Antigen Capture and Presentation Flashcards
How does the Innate IR initiate the Adaptive IR?
Two signals:
- Process and present Ag to T-cells
- Generation of surface molecules that function as co-stimulatory signal with Ag to activate T and B cells
What are MHCs?
Major histocompatibility complex
They are antigen presenting molecules to T-lymphocytes
Also referred to as HLA (human leukocyte Ag)
What are the 2 classes of MHC? Where are they found?
Class 1 and Class 2
Class I is found on all nucleated cells
Class II is found on professional APCs: dendritic cells, macrophages, B-lymphocytes and some thymocytes
There is a Class III but it is not an APC
How were MHCs found?
During transplantations. There was histocompatibility if the transplanted tissue was accepted at self.
Describe MHC location and organization.
The allele is found on chromosome 6
Highly polymorphic - more than 150 separate alleles
Describe the MHC/HLA haplocyte.
Total set of MHC/HLA alleles present on each chromosome
Heterozygous - one from mom and dad
Codominant expression
Why is the codominant expression of the MHC/HLA allele important?
Increases number of different MHC molecules that can present peptides to T-cells
Why is the polymorphism of the MHC/HLA allele important?
Ensures that different individuals are able to present and respond to different microbial peptides
Which class of MHC is more variable?
Class 2
Describe Class 1 MHC.
Membrane bound glycoproteins
Expressed on all nucleated cells
Present Ag to CD8 cytotoxic Ts
Inhibitory receptor for NK cells
What genes encode for Class 1 MHCs?
HLA-A, HLA-B, HLA-C
What is the structure of Class 1 MHCs?
4 extracellular globular domains Heterodimer of 2 proteins: a chain (a1, a2, a3) - HLA encoded B-microglobulin - nonHLA encoded covalently linked
What is the peptide binding groove on Class 1 MHCs?
Area b/w the a1 and a2 domains - site with the greatest polymorphism
Binds peptides of about 8-10 aa - the closed ends limit size
Each allele has different range of peptide that can bind
If its hydrophilic, only hydrophilic molecules bind.
How are Class 1 MHCs synthesized?
a chain translated into ER where it interacts with B2 micro globulin
Associates with peptides derived form cytosolic (intracellular) proteins
Transported to surface
What is a key features of Class 1 MHCs?
All alleles of Class 1 can be expressed at same time, so you can have up to 6 different class 1 MHCs being expressed that present slightly different shapes and sizes of peptides.
What genes at Class II MHCs encoded by?
HLA-D region:
HLA-DP, HLA-DQ, HLA-DR
Describe Class II MHCs.
Membrane bound glycoproteins
Primarily on APCs - DCs, macrophages, B-cells
Present Ag to CD4 T-cells
What is the structure of Class II MHCs?
Heterodimer - a (a1 and a2) & B chains (B1 and B2)
Strongly associated instead of covalent linkage
What is the peptide binding groove on Class II MHCs?
Formed by a1 and B1.
Binds peptides b/w 13 and 18 aa - open ends
a1 and B1 domains - greatest polymorphism
How are Class II MHCs synthesized?
a and B chains synthesized in ER and interact with a third protein (invariant chain) that blocks the peptide binding domain
Invariant chain is degraded in an endocytic compartment and peptides can bind that have entered cell via endocytosis
What is the key feature of Class II MHCs?
All alleles of a and B chain are expressed. So you can have 6 a chains and 6 B chains that can all bind with each other (a to B). Makes it very diverse and a greater range of peptides can bind.
What is characteristic about peptide binding to the MHCs?
Low affinity interaction
So it has a slow “on” rate and and slow “off” rate, which allows the peptide to be bound long enough for T-cell to interact with it.
One peptide at a time
How does a peptide bind to the MHC?
Side chains of the peptide will fit into pockets on the floors of the peptide binding cleft. This anchors the peptide. There are residue of peptide that bow upwards and are recognized by Ag receptors on T-cells.
What is significant about minor MHCs in males?
Ags can be encoded on Y chromosome. This results in acute rejection of male grafts in females.
Explain the Dendritic Cell expression of Class 2 MHCs.
Constitutive expression, increased by IFN-y
Ag presentation to naive T in initiation of T-cell response to protein Ags.
Explain the Macrophage expression of Class 2 MHCs.
Low or negative, induced by IFN-y
Ag presentation in CD4 effector T cells in effector phase of cell-mediated response
Explain the B-cells expression of Class 2 MHCs.
