Antigen Presentation, T Cell Development/Fx

Question 1

Cytosolic vs. Intravesicular vs. Extracellular paths/toxs

Answer 1

Question 2

MCH class I genes

Answer 2

HLA-A, -B, and -C

Multiallelic and are co-dominantly expressed

Up to six different MHC class I isoforms can be expressed on a single cell

Question 3

β2-
microglobulin

Answer 3

Part of the heterodimeric protein complex of the MHC class I alpha-chain

Required for MHC I function

Invariant = does not vary from person to person

MHC class I proteins are expressed on all nucleated cells

Question 4

MHC Class I α-chains

Answer 4

Three extracellular domains and one intracellular domain

EC domains - contain peptide-binding pocket (holds peptide being presented)

Present endogenous antigen peptides to CD8+ T cells

Question 5

Where is the most sequence variability between HLA-A, -B, and -C allotypes?

Answer 5

In the peptide-binding groove

Question 6

Endogenous pathway of antigen presentation

Answer 6

1. Ubiquitination
2. Proteosome degrades ubiquinated targets into peptides
3. TAP (transporter associated w/ antigen processing) transports antigen into the ER
4. Antigen is loaded into the MHC class I binding groove in the ER
5. Transport of MHC/peptide complex to cell surface (will not transport to cell surface w/o peptide in binding pocket)

Question 7

MHC class II genes

Answer 7

HLA-DR, -DQ, and -DP

Each locus (DR, DQ, and DP) expresses one α chain and at least one β chain

MHC class II genese are multi-allelic and co-dominantly expressed

A single cell can express many (3-16) different MHC class II isoforms

Question 8

MHC Class II proteins

Answer 8

Only expressed in B cells, macrophages, and DCs (professional APCs) and thymic epithelial cells

Present to CD4+ T helper cells

Question 9

Exogenous pathway of antigen presentation

Answer 9

1. APC ingests whole organism, Ab/Ag complex
2. Class II invariant chain (Li) associated w/ newly synthesized MHC class II in the ER to prevent cytosolic peptides from binding - MHC class II + Li complex is exported into an exocytic vesicle
3. Exocytic vesicle containing MHC class II + Li fragment fuses w/ endocytic vesicle containing Ag peptides - forming the MHC class II compartment - Li is then released from MHC class II, allowing Ag peptides to bind (complex then transported to cell surface)

Question 10

Clinical consequences of MHC variability in the population

Answer 10

Major cause for transplant rejection

The better an Ag peptide binds to MHC determines how well the adaptive immune response activates

For example, individuals with specific MHC haplotypes do not respond well to the Hepatitis B Virus vaccine because the HBV antigen protective epitope peptides do not “fit” well into the grooves of the specific MHC isoforms that they express

Question 11

To activate a T cell, what must the TCR bind?

Answer 11

The TCR must bind specifically to both the MHC protein and the peptide Ag being presented

Further - once TCR binds to correct MHC allotype/peptide complex - TCR requires CD3 complex to transduce an activating signal into the T cell

Question 12

What happens to the TCR if CD3 is not present/functional?

Answer 12

The TCR will not be expressed on the T cell surface

Question 13

Superantigens

Answer 13

Bacterial toxins that bind MCH class II and TCR (CD4+ T cell) at the same time - activates T cell and APC regardless of the antigen peptide presented or the TCR antigen or MHC specificity

Superantigens stimulate APCs as well as 2-20% of all mature CD4+ T cells - results in massive, systemic pro-inflammatory cytokine release

Produces SIRS, shock, and death

Toxic shock syndrome - if survived may experience compensatory anti-inflammatory response syndrome

Question 14

TCR alpha/beta chain gene segments

Answer 14

V - Variable: These segments are varied, giving the TCR its diversity

D - Diversity: Found only in the beta chain, adding more variability

J - Joining: Connects segments together, found in both chains

C - Constant: This part is the same across all humans and is essential for TCR function.

Question 15

What encodes the variable region of a TCR beta chain?

