Antigen Capture and Presentation

Question 1

How does the Innate IR initiate the Adaptive IR?

Answer 1

Two signals:

1. Process and present Ag to T-cells
2. Generation of surface molecules that function as co-stimulatory signal with Ag to activate T and B cells

Question 2

What are MHCs?

Answer 2

Major histocompatibility complex
They are antigen presenting molecules to T-lymphocytes
Also referred to as HLA (human leukocyte Ag)

Question 3

What are the 2 classes of MHC? Where are they found?

Answer 3

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

Question 4

How were MHCs found?

Answer 4

During transplantations. There was histocompatibility if the transplanted tissue was accepted at self.

Question 5

Describe MHC location and organization.

Answer 5

The allele is found on chromosome 6

Highly polymorphic - more than 150 separate alleles

Question 6

Describe the MHC/HLA haplocyte.

Answer 6

Total set of MHC/HLA alleles present on each chromosome
Heterozygous - one from mom and dad
Codominant expression

Question 7

Why is the codominant expression of the MHC/HLA allele important?

Answer 7

Increases number of different MHC molecules that can present peptides to T-cells

Question 8

Why is the polymorphism of the MHC/HLA allele important?

Answer 8

Ensures that different individuals are able to present and respond to different microbial peptides

Question 9

Which class of MHC is more variable?

Answer 9

Class 2

Question 10

Describe Class 1 MHC.

Answer 10

Membrane bound glycoproteins
Expressed on all nucleated cells
Present Ag to CD8 cytotoxic Ts
Inhibitory receptor for NK cells

Question 11

What genes encode for Class 1 MHCs?

Answer 11

HLA-A, HLA-B, HLA-C

Question 12

What is the structure of Class 1 MHCs?

Answer 12

4 extracellular globular domains
Heterodimer of 2 proteins:
a chain (a1, a2, a3) - HLA encoded
B-microglobulin - nonHLA encoded 
covalently linked

Question 13

What is the peptide binding groove on Class 1 MHCs?

Answer 13

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.

Question 14

How are Class 1 MHCs synthesized?

Answer 14

a chain translated into ER where it interacts with B2 micro globulin
Associates with peptides derived form cytosolic (intracellular) proteins
Transported to surface

Question 15

What is a key features of Class 1 MHCs?

Answer 15

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.

Question 16

What genes at Class II MHCs encoded by?

Answer 16

HLA-D region:

HLA-DP, HLA-DQ, HLA-DR

Question 17

Describe Class II MHCs.

Answer 17

Membrane bound glycoproteins
Primarily on APCs - DCs, macrophages, B-cells
Present Ag to CD4 T-cells

Question 18

What is the structure of Class II MHCs?

Answer 18

Heterodimer - a (a1 and a2) & B chains (B1 and B2)

Strongly associated instead of covalent linkage

Question 19

What is the peptide binding groove on Class II MHCs?

Answer 19

Formed by a1 and B1.
Binds peptides b/w 13 and 18 aa - open ends
a1 and B1 domains - greatest polymorphism

Question 20

How are Class II MHCs synthesized?

Answer 20

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

Question 21

What is the key feature of Class II MHCs?

Answer 21

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.

Question 22

What is characteristic about peptide binding to the MHCs?

Answer 22

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

Question 23

How does a peptide bind to the MHC?

Answer 23

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.

Question 24

What is significant about minor MHCs in males?

Answer 24

Ags can be encoded on Y chromosome. This results in acute rejection of male grafts in females.

Question 25

Explain the Dendritic Cell expression of Class 2 MHCs.

Answer 25

Constitutive expression, increased by IFN-y | Ag presentation to naive T in initiation of T-cell response to protein Ags.

Question 26

Explain the Macrophage expression of Class 2 MHCs.

Answer 26

Low or negative, induced by IFN-y | Ag presentation in CD4 effector T cells in effector phase of cell-mediated response

Question 27

Explain the B-cells expression of Class 2 MHCs.

Answer 27

Constitutive expression, increased by cytokines (IL-4) | APC to CD4 helper Ts in humoral immune response

Question 28

Explain the process of when an antigen is captured.

Answer 28

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)

Question 29

What are the two processing pathways?

Answer 29

Class 1 MHCs - process and express INTRAcellular pathogens and self Ags Class 2 MHCs - process and express EXTRAcellular pathogens

Question 30

Describe the Class 2 MHC pathway.

Answer 30

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

Question 31

In the Class 2 MHC pathway, how is the invariant chain removed.

Answer 31

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

Question 32

Why is the invariant chain important?

Answer 32

Because cytosolic proteins or self proteins can attach to the receptor, which you don't want to happen.

