L13: Cell Mediated Immunity

Question 1

What does the fate of antigens in vivo and the type of immune response that develops depend on?

Answer 1

1. Nature of antigen
2. Route of antigen entry
3. Immunological status of individual

Question 2

What is the purpose of cell-mediated immunity?

Answer 2

Defense mechanisms against microbes that survive within phagocytes or infect non-phagocytic cells

Question 3

What cells is CMI mediated by?

Answer 3

1. Mediated by cells of the innate immune system (T cell independent): phagocytosis, natural killer cells, cytokines, complement
2. T cell mediated: initiated by T cell recognition of MHC-bound peptide antigens expressed on the surface of cells infected w/ viruses and intracellular bacteria

Question 4

What are sequential events in the progression of the immune response?

Answer 4

1. Antigen capture and presentation
2. Recognition of antigen by T and B cells
3. Clonal expansion and differentiation of antigen-activated lymphocytes
4. Migration of effector cells and molecules to sites of antigen in tissues

Question 5

What are T cell independent CMI responses?

Answer 5

Phagocytosis

NK cell cytotoxicity

Cytokines (e.g. TNF: IL-12)

Complement

Question 6

What is the CMI response to phagocytized microbes?

Answer 6

Response is mediated by T cells that recognize microbial antigens, and produce cytokines, such as IFN-γ and TNF, that enhance the ability of phagocytes to kill the microbes and stimulate inflammation

Antigen-specific CD4+ Th1 cells recognize antigenic peptide-MHC II complex and produce phagocyte-reactivating IFN-γ

Antigen-specific CD8+ recognize antigenic peptide-MHC I complex and also produce IFN-γ

Antigen-activated T cells also produce cytokines, such as TNF and lymphotoxin (LT) that stimulate inflammation

Question 7

What is the downside to CMI response to phagocytized microbes?

Answer 7

Tissue injury due to delayed type hypersensitivity (DTH) reaction may occur

Question 8

What is the response to microbes that infect and replication in various cell types mediated by?

Answer 8

CD8+ cytotoxic T lymphocytes (CTLs) that kill infected cells and eliminate the reservoirs of infectious organisms

CTLs are the principal defense mechanism against viruses that replicated in the cytoplasm of infected cells

CTL killing is also a mechanism to eliminate microbes taken up by phagocytes, but exist in the cytosol outside of the phagosomes

Question 9

What is the response to helminthic parasites mediated by?

Answer 9

TH2 cells which stimulate IgE antibody production and activate eosinophils that bind and destroy IgE coated helminths

Question 10

Describe the induction phase of CMI

Answer 10

Antigen recognition in lymphoid organs → T cell expansion and differentiation → differentiated effector T cells enter circulation

Question 11

Describe the effector phase of CMI

Answer 11

Differentiated effector T cells enter circulation → effector T cells encounter antigens in peripheral tissues → effector T cells encounter antigens in peripheral tissues → secrete their products (such as TNF and IFN-γ) locally

Question 12

What are the subsets of T helper cells?

Answer 12

Th1
Th2
Treg
Th17

Question 13

What is/are the prototypic cytokine(s) produced by Th1? What is the primary function of these?

Answer 13

IFN-γ

Macrophage activating factor

Question 14

What is/are the prototypic cytokine(s) produced by Th2? What is the primary function of these?

Answer 14

IL-4, IL-5, IL-13 (and IL-10)

Function in switch factor (in isotype switching)

IL-4 in particular is the switch factor for IgE

Question 15

Describe the development of Th1 subset

Answer 15

Th1 differentiation pathway is the response to microbes that infect or activate macrophages and NK cells

IL-12 production during the initiation of response factors Th1 cell differentiation →
IL-12 binds to receptors on antigen-stimulated CD4+ T cells and activates the transcription factor STAT4, which promotes Th1 differentiation →
another transcription factor, T-bet, provides an amplificaiton mechanism for Th1 differentiation →
IFN-γ is the signature cytokine produced by Th1 differentiation

Question 16

What stimulates IL-12 gene transcription and production in APC?

Answer 16

APC has receptor for IFN-γ which when activated stimulates IL-12 production

CD40:CD40L engagement by a T helper cell drives IL-12 production

Question 17

What are the effector functions of Th1 cells?

Answer 17

General rule is that if Th1 cells are involved, think CMI responses

IFN-γ stimulates macrophage activation (enhanced microbial killing)

IFN-γ also stimulates complement-binding and opsonizing by antibodies

IFN-γ also helps naive CD8 cells to engage antigen and differentiate to become cytolytic T cells

LT and TNF stimulates activation of neutrophils (enhanced microbial killing)

Question 18

When macrophage is activated by Th1 cells, what does this cause?

