Lecture 7 - Adaptive Immunity: Lymphocyte Activation

Question 1

How does a naive CD4 T cell get activated during infection?

Answer 1

Integration of both Innate and Adaptive Immunity

Adaptive Immunity

• DCs phagocytose microbe and migrate to lymph node to present to CD4 T Cell w/ MHC II
• This is how the CD4 T cell knows the antigen is foreign

Innate Immunity

• Innate immune cells recognize PAMPs on the pathogen and release cytokines
• Cytokine receptors on Naive CD4 T cell bind cytokines
• This is how CD4 T cell knows the pathogen is harmful

Question 2

Go through the steps of Naive CD4/CD8 T Cell activation?

Answer 2

1. Antigen Recognition: APC presents to T cell w/ specific receptor for its antigen
   - APC uses MHC I for Endogenous w/ CD8 co receptor and MHC II for Exogneous w/ CD4 co receptor
2. Activation of the T cell through autocrine/paracrine cytokine stimulation
3. Clonal Expansion: rapid mitosis of T cell
4. Differentiation of the two T cell type, T_H and T_C
5. Effector Response: T_H directs other immune cells and T_C kills infected cells
6. Memory: After infection over, effector cells apoptosed and CD4/CD8 memory T cells stored to speed up response 2nd time around

Question 3

What are the types of effector functions for Mature Naive CD4/CD8 T cells?

Answer 3

Mature Naive CD4 T Cell

-Effector CD4 T Cell (T_H): activation of macrophages, B cells, other immune cells

Mature Naive CD8 T Cell

-Effector CD8 T Cell (T_C): Killing of infected “target cells” and macrophage activation

Question 4

What are the interactions between T cells and APCs?

Answer 4

T cell__APC__Function

TCR MHC + Antigen Antigen Recognition

CD4/CD8 Coreceptor MHC I or MHC II MHC Specificity

CD28 B7 Positive co-stimulation

Question 5

What do the various receptors on the T Cell (TCR, CD4/8, CD28) do when they bind the APC?

Answer 5

Produce intracellular signals that stimulate activation of the T cell

-Differentiation + Mitosis

Question 6

What is the co-stimulatory receptor?

Answer 6

When APCs present antigens to T Cell:

• B7 on the APC binds to CD28 on the T Cell
• stimulates intracellular response

Question 7

What is the CD3 Complex?

Answer 7

Stabilizes TCR in membrane and mediates the intracellular signalling from TCR/Co-receptor w/ MHC complex

Question 8

What signals do T Cells require in order to reach threshold and induce activation?

Answer 8

1. TCR Signalling

• MHC-Antigen Complex / TCR-CD4
• Tells T Cell that antigen is nonself

2. Costimulatory interaction

• B7/CD28
• Only expressed when inflammation present
• PRRs on innate immune cells bind to PAMPs on pathogens and release pro-inflammatory cytokines to induce inflammation
• How T Cell knows antigen is harmful

3. Cytokine Signaling

• Cytokines/Receptors
• Autocrine self stimulation and paracrine signalling from APC and other innate immune cells
• How T Cell knows what kind of T cell to differentiate into

Question 9

How does a T Cell know food is nonself but nonharmful?

Answer 9

Food don’t have PAMPs⇒No binding to PRR = No Inflammation

• No inflammation = no B7 costimulatory molecule expression
• No B7 = No costimulation = No Cytokine signalling

This is an example of further development of tolerance

-tolerance to nonself and nonharmful

Question 10

What would happen to a T cell that binds a food antigen?

Answer 10

Binding of the MHC-Antigen Complex / TCR-CD4 without activation due to no inflammation generates intracellular signalling that leads to:

• Apoptosis of the T Cell
• The T Cell become anergic (cannot become activated)
• Some of those T Cells become T_REG cells that will recognize this food in the future and inhibit further immune responses

Question 11

What interleukins (cytokines) stimulate clonal expansion?

Answer 11

Autocrine activation with IL-2 and IL-2R stimulates mitosis

• This expands only population that has TCR that recognizes the antigen of interest
• Next step is differentiation

Question 12

What influences T Cells to differentiate into the different T_H subtypes?

Answer 12

PRRs on Innate Immune Cells bind PAMPs on the pathogen

• they release polarizing cytokines
• these cytokines tell the T Cell what kind of T_H Cell is needed to fight this type of pathogen

Question 13

What are the types of T_H Cells?

Answer 13

T_H1

T_H2

T_H17

T_REG

T_FH

Question 14

What is the job of a Helper T Cell?

