Antibody and cell-mediated effector functions Flashcards
CD4+ T helper (Th) cells
The key to adaptive immunity
- Must be activated by an antigen- presenting DC
- Activated CD4+ T cells activate many responses
CD8+ T cytotoxic (Tc) cells
Must be activated by an antigen-presenting DC
- Activated T cell KILLS the cell presenting the antigen (Ag)
B cells
Recognize antigen directly
- Need help from activated CD4+ T cell to optimize antibodies and for memory
- Activated B cells differentiates to a plasma cell that produces antibodies that recognize the same Ag as the BCR on the mother cell
Th cells
–>cytokines–>indirectly contribute to the activation of phagocytic cells, B cells and cytotoxic T cells Th1, Th2, Th17, TFH, (Treg)
Cytotoxic cells
–> directly attack infected cells and certain pathogens - CD8+ CTL
- NK cells
- NK T cells
Antibody-mediated immunity
Mediated by antibodies secreted
by plasma B cells
Ab
detect antigens from pathogens found in extracellular spaces and
clear infection by;
* Neutralization
* Agglutination
* Opsonization
* Complement activation
* Antibody-Dependent Cell-mediated Cytoxicity
* NK cells
* Antibody-mediated Degranulation and Mediator Release * Granulocytes
Effector T cells:
Activated CD4+ Th cells Cytokines -> activate other cells Aktivert CD8+ Tc cells Cytotoxins -> kill virus-infected cells
Cytokines
– signal proteins
Cytotoxins
– death proteins
Cytotoxic effector cells include three subsets
- Cytotoxic T Lymphocytes (CTL)s
- Natural Killer (NK) cells
- Natural Killer T (NKT) cells
apoptosis
is the process of programmed cell death. It is used during early development to eliminate unwanted cells; for example, those between the fingers of a developing hand. In adults, apoptosis is used to rid the body of cells that have been damaged beyond repair.
Precursor Cytotoxic T lymphocytes need to be activated by
CD4+ Th cells or licensed DC
Precursor (naïve) CTL
In order to be activated, a naive T cell must recognize a foreign peptide bound to a self MHC molecule. But this is not, on its own, sufficient for activation. That requires the simultaneous delivery of a co-stimulatory signal by a specialized antigen-presenting cell
-no high-affinity IL-2R, or IL-2 production
* No cytotoxic activity
Activated CTL
-Expres scytotoxins packaged into lytic granules
* UpregulateIL-2R
* ProduceIL-2(proliferation and differentiation)
Cross-presentation
allows presentation of exogenous antigens on MHCI and priming of CD8+ T cells
Dendritic cells (DC)s primary cross-presenting cell type
Exogenous antigens are redirected to the endogenous presentation pathway
– Allows presentation on MHC class I molecules, priming CD8+ T-cell responses
– Licensed DC are resistent to the cytotoxic effects of Tc.
Antigen presentation to T cells
All cells can present antigen from inside cytoplasm on MHCI
- Signals that the cell is infected or cancerous
- Aktivated CD8 T cells that recognize Ag will kill
cell
Professional antigen-presenting cells
Can take up Ag from outside cell and present on MHCII –Activate CD4+ Th cells
- Can activate T cells without being infected
- Dendritic cells (DCs) -naïve T cells –> activated
T cell
- Macrophage
- Bcell
Exception Cross-presentation
DCs kan present antigen from outside the cell on
MHCI to CD8 T cells without being infected and
without getting killed
-> activate CD8+ Tc cells
Cross presentation activates naïve CTL to become effector CTLs
Naïve Tc must be activated by APC to become effector Tc (CTL)
APC licensing Th1 express CD40L binds CD40 CD80/86 up + cytokines
– CD4+ T cells are required for CD8+ T-cell memory and optimal expansion.
Signal 1
―TCR binds peptide on MHC classI on APC
Signal 2
―co-stimulatory signa lCD28-CD80/86
– APCs get help from Th1\Th17 cells to upregulate stimulation molecules
Signal 3
IL-2–>proliferation and differentiation into CTL form
Cross-presentation is important for CTL activation
Best CTL activation is achieved when the APC can present peptides on both types of MHC molecules in response to intracellular abnormality.
– Ex Ag + MHCIIActivates CD4+
– Int Ag + MHCIActivates CD8+
– Crosspresentation- presentation of Ex Ag on MHCI
* APC not necessarily infected.