Answer 15

VDJ gene segements

Question 16

What encodes the variable region of a TCR alpha chain?

Answer 16

VJ gene segments

Question 17

Somatic recombination

Answer 17

RAG enzyme binds to recombination signal sequences (RSS) flanking gene segments. It loops out and excises the DNA between gene segments. RAG and other DNA repair enzymes then ligate the segments to form a contiguous DNA sequence in the TCR α or β chain gene.

Each developing thymocyte has two alleles of both α/β genes. Once one allele is successfully recombined, the recombination of the other allele is inhibited (allelic exclusion), ensuring a single T cell clone bears TCRs that react with the same MHC/antigen peptide combo.

Question 18

Continuous autocrine stimulation with what is required for naive T cell survivial?

Answer 18

IL-2 via IL-2 receptor

Question 19

Double negative thymocyte first undergoes _______

Answer 19

TCR beta chain and alpha chain gene rearrangement

Question 20

What happens if rearrangement of alpha/beta TCR segments is successful?

Answer 20

TCR alph/beta proteins complex w/ CD3 to form fully functional TCR -

signals developing thymocyte to survive and express both CD4 and CD8 - producing a double positive thymocyte (can undergo + and - selection)

Question 21

Where does positive selection of developing T cells occur?

Answer 21

Thymic cortex

Double-positive thymocytes that have moderate-strong binding affinity to either MHC class I or MHC class II and self-antigen peptides presented on thymic cortical epithelial cells will survive

Question 22

T cell maturation

Answer 22

Question 23

Tigh vs. moderate binding of TCR maturation

Answer 23

Question 24

IL-2 receptor defects

Answer 24

IL-2 autocrine stimulation is required for thymocytes to survive development, as well as for survival of naive T cells

Can result in complete CD4+/CD8+ T cell deficiency

Question 25

Only APC that activates naive CD4+ and CD8+ T cells

Answer 25

Dendritic cells

Naive T cell activation requires two contact dependent signals initially, with other contact-independent signals (cytokines) coming later in the differentiation process

Question 26

T cell activation signal and co-stimulation

Answer 26

Activation signal - TCR binding to MHC-Ag peptide complex

Co-stimulation - naive T cell CD28 binds to the B7 co-stimulatory molecule (CD80/86) on the DC

Proliferation and survival signal for the naive T cell - must be delivered simultaneously (if abnormal - i.e. mistimed - T cell apoptosis)

Question 27

Naive T cell activation

Answer 27

Naive T cell’s TCR recognizes an MHC/Ag complex on DC - two cells become tightly bound together via LFA-1 and ICAM-1

Activates CD3 signaling complex and DC B7 binding to T cell CD28 activates mTOR and the transcription factor NF-kB

Question 28

TCR/CD3 stimulation

Answer 28

Results in phosphorylation of CD3 complex and recruitment of tyrosine kinases that ultimately activate phospholipase C gamma-1 - produces secondary messengers DAG and IP₃

IP₃ activates calcineurin and leads to activation of transcription factor NFAT

DAG activates kinase cascades leading to activation of trancription factors AP-1 and NF-kB

Question 29

Role of AP-1 and NF-kB in T cell gene expression

Answer 29

Leads to T cell proliferation and differentiation of effector T cells

1. IL-2
2. Alpha subunit of the IL-2 receptor

Two most important genes activated

Because IL-2 and IL-2R a subunit
expression requires NFAT, NF-kB, and AP-1, any defect or inhibition of the T cell activation signaling process will similarly cause activated T cells to die

Question 30

Sirolimus (rapamycin)

Answer 30

Inhibits mTOR activation - antitransplant rejection drug

Question 31

Cyclosporine and tacrolimus

Answer 31

Inhibit calcineurin - immunosupressive

Question 32

What drug class inhibits NF-kB activation of cytokine production?

