Lecture 5: Antigen Capture and Presentation

Question 1

Lymphocytes constantly recirculate between tissues to kill microbes. What are the steps on how they do this?

Answer 1

1. Lymphocytes enter lymph nodes, where they encounter APCs.
2. Lymphocytes are activated and differentiate in the lymph nodes.
3. Lymphocytes exit lymph nodes and enter circulation, head to inflamed tissues where they mediate microbial destruction.

Question 2

What activates the T cells?

Answer 2

Antigen Presenting Cells (APC)

Question 3

How does the antigen presenting cells (APC) activate T cells?

Answer 3

APCs capture antigens (Ags) in tissues and transport these Ags to peripheral lymphoid tissues (lymph nodes usually) where lymphocytes are concentrated, to present those Ags to T cells.

Question 4

Major Histocompatibility Complex (MHC) class I and II molecules

Answer 4

APC present peptide antigens to T cells on MHC class I and class II molecules

Question 5

These antigens are linear peptides bound and presented by MHC molecules

Answer 5

T cell antigens

Question 6

MHC locus

Answer 6

• collection of genes found in all mammals that code for MHC molecules. The locus contains two sets of highly polymorphic genes (Class I and Class II)
• originally discovered as principle determinant of graft rejection.
• people with the same MHCs (e.g. twins) accept grafts.
• people with different MHCs reject grafts.

Question 7

Polymorphism

Answer 7

multiple alleles of a gene within a population

Question 8

What does the Class I and Class II genes code for?

Answer 8

Class I and Class II genes code for the Class I and Class II molecules that display peptides to T cells

Question 9

Most MHC peptide binding clefts have ______

Answer 9

pockets

Question 10

What fits into the MHC pockets of MHC binding clefts and what do they do?

Answer 10

Side chains of two or three amino acids of antigenic peptides fit into the MHC pockets and “anchor” the peptide in the MHC peptide-binding cleft.

Additional amino acid residues of the antigenic peptide extend upwards and are recognized by TCR.

Question 11

Why does MHC molecules have a broad specificity?

Answer 11

• any given MHC can present any peptide with the correct anchor residues.
• Thus, MHC molecules have a broad specificity, allowing a small number of MHC molecules to display a large array of peptide antigens.

Question 12

What happens to when peptides are bound to the MHC?

Answer 12

When bound, peptides remain on display for up to days maximizing the likelihood of encountering the correct T cell.

Question 13

What are the MHC-expressing cell types of Class II MHC molecules?

Answer 13

• dendritic cell
• macrophage
• B cell

Question 14

What are the MHC-expressing cell types of Class I MHC molecules?

Answer 14

All nucleated cells

• leukocytes
• epithelial cells
• mesenchymal cells

Question 15

CD4+ helper T lymphocytes

Answer 15

interact with dendritic cells, macrophages, B lymphocytes

Question 16

CD8+ CTLs

Answer 16

can kill any virus-infected cell

Question 17

Structure of Class I MHC
What is it made of?

Answer 17

• an α chain, noncovalently linked to a β2-microglobulin chain.
• α1 and α2 domains form a peptide binding groove that holds peptides of 8-11 amino acids.

-TCR on top of the MHC molecule

• Polymorphic residues located in the α1 and α2 domains of Class I MHC molecules
• α3 domain

Question 18

Where does the TCR stay on the Class I MHC molecule and what does it do?

Answer 18

The TCR sits down on the top of the MHC molecule and contacts residues extending up out of the peptide binding groove on the antigenic peptide.

Question 19

Where are polymorphic residues located in the Class I and II MHC molecule?

Answer 19

Polymorphic residues are located in the α1 and α2 domains of Class I MHC molecules, thereby affecting peptide binding and T cell recognition.

Question 20

α3 domain of Class I MHC molecule

Answer 20

The α3 domain is invariant and contacts the T cell CD8 co-receptor. Thus, only CD8+ T lymphocytes (cytotoxic T cells) respond to Class I MHC-bound antigens.

Question 21

Antigen Processing- Converting microbial proteins into peptides for the MHC

Steps (Class I)

Answer 21

1. Microbes (viruses, bacteria) grow and reproduce in the cytoplasm producing their microbial proteins.
2. This results in microbial proteins in the cytoplasm of infected cells.
3. Proteins are cleaved into peptides of varying size and composition by the cytoplasmic proteasome complex.
4. Newly synthesized Class I MHC molecules are loosely attached to the Transporter associated with Antigen Presentation (TAP), a cellular pump that drives transport of cytoplasmic peptides into the E.R.
5. Microbial peptides are pumped into the E.R. lumen by TAP, where they associate with Class I MHC molecules.
6. If a Class I molecule stably binds a microbial peptide, the complex is sent to the cell surface, via the Golgi apparatus and exocytic vesicles.
7. Class I MHC- peptide complexes are delivered to the cell surface where they interact with CD8+T lymphocytes.

