Immunology IV: Antigen presentation

Question 1

T cells recognize antigens in a specific fashion:

Answer 1

Antigens must be presented to a T-cell in order for them to recognize the antigen.

Question 2

Antigens are presented by being bound to a particular protein found on other cells:

Answer 2

In mice & rats, these are called MHC (major histocompatibility) proteins.

In humans they’re called the HLA (human leukocyte antigen) proteins

A wide variety of cells can present antigens using HLA proteins.

Question 3

The HLA protein can help a T-cell distinguish between foreign antigen and self-antigen and thus:

Answer 3

should know to only mount a response against foreign antigens.

Question 4

Although HLA proteins are bound to antigens, they are not genetically “shuffled” like lymphocyte receptors:

Answer 4

Meaning that the antigen is not bound particularly tightly to these proteins.

This also implies that these proteins can present a wide variety of antigens to a wide variety of lymphocytes.

There are also a wide variety of genes/proteins called “MHC-like” (have a wide range of functions beyond simply presenting antigens)

Question 5

Two types of HLA proteins:

Answer 5

HLA class I: Interact with cytotoxic T-cells and binds intracellular antigens.
-Interact with a CD8 co-receptor on the cytotoxic T-cell.

HLA class II: interact with T-helper cells and binds extracellular antigens.
-Interacts with CD4 co-receptor on the T-helper cell

Question 6

HLA I structure:

Answer 6

Question 7

HLA II structure:

Answer 7

Question 8

Each individual expresses about __ different class I molecules and ___ different class II molecules.

Answer 8

6 = class I
12 = class II

Question 9

HLA type I molecule subtypes are all indicated by the designation:

Answer 9

A, B, or C

Question 10

HLA type II molecule subtypes are all indicated by the designation:

Answer 10

D

Question 11

Polymorphism clinical aplication:

Answer 11

There are hundreds and hundreds of different allelic variants of class I and class II molecules in the human population.
-Differing allelic variants of class I and II HLA proteins is the main reason why we usually reject organs from randomly-matched organ donors.

The presence of particular allelic variants have been associated with increased risk of some autoimmune diseases.
-ex: HLA-B27 predisposes a person to ankylosing spondylitis
-ex: HLA-DQ8 predisposes a person to celiac disease

Question 12

HLA I types are expressed on almost all nucleated cells:

Answer 12

Level of expression differs: highest expression level found on lymphocytes.
-50,000 HLA I proteins on lymphocyte cell membranes
-Much, much less on fibroblasts, muscle cells, hepatocytes, and most neurons
-Other cells have intermediate levels of expression

Cells without nuclei do not express HLA I.

Question 13

HLA I proteins bind intracellular antigens via the endogenous pathway:

Answer 13

-Most of the time these are self-antigens
-In the case of infection or malignancy, the peptide can be foreign (ex: not a self antigen)
(During viral infection, some HLA I molecules on a cell will express viral peptides, while some will express host peptides)

Question 14

Antigen processing step 1:

Answer 14

Endogenous pathway (the basics):
Source of the antigenic peptide is from the cytosol;
-The peptide is derived from proteasomal degradation of foreign/altered proteins (or normal self-antigens)

Question 15

Antigen processing step 2:

Answer 15

The peptide is then transported into the RER and loaded onto the HLA 1 protein.

Question 16

Antigen processing step 3:

Answer 16

The loaded HLA-1 is then expressed on the cell surface.

Question 17

Cells that are very good at presenting HLA-1 bound peptides to T-cells have specialized proteosomes, called:

Answer 17

immunoproteasomes

cell types: antigen presenting cells & some other cell types

Question 18

Immunoproteasomes have slightly different subunits that are substituted into the:

Answer 18

“regular” proteosome
This can be induced by cytokines-IFN-gamma and TNF-alpha

Question 19

The peptides that are produced by immunoproteasomes bind with better affinity to ______

Answer 19

HLA-1

Question 20

The protein that translocates the peptide fragment into the RER for loading onto the HLA-1 is called:

Answer 20

TAP

Question 21

There are a variety of other proteins that help with loading onto HLA-1 once in the ____

Answer 21

RER

Question 22

Once the peptide is loaded onto the HLA-1, what happens?

