Antigen Presentation Flashcards
How do T-cells recognize antigens?
T-cells recognize antigens in a specific fashion:
Antigens must be presented to a T-cell in order for them to recognize the antigen
Antigens are presented by being bound to particular proteins found on other cells
In mice and 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
The HLA protein can help a T-cell distinguish between foreign antigen and self antigen
And thus, should know to only mount a response against foreign antigens
Are HLA proteins genetically “shuffled” like lymphocyte receptors?
Although HLA proteins are bound to antigens, they are not genetically “shuffled” like lymphocyte receptors
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”
They have a wide of range of functions beyond simply presenting antigens.
What are the 2 types of HLA proteins? What is the difference?
HLA class I
HLA class II
HLA class I proteins interact with cytotoxic T-cells and binds intracellular antigens
Interact with a CD8 co-receptor on the cytotoxic T-cell (One is toxic (cytotoxic T-cells) inside (intracellular) because of what you ate (8)
HLA class II proteins interact with T-helper cells and binds extracellular antigens
Interacts with CD4 co-receptor on the T-helper cell (I <3 2 help extra 4ever)
HLA-I Structure
HLA- II Structure
Polymorphism- Each individual expresses about how many different class I molecules and how many different class II molecules?
HLA type I molecule subtypes are all indicated by the designation A, B, or C
HLA type II molecule subtypes are all indicated by the designation “D”
Clinical application of allelic variants?
Clinical Application:
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
Eg. HLA-B27 predisposes a person to ankylosing spondylitis
Eg. HLA-DQ8 predisposes a person to celiac disease
HLA-1 expression and function
HLA-1 types are expressed on almost all nucleated cells
Level of expression differs – highest expression level found on lymphocytes
50,000 HLA-1 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-1
For the most part, HLA-1 proteins bind intracellular antigens via the endogenous pathway
Most of the time these are self-antigens
In the case of infection or malignancy, the peptide can be foreign (ie not a self antigen)
During viral infection, some HLA-1 molecules on a cell will express viral peptides, while some will express host peptides
HLA-1 expression: endogenous pathway
Antigen processing: 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)
How to proteins destined for degradation by proteosomes get tagged?
Ubiquitylation
Antigen processing: endogenous pathway (the details)
Cells that are very good at presenting HLA-1 bound peptides to T-cells have specialized proteosomes, called immunoproteosomes.
Cell types: antigen presenting cells & some other cell types
Immunoproteosomes have slightly different subunits that are substituted into the “regular” proteosome
This can be induced by cytokines -IFN-gamma and TNF-alpha
The peptides that are produced by immunoproteosomes bind with better affinity to HLA-1
The protein that translocates the peptide fragment into the RER for loading onto the HLA-1 is called TAP
There are a variety of other proteins that help with loading onto HLA-1 once in the RER
What will the presence of intracellular invaders (ie. viruses) trigger? An increase or decrease in transcription of HLA-1 proteins?
Presence of intracellular invaders (eg. viruses) will trigger an increase in transcription of HLA-1 proteins
This occurs via binding to NOD-like Receptors (NLRs)
Review - NLR are PRRs found within the cytosol
complex PAMP and DAMP receptors
Binding of NLR will upregulate the expression of HLA-1
NODs activate NF𝜅B
NF𝜅B pathway
Note - in this case it is the intracellular NLR that triggers the NF-𝜅B pathway rather than a cell surface receptor
Points to remember:
Upon activation of the pathway, an inhibitory protein (I𝜅B) is phosphorylated and degraded
NF𝜅B is free to travel to the nucleus and promote the transcription of a NF𝜅B target gene
Do cytokines increase or decrease the expression of HLA-1 molecules? Which types? Source? First to produce? Later to produce?
Cytokines can also increase the expression of HLA-1 molecules
Both type 1 and type 2 interferons (IFN)
Tumour necrosis factor alpha (TNF⍺)
The source of these cytokines is typically a cell that has either been activated or infected (or both) in the nearby neighbourhood
The first to produce them are local antigen-presenting cells
TNF-alpha and type-1 interferons
Later activated T-helper cells (Th) cells will contribute to the cytokine production
Source of IFN-gamma
Once a peptide-bound HLA-1 is expressed on the surface of a cell, what happens next?
What is the basic function of a cytotoxic T cell?
It can bind a CD8+ T-cells Cytotoxic T cell and activate it.
What is the basic function of a cytotoxic T cell?
Once activated, a cytotoxic T-cells can kill infected cells by inducing apoptosis
HLA-2 expression and function. HLA-2 are expressed EXCUSIVELY on what cells? What are the 2 types and what kind of cells are in each?
HLA-2 types are expressed exclusively on Antigen presenting Cells
Antigen presenting cells (APCs)
“Professional”
Dendritic cells
Macrophages
B-cells
“Non-professional”
Fibroblasts (skin)
Glial cells
Pancreatic beta cells
Thymic epithelial cells
Intraepithelial lymphocytes
Vascular endothelial cells
Some of these APCs won’t even express HLA-2 unless they’ve been activated, but some always express high levels. What is the expression of these cells?
1.) Dendritic cells?
2.) Macrophages?
3.) B-cells?
4.) Non-professional APCs?