Answer 18

Activated macrophage has:

Increased expression of costimulators (B7 molecules) → Increased T cell activation (amplification)

Secretion of cytokines (TNF, IL-1, IL-12) → increased leukocyte recruitment (inflammation), more Th1 differentiation, and IFN-γ production

Increased expression of MHC molecules → increased T cell activation (amplification)

Production of ROI and NO → killing of microbes in phagolysosomes (effector function of macrophages)

Question 19

Describe the development of Th2 subset

Answer 19

Th2 differentiation pathway is the response to helminths and allergens, which cause chronic T cell stimulation

Th2 differentiation is dependent on IL-4, which functions by activating STAT6 transcription factor (IL-4 initially produced by antigen-activated naive CD4+ cells are though to trigger Th2 differentiation) → GATA-3, another transcription factor, provides an amplification mechanism for Th2 differentiation → IL-4, 5, 10, and 13 are the defining cytokines produced

Question 20

What are the effector functions of Th2 cells?

Answer 20

General rule is that if Th2 cells are involved, think humoral immune responses

IL-4 → enhances production of IgG as well as IgE (involved in allergic reactions and parasitic infestations)

IL-5 → eosinophil promoting factor since in a parasitic infection, will need more eosinophils at site of infection

IL-10 and IL-4 → suppression of macrophage activation

Question 21

Th1 - Th2 crosstalk

Answer 21

Production of IL-10, IL-4, IL-13 (anti-inflammatory cytokines) by Th2 inhibits macrophage activation

Production of IFN-γ by Th1 causes macrophage activation

Th1 cells won’t be involved in responses that are Th2 dominant and vice versa

Question 22

Describe the development of Treg subset and its role

Answer 22

Produced in response to TGF-β and act to negatively regultae immune responses by production of cytokines (such as IL-10) that suppress immune function

Foxp3 transcription factor also involved

Treg cells are thought to inhibit development of autoimmune diseases

Question 23

Describe the development of Th17 subset and its role

Answer 23

Arise when IL-6 is produced along w/ TGF-β, which occurs when APC are activated during an inflammatory response

RORγt transcription factor also involved

Production of Th17 appear important for immune protection against certain pathogens They produce pro-inflammatory cytokines (such as IL-6, IL-17, and others)

However, Th17 cells also promote development of autoimmune diseases, such as multiple sclerosis and rheumatoid arthritis

Question 24

What is the pathway of development of the 4 T cell subtypes dermined by?

Answer 24

Interaction b/w APC and antigen-specific T cells and cytokines that are being produced as a result of that interaction

Question 25

Name the cytokines and transcription factors that drive the production of each of the T cell subtypes

Answer 25

Th1
Cytokine: IL-12
TFs: STAT4 & T-bet

Th2
Cytokine: IL-4
TFs: STAT6 & GATA-3

Treg
Cytokine: TGF-β
TF: Foxp3

Th17
Cytokine: TGF-β & IL-6
TF: RORyt

Question 26

What are the cytotoxic effector cells?

Answer 26

CTLs

NKs

Question 27

What are the different pathways of differentiation of CD8+ CTLs?

Answer 27

1. Role of co-stimulation: CD8+ T cells recognize antigen + costimulators on professional APCs → CTL differentiation w/o helper T cells
2. Role of TH cells via cytokine production: CD4+ helper T cells produce cytokines that stimulate CTL differentiation; this occurs when peptide antigen can be engaged, but there is no co-stimulation (such as in liver cells which don’t have that)
3. Role of TH cells via CD40-CD40L interaction: CD4+ helper T cells enhance the ability of APCs to stimulate CTL differentiation; with a CD4+ cell that can engage CD40-CD40L, which causes upregulation of costimulatory molecules

Question 28

Describe the steps in CTL-mediated lysis of target cells

Answer 28

Antigen recognition and conjugate formation (via MHC class I) → CTL activation → delivery of lethal hit (granules are exocytosed) → detachment of CTL → target cell death (granule components cause activation of caspases, leading to apoptosis)

OR

FasL-Fas mediated target cell apoptosis: FasL on CTL interacts w/ Fas on target cell → apoptosis of target cell

Question 29

When do CTLs stop apoptosing?

Answer 29

When they run out of granules

Question 30

NK cell mediated cytoxicity

Answer 30

Receptors are different, by killing mechanism is pretty much the same as CTL

Question 31

Antibody-dependent cell-mediated cytotoxicity (ADCC)

Answer 31

Antibody binds to surface antigen on a viral-infected cell → creates antibody-coated cell → Fc receptors on NK cells can bind via the Fc region of the antibody molecules → lysis of antibody-coated cell

This is not unique to NK cells

ADCC effector cells: NK cells, macrophages, eosinophils (these all have receptors for Fc portion of antibody molecules and can kill under certain conditions)