Constitutive expression, increased by cytokines (IL-4)
APC to CD4 helper Ts in humoral immune response
Explain the process of when an antigen is captured.
Microbe phagocytosed by APC. Travel to the closest lymphoid tissue. On the way, process antigen and ready to express. Lose adhesive markers and up regulate CCR7. Increase expression of MHCs and CD80 (DC)
What are the two processing pathways?
Class 1 MHCs - process and express INTRAcellular pathogens and self Ags
Class 2 MHCs - process and express EXTRAcellular pathogens
Describe the Class 2 MHC pathway.
Exogenous proteins are ingested and degraded in phagosome
At same time a & B chains + invariant synthesized in ER - transported to endosome
Peptide not loaded until LATE endosome –> expression
In the Class 2 MHC pathway, how is the invariant chain removed.
In the phagosome, the invariant chain is degraded to the CLIP protein still in the peptide groove by lysosomal enzymes. HLA-DM acts as a peptide exchanger - removes CLIP and exchanges it with peptide
Why is the invariant chain important?
Because cytosolic proteins or self proteins can attach to the receptor, which you don’t want to happen.
Describe the Class 1 MHC pathway.
Ag can be cytosolic or ingested into phagosome. The protein of the degraded microbe is then ubiquinated in the cytosol. A proteasome will degrade the protein. Peptide fragments will then be transported into the ER with the help of TAP. a chain of MHC is being made at same time. The enzymatic activity of TAP + tapasin, will cut peptide into right length and load it into peptide groove. B-chain is added and the complex is exocytosed.
Everyday, Class 1 MHCs are presenting self-peptides. Why is this important?
Because the MHCs are trained to not kill your cells. They are checking to see if your proteins are still you. If they detect a misfolded or wrong protein, they invoke an immune response. So, it is kind of a regulatory process.
What are the outcomes of Ag presentation?
Class 2:
Macrophage activation - killing of phagocytksed microbe
B-cell Ab secretion
Class 1:
Killing of Ag-expressing target cell by cytotoxic Ts
What is Cross-presentation?
DCs ingest microbes and can present Ag to CTLs (requires IL2 for activation) with Class 1 MHC and to Th with Class 2 MHC. This pathway of antigen presentation violates the presumption that internalized proteins are displayed only by class II MHC molecules to CD4 + T cells.
HLA associated disease: Ankylosing Spondylitis. Describe what is happening.
Inflammation of the spine
88% of people with HLA-B27 get the disease. This may be because they express a protein that is being reacted to that should not be expressed or they can’t express a protein that they need to in order to not get this disease.
HLA associated disease: Ankylosing Spondylitis. Describe what is happening.
Sequelae of the streptococcus pyrogenous infection
Generation of Abs against streptococci cross reacts with the cardiac tissue. This causes inflammation of valves and they end up in cardiac failure.
Pt with HLA-DR4 allele more susceptible.
What are some other HLA associated diseases?
Sjogren’s Syndrome (associated with HLA-DR3)
Insulin-dependent DM (HLA-DQw8)
Psoriasis (HLA-B3)
What happens in Renal cell carcinoma in relation to Ag processing?
The TAP protein is down regulated so we are not able to get peptides loaded into the Class 1 MHCs. Also with neuroblastomas.
Describe the Bare Lymphocyte Syndrome (Class 1).
TAP protein in nonfunctional, so peptides can’t enter ER and cannot be presented on the cell surface (“bare”)
Sx: chronic respiratory infections, poor response to virus
Describe the Bare Lymphocyte Syndrome (Class 2).
HLA II genes turned on by transcriptional activator - HLA class II transactivator (CIITA), which is induced by IFN-y. There is a defect in CIITA leading to deficiency in HLA class II expression on cells and non-functioning T-cells.
What are the 2 membrane bound lymphocyte receptors? Where are they found? What are the soluble receptors?
B-cell receptors - B-cells
T-cell receptors - T-cells
Soluble receptors = Antibodies
What is an Epitope?
Specific part of the Ag that contacts the Ag-binding sitss of an Ab or TCR
What are Haptens?
Small molecules that can’t induce an immune response alone. Can induce an response when couple with a self or carrier protein.
They are clinically important for drug allergies (Penicillin) and vaccine design
Describe the B-cell Receptor.