Question 33

Describe the Class 1 MHC pathway.

Answer 33

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.

Question 34

Everyday, Class 1 MHCs are presenting self-peptides. Why is this important?

Answer 34

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.

Question 35

What are the outcomes of Ag presentation?

Answer 35

Class 2: Macrophage activation - killing of phagocytksed microbe B-cell Ab secretion Class 1: Killing of Ag-expressing target cell by cytotoxic Ts

Question 36

What is Cross-presentation?

Answer 36

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.

Question 37

HLA associated disease: Ankylosing Spondylitis. Describe what is happening.

Answer 37

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.

Question 38

HLA associated disease: Ankylosing Spondylitis. Describe what is happening.

Answer 38

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.

Question 39

What are some other HLA associated diseases?

Answer 39

Sjogren's Syndrome (associated with HLA-DR3) Insulin-dependent DM (HLA-DQw8) Psoriasis (HLA-B3)

Question 40

What happens in Renal cell carcinoma in relation to Ag processing?

Answer 40

The TAP protein is down regulated so we are not able to get peptides loaded into the Class 1 MHCs. Also with neuroblastomas.

Question 41

Describe the Bare Lymphocyte Syndrome (Class 1).

Answer 41

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

Question 42

Describe the Bare Lymphocyte Syndrome (Class 2).

Answer 42

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.

Question 43

What are the 2 membrane bound lymphocyte receptors? Where are they found? What are the soluble receptors?

Answer 43

B-cell receptors - B-cells T-cell receptors - T-cells Soluble receptors = Antibodies

Question 44

What is an Epitope?

Answer 44

Specific part of the Ag that contacts the Ag-binding sitss of an Ab or TCR

Question 45

What are Haptens?

Answer 45

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

Question 46

Describe the B-cell Receptor.

Answer 46

Composed of surface immunoglobulin and 2 invariant chains (Iga and IgB). Iga/B ensure surface expression of immunoglobulin and send cytoplasmic signals.

Question 47

Describe the structure of an Ab.

Answer 47

4 polypeptide chains - 2 heavy, 2 light Held together by disulfide bonds Constant region (Fc) defines the class, variable region (Fab) is antigen binding

Question 48

Name the classes of Abs.

Answer 48

m, g, a d, e | Based on the heavy chain

Question 49

Name the types of Abs.

Answer 49

``` k and (upside-down) y Based on the light chain ```

Question 50

Describe Ab digestion with Papain and Pepsin.

Answer 50

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.

Question 51

What are the Hypervariable regions?

Answer 51

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

Question 52

Define Allotype.

Answer 52

Allelic differences leading to differences in the constant regions of the Ig

Question 53

Define Idiotype.

Answer 53

Antigenic determinant on the V regions differ. We all will see the Ag, but may see it differently.

Question 54

Describe IgM.

Answer 54

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.

Question 55

What is the J-chain on the IgM?

Answer 55

Polypeptide linked to the Fc region by disulfide bonds. Bonded to 2 of 10 mu chains Binds to secretory cells

Question 56

Describe IgG.

Answer 56

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

Question 57

Describe serum IgA.

Answer 57

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

Question 58

Describe secretory IgA.

Answer 58

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

Question 59

Describe IgE.

Answer 59

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

Question 60

Describe IgD

Answer 60

>1% in serum | Most are expressed as membrane bound B-cell receptors

Question 61

Relationship b/w IgD and IgM as B-cell receptors.

Answer 61

They bind to SAME Ag. Difference is IgD does not lead to activation of B-lymphocytes, may be regulatory.

Question 62

By what interactions do Ab and Ag interact?

Answer 62

``` Non-covalent bonds: Hydrogen bonds Electrostatic bonds Van der Walls Hydrophobic forces ``` Each bond is weak and reversible

Question 63

Describe Affinity vs. Avidity.

Answer 63

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.

Question 64

In what ways van epitopes be recognized by B-cells?

Answer 64

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

Question 65

Describe T-dependent Ags.

Answer 65

Theses are Ag that require both Th-cells and B-cells to stimulate an Ab response. These Ags are PROTEINS.

Question 66

Describe T-independent Ags.

Answer 66

NON-proteins = LIPIDS or POLYSACCHARIDES Can stimulate Ab response without Th-cells - multiple epitopes crosslink several BCRs

Question 67

Describe T-cell receptors (TCR).

Answer 67

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 ```

Question 68

Through what receptor does T-cell signaling happen?

Answer 68

CD3-zeta

Question 69

What do y/d chain TCR recognize?

Answer 69

Lipid Ags - b/c a lot of bacteria goes through mucosa every day DAMPs

Question 70

Describe Superantigens.

Answer 70

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.