* Cross-presentation allows DC to acquire antigens from non-APCs and present them on both types of MHC molecules
Effector CTLs recognize and kill infected or tumor cells via TCR activation
CTLs induce apoptosis (programmed cell death) in infected or malignant target cells by;
1. Directional release of granule contents
2. Fas-FasL interactions
– Ability to bind target cells well is critical
* Central ring of TCR surrounded by a peripheral ring of adhesion molecules
* Adhesion molecule LFA converted to a high affinity statebinds ICAM
Granzyme/perforin-mediated cytolysis
When stimulated, CTLs release granule contents
* Perforin is a 65 kDa pore-forming protein
* Granzymesareserineproteases
* Both taken up by endocytic processes,
* Perforin punch holes in the membranes and release GranzymeB
* Granzymes activates apoptotic pathways that lead to fragmention of target cell DNA
–>Induce apoptosis from the inside out
Granzyme/perforin-mediated cytolysis activates apoptotic pathways leading to apoptosis (Programmed cell death)
- Cleaves Caspase 3–>Apoptosis
- Proapoptotic BID–>Cytochrome c–>Caspase 9–>
Caspase 3 –> Apoptosis - Need to activate both paths for optimal pro-apoptotic activity
Fas/FasL pathways
- FasL expressed on CTLs
- crosslinks Fas (CD95) on
target
FADD- delivers a death signal
EXTRINSIC PATHWAY
CTL - FADDCaspase 8 Caspase 3Apoptosis
- BIDCytochrome c Caspase 9Caspase 3 Apoptosis
CTLs recognize and kill infected or tumor cells via TCR activation
CTLs activate dormant death pathways in target cell.
“Persuade target cells to commit suicide”.
Two pathways:
- Perforin/granzyme pathways
- Fas/FasLpathways
Natural Killer (NK) cells
Make up 5–10% of circulating lymphocytes
– Lack specific Ag receptors (no TCR)
– Innate immune cells
– Help to regulate innate/adaptive immunity by cytokine secretion
– Recognize and destroy pathogen-infected cells and abnormal tumor cells
– Proliferate earlier in infection than CTLs
NK cells recognize
and kill infected cells and tumor cells by their absence of MHC class I
How NK cells recognize targets:
The missing self model
Normal cells:
present a ligand for the activating (killing) receptor on NK cells AND
* a ligand for the inhibitory receptor (MHCI))
* MHCIexpressedon~nucleatedcells
* FailuretoexpressMHCInormallyactivatesNKcells
* Altered MHCI expression–>something is wrong with the cell–> triggers NK cells to kill the cell.
Viral infection, malignant cells
Cell-mediated effector responses
How NK cells recognize target
– The balance of inhibitory vs activating signals determines whether NK is activated or not
NK cell receptors
- Inhibitory NK receptors and ligands
- Activating NK receptors and ligands
– Whether a cell is killed or not, depends on the BALANCE between inhibiting and activating signals the NK cell receives
NK cells induce apoptosis of their targets
Once activating signal molecules are engaged, NK cells use mechanisms very similar to CTLs to induce target cell death
NKT cells bridge innate/adaptive immune systems
ossess a TCR, but it is invariant (constant, doesn’t vary)
– This TCR recognizes glycolipids presented by nonpolymorphic CD1d
* Can act as helper cells (secreting cytokines) or killer cells – Killing seems dependent on Fas-FasL interactions
* Include both CD4+, CD4–. CD8+ and CD8- cell types
* Don’tformmemorycells
* Possess NK surface proteins rather than T-cell varieties
NKT cells
May recognize lipid antigens specific to tumor cells
– Appear to play a role in viral immunity, although virus don’t usually express
glycolipids.
* May play an indirect role in shaping the viral immune response
* IFNg, IL-2, TNF and IL-4
Cell-mediated immunity:
Comparison of ways effector cells of the immune system recognize their targets
Antibody-mediated immunity
Immunity mediated by antibodies secreted by plasma B cells
B cells
are part of the immune system and develop from stem cells in the bone marrow. Also called B lymphocyte. Enlarge. Blood cell development. A blood stem cell goes through several steps to become a red blood cell, platelet, or white blood cell.
B cells kan bind Ag directly, but need Th cells for memory
* B cells take opp and present Ag om MHCII to CD4+ Th cells that recognize the
same antigen in the lymphnode
* Some B cells produce IgM Ab quickly in primary foci
* Other B cells form germinal centers and go through somatic hyper mutation (SHM)
and Class switch recombination (CSR) and compete for survival signals from Th
Antigen binding site
Antibody/Immunoglobulin («soluble BCR»)
cells
* B cells that bind Ag best
–> plasma cells that produce Ab –> memory B cells
Two types of B-cell responses elicited by distinct Ag types
T-dependent (TD) and T-independent (TI)
T-dependent (TD)
responses require help from T cells * Are typically generated upon recognition of protein Ag
T-independent (TI)
responses do not require T-cell help
* Generated upon exposure to multivalent/polymerized Ag
– TI-1 Ag bind to B cells through PRRs and mIgs
– TI-2 Ag cross-link BCRs
Effects of antibodies
Antibodies promote removal and destruction of pathogens in several ways
* Effector function is determined by antibody isotype, FC receptor and which cell expressed the given FC receptor
Neutralization:
protects against viral or bacteria infection or the damaging effects of toxins
Agglutination
enhances neutralization and more efficient clearance of pathogens from the body
Opsonization
promotes and/or enhances the engulfment of antigens by phagocytes.