Answer 22

Presence of intracellular invaders (ex: viruses) will trigger an increase in transcription of HLA-1 proteins:

This occurs via binding to NOD-like receptors (NLRs)- complex PAMP and DAMP receptors

Binding of NLR will upregulate the expression of HLA-1.
-NODs activate NFkB

Question 23

Upon NFkB activation of the pathway:

Answer 23

An inhibitory protein (IkB) is phosphorylated and degraded:

Question 24

NFkB is free to travel to the nucleus and promote the transcription of a NFkB target gene:

Answer 24

Promoting the transcription of HLA-1

Question 25

Cytokines can also _________ the expression of HLA-1 molecules:

Answer 25

increase -Both type 1 & type 2 interferons (IFN) -Tumour necrosis factor alpha (TNFalpha) The source of these cytokines is typically a cell that has either been activated or infected (or both) in the nearby neighborhood. -The first to produce them are local antigen-presenting cells: TNF-alpha & type-1 interferons -Later activated T-helper cells (Th) cells will contribute to the cytokine production: source of IFN-gamma

Question 26

Once a peptide-bound HLA-1 is expressed on the surface of a cell, what happens next?

Answer 26

It can bind a CD8+ T-cells cytotoxic T cell and activated it. Once activated, a cytotoxic T-cells can kill infected cells by inducing apoptosis.

Question 27

HLA-2 types are expressed exclusively on antigen presenting cells:

Answer 27

Professional: -Dendritic cells -Macrophages -B-cells Non-professional: -Fibroblasts (skin) -Glial cells -Pancreatic beta cells -Thymic epithelial cells -Intraepithelial cells -Vascular endothelial cells

Question 28

Some of the APCs wont even express HLA-2 unless they've been activated:

Answer 28

Dendritic cells constitutively (aka always) express high levels of HLA-2 and are very good at co-stimulating helper T-cells Macrophages need to be activated before they express HLA-2, but they are also good at co-stimulating helper T-cells B-cells constitutively express HLA-2 at low levels and are good at co-stimulating helper T-cells Non-professional APCs will only express HLA-2 under particular conditions ex) sustained inflammation

Question 29

HLA-2 proteins bind extracellular antigens via the exogenous pathway:

Answer 29

For some APCs, first we need to up-regulate the expression of HLA-2: -HLA-2 expression is upregulated by particular cytokines: Interferon-gamma is particularly good at this In B cells, IFN-gamma actually fown-regulates HLA-2 IL-4 is good at upregulating HLA-2 on B-cells

Question 30

HLA-2 proteins bind extracellular antigens via the _______________

Answer 30

Exogenous pathway: Phagocytosis needs to be upregulated in concurrence with HLA-2 expression: Phagocytosis is the source of the peptides that are loaded onto the HLA-2 -Phagocytosis can occur through the "regular" pathway -Or, phagocytosis can also be antibody mediated in the case of B-cells

Question 31

The phagocytosed antigen is processed in a phagosome and then merges with a vesicle containing the:

Answer 31

HLA-2

Question 32

The antigen is loaded onto the HLA-2 and expressed on the surface of the cell:

Answer 32

Question 33

How do we ensure a cytosolic antigen isn't loaded onto an HLA-2 (rather than an HLA-1) in the RER?

Answer 33

The HLA-2 protein associates with the invariant chain (aka: CD74) -This prevents a cytosolic antigen from being loaded onto the HLA-2 while in the RER.

