T Cell Activation

Question 1

Briefly description of the start of T Cell Activation:

Answer 1

-Maturation of T cells in the Thymus (pos/neg selection) and B cells in Bone marrow
-Migration to secondary lymph organs
-Antigen presentation (MHC-II) from APC (dendritic cells) to naive T helper cells

Question 2

What are the signals required to activate a naive T cell?

Answer 2

Signal 1: APC: MHC-II + peptide with - -> on T helper: TCR + CD4 (helping stabilize MHC interaction)

Signal 2: APC: CD80/86 =B7 with - -> on T helper: CD28 (costimulatory, security signal bc we really need to be sure that it is an APC with a foreign peptide)

-Adhesin proteins

Signal 3: APC: Cytokines IL-2 from APC (Paracrine) and from T helper (self -> Autocrine IL-2)

Question 3

The function of IL-2:

Answer 3

-Increased proliferation
-Increased differentiation, and generation of memory cells at each step of differentiation
-Final differentiation into effector cells through activation to different helper T cell categories

Question 4

What is the function of Signal 3?

Answer 4

-Required for T cell differentiation

-Defines the path a T cell goes, depending on the cytokine

Is IL-2 the primary cytokine for T-cell activation?? -> No it is one of many effector cytokines and it depends on the pathogen which one is secreted
Does it activate T helper and cytotoxic cells? -> mainly through cross-presentation (license to present through MHC-I to cytotoxic T cells)

Question 5

Which of the signal for T cell activation is responsible for activation?

Answer 5

CD28 on T helper to check if it is really an APC

-Failure to make co-stimulatory interactions CD80 to CD28 leads to T cell death or conversion to T(reg) -> (peripheral tolerance: counteract self-reactive T-cells)

Question 6

What happens if a T helper cell binds to an APC but doesn’t get Signal 2 (verifying it is an APC)?

Answer 6

It will undergo apoptosis, bc the failure of signal 2 might be due to binding to a non-APC, which probably has a self-peptide on MHC-II
(if it is a non-APC with self -> that T cell shouldn’t be able to bind - so maturation wasn’t efficient)

or Clonal anergy

PERIPHERAL TOLERANCE

Question 7

Why is signal 2 important, which is making sure T helper cells are actually binding to an APC and not to a non-APC?

Answer 7

Because the odds of a non-APC showing a pathogen peptide is very low -> non-APC probably have self-peptides on their MHC-II

Question 8

What is the purpose of Superantigens?

Answer 8

Produced by viruses and bacteria (Toxins)
-They override the costimulation of Signal 2

-Bind to MHC-II and TCR and activate any TCR that binds any peptide - could be self (most TCR wouldn’t be activated upon binding to a peptide bc it is too unspecific)

-cause a lot of cytokine secretion by activated T cells to distract from itself

Question 9

What is the role of CTLA-4 and how is it different from CD28?

Answer 9

Located at T helper cell and like CD28 it binds B7 (CD80/86)

-Turn CD28 activation off after 2-3 days -> so that the inflammatory response will stop causing tissue damage or stop the auto-immune response

-It binds with higher affinity than CD28 and has inhibitory effects
-It works by outcompeting CD28 or blocking CD28 signal pathways (while both are bound to B7)

Question 10

How can CTLA-4 be used to fight cancer?

Answer 10

By blocking CTLA-4 with an antibody
-it extends the life span of the T cell
-for cancer treatment, where the immune system activity is low bc in cancer it fights against self (we do not have much self fighting T-cells)
-not good if an actual infection is around

Question 11

How does T-Cell Differentiation work?

Answer 11

-Through polarizing cytokines: from APCs binding to PAMPS with their PRRs (depending on which pathogens PAMP they bind they will send different cytokines leading to different T cell types)
Does one PRR bind to any PAMP, or is there a specific PRR for a specific PAMP??

-IL-2: T-cells produce autocrine IL-2 stimulating self and surrounding T-cells, and they produce IL-2 receptors
(is it always IL-2, or are they producing a specific IL depending on the pathogen??)

Question 12

How do T cells differentiate into TH1 and TH2?

