12 - Effector Responses: Ab and Cell-Mediated Immunity

Question 1

FcRs role in immunity

Answer 1

the class of an Ab determins which receptors can bind to it. Ab-binding receptos (which binds to the constant region of Abs, and therefore are called FcRs) determine which cells an Ab can activate to aid in its protective mission, as well as ehterher it can gain access to certain locations in the body.

Question 2

The six broad categories of Ab effector functions

Answer 2

1) neturalization of pathogens and toxins
inactivates, prevernts binding to cells

2) Agglutination of particulate Ags
Prevents binding to cells, enhances
clearance

3) Opsonization
Phagocytosis

4) complement activation
Lysis

5) Ab-dependent cell-mediated cytotoxicity
(ADCC)
NK cell-induced apoptosis

6) Degranulation

Question 3

Neutralizing Abs

Answer 3

Abs can bind pathogen proteins that allow entry into cells/tissues. If this is blocked, the pathogen may be prevented from initiating an infection. these are called neutralizing Abs.

Can also prevent entry of toxins into cells.

Physiologically most effective mode of protection against infection or the damaging effects of toxins.
Abs with all Ig isotypes can mediate neutralization.

Mutations in the pathogen can prevent the Abs binding them

Question 4

Aggultination

Answer 4

Abs have multiple Ag-binding sites.

IgM has 2, IgA = dimer, has 4 total, IgM = pentamer, 10 total.

This menas that one Ab can bind and cross-link multiple pathogens, resulting in agglutination. This can result in enhanced neutralization (harder for clumps of pathogens to enter cells) and more efficient clearance of the pathogens.

Example: agglutinated bacteria in the respiratory tract are more susceptible to being transported by cilary movement.

Question 5

Opsonization and phagocytosis

Answer 5

opsonization = Abs promote/enhance the engulfment of Abs by phagocytosis.

Opsonizing Ags bind their Ag via Ag-binding sites and are then bound by FcRs expressed on phagocytes.

phagocytes can thus bind Ags directly or via receptors that recognize opsonins.

If the Ag in question is a cellular Ag (virus-infected cell, tumor cell, ++) the process is often referred to as ADCP (antibody-dependent cellular phagocytosis).

Example of opsonin receptors: FcRs that rec Fc regions of IgG Abs

Question 6

Complement activation

Answer 6

formations of MAC (membrane attack complexes)

the capaxity to stimulate complementmediated membrane fdamage is the property of specific AB classes, including IgM and IgG.

The early part of the complement cascade can also work as opsonization. C1q, C3b (and various fragments of it) and C4b can be rec by various complement receptors on phagocytes when they are bound to pathogens.

Question 7

Antibody-dependent Cell-mediated cytotoxicity

Answer 7

Abs bound to foreign Ags on self-cells (like virus proteins on infected cells, or tumor antigens) can activate the killing activity of several types of cytotoxic cells (NK, granulocytes). NK don’t have TCRs, but they have IgG Fc receptors that can bind to Abs on Ags.

Question 8

Antibody-activated degranulation and mediator release

Answer 8

Granulocytes like neutrophils, mast cells, basophils and eosinophils express Fc receptors for various immunoglobulin classes.

When Abs are bound by granulocyte Fc receptors and then are corss-linked by some pathogens (such as parasitic worms), fusion of the secretory granules with the plasma membrane can be triggered, releasing the contents of the granules.

Question 9

Antibody classes

Answer 9

Ab specificity is determined by the H- and L-chain Variable regions in the Fab fragments

Isotype/class is defined by the H-chain C-regions.

5 classes (GAMED). Plasma cells and B cells will synthetize one of these. Also there are subclasses.

Question 10

IgM

Answer 10

first to be produced in a primary immune response.

low affinity, the B cells that produce them have not gone through affinity maturation.

