Immunology IV: Antigen presentation Flashcards
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 a particular protein found on other cells:
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.
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.
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” (have a wide range of functions beyond simply presenting antigens)
Two types of HLA proteins:
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
HLA I structure:
HLA II structure:
Each individual expresses about __ different class I molecules and ___ different class II molecules.
6 = class I
12 = class II
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
Polymorphism clinical aplication:
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
HLA I types are expressed on almost all nucleated cells:
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.
HLA I 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 (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)
Antigen processing step 1:
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)
Antigen processing step 2:
The peptide is then transported into the RER and loaded onto the HLA 1 protein.
Antigen processing step 3:
The loaded HLA-1 is then expressed on the cell surface.
Cells that are very good at presenting HLA-1 bound peptides to T-cells have specialized proteosomes, called:
immunoproteasomes
cell types: antigen presenting cells & some other cell types
Immunoproteasomes 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 immunoproteasomes 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
Once the peptide is loaded onto the HLA-1, what happens?
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
Upon NFkB activation of the pathway:
An inhibitory protein (IkB) is phosphorylated and degraded:
NFkB is free to travel to the nucleus and promote the transcription of a NFkB target gene:
Promoting the transcription of HLA-1
Cytokines can also _________ the expression of HLA-1 molecules:
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
Once a peptide-bound HLA-1 is expressed on the surface of a cell, what happens next?
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.
HLA-2 types are expressed exclusively on antigen presenting cells:
Professional:
-Dendritic cells
-Macrophages
-B-cells
Non-professional:
-Fibroblasts (skin)
-Glial cells
-Pancreatic beta cells
-Thymic epithelial cells
-Intraepithelial cells
-Vascular endothelial cells
Some of the APCs wont even express HLA-2 unless they’ve been activated:
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
HLA-2 proteins bind extracellular antigens via the exogenous pathway:
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
HLA-2 proteins bind extracellular antigens via the _______________
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
The phagocytosed antigen is processed in a 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:
How do we ensure a cytosolic antigen isn’t loaded onto an HLA-2 (rather than an HLA-1) in the RER?
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.
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 the 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.
________ helps with loading onto an HLA-2
HLA-DM
How do you keep antigens separate?
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.
Exogenous antigens can be presented by HLA-1, and endogenous antigens can be presented by HLA-2 in some circumstances:
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
Once a peptide-bound HLA-2 is expressed on the surface of a cell, what happens next?
It can bind a CD4+ T-cells, ex: a helper T-cell, and activate it.
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.
-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.
Although most T-cells express CD28, most cells in the body do not express its ligands (CD80 or 86):
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
How does CD28 signaling add to the effects of TCR signaling described previously?
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
The interactions between the T-cell and the antigen presenting cell can be divided into two categories:
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
Once the T-cell receptor (TCR) interacts with the antigen bound to HLA-2:
-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 signaling between the antigen presenting cell and the T-cell:
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