Question 22

The Class I pathway responds to ___________ microbes

Answer 22

intracellular

Question 23

Intracellular microbes are presented by Class I MHC molecules on the _______________

Answer 23

surface of all nucleated cells

Question 24

Class I MHC- peptide complexes are recognized by _____________

Answer 24

CD8+ T lymphocytes

Question 25

Class I MHC-peptide complexes are recognized by CD8+ T lymphocytes. What happens when TCR is activated?

Answer 25

TCR activation triggers T lymphocyte differentiation to cytotoxic T lymphocytes (CTL). CTLs kill target infected cells, thereby eradicating the infection.

Question 26

Class II MHC molecule structure What is it made of?

Answer 26

- an α chain, noncovalently linked to a β chain. - α1 and β1 domains form a peptide binding groove that holds peptides of 10-30 amino acids. - polymorphic residues of Class II MHC are in the α1 and β1 domains, affecting peptide binding and T cell recognition. - β2 domain

Question 27

β2 domain of Class II MHC molecules

Answer 27

- β2 domain is invariant and contacts the T cell CD4 co-receptor. Thus, only CD4+ T lymphocytes (helper T cells) respond to Class II MHC-bound antigens.

Question 28

What does the TCR does in the MHC II molecules?

Answer 28

The TCR contacts residues of the antigenic peptide that extend up out of the peptide binding groove.

Question 29

Antigen Processing- Converting microbial proteins into peptides for the MHC Class II Steps

Answer 29

1. APCs have several means of ingesting microbes a. PRR bind microbes b. Receptors bind antibodies bound to microbes c. APCs sample their environment through pinocytosis 2. Internalized microbes are delivered to lysosomes 3. Microbial proteins are cleaved by lysosomal enzymes, yielding numerous microbial peptides of different sizes and configuration 4. In the ER, a protein called the invariant chain, blocks the peptide binding groove of MHC II, preventing peptides associating with MHC II in the ER. 5. Class II molecules are transported to the cell surface in exocytic vesicles. 6. Endosomal vesicles with microbial peptides fuse with exocytic vesicles containing MHC II molecules. Invariant chain is degraded and MHC II binds microbial peptides. 7. If MHC II stably binds a microbial peptide the complex is transported to the cell surface

Question 30

What are the several means that APCs ingest microbes (Class II)

Answer 30

- PRR bind microbes - Receptors bind antibodies bound to microbes - APCs sample their environment through pinocytosis

Question 31

The Class II pathway responds to __________ microbes

Answer 31

extracellular

Question 32

In the Class II pathway, microbes are captures by ________, __________, and __________.

Answer 32

APCs, B lymphocytes, and phagocytes

Question 33

In class II pathway, microbial antigens are presented by _____________

Answer 33

Class II MHC

Question 34

Class II MHC-peptide complexes are recognized by ____________.

Answer 34

CD4+ T lymphocytes

Question 35

What happens when CD4+ T lymphocytes stimulate B lymphocytes?

Answer 35

CD4+ T lymphocytes stimulate B lymphocytes to produce antibodies and phagocytes to ingest and destroy microbes. These pathways are most suited for the destruction of extracellular microbes.

Question 36

Two properties of the MHC

Answer 36

- Co-dominance - Polymorphism

Question 37

Co-dominance

Answer 37

Property of MHC Both parental alleles are expressed equally. This means that each individual can express up to 6 different Class I molecules and 10 to 20 different Class II molecules.

Question 38

Polymorphism

Answer 38

Property of MHC Multiple MHC alleles means that at least some members of the population will be able to present any given microbial antigen.

Question 39

features of peptide binding to MHC

Answer 39

1. Each MHC molecule displays one peptide at a time 2. Broad specificity 3. Very slow off-rate 4. Stable expression requires peptide 5. MHC molecules bind only peptides

Question 40

What is the significance of each MHC molecule displaying one peptide at a time?

Answer 40

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

Question 41

What is the significance of peptide binding to MHC molecules having broad specificity?

Answer 41

Many different peptides can bind to the same MHC molecule

Question 42

One feature of peptide binding to MHC is it has very slow off-rate. What is the significance of this?

Answer 42

MHC molecule displays bound peptide for long enough to be located by T cell.

Question 43

One feature of peptide binding to MHC is stable expression requires peptide. What is the significance?

Answer 43

Only MHC molecules that are displaying peptides are expressed for recognition by T cells

Question 44

One feature of peptide binding MHC is MHC molecules bind only peptides. What is the significance?

Answer 44

MHC-restricted T cells respond only to protein antigens, and not to other chemicals.

Question 45

Co-stimulatory functions of APCs

Answer 45

- APCs present antigens and provide the second signal for T cell activation. - APCs present microbial peptides via Class II MHC molecules. - CD4+ T lymphocytes recognize Class II MHC-peptide complexes. - Microbial substances stimulate APCs to express costimulators on their surface and secrete cytokines. - Costimulators and cytokines provide the second signal that triggers T lymphocyte differentiation.