecules enter the same vesicles and lose the CLIP peptide that occupies the cleft of newly synthesized class II molecules.

3. These class II molecules are able to bind peptides derived from the endocytosed protein.

4. The peptide-class II MHC complexes are transported to the cell surface and are recognized by CD4+ T cells

What are MHC Class I recognized by?

CD8⁺ cytotoxic T cells (CTLs)

What are MHC Class II recognized by?

CD4⁺ helper T cells

What is the main function of MHC Class I?

MHC Class I displays peptides from proteins inside a cell to CD8⁺ cytotoxic T cells

What does a CD8⁺ cytotoxic T cell do when it recognizes an infected cell through MHC Class I?

It kills the infected cell.

Why would a CD8⁺ cytotoxic T cell need to kill an infected cell?

The cell may contain an intracellular virus, and killing the infected cell helps eliminate the infected cell.

How many transmembrane regions does MHC Class I have?

One transmembrane region.

What does transmembrane mean?

A part of a protein that passes through the cell membrane.

What is the main function of MHC Class II?

MHC Class II displays peptides from proteins taken up from outside the cell to CD4⁺ helper T cells.

What do CD4⁺ helper T cells do when activated?

They release cytokines that help coordinate and stimulate immune responses from other cells:

• Dendritic cells (DCs).

• Macrophages (Mφ).

• B cells.

How many transmembrane regions does MHC Class II have?

Two transmembrane regions, one from each chain.

What does the endosome in the MHC Class II pathway represent?

The endosome represents a compartment inside the cell where proteins taken in from outside the cell are broken down into smaller peptides. These peptides can then bind to MHC Class II and be displayed to CD4⁺ helper T cells.

What enzymes are responsible for peptide generation in Class I MHC?

Protease components of the cytosolic proteasome

What enzymes are responsible for peptide generation in Class II MHC?

Endosomal and lysosomal proteases (e.g., cathepsins)

What is the site of peptide loading in Class I MHC?

Endoplasmic reticulum

What is the site of peptide loading in Class II MHC?

Late endosomes and lysosomes

Class II MHC pathway: antigen presentation to helper T cells

• A macrophage recognizes an extracellular microbe and absorbs it

• A macrophage presents an antigen to a CD4+ helper T cell

• T cell releases cytokines, activating the macrophage

• This activation allows the macrophage to do its job better and kill the phagocytosed microbe

Class I MHC pathway: Antigen presentation to cytotoxic T cells

• A cell contains a cytosolic antigen.

• The antigen is displayed by MHC Class I.

• A CD8⁺ cytotoxic T lymphocyte recognizes the antigen.

• The CD8⁺ cytotoxic T cell kills the antigen-expressing target cell.

Cross-presentation of Microbial Antigens by Professional APCs

• A cell becomes infected with a virus

• A dendritic cell captures the infected cell's fragments.

• The viral antigen enters the dendritic cell's cytosol.

• The dendritic cell presents the viral antigen using MHC Class I.

• The dendritic cell also displays costimulators.

• A virus-specific CD8⁺ T cell recognizes the viral antigen

• The CD8⁺ T cell recognizes the costimulator

• The CD8⁺ T cell becomes activated.