Answer 14

Coordinates other immune cells to help fight infection by using cytokines

• Type of T_H cell influences the types of cytokines it releases
• The type of cytokines the T_H cell releases determines the type of immune cells it recruits to fight infection

Question 15

How does a T_H1 Cell fight infection?

Answer 15

Cytokines Produced

• IFN_γ = B Cell and Macrophage (Extracellular)
• IFN_γ + IL-2 = NK and CD8 T Cells (Intracellular)

Types of Cells Affected

• B Cell → Plasma Cell → IgG
• Macrophage → Activated (better eater) macrophage
• NK / CD8 T Cell → Kill infected Cells

Kinds of Pathogens it Fights

• Combats both intra/extracellular pathogens
• Good for viral response or bacterial infection

Question 16

What is different about the B Cells that are activated by T_H1 vs T_H2 Cells?

Answer 16

The B Cells that are activated by T_H1 and T_H2 Cells undergo:

• BOTH undergo clonal expansion then differentiation into Plasma Cells
• The Plasma cells activated by T_H1 cells produce IgG antibodies
• The Plasma cells activated by T_H2 cells produce IgA and IgE antibodies

Question 17

What is different about IgG, IgE and IgA antibodies?

Answer 17

IgG

• Extracellular pathogen
• Opsonizes (coats) Ag w/ IgG antibodies making it harder to stick to surface
• Opsonization flags the pathogen for phagocytosis by macrophages

IgE

• Binds to cells that release inflammatory cytokines (MAST cells)
• When IgE receptors bind antigen it causes inflammatory cell to release histamine
• Inflammatory resposne

IgA

• Mucosal antibody
• Opsonizes the pathogen for later phagocytosis or mechanical expulsion

Question 18

During primary exposure to a pathogenic microbe, what is the role of innate immune cells?

Answer 18

1. trigger the inflammatory response to recruit immune cells
2. eliminate microbes by phagocytosis and antimicrobial proteins
3. limit growth and spread of microbes with antimicrobial proteins
4. activate adaptive immunity with antigen presentation and cytokine

Question 19

When professional APCs present antigens to mature naive T cells, intracellular signals cause the antigen-specific T cells to undergo rounds of mitosis. This is called….

Answer 19

clonal expansion

Question 20

When naive T cells are activated, they secrete a cytokine called interleukin-2 (IL-2). IL-2 binds to receptors (IL-2R) on the T cell, which stimulates the secretion of more IL-2 and an increased expression of IL-2R. What type of signalling is responsible for this positive-feedback loop that leads to proliferation of activated T cells?

Answer 20

autocrine

Question 21

What is the ligand on the APC that binds to each of the following T cell receptor molecules?

• CD28
• CD4
• CD8
• TCR

Answer 21

• CD28 = B7
• CD4 = MHC II
• CD8 = MHC I
• TCR = Peptide Antigen + MHC

Question 22

In response to inflammation, professional APCs express a co-stimulatory molecule called (FILL-IN-THE-BLANK) that binds to the CD28 molecule on T cells during antigen presentation.

Answer 22

B7

Question 23

Compare and contrast an effector lymphocyte and a memory lymphocyte.

Answer 23

Effector lymphocytes are used for the immediate defense against foreign microbes. Once the infection has cleared, the body apoptoses these effector cells.

Memory Lymphocytes are generated following encounters with an antigen and are characteristically long lived and more easily stimulated than naïve lymphocytes in order to mediate a secondary response to subsequent encounters with the same antigen.

Question 24

How does a T_H2 Cell fight infection?

Answer 24

Cytokines Produced

• IL-5

Types of Cells Affected

• B Cell → Plasma Cell → IgE and IgA
• Macrophage → Alternatively activated (inflammatory) macrophage
• Eosinophil and Basophils/MAST cells → Inflammation

Kinds of Pathogens it Fights

• Parasites

Question 25

How does T_H17 fight infections?

Answer 25

Cytokines Produced

• IL-17

Cells Affected

• Indirectly affects neutrophils
• Stimulates stromal and endothelial cells to release cytokines that recruit neutrophils

Types of Infection it Fights

• Bacterial and fungal infections

Question 26

What are the effects of T_H1 cell differentiation?

Answer 26

Question 27

What are the effects of T_H2 cell differentiation?

Answer 27

Question 28

What are the effects of T_H17 cell differentiation?

Answer 28

Question 29

A 2013 study by Warfel et al. demonstrated that the acellular pertussis vaccine protects against disease but not infection and transmission (in an animal model). Why is this distinction important to understand in regards to vaccine effectiveness?