Question 34

As the HLA-2 containing vesicle merges with the phagosome/endosome containing the antigen, the invariant chain is chopped up:

Answer 34

the chopped version is called CLIP, and it will remain bound to the HLA-2 peptide binding region until displaced

Question 35

When the peptide binds with sufficient affinity to HLA-2:

Answer 35

CLIP is displaced

Question 36

HLA-2 with bound extracellular antigen is then expressed:

Answer 36

on the surface of the cell.

Question 37

________ helps with loading onto an HLA-2

Answer 37

HLA-DM

Question 38

How do you keep antigens separate?

Answer 38

The mode of antigen entry into cells and the site of antigen processing determine whether antigenic peptides associate with: -HLA I molecules: in the rough endoplasmic reticulum. -HLA II molecules: in endocytic compartments Under certain circumstances however, exogenous antigens may be assembled with MHC class I molecules via cross-presentation and vice versa.

Question 39

Exogenous antigens can be presented by HLA-1, and endogenous antigens can be presented by HLA-2 in some circumstances:

Answer 39

1) An infected cell dies and is phagocytosed -Viral particles int he cytosol of the infected cell will be presented on HLA-2 of the phagocyte (ex. Macrophage): Thus and "intracellular" antigens becomes extracellular antigens -Autophagy can also results in peptides from the cytosol being presented on HLA-2 2) Cross-presentation-not fully understood: Likely a blend of the exogenous and endogenous pathways -Dendritic cells are best at cross presentation, but all antigen presenting cells can do it -Allows antigen presenting cells to either sequentially or simultaneously active both T-helper and cytotoxic T cells

Question 40

Once a peptide-bound HLA-2 is expressed on the surface of a cell, what happens next?

Answer 40

It can bind a CD4+ T-cells, ex: a helper T-cell, and activate it.

Question 41

T-cell activation: An antigen presenting cell is "presenting" an antigen via HLA-2 on its surface:

Answer 41

Helper T-cells have randomly-reorganized T-cell receptors that are highly specific for a particular peptide. -They only recognize the antigen in the context of an HLA-2 molecule. -They also need a co-receptor to bind to the HLA-2 molecule: known as CD4; major CD that distinguishes a Th from a cytotoxic (Tc) T-cell. Finally, they also need co-stimulatory activation. A key co-stimulatory interaction is CD28 (on T-cell) with CD80/86 on the APC.

Question 42

Although most T-cells express CD28, most cells in the body do not express its ligands (CD80 or 86):

Answer 42

Only professional APCs have the capacity to express CD80/86: -Mature dendritic cells, the best activator of naive T cells, appear to constitutively express CD80/86 -Macrophages and B cells have the capacity to up-regulate CD80/86 after they are activated by an encounter with a pathogen

Question 43

How does CD28 signaling add to the effects of TCR signaling described previously?

Answer 43

1) Enhances the strength of signal between the T-cell and the antigen presenting cell -Initiates a cell-signaling cascade to promote T cell survival and proliferation: CD28 will recruit PI3 Kinase

Question 44

The interactions between the T-cell and the antigen presenting cell can be divided into two categories:

Answer 44

cSMAC (central supramolecular activation complex): -TCR & HLA-2 (w/ co activator CD4) -Co-simulator interaction (CD28 & CD80 or 86) pSMAC (peripheral supramolecular activation complex): -Includes other receptor-ligand interactions that help to strengthen the connection between the T-cell and APC to sustain the signal FYI: ex: LFA-1 & ICAM-1

Question 45

Once the T-cell receptor (TCR) interacts with the antigen bound to HLA-2:

Answer 45

-Initiates cell signaling cascades to promote T-cell function, survival, and proliferation. Note: activation of PLC cascade & Ras cascade

Question 46

In addition to TCR and HLA-2 (with CD4+ co-receptor) interactions and co-stimulatory interactions, there will also be paracrine signaling between the antigen presenting cell and the T-cell:

Answer 46

These signals will help polarize the helper T-cells: -Polarization depends on: A) The type of APC interacting with the T-helper cell B) the cytokines present during the time of activation