Answer 12

-Intracellular pathogens (ie viruses PAMPs) ->IL-12 production, which leads to a TH1 response -> IFN-γ to activate macrophage killing + NK upregulation

INTRACELLULAR: IL-12 -> TH 1 -> IFN γ

-Extracellular pathogens (ie worms PAMPs) ->IL-4 production, leading to TH2 response -> IL-4, IL-5, and IL-13 to activate B cell antibody production and granulocyte activity (Neutrophiles, Mast cells, Basophils)

EXTRACELLULAR: IL-4 - TH2 - IL-4,5, 13

Question 13

Other T cell subtypes:

Answer 13

-TH9 for helminth response - produce IL-9

-TH17 extracellular pathogens in barrier site - produce IL-17

-TH22 extracellular pathogens in the skin - produce IL-22

-TFH hangs out with B-cells in follicles, B cells present antigens to Follicle-helper cells ?? or does TFH present antigens to B cells? -> help to activate B cells within secondary lymphoid organ follicles

What exactly is the role of TFH?

Question 14

What is T helper subset cross-regulation?

Answer 14

-It ensures that T helper develops along one path

-IFN γ from TH1 inhibits IL-4 (induce TH2)
-IL-4 inhibits IFN γ (induce TH1)

Question 15

REMINDER: What are good indicators for PAMPs?

Answer 15

-It should be broad (for many different pathogens, f.e. there are different viruses)

-and it should not be self (viruses use human lipids, so lipids of viruses are not good PAMPs)

Question 16

How are cytotoxic cells activated?

Answer 16

-First, a T helper Cell needs to bind with 3 signals (MHC-II) to an APC
-The T helper cell will license the APC to present through MHC-I
-Binding and activation of cytotoxic T cells through 3 signals (MHC-I, CD28 - B79; CYTOKINES)

3 signals also for cytotoxic T cells to avoid interacting with self cells, bc self cells don’t have CD80/86 on their surface

Question 17

What happens after a T cell gets activated?

Answer 17

-It is turned into an effector cytotoxic T lymphocytes (CTLs) or memory cytotoxic T cell

-CTLs generate granules, used to lyse infected cells

-CTLs don’t need additional signals anymore to target infected cells only TRC - MHC-I + peptide
-> makes sense bc those infected self-cells don’t have B7 (CD80/86)

Question 18

IN CLASS QUIZ: Which molecule is missing from epithelial cells that would prevent cytotoxic T cells from activating on their MHC-I?

Answer 18

B7 (CD80/86)

Question 19

What are the benefits of T-memory cells?

Answer 19

-They respond faster to antigens
-CD4+ memory T-cells can respond to a greater range of APC (naive CD4 cells are primarily activated by dendritic cells)
-there are more CD8 than CD4 memory T cells
-they can last for years (vaccination)

Question 20

What is the location of different types of memory cells?

Answer 20

-T(CM): central memory in secondary lymphoid tissue - early in development -> they can differentiate in different subset types

-T(EM): effector memory T cells circulating to peripheral tissues - differentiated, activate effector functions quickly on second Ag exposure

-(RM): permanent residents of previously infected tissue - respond immediately upon reinfection (hang out at infected site bc it could happen again)

Why do we keep memory cells that are naive, since when the antigen comes again it has to undergo that differentiation step anyway -> Wouldn´t it be faster to have memory cells that are differentiated already??

Question 21

Explain the NFκB pathway:

Answer 21

-NFκB is sequestered (separated) in the cytoplasm by the inhibitor IκB
-Protein kinase C phosphorylates IκB leading to its degradation and release of NFκB
-NFκB goes into the nucleus and acts as a transcription factor for IL-2

Question 22

Explain the NFAT pathway:

Answer 22

-NFAT in the cytoplasm is in a phosphorylated state (NFAT-P)
-Calcium binds to calmodulin, which in turn then binds to Calcineurin
-Calcineurin dephosphorylates NFAT-P
-NFAT then goes to the nucleus to activate IL-2 transcription

Question 23

Explain the AP-1 pathway:

Answer 23

-The G proteins Ras and Rac are converted to their active, GTP-bound state
-These activate kinases like ERK and JNK

-ERK activates ELK -> ELK is a transcription factor for Fos
-JNK activates JUN -> JUN-P moves into the nucleus and binds to Fos -> building AP-1
-AP-1 transcribes IL-2