Answer 29

In animal models, they showed that the severe symptoms did not present, although there was colonization of B.pertussis. This is important because while symptoms in vaccinated individuals are not expressed, the colonization of these cells will decrease herd immunity because it can still be spread to vulnerable members f the population.

Question 30

Warfel et al. studied immune responses (Fig. 5) in animals vaccinated with acellular vaccine (aP), whole cell vaccine (wP), and animals who had never been infected or vaccinated (naive). They also looked at convalescent (conv.) animals. What does convalescent mean?

Answer 30

infected with pertussis but now recovered

Question 31

In figure 5 of the Warfel et al. 2013 paper, they describe measurements of three cytokines - interleukin-5, interleukin-17 and interferon-gamma - when studying immune responses in animals. Why did they look at CD4+ cells for the production of these cytokines?

Answer 31

They are helper T cells that produce these cytokines

Question 32

In figure 5 of the Warfel et al. 2013 paper, they describe measurements of three cytokines - interleukin-5, interleukin-17 and interferon-gamma - when studying immune responses in animals. Match the cytokine with the cell type from which it is produced.

Answer 32

IL-5 ⇒ T_H2

IL-17 ⇒ T_H17

IFN-_Y ⇒ T_H1

Question 33

In the absence of inflammation, what is the fate of a mature naive CD4 T cell if a DC presents nonself antigen with MHC II?

Answer 33

It differentiates into a regulatory T cell (T-REG) or It dies or becomes anergic

Question 34

Polarizing cytokines influence the the type of T helper response (e.g. TH1 vs. TH2) created by CD4 T cell activation. What causes polarizing cytokines to be secreted by immune cells?

Answer 34

PAMPs from microbes bind to PRRs on immune cells

Question 35

Based on experimental evidence of T helper responses (Warfel et al. 2013, Fig. 5; asterisks denote statistically significant differences between negative control and experimental group), what is the polarized response produced by convalescent animals (i.e. those infected with pertussis, survived and recovered)?

Answer 35

TH1 and TH 17

Question 36

Based on experimental evidence of T helper responses (Warfel et al. 2013, Fig. 5), why does the wP vaccine seem to provide a better response against B. pertussis than the aP vaccine?

Answer 36

aP vaccination induces a TH2 response, while wP vaccination and natural infection induced a TH1 response, and a partial TH17 response. The response by TH17 and TH1 cells allows for the development of memory by the cells and protective immunity against future infections. This is compared to the aP vaccine that simply treats the symptoms and does not induce imunnologic memory, prevent colonization or transmission.

aP vaccine may protect abainst symptoms, but it can still spread from person to person, but with the wP vaccine however it can not

Question 37

What was the importance in Warfel et al. 2013, Fig. 5 (right-side graphs) of showing that CD95-negative/CD4-positive (CD95- CD4+) cells did not produce cytokines (i.e. cytokines were only produced by CD95+ cells)?

Answer 37

When stimulated with heat-killed B. pertussis, both total nonadherent cells and CD4+ cells from convalescent animals secreted high levels of IL-17, some IFN-γ, and no IL-5.

When the CD95+ memory cells were depleted, the CD95–CD4+ cells did not secrete IL-17 or IFN-γ, consistent with induction of B. pertussis-specific Th17 and Th1 memory cells (Fig. 5). Stimulated total nonadherent cells and CD4+ cells from aP-vaccinated animals secreted significant IFN- γ, but the response was weaker than convalescent cells (P = 0.01), and there was no significant increase in IL-17 secretion. However, there was a significant IL-5 response, consistent with skewing toward Th2 and Th1 memory (Fig. 5).

Question 38

Based on experimental evidence of T helper responses (Warfel et al. 2013, Fig. 5; asterisks denote statistically significant differences between negative control and experimental group), what is the polarized response produced by the acellular pertussis (aP) vaccine?

Answer 38

T_H1 and T_H2

Question 39

Warfel et al. 2013 showed that the wP and aP vaccines induce different T cell responses with the wP vaccine being more similar to infection/recovery (convalescent animals). How can the aP vaccine be modified to produce a response similar to wP but without the adverse reactions?

Answer 39

If they were able to tailor the aP vaccine to stimulate a T-17 response, similar to that of the wP vaccine, they may see comparable effects. Using the aP vaccine that has the ability to stimulate that response, would decrease the likelihood that a person would have an adverse reaction due to the whole cell not being presented.

Question 40

Which intracellular signalling complex is associated with the B cell receptor (BCR) and serves a similar function to the CD3 complex for TCR?

Answer 40

Ig-alpha and Ig-beta