4: General Responses of Cell-Mediated vs Humoral Activity

Effector CTL activation (3 signals)

1. TCR:MHC class I (via w/ licensed APC)

2. costimulatory signal through CD28/CD80/86 on DC

3. high affinity IL-2R binding to IL-2

Signal one of CTL activation

1. DC binds to TH1 w/ CD40 and MHC-II → DC becomes licensed by TH1

2. DC becomes licensed, performs cross-presentation: presenting peptides on MHC-I that would normally go on MHC-II

3. CTL differentiation/proliferation: naive CD8 T cell & DC bind via MHC-I and TCR; CD80/86 & CD 28

(steps occur simultaneously)

What are the two ways a CTL kills a target?

1. Granule exocytosis (granzyme & perforin-mediated cytolysis)

2. Fas:FasL signaling

Granzyme and Perforin-mediated cytolysis

intrinsic pathway of apoptosis

after CTL forms conjugate w/ target cell, releases perforce and granzymes into space between cells → pokes holes in target cell membrane and leads to cell death

Fas/FasL-Mediated Cytolysis

extrinsic pathway of apoptosis

FasL/Fas binding delivers death signal

Fas mutations result in what?

lymphoproliferation disorders; swollen lymph node tissue leading to autoimmunity

how are NK cells similar/different to CTLs?

similar: similar apoptotic pathways in target cells

different: NKs don’t need to be activated (activate rapidly by INF-a/b), controls pathogen within first 7 days (time it takes CTLs to develop), activity peaks after 3 days of infection

2 types of receptors on NK cells

1. activating receptors (bind to cell surface ligands that indicate infection, cancer, stress)

2. inhibitory receptors (recognize membrane proteins like MHC on normal cells and inhibit killing)

missing self model

if cell has no MHC-I, NK will kill it

(virally infected and tumor cells down regulate MHC-I to avoid CD8 activation)

balanced signals model

balance of inhibitory/activating signals from receptors recognizing ligands on target cell determines if NK will kill it

3 signals of T-dependent B-cell responses (how B cells get activated to protein antigens)

1. BCR binds antigen & expresses it on MHC-II, recruiting T-cell help

2. Activated TH cell binds to B-cell (TCR/MHCII, CD40/CD40L on TH cell)

3. T cell releases cytokines that activate B-cell

What makes a B cell more effective at eliciting T-cell help?

more antigen taken in (b/c more peptide expressed on MHC-II)

BCR signaling also increases expression of costimulatory molecules (CD80/86/40)

What does T cell help ensure?

only B-cells taking in a high level of antigen can get T-cel help and make antibodies to that antigen

what do B cells differentiate into after T-dependent antigen activation?

plasma cells

memory cells

germinal center B cells

What are the first B-cells to produce antibodies after T-cell help?

antibody-forming cells (AFCs): plasma cells and plasmablasts

(useful b/c immediately have antibodies to target pathogen, but low affinity antibodies)

plasmablasts

state between naive B cell and plasma cell

express some BCRs and secrete antibodies; become fully differentiated after taking in more antigen

AFC foci produce what antibodies? How? Do they undergo somatic hypermutation?

initially produce only IgM, after 5-6 days, class switch recombination occurs and IgG antibodies are produced instead

(these antibodies do not under go SHM → lower affinity)

how are germinal centers formed?

activated B cells enter the follicle, divide rapidly and undergo further differentiation

How are GC B-cells so high affinity?

somatic hypermutation

better affinity cells go through more rounds of mutation/selection (no or reduced affinity → apoptosis)

CSR/class switch recombination

RNA splicing that changes constant region of antibody, changing antibody class

CSR in GC B-cells

IgM constant region becomes IgG, IgA, IgE

how do GC B cells proliferate?

enter follicles (secondary lymphoid structures) and colocalize with FDCs → original naive B cells pushed to periphery (mantle zone)

What cell provides anti-apoptotic signals to GC B cells?

T follicular helper cells (TFH), since GC B cells are highly susceptible to apoptosis

early memory B cells

formed before germinal center formed, mostly form IgM

low affinity for antigen

GC memory B cells

mutated, high-affinity receptors (SHM)

gradually sift from IgM to mostly other classes of antibodies via CSR

6 effector mechanisms of antibodies

1. neutralization

2. agglutination

3. opsonization

4. complement activation

5. ADCC (antibody-dependent cell-mediated cytotoxicity)

6. degranulation

neutralization by antibodies definition. when is it ineffective?

antibodies bind to cell-surface proteins, preventing virus/bacteria from binding to cell and internalizing

also neutralize toxins

ineffective in stopping pathogens that mutate rapidly

agglutination by antibodies

antibodies bind multiple pathogens using their multiple binding sites, allows clearance of pathogens (such as mucus or pushing out from the lungs)

Opsonization, antibody dependent cellular phagocytosis

antibodies bind antigen and FcRs on phagocytic cells to trigger phagocytosis

Complement Activation

antigen-antibody complexes (IgM, IgG) initiate complement cascade

ADCC (antibody-dependent cell-mediated cytotoxicity)

antibodies bind to foreign antigens on cells and recruit cytotoxic cells (NK cells have Fc receptor that triggers release of perforin, granzyme)

IgM

(pentameter = 10 binding sites)

natural antibodies (can be prod. w/o antigen)

effective at activating complement and pathogen lysis

good at agglutinating pathogens

IgG

most common antibody in serum

bind to FcR and enhance phagocytosis by macrophages

IgG1, IgG3 activate complement

IgG1 induces ADCC by NK cells

IgA

major antibody class found in secretions, agglutination

stimulate phagocytosis, ADCC, degranulation

forms dimers to neutralize toxins/pathogens

does NOT activate complement → won’t induce inflammation

IgE

major role in allergy and asthma

important in protection against parasitic worms/protozoa

(degranulation of eosionphils/basophils → histamine released in blood)

IgD

smallest amount of antibodies in blood

triggers secretion of antimicrobial peptides & cytokines

Fc receptors

exposed on immune cells, bind to antibodies (use antibodies to activate cell-mediated response); only signal if antibody is bound to pathogen

What classes of antibodies neutralize pathogens

all

What classes of antibodies agglutinate pathogens

IgA, IgM

what antibody class initiates the complement cascade to lyse pathogens directly?

IgM

What antibody activates complement, opsonizes pathogens, and enter tissues to interact with innate cells to promote inflammation

IgG

Ab for extracellular pathogens

IgG (fix complement, opsonize pathogen)

Ab for intracellular pathogens

IgG subtypes that activate cytotoxic cells (ADCC)

what Abs provide protection in secretions of mucosal/glandular tissues

IgA, IgM

What Abs promote degranulation of mast cells and basophils to release mediators?

IgE, IgD

Chimeric antibodies

mouse variable regions w/ human constant regions (therapeutically used)

humanized antibodies

replaces most of mouse variable region w/ human regions

Why is IgG used to treat so many diseases (cancer therapies, cytokine modulators, infectious diseases)

wide range of effector functions, long half life

used in cancer therapies to activate complement and mediate ADCC by NK cells