Answer 32

Glucocorticoids

Question 33

Most common cause of X-linked SCID

Answer 33

IL-2R_γ subunit defects

Severe defects in T and B cell Fx - b/c continous autocrine IL-2 stimulation via the IL-2 low affinity receptor is required for T cells to survive development

Completely lack T cells

Low-affinity IL-2 receptor consists of beta+gamma subunits

Most common form of SCID

Question 34

Th₁ cells

Answer 34

Promote macrophage and effector CD8+ CTL-mediated immune responses

Question 35

What cytokines drive Th₀ to Th₁ differentiation?

Answer 35

IL-12 and IFN-γ

Th₁ cells then leave secondary lymphoid tissues and enter sites of infection to carry out effector functions that activate macrophages and effector CD8+ CTLs

Question 36

Macrophage activation by Th₁ cells

Answer 36

Th₁ cell CD40L binds macrophage CD40 - in combination with the Th₁ secreted IFN-γ activated the macrophage and increases microbial killing via NADPH oxidase up-regulation and inflammatory cytokine production

Th₁ secreted IL-2 and IFN-γ also activate effector CD8+ CTLs

Th₁ response is pro-inflammatory and can damage tissues

IL-4 and IL-10 made by CD4+ Th₂ cells inhibit the differentiation of Th₀ to Th₁

Thus Th₁ are critical for eliminating intracellular paths that survive and replicate in macrophages (such as Mycobacteria)

Question 37

What T cells promote a neutrophil mediated inflammatory response?

Answer 37

CD4+ Th₁₇ cells

Differentiation is induced by TGF-β and IL-23

Question 38

What is produced by Th₂ cells at the site of infection that does not require additional TCR-mediated stimulation

Answer 38

IL-17 - promotes TNF-alpha, IL-1β, IL-6, and IL-8 production in epithelial cells, endothelial cells, and macrophages

Highly inflammatory and rapidly recruit neutrophils

Th₁₇ are integral in response to extracellualr, unicellular pathogens such as Candida and Klebsiella

Question 39

Effector CD4+ Th₂ cell differentiation

Answer 39

Question 40

What T cell does not migrate to infection sites?

Answer 40

Th₂ only migrates between different secondary lymphoid tissues

Question 41

DC secreted cytokine that causes differentiation of Th₀ into Th₂ cells

Answer 41

IL-4

Question 42

DC secreted cytokine that causes differentiation of Th₀ into Th₁₇ cells

Answer 42

TGF-β

IL-23

Question 43

Target cells of Th₂ cells

Answer 43

T-dependent B cells

Th₂ cells secretes IL4, IL5, IL10, and IL13 to help activate B cells

Question 44

What induces B cell class switching to make IgE?

Answer 44

Th₂ secretion of IL4 or IL13

Question 45

What induces B cell class switching to make IgA and also activates eosinophils?

Answer 45

IL5 secreted by Th₂ cells

Question 46

How does Th₁ activity inhibit Th₂ development?

Answer 46

Th₁ secretion of IFN-γ inhibits Th₂ development

Question 47

What induces B cell class switching to make IgG?

Answer 47

IL4 secretion by Th₂ cells

Question 48

How do Th₂ cells reduce Th₁ differentiation?

Answer 48

By secreting IL-4 and IL-10

Question 49

CD8+ cell induced apoptosis

Answer 49

Question 50

A patient is prone to infections with extracellular bacteria (like Staphylococcus aureus) and fungi (like Candida albicans). Which effector T cell type are they MOST LIKELY deficient in? (choose the SINGLE BEST answer)

Answer 50

CD4+ Th₁₇

Question 51

Dendritic cell secreted cytokines determine which type of effector T cell that a CD4+ Th0 cell will become. Which cytokine/effector cell combination is NOT CORRECT?

Answer 51

CD4+ Th₁₇ differentiation occurs in response to IL-17

Th17 cells secrete IL-17 to activate target cells

Question 52

Answer 52

A. Expression of the IL-2alpha subunit

and

D. Expression of IL-2

Question 53

Answer 53

C. VLA4 on T cell and VCAM-1 on endothelium

Reference table in first aid