T-cell (will rename)

Question 1

What are the major types (subsets) of T-cells?

Answer 1

T-cells:

1. CD 8 (Cytotoxic T-cells)
2. CD 4 (T-helper cells)
   
   • Th1 = fight intracellular pathogens
   • Th2 = allergy & helminths
   • Th17 = extracellular bacteria & fungi
   • T-regs = control / calm immune response to self and foreign antigens

Question 2

What is the function of CD8 CTL cells?

Answer 2

Kill viral infected and tumor cells presenting Ag via MHC1

Question 3

What is the function of CD4 Th1 Cells?

Answer 3

Major action is killing of intracellular microbes that have been phagocyzed by macrophages

• Microbe activates dendritic cell or macrophage
• Dendritic cell or macrophage release IL-12
• NK cells get excited b/c of the identified intracelluar microbe (or the IL-12 release), then start making INFy
• IL-12 & IFNy cause the CD4 cell to turn into a Th1 specific T-helper cell (via STAT1, Tbet, & STAT4)
• Th1 cell now starts dumping out IFNy

Th1 activation leads to:

1. IFN-y production
2. Macrophage activation (intracellular killing)
3. B-cell activation for additional help
4. Decreased Th2

Question 4

What is the function of CD4 Th2 cells?

Answer 4

Killing helminths and allergic responses

•Helminths are too large to be phagocytosed

• IL-4 is released when the helminth is recognized by the innate immune system
• IL-4 stimulates Th2 development by activating JAK1 & JAK3 to stimulate STAT6
• STAT6 induces expression of GATA3 → Th2 production

Th2 cell makes:

1. IL-4
2. IL-13
3. IL-5

Th2 activation leads to:

• Killing of helminths
• Eosinophil activation and production (via IL-5)
• B-cell activation & enhancement
• Class switching to IgE (via IL-4)
• “Allergy” via IgE interaction w/ mast & basophils
• Mucus production & smooth muscle changes (IL-13)

Question 5

What is the function of CD4 Th17 cells?

Answer 5

Killing of extracellular bacteria and fungi by

a. Enhancing neutrophil recruitment & activation
b. Promoting barrier integrity (skin, intestine)

Bacteria or fungi act on dendritic cells, causing production of:

• IL-6
• IL-1
• IL-6 & IL-1 work with TGF-B to activate RORγϯ and STAT3 •
• This leads to increased production of IL-21 & IL-23, which amplifies Th17 production
• Active Th17 cells release IL-17 & IL-22 which drive inflammatory responses in the fight against infection in the GI tract and skin especially

Question 6

What is the function of T-reg cells (CD4/CD25+)

Answer 6

T-reg cells:

• modulate the immune system
• maintain tolerance to self-antigens
• prevent autoimmune disease

Tregs are generally immunosuppressive and often suppress or downregulate induction and proliferation of effector T cells

• Tregs express the biomarkers CD4, FOXP3, and CD25

The cytokine TGFβ is essential for Tregs to differentiate from naïve CD4+ cells and is important in maintaining Treg homeostasis

• TGF-B & IL-2 activate STAT5 which stimulates FOXP3
• The activated T-reg cell releases IL-10 & TGF-B

•IL-10 ⇒ decreased Th1 response

Question 7

How do CD8 CTL destroy cells?

Answer 7

a. FasL activates apoptosis
b. Secretes cytokines: IFN-γ, TNF-α
c. Secrete cytotoxic granules perforin to form pores and granzymes to activate caspases

Question 8

Where does CD8 CTL action take place?

Answer 8

At immune synapse, interface btw CTL and target

LFA-1 on T cell binds tightly ICAM-1 on target

TCR/CD8-MHC/AG cluster centrally

Question 9

How do CD8 CTL cells form immune synapse?

Answer 9

• LFA-1 on CTL binds ICAM-1 on target
• Keeps cytotoxic molecules within synapse

Question 10

How are NK cells different from T & B cells?

