Antigen Capture and Presentation Flashcards
What are the two steps taken by the innate immune system to signal the adaptive immune response?
- Process and present Ag to T cell
2. Generation of surface molecules that function as costimulatory signals w/ the Ag to activate T and B cells
What types of cells express HLA I?
How about HLA II?
HLA I found on all nucleated cells
HLA II found on professional APCs (dendritic cells, macrophages, B cells and some thymocytes [for testing immature T cells])
What are the two receptors on B and T cells which receive Ag presentation from HLA?
B cell receptors (BCRs)
T cell receptors (TCRs)
What’s the function of HLA proteins?
Function as antigen-presenting structures to T cells
Do T cells recognize free or soluble Ags?
No
What chromosome is responsible for all HLA proteins
Chromosome #6
What are the three distinct classes of HLA and their associated loci?
Class I: A, B, C
Class II: DP, DQ, DR
Class III: H-Y
What HLA class has the highest number of allotypes in the body (has the largest diversity)?
HLA I
What cells do HLA I present to?
It also acts as an inhibitory receptor for what?
CD8+ (cytotoxic) T cells
NK cells
What’s the structure of HLA I?
Heterodimer of two proteins. The α chain forms 3 of the 4 globular domains (HLA encoded) which are numbered α1, α2, and α3. The β2-microglobulin is the non-HLA encoded fourth globular domain which associates non-covalently with the α3 domain.
Why can HLA I only bind peptides that are about 8-10 amino acids long?
Because the peptide binding groove has closed ends on both sides
What are the steps that occur in order to synthesize HLA I?
- α chain translated into ER as glycoprotein
- In the ER, α chains interact with β2 microglobulin
- Class I associates with peptides derived from cytosolic proteins
- HLA I-peptide complex are transported to the cell membrane via the normal passage of glycoproteins through ER and golgi
What are some key features of HLA I?
All alleles of HLA I can be expressed at the same time on each cell totaling 6 different HLAs
Are a slightly different shape and present a different set of peptides
HLA II contain three sets of genes, each capable of producing α and β chains, what are they?
HLA-DP
HLA-DQ
HLA-DR
What’s the primary cell type we would find these located?
APCs: macrophages, dendritic cells, and B cells
Which type of cell does HLA II present to?
CD4+ (helper) T cells
What is the structure of HLA II?
Composed of two proteins (α and β chains) which are both encoded by the HLA-D gene region
There are 2 α globular domains and 2 β globular domains, which are strongly associated, but not covalently linked.
How big can a peptide be in order to bind to the peptide binding groove of HLA II?
Between 13 and 18 amino acids
What allows the larger peptides to bind to HLA II?
Opens ends of the peptide binding groove.
What are the steps that occur in order to synthesize HLA II?
- Both α and β chains are synthesized in the ER, where they interact
- The invariant protein then is added in order to cover up the peptide binding groove, preventing any binding . of unintentional peptide
- In the endocytic compartment, the invariant chain is degraded which frees the the peptide binding groove, and Class II molecules bind peptides that have entered the cell via endocytosis
- HLA II-peptide complex then transported to the cell surface.
What are some key features of HLA II?
All alleles from the α and β chains are expressed (6 α chains and 6 β chains).
Since any α chain can interact with any β chain, we get increased diversity in the peptide binding groove, which means a greater range of peptides that can bind to class II
What are some key features of peptide binding to HLA proteins?
It is a low-affinity interaction
Slow “on rate” and even slower “off rate”
Only one peptide can bind to HLA at a time, however, Each HLA protein can bind multiple peptides
What is antigen processing?
Converting proteins to peptides for display and presentation.
What are the two processing pathways of Ags?
Class I MHC pathway
Class II MHC pathway
What 4 Ag characteristics will determine the pathway?
- Chemical nature of the fragment
- Density of the protein
- Specific MHC and it’s binding site
- Self vs. non-self
Describe the interaction between MHC II and CD4+ T cells
First, CD4+ receptor will check the MHC molecule to ensure it is a class II MHC being expressed. If it checks out, then the TCR will check the Ag (the TCR binding groove also only fits certain Ags). If it fits, then we trigger a signal transduction cascade which will eventually lead to the activation of the naive CD4+ cell
Describe the MHC I processing pathway
- Proteasome degrades damaged or ubiquitinated proteins into peptide fragments.
- Peptide fragments move from the cytosol into the ER via TAP
- MHC I, which is bound to TAP via tapasin, picks up incoming peptides
- The MHC I-peptide complex is then shuttled to the Golgi and loaded to an exocytic vesicle where it migrates to plasma membrane to present Ag.