10 Ag-binding sites (pentamers).

bind Ag with high avidity.

many come from B-1 cells (express them constitutively)

appear to protect from common pathogens, esp those from the gut/mucosal areas

vary good at activating complement and efficiently induce lysis of the pathogens to which they bind.

Also good at aggluantion

Question 11

IgG

Answer 11

most common Ab class in serum.

Most diverse (many subclasses (IgG1-4 in humans)).

Every subclass binds to FcRs and can enhance phagocytosis (opsonization)

Human IgG1 and IgG 3 are very good at activating complement.

Human IgG1 is very good at mediating ADCC by NK cells.

Question 12

IgA

Answer 12

found in circulation, and are the major class in secretions (incl musuc in gut, resp and reprod tracts + tears, saliva and milk from mammary glands).

2 subclasses in humans (IgA1 (serum) and IgA2(secretions))
In blood both classes stimulate phagocytosis, mediate ADCC and drigger degranulation.

in secretions, IgA can neutralize toxins and pathogens + enhance their clearance, continously binding to commensal bacteria and preventing them from entering blood stream.

Cannot fix complement, don’t induce inflammation -> good because they typically bind “good” bacteria.

IgA in secretions have an extra polypeptida chain, secretory component (SC) (not in blood), mediated IgA2 entry into mucosal and glandulary secretions.The receptor for this (poly-Ig receptor) on epithelial cells allow IgA to cross multiple barriers via endocytosis and transport (transcytosis). IgM is also polymeric and can bind to these receptors.

Question 13

IgE

Answer 13

best known for role in allergy and asthma. they could also play an important role in protection against parasitic worms + immunity to a variety of venoms.

made in small quantities, but has very potent effects, incl degranulation of eosinophils and basophils and release of molecules like histamine and proteases that damage pathogens. more in chap 15

Question 14

IgD

Answer 14

0,2% of circulating Abs.

present at higher levels in secretions in upper resp tract, where it reacts with resporatory bacterial and viral pathogens.

recently found to bind to basophils and mast cells and, after Ag binds to IgD, these cells are activated to release antimicrobial peptides, which attack pathogens. The activated cells also release cytokines (IL-4, TNF ad IL-1), BAFF (B cell survival factor) and chemokines.

Question 15

How do FcRs differ?

Answer 15

1) Ab class(es) they bind
2) the cells that express them
3) the responses they activate

Question 16

FcR signaling

Answer 16

One Ab cannot activate a receptor, the FcRs need to be cross-linked or oligomerized by the binding of multiple Abs coating a single pathogen.

The receptor can have ITAMs or ITIMs, and can generate positive or negative signals based on which it has.

FcRs induce many effector functions of Abs;

• phagocytosis of Ab/Ag complexes by macrophages
• transcytosis of Abs through epithelial cell layers
• lysis of Ab-bound infected cells by NK cells (ADCC)
• protection of serum Abs from degradation
• induction of degranulation¨- regulation of the activities of innate immune cells.

Question 17

Theraputic Abs

Answer 17

To reduce their immunogenicity in vivo, monoclonal Abs are being generated that are chimeric, humanized, or fully human, so that their sequences are partly or fully human in origin

Theraputic Abs are used to treat a growing set of conditions, including cancer, autoimmune and inflammatory diseases, and infectious diseases.

Theraputic ABs are being modified to optimize their functions in vivo, including complement activation and binding to Fc receptors that activate phagocytosis, ADCC, inhibitory signaling, and enhanced lifetimes in blood

Question 18

Cell-mediated effector responses

Answer 18

CD8+ cytotoxic T cells

NKT cells

NK cells

they all trigger apoptosis in their targets.

important for infected cells and also tumors.

Also the ceclls that fuck up organ transplants.

Question 19

Cytotoxic T lymphocytes (secreted cytokines + killing mechanism)

Answer 19

effector molecules produced: cytotoxins (perforins and granzymes), IFN-γ, TFN, Fas Ligand (FasL)

Mechanism of killing: Cytotoxic granule release; and FasL-Fas interactions.