Complement Activation:
results in the generation of the membrane attack complex (MAC), creating pores in pathogen membranes and killing the microbe
Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)
activates the killing activity of several types of cytotoxic cells, e.g., NK cells
Antibody-Dependent Degranulation and Mediator Release:
triggers mediator release from granulocytes
IgM
First Ab produced in a primary response
– Tend to be lower affinity
– Pentavalent (10 total Ag binding sites)
– Very good at complement fixation leading to MAC formation and target lysis
– Also efficient at forming dense Ab-pathogen complexes that are efficiently engulfed by macrophage (agglutination)
IgGs
Include several subclasses, each with distinct effector
capabilities
– Human IgG1/IgG3 effective at complement fixation
– Human IgG1 (Mouse IgG2a) mediates ADCC by NK cells
* All variants bind to FC receptors, enhancing phagocytosis by macrophages (opsonization)
IgA
- Majorisotypefoundinsecretions
– Mucus in gut
– Milk from mammary glands – Tears
– Saliva - Effective at neutralizing toxins and pathogens
- Protects epithelial surfaces from infectious agents
- Does not fix complement, so does not drive inflammation
- Long half-life in secretions due to protease-resistant amino acid sequence in Fc region
- IgA1 monomer serum
- IgA2 dimer secretions
- MediatesADCC
- Triggers degranulation of granulocytes
IgE
Best known for role in allergy and asthma
* May also play a role in protection against parasitic helminths (worms) and protozoa
* Made in very small quantities, but induce potent effects
– Degranulation of eosinophils/basophils
– Release of molecules such as histamine from mast cells to damage large pathogens
Antibody isotypes bind different Fc receptors
FcγRs–>IgG
- FcαR–>IgA
- FcεR–>IgE
- pIgR –Polymeric Ig Receptor –> IgM and IgA
- FcRn –Neonatal Fc Receptor –> IgG and Albumin
Ab-binding can activate FC receptors on effector cells
Fc region of Ab’s – bind Fc receptors expressed by accessory cells an trigger;
– Phagocytosis of Ab-bound extracellular pathogens (Macs, DC,Neutrophils) – Secretion of stored mediators (NK cells, Eosinophils, Basophils and Mast
cells)
–>Destruction of Ab-coated pathogens
FcR signaling
Multiple FcRs need to be cross-linked to initiate a signal
FcR signaling positive
enhancing effector function
FcR signaling negative
inhibiting effector function
ITAM/ITIM
(Immunereceptor Tyrosine-based Activating/Inhibitory Motifs)
FcγRs –> IgG
Most are activating receptors (three activating, one inhibiting family)
– Will induce phagocytosis if expressed by macrophages
– Will induce degranulation if expressed by cytotoxic cells
FcαR –> IgA
Expressedbymyeloidcells
– Monocytes/macrophages – Granulocytes
– Dendritic cells
* triggersphagocytosis
* MediatesADCC
*Stimulates myeloid cells to release inflammatory cytokines and generate superoxide free radicals to help kill internalized pathogens
FcεR –> IgE
Expressedby
– Mast cells/basophils
– Eosinophils
* Twotypes
– High-affinity FcεRI
– Low-affinity FcεRII (on B cells and eosinophils)
* Triggers a signaling cascade that releases histamines, proteases, and other inflammatory mediators
* Most often associated with allergy symptoms
* Defendsagainstparasiticworms
pIgR
Polymeric immunoglobulin receptor
* Expressedbyepithelialcells
* Initiates transport of IgA and IgM from blood to the lumen of multiple tissues
– Gastrointestinal tract
– Respiratory tract
– Reproductive tract
* Responsible for carrying Ab into tears and milk
* populates gut mucosa with IgA Ab to protect against ingested microbes and toxins
FcRn
- Neonatal Fc receptor
- Related to MHC class I
- Expressed on many different cell types early in an organism’s lifespan
– Epithelial/endothelial cells - Helps to carry Ab ingested in milk in newborn across epithelial cells of the intestine into the bloodstream
- In adults, can help recycle IgG taken up through endothelial cell pinocytosis processes back into blood
High affinity IgG and IgA can inhibit the infectivity of viruses
Viruses infect cells by binding to a particular cell-surface receptor
- Ex Hemagglutinin of influenza virus
–> binds terminal sialic acid residues on glycoproteins on epithelial cells of the respiratory tract
–>Ab against hemagglutinin prevent infection
adhesins
are virulence factors that allow bacteria to attach to host cells. Although many pathogenic bacteria express various kinds of adhesins, often they are encoded on the bacterial backbone DNA (such as S fimbriae and Type 1 fimbriae expressed by E.