Answer 10

b. Born ready to fight; no clonal expansion needed
c. Non specific like other innate cells (macro, DC or PMNS)
1. Kills cells that CTL can’t see

• Foreign proteins in cell membrane without MHC I
• Cell surface distress proteins - MICA/B
• Loss of MHC I expression

2. Also has quicker response than the naive CD 8 T cell: immediate vs. 1 week

Question 11

How does NK cells execute its action?

Answer 11

Synapse, cytokines and killing like CD8 CTL

Perforin/granzymes and FasL

Not antigen specific

Question 12

What are two types of NK cell receptors?

Answer 12

a. Inhibitory receptors - stand off - cell lives
* Killer Ig-like receptor (KIR)
b. Excitatory receptors - favor attack - cell apoptose
* NK revived up to kill
- MICA and MICB - markers of cell stress
- IgG coating target cell
- Detect various patterns of viral DNA in the cell membrane

Question 13

What is the function of Killer Ig-like receptor (KIR)?

Answer 13

a. Detects the presence of MHC1 on target
b. Cytoplasmic tail contains immunotyrosine based inhibitory motifs (ITIMs)
c. ITIM recruit phosphatase which dampens activating signals

Question 14

What are other NK cell excitatory receptors?

Answer 14

1. NKG2D (activating)
   
   • a. Recognizes MICA/B, markers of cell stress (MHC1 homologs w/o peptide)

b. Provides an activating signal similar to CD28 on a T cell

c. One step closer to killing the target
2. Fc(gamma) Receptor III (FC-gamma-RIII)
* a. Binds IgG coating the target cell -viral env proteins, mutated tumor proteins

b. Its ITAM’s favor NK activation
3. TLR-3, TLR-7
* a. Detect viral RNA

b. Contribute to NK activating signals

Question 15

What is the fate of target if NK’s activating signals outweigh inhibitory signals?

Answer 15

a. Immune synapse formation
b. Expression of FasL (Fas ligand)
c. Exocytosis of perforin and granzyme B

Question 16

Describe the layout of a T-cell receptor

Answer 16

Question 17

How strong is the general affinity of a T-cell receptor for the average antigent?

Answer 17

TCR’s have low affinity for Ag

– they are very selective and need accessory molecules to increase / strengthen the interaction

• The CD4 or CD8 receptor of the T-cell interacts with membrane proximal protein to strengthen it

Question 18

Where do CD8 & CD4 T-cells bind to their respective MHC molecule?

Answer 18

CD8 receptor binds to the alpha-3 protein chain of the MHC I

CD4 binds the beta-chain of the MHC II

**researchers have figured out they can purify Class I molecules, then add an immunoflorescent reagent that works as marker for a specific Ag. That way they can monitor how many CD8 cells are reacting to the specific Ag during an infection (say influenza) or to an autoimmune condition Ag. Then they can use this information to guide treatment / development of therapies by how many T-cells are responding to the particular Ag

Question 19

What are the major genes involved in MHC class I & II molecules?

Answer 19

Class I = major genes are HLA A, B, & C →display peptides from components of intracellular pathogens

Class II = major HLA DR, DP, DQ → display peptides from phagocytes (CD4)

Question 20

What are the major proteins / buzz words / molecules to know about MHC class I antigen development / presentation?

Answer 20

**TAP1 & TAP2 (transporter proteins)

**Tapasin / Calnexin / Erp57

Class I Ag processing and presentation

• the protein Ag is already w/in a cell (endogenous) → proteasome breaks down endogenous protein Ags into small peptides → transporter protein called TAP1/TAP2 take peptides to endoplasmic reticulum → chaperone proteins hold the peptides together while it is being formed (Tapasin / Calnexin / Erp57) →moves to cell surface to be displayed as MHC I

Question 21

What are the major proteins / buzz words / molecules to know about MHC class II antigen development / presentation?