Composed of surface immunoglobulin and 2 invariant chains (Iga and IgB). Iga/B ensure surface expression of immunoglobulin and send cytoplasmic signals.
Describe the structure of an Ab.
4 polypeptide chains - 2 heavy, 2 light
Held together by disulfide bonds
Constant region (Fc) defines the class, variable region (Fab) is antigen binding
Name the classes of Abs.
m, g, a d, e
Based on the heavy chain
Name the types of Abs.
k and (upside-down) y Based on the light chain
Describe Ab digestion with Papain and Pepsin.
Papain digestion gives a functional Fc region, but the Fab region fall apart.
Pepsin digestion gives a functional F(ab’)2 region, able to bind Ag, but Fc falls apart.
What are the Hypervariable regions?
Each V domain has 3 Hv regions - involved in Ag binding by creating interaction site that is complementary in shape, charge, and hydrophobicity to epitope that binds
Epitope binds to idiotope
Define Allotype.
Allelic differences leading to differences in the constant regions of the Ig
Define Idiotype.
Antigenic determinant on the V regions differ. We all will see the Ag, but may see it differently.
Describe IgM.
5-10% of serum Abs
First produced
Pentamer - 10 Ag binding sites
Efficient binding of Ags with multiple repeating epitopes
Efficient binding C’ (classical)
Expressed on B-cells as a monomer B-cell receptor
Has a J-chain.
What is the J-chain on the IgM?
Polypeptide linked to the Fc region by disulfide bonds. Bonded to 2 of 10 mu chains
Binds to secretory cells
Describe IgG.
80% of serum Abs.
Predominant Ab of the secondary immune response
4 subclasses: 1-4
Differences on H chains, with functional differences
Only class to cross placenta
Involved in Classical pathway
Describe serum IgA.
10-15%
Part of secondary immune response
Exists usually as a monomer, but can be a polymer with a J chain binding a Fc region
Describe secretory IgA.
Predominant Ab class in external secretions
Secretory component - polypeptide produced by epithelial cell of mucosa membranes
Found a lot in entry points for Ag
Has 4 binding sites - efficient
involved in Newborn immunity
Describe IgE.
Low concentration in serum
Secondary immune response
Binds to blood basophils and tissue mast cells by Fc receptor with high affinity
Powerful pharmacologic Rxns (asthma, allergies)
Thought to play role in Helminth infections
Describe IgD
> 1% in serum
Most are expressed as membrane bound B-cell receptors
Relationship b/w IgD and IgM as B-cell receptors.
They bind to SAME Ag. Difference is IgD does not lead to activation of B-lymphocytes, may be regulatory.
By what interactions do Ab and Ag interact?
Non-covalent bonds: Hydrogen bonds Electrostatic bonds Van der Walls Hydrophobic forces
Each bond is weak and reversible
Describe Affinity vs. Avidity.
Affinity - strength of interaction b/w one epitope and idiotope
Avidity - strength of interaction b/w multiple epitopes and multiple idiotypes (Abs interacting with multiple Ags or other Abs.
In what ways van epitopes be recognized by B-cells?
As conformational determinants - Recognizes the Ag in its 3D structure, determinant lost with denaturation
As linear determinants - binds to accessible determinant, Ag denatures, and can bind to inaccessible determinant
Neoantigenic determinant - determinant is near site of proteolysis - needs to happen to bind
Describe T-dependent Ags.
Theses are Ag that require both Th-cells and B-cells to stimulate an Ab response.
These Ags are PROTEINS.
Describe T-independent Ags.
NON-proteins = LIPIDS or POLYSACCHARIDES
Can stimulate Ab response without Th-cells - multiple epitopes crosslink several BCRs
Describe T-cell receptors (TCR).
a-chain + B-chain
Small subpopulation of delta/gamma chains existing in epithelial and mucosal compartments
Variable and constant regions
Di-sulfide bonds
Need co-receptor for cell-signaling
Free peptides are not recognized.
Th = release cytokines Cyt-T = kill infected cells
Through what receptor does T-cell signaling happen?
CD3-zeta
What do y/d chain TCR recognize?
Lipid Ags - b/c a lot of bacteria goes through mucosa every day
DAMPs
Describe Superantigens.
Bind directly to MHC Class II molecules and Vb of TCR.
Diseases associated with Sags are sue to hyper activation of immune system, releasing a lot of cytokines by activated T-cells.
Ex: food poising, shock, etc.