Ab binding of adhesins blocks bacteria entry into the cell.
Agglutination
Agglutination, which refers to the clumping of particles together, is an antigen-antibody reaction that occurs when an antigen (i.e., a molecule capable of triggering the adaptive immune response) is mixed with its corresponding antibody at a suitable pH and temperature.
enhances neutralization and more efficient clearance of pathogens from the body
Impairs uptake of antigen into cells (large complexes can’t be taken up)
Ab-coated pathogens are recognized
by effector cells through Fc receptors that bind to Fc portion of Ab.
Opsonization
«To prepare for eating»
- Involves the coating of the surface of a pathogen by antibody and/or
complement C3b, making it more easily ingested by phagocytes
Fc receptors activate macrophages
to engulf and degrade (phagocytose) antibody-coated bacteria
- Antibodies must be aggregated on a surface to cross-link Fc receptors and activate phagocytes
- Free Ig bind Fc receptors with low affinity (control mechanism).
Fc-dependent pathogen destruction by triggering granule release
Ab can target effector cells to release granule contents.
* Effector cells express Fc receptors that bind Ab crosslinking
induces activation of effector cells.
* Fc-dependent pathways for destruction by granule release include;
* Mast cell and basophil activation
* Antibody-Dependent Cell-mediated Cytotoxity (ADCC)
* NK cells
* Eosinophils ++)
Mast cells
Defend against pathogens that penetrate the epithelial barriers.
The high affinity receptor for IgE is expressed constitutively by mast cells and basophils.
Can bind free monomeric IgE
IgE bound to the surface of mast cells is
aggregated by binding to antigen
histamine and many other mediators
increased blood flow to the site of infection
recruits Ab and effector cells
coughing, sneezing, rhinitis, vomiting
Fc receptors on Eosinophils bind Ab-coated parasites
Defence against large parasites such as helminths that are too large to be phagocytosed.
* Eosinophil binds IgE-coated parasite through Fc receptors triggers the release of secretory granules containing anti- parasitic toxins by exocytosis.
Antibody-Dependent Cell-mediated Cytotoxicity
A type of immune reaction in which a target cell or microbe is coated with antibodies and killed by certain types of white blood cells. The white blood cells bind to the antibodies and release substances that kill the target cells or microbes. Also called ADCC and antibody-dependent cellular cytotoxicity.
-The killing of Ab-coated target cells by cells with Fc receptors that
recognize the constant region of the bound antibody.
* ADCC is generally mediated by NK cells
NK cells express FcγRIII/CD16 and recognize bound Ab (IgG1/IgG3) on
virus-infected host cells
Negative regulation of B cells
Shutting down B cell receptor (BCR) signaling when
proliferation is no longer required
Negative signaling through FcγRIIb receptor inhibits B-cell activation
– Possesses ITIMs
– Antibodies bind antigens efficiently and are normally not in high concentrations in circulation as long as Ag is present.
– When Ag is no longer presentExcess circulating IgG can bind this receptor and shut down B-cell activation
– Phosphatases are recruited to phosphorylated ITIMS, stripping phosphates from signaling molecules
antibody isotypes
antibodies are classified into five main classes or isotypes – IgA, IgD, IgE, IgG and IgM. They are classed according to the heavy chain they contain – alpha, delta, epsilon, gamma or mu respectively.
Describe two ways by which antibodies can induce effector responses that promote phagocytosis.
Another mechanism by which antibodies can response to pathogens is known as “opsonization.” By opsonization, antibodies enable phagocytes for ingesting and destroying the extracellular bacterium. The phagocytes recognize the Fc region of the antibodies coating the pathogen and foreign particles (Fig.
What is antigen presentation?
Antigen presentation, activation of both CD4+ cells (such as TH1 and TH17s) and CD8+ T cells, increases in the inflammatory cytokines, IL-6, IL-12, TNFα, and IFNγ, and NFκB activation, in addition to decreased pro-apoptotic proteins such as caspase 8 and 10, lead to enhanced immune cell survival and activation of pro-inflammatory mechanisms