Answer 21

invariant chain

HLA-DM and chaperone proteins (Calnexin)

Class II Ag processing & presentation

• these are able to be taken from outside the cell (i.e. phagocytosed / endocytosis / ect by APC).
• uses the phagocytosed Ag → uses the HLA 2 invariant chain to hold the Ag in place → allows the Ag to be broken down into peptides by the lysosome → HLA-DM and chaperone proteins (Calnexin) hold the peptides in place while the invariant chain is dissolved → then is transported to the surface to be presented

Question 22

What does polymorphic mean?

Answer 22

-MHC molecules are polymorphic → meaning each person’s MHC molecules are different → this prevents microbes from being able to avoid the MHC of another individual’s immune system (even if it has tricked the original host persons immune system completely)

Question 23

What cell surface molecules and co-receptor molecules are present on a T-cell when it is trying to be activated? (5)

Answer 23

TCR must have:

1. 2 alpha chains & 2 beta chains
2. the CD3 complex
   
   • ​CD3 has 2 chains = eδ & eγ (have ITAMs)
   • CD3 & ζζ chains are identical in all T-cells
3. ζζ chains (have ITAMs)
4. the CD4 or CD8 co-receptor
5. the CD28 – co receptor

AND…

• In order for the T-cell to be activated, the APC must also present:
  
  • CD80/CD86 (aka B7-1 or B7-2)
  • which tells the T-cell that the antigen is a foreign Ag that can allow activation

Question 24

What determines whether a T-cell will be activated?

Answer 24

The APC determines whether the Ag it is presenting is dangerous or foreign. It does this with PAMPs.

• If it determines it to be dangerous, it will also present the B7-1 / B7-2 (aka CD80/CD86) on it’s surface

If it doesn’t express the CD80 / B7-1 co-receptor (just presents the Ag), the T-cell wont activate to it & will go into anergy

**HOWEVER… super-antigens can (Staph) can activate the T-cell w/o the co-stimulatory receptors

Question 25

What MHC class molecules do B-cells contain?

How can a B-cell activate a T-cell?

Answer 25

B-cell interaction with T-cell

• B-cells have immunoglobulins on their surface, but also express class II HLA (MHC) receptors separately from the immunoglobulin area.
• If the B-cell HLA class II receptors identify an Ag, it will then endocytose the Ag and process it. This tells the B-cell that the Ag is potentially dangerous, so it will then express CD80/CD86 (B7-1 / B7-2) on it’s surface for the T-cell to recognize
• the T-cell recognizes the Ag, as well as the CD80 / CD28 interaction, which tells the T-cell this is a big deal.

Question 26

When a B-cell activates a T-cell, what does the T-cell do in response?

Answer 26

• When a T-cell recognizes an Ag from a B-cell (which is also presenting the CD80 receptor for the T-cell’s CD28 co-receptor to acknowledge), it tells the T-cell = GAME ON
• The T-cell then expresses CD40L back toward the B-cell, which activates the B-cell CD40 co-receptor.
• This tells the B-cells to undergo isotype switching to change it’s initial IgM antibody subtype to whatever particular antibody is necessary to address the Ag (IgG / IgE / IgA)
• This process is regulated by cytokines to determine which Ig the B-cell should make

B-cell interaction with T-cell

• B-cells express class II HLA (MHC) receptors on surface
• B-cell HLA class II receptors identify an Ag, it will then endocytose the Ag and process it. This tells the B-cell that the Ag is potentially dangerous, so it will then express CD80/CD86 (B7-1 / B7-2) on it’s surface for the T-cell to recognize

Question 27

What interaction b/w a T-cell and B-cell causes isotype switching?

(i.e., what co-receptor leads to B-cell isotype switching)

Answer 27

T-cell = expresses CD40L

B-cell = CD40 activated by CD40L

This tells the B-cells to undergo isotype switching to change it’s initial IgM antibody subtype to whatever particular antibody is necessary to address the Ag (IgG / IgE / IgA)