Question 20

NKT cell (secreted cytokines + killing mechanism)

Answer 20

Effector molecules produced: IFN-γ, IL-4, GM-CSF, IL-2, FasL

mechanism of killing: FasL interactions predominantly. Can also activate NK cells indirectly via cytokines

Question 21

NK cells (secreted cytokines + killing mechanism)

Answer 21

effector molecules produced: cytotoxins (perforins and granzymes), IFN-γ, TNF, FasL.

Mechanism of killing: Cytotoxic granule release and FasL-Fas interactions

Question 22

Cytotoxic T cells key concepts (6)

Answer 22

easier to activate than naïve TC and TH cells, express higher levels of cell-adhesion molecules, exhibit different trafficking patterns, and produce both soluble and membrane effector molecules.

In order to become functional CTLs, naïve TC cells must engage with APCs (usually DCs) that have been previously activated (licensed), Licensing is often carried out by CD4+ helper T cells that recognize MHC class II with bound peptide presented by the DC (DCs can also be activated through PRRs).

T cell help is not absolutely necessary for this first activation step, but is required for optimal proliferation and generation of CTL and memory cells.

Ag-specific T cell populations, incl CTLs, can be identified and tracked by labeling with MHC tetramers containing bound Ag-derived peptides

CTLs induce cell death via two mechanisms: the perforin-granzyme pathway and the Fas-FasL pathway. Both trigger apoptosis

The cytotoxic effector function of CTLs involves several steps:

• TCR/MHC-mediated recognition of target cells
• formation of CTL-target cell conjugates and immune synapse formation
• repositioning of CTL cytoplasmic granules toward the target cell
• granule fusion with the plasma membrane, release of mediators, and insertion of FasL into the CTL plasma membrane where it can bind Fas on the target cell
• initiation of apoptosis
• dissociation of the CTL from the target
• death of the targeted cell.

Question 23

NK cells Key concepts

Answer 23

NK cells induce apoptosis of tumor cels and virus infected cells by mechanisms similar to those of CTLs (involving perforin/granzymes and FasL-Fas interactions), but are regulated by distincts receptors.

stimulated by IFN-α and IFN-β and IL-12 (secreted during viral attack)

inhibited by KIR2DL1 (killer-cell immunoglobulin-like receptor)

In general, NK cell killing is regulated by a balance between positive signals generated by the engagement of activating NK receptors and negative signals from inhibitory NK receptors. Only if the level of activating signal exceeds the level of inhibitory signals will the NK cell kill.

NK cell receptors fall into two major structural groups based on their extracellular regions: the lecine-like receptors and the Ig-like receptors. Whether receptors are activating og inhibitory depends on their intracellular regions (ITAMs and ITIMs).

many NK inhibitory receptors bind MHC class I proteins. The expression of relatively high levels of MHC class I molecules on normal cells protects them against NK cell-mediated killing by engaging inhibitory receptors. NK cells kill those cells that have lost or reduced their MHC I levels, and ezpress ligands for NK activating receptors that are upregulated on infected or stressed cells.

NK cells also kill target cells via ADCC, which results from the binding of cell-associated IgG Abs to FcγRIII (CD16), which functions as an activating receptor.

The capacity of NK cells to distinguish between normal and altered and potentially dangerous cells develops through the process of NK cell licensing, in which only those cells that express inhibitory receptors for seld MHC class I proteins develop the capacity to kill targets.

NK cells have recently been shown to have memory responses, the expansion of NK cells capable of responding to cells expressing particular ligands for activating receptors.

Question 24

NKT cells key concepts

Answer 24

NNKT cells have characteristics common to both T lymphocytes and NK cells. Many express an invariant TCR that recognizes lipid AGs bound to CD1 proteins, as well as markers common to NK cells

NKT cells exhibit both helper and cytotoxic activity and kill cells predominantly via FasL-Fas interactions.