1.) Dendritic cells constitutively (aka always) express high levels of HLA-2 and are very good at co-stimulating helper-T-cells
2.) Macrophages need to be activated before they express HLA-2, but they are also good at co-stimulating helper T-cells
3.) B-cells constitutively express HLA-2 at low levels and are good at co-stimulating helper-T-cells
4.) Non-professional APCs will only express HLA-2 under particular conditions
Eg. Sustained inflammation
HLA-2 proteins bind extracellular antigens via what pathway?
the exogenous pathway
For some APCs, first we need to up-regulate the expression of HLA-2:
HLA-2 expression is up-regulated by particular cytokines
Interferon-gamma is particularly good at this
Note - In B-cells, IFN-gamma actually down-regulates HLA-2
IL-4 is good at upregulating HLA-2 on B-cells
HLA-2 expression: exogenous pathway (basics)
HLA-2 proteins bind extracellular antigens via the exogenous pathway (the basics)
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
The phagocytosed antigen is processed in an phagosome and then merges with a vesicle containing the HLA-2
The antigen is loaded onto the HLA-2 and expressed on the surface of the cell
HLA-2 proteins bind extracellular antigens via the exogenous pathway (the details)
Phagocytosis by B-cells can be antibody-mediated
Antigen binds to specific antibody on the surface of the B-cell (aka B-cell receptor)
Antigen and the antibody are phagocytosed together (process is called receptor-mediated endocytosis)
The antibody is the recycled back to the surface of the cell and the antigen is processed in an endosome.
How do we ensure a cytosolic antigen isn’t loaded onto an HLA-2 (rather than an HLA-1) in the RER?
In the RER, the HLA-2 protein associates with the invariant chain (FYI - aka CD74)
This prevents a cytosolic antigen from being loaded onto the HLA-2 while in the RER
As the HLA-2 containing vesicle merges with the phagosome/ endosome containing the antigen, the invariant chain is chopped up
The “chopped version” is called CLIP, and it will remain bound to the HLA-2 peptide binding region until displaced
When a peptide binds with sufficient affinity to HLA-2, CLIP is displaced
HLA-2 with bound extracellular antigen is then expressed on the surface of the cell
How do you keep antigens separate?
In summary:
The mode of antigen entry into cells and the site of antigen processing determine whether antigenic peptides associate with:
HLA class I molecules in the rough endoplasmic reticulum
HLA class 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.
Antigen-processing exceptions
Exogenous antigens can be presented by HLA-1, and endogenous antigens can be presented by HLA-2 in some circumstances:
- An infected cell dies and is phagocytosed
Viral particles in the cytosol of the infected cell will be presented on HLA-2 of the phagocyte (eg. Macrophage)
Thus an “intracellular” antigens becomes extracellular antigens
Autophagy can also results in peptides from the cytosol being presented on HLA-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.
Antigens can leak out of the endosome after phagocytosis into the cytosol, then the antigen can be presented to HLA-1
Allows antigen presenting cells to either sequentially or simultaneously activate both T-helper and cytotoxic T cells
Once a peptide-bound HLA-2 is expressed on the surface of a cell, what happens next?
It can bind a CD4+ T-cells, ie. a helper T-cell, and activate it.
Let’s take a closer look!
T-cell activation
An antigen presenting cell is “presenting” an antigen via HLA-2 on its surface
Helper T-cells have randomly-reorganized T-cell receptors that are highly specific for a particular peptide
However, remember, they will 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
This co-receptor is known as CD4
It is the major CD that distinguishes a Th from a cytotoxic (Tc) T-cell
What was the co-receptor for a cytotoxic 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.
Although most T cells express __________, most cells in the body do not express its ligands (CD80 or 86)
CD28
Only professional APCs have the capacity to express CD80/86.
Mature dendritic cells, the best activator of naïve 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 pathogen
How does CD28 signaling add to the effects of TCR signaling described previously?
- 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
The interactions between the T-cell and the antigen presenting cell can be divided into what two categories?
cSMAC (central Supramolecular Activation Complex)
TCR & HLA-2 (with co-activator CD4)
Co-stimulator interaction (CD28 & CD80 or 86)
pSMAC (peripheral Supramolecular Activation Complex)
Includes other receptor-ligand interactions that help to strength the connection between the T-cell and APC to sustain the signal
Once the T-cell Receptor (TCR) interacts with the antigen bound to HLA-2 what happens?
(With the help of it’s CD4 co-receptor & costimulatory interactions)
Initiates cell signaling cascades to promote T-cell function, survival, and proliferation
Note: activation of PLC cascade & Ras cascade
In addition to TCR and HLA-2 (with CD4+ co-receptor) interactions and co-stimulatory interactions, there will also be paracrine signalling between the antigen presenting cell and the T-cell.
What will these signals do?
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
Study Guiding Questions
Build a table comparing the endogenous antigen presentation pathway to exogenous antigen presentation pathway.
In your table, include:
HLA types
Antigen type (intracellular or extracellular)
Description of the steps of the pathway
When is the antigen loaded onto the HLA protein?
Describe how T-cells are activated:
List all the interactions needed to activate a T-cell
How are cytotoxic T cells activated?
Which type of HLA is used to present the antigen?
What types of cells might present the HLA bound antigen?
How are helper-T cells activated?
Which type of HLA is used to present the antigen?
What types of cells might present the HLA bound antigen?
Review: PI3 Kinase pathway
Review: Ras-MAP Kinase pathway
Review: PLC pathway