Does Ag presentation occur in the absence of an infection?
Yes. Protein degradation and peptide transport occur continuously.
What is cross-presentation, and what cells are capable of it?
Dendritic cells ingest virally infected cells or transformed cells and are capable of display Ag to CD8+ cells or CD4+ cells.
How do you FULLY activate a CD8+ cell?
In order to fully activate a CD8+ cell, CD4+ cells need to secrete IL-2 to trigger CD8+ cells along with Ag presentation to CD8+ cells.
What is the most common HLA associated disease? What do they think may be causing the disease?
Ankylosing spondylitis
It is possible that the HLA-B27 allele can not bind a critical antigenic peptide.
What disease can cause antibodies which cross-react with cardiac tissue, resulting in cardiac failure?
What type of patients are more prone to developing said disease?
Rheumatic fever, since we get the generation of Abs against the streptococcus pyogenes infection.
Patients who have the HLA-DR4 allele.
What are the soluble Ag receptors?
Abs
Define pathogen
Organism that can cause disease
Define epitope
Specific part of the Ag that contacts the Ag binding sites of an Ab or TCR.
What is a hapten?
Why is this clinically relevant?
Small molecules which cannot . induce an immune response alone, however, when coupled with a carrier protein, they do.
This is why we have drug allergies and how vaccines work
What is the structure of the BCR?
Composed of a surface immunoglobulin (Ig) and two invariant chains (Igα and Igβ) which ensure surface expression of Ig and also function in signal transduction.
What is the role of Abs
They promote killing and/or removal of the immune complex. Also function as membrane bound surface Ag receptors for B cells and play a key role in B cell differentiation.
Explain an Abs structure
Consists of 4 polypeptide chains. The heavy chains (2) form the class of Ig. The light chain forms the type of Ig. Both chains have a variable region (Fab) and a constant region (Fc).
Explain how immunologists used to “digest” Abs
To isolate a functioning Fab region, they can use papain digestion which will separate the two Fab regions on a single Ab, or they can use pepsin digestion which would keep both Fab regions on an Ab in-tact, but would digest the Fc region (called F(ab’)2).
To isolate the Fc region, they just use papain digestion
Define idiotope
The hypervariable region on Abs is known as the idiotope (thus epitopes bind to idiotopes)
How do the five isotypes of Abs differ?
By size, charge, aa sequence, and carbohydrate content
What determines which isotype an Ab is?
The Fc region of the H chain defines the class and subclass
How many isotypes of Abs are in humans?
9 (need to include subtypes of IgG)
What is the first Ab produced in a primary immune response?
IgM
Is IgM a monomer, dimer, or pentamer?
Pentamer: 10 Ag-binding sites
What is the predominant Ab of secondary immune responses?
IgG
What are the four subclasses of IgG and how do they differ?
IgG1, IgG2, IgG3, IgG4
Subtle differences are on the H chain, but this causes important functional differences
What is the most abundant Ab in serum?
IgG (80% of total serum Abs)
What is the only class of Abs capable of crossing the placenta?
IgG
What is the most abundant Ab in the body?
IgA since it’s responsible for mucosal immunity
What Ab is a constituent of secondary immune responses?
IgE
What monomeric Ab works as a Ag-specific receptor on mature B cells, even though it does not lead to activation of B cells?
IgD
Explain the difference between affinity and avidity
Affinity: The strength of interaction between univalent Ag and univalent Ab
Avidity: The strength of interaction between multivalent Ag and multivalent Ab
What are T-dependent Ags
A protein Ag that requires both CD4+ cells and B cells to stimulate Ab response
What are T-independent Ags
Non-protein Ags, such as polysaccharides and lipids that can stimulate Ab responses without T help.
What are the Ag receptors in cell-mediated immunity?
TCRs
What is the response of CD4+ and CD8+ T cells upon binding an Ag?
CD4+ T cells: Secrete cytokines
CD8+ T cells: Kill altered or infected cells.
Are free peptides recognized by TCRs?
No
Where are γ and ẟ T cells located, and what do they bind?
They are located in the epithelial/mucosal compartments and they can recognize lipid Ags along with DAMPs
How does superantigen and antigen binding differ?
Antigens are first processed and then bind to the peptide binding groove which interacts with the TCR
Superantigens are not processed. Instead, they bind directly to an expressed HLA class II molecule and the β portion of the TCR.
