Antibodies contribute to immunity through three key mechanisms:
- neutralization
- opinization
- complement activation
There are three main ideas here:
- Affinity maturation + class switching are the core programs executed by B-cells, facilitated by T-cells
- Distribution and function of antibody types
- How antibodies engage various effector functions
T_FH -> CD40L CD40 -> B-cell marker
non-canonical NfKb -> main pathway for proliferation + differentiation ()
CD21 / CD19 / CD81
B-cell coreceptor complex CD19 CD21 CD81
response
B cells that encounter their antigens migrate towards the boundaries between B cell and T cell zones
It is remarkable that both these cells can be specific for the same type of antigen when the odds are 1/10000.
T-cells - CCR7 -> CCL19 + CCL21 expressed by stromal + dendritic cells in the T-cell zones
B-cells - CXCR% -> CXCL13 expressed by specialized follicular dendritic cell in the primary lymphoid follicles / B-cell zones.
B-cells encounter BAFF in the follicles, an important TNF-family cytokine that acts through BAFF-R, but also BCMA + TACI, to activate the non-canonical NFkB pathway similar to CD40.
Antigens are trapped by complement receptors on macrophages in specialized zones (subcapsular sinus in lymph nodes + marginal sinus in spleen). These macrophages do not swallow them for some reason. B cells shuttle between sinus zones and follicle, bringing antigens to the FDCs there.
Movements:
- B-cells first position in the primary follicles for 6-24 hours with EBI2 expression.
- They then express CCR7 and move towards the interface of follicles + T-cell zones where they encounter T-cells for crosslinking.
T cells express surface molcules and cytokines that activate B-cells that in turn promote T_FH development
T-cells (early FH linaeage subtypes) express:
- IL-21 -> STAT3 activation in B-cell
- CD30L -> B-cell activation + proliferation
- CD40L -> B-cell activation and proliferation
Prolong and stable physical contact mediated primarily by SLAM (IgG superfamily protein expressed on both cells)
Some reciprocal signaling from B-cells - ICOSL binds to ICOS and leads to T_FH differentiation. ICOS also involved in expressing Bcl-6 and c-Maf
2-3 days after initial activation, B-cells downregulate CCR7 and once again express EBI2 to move into follicles. There they form clumps called the primary focus.
The primary focus is mostly made up of plasmablasts. Not all T-cells become plasmablasts in the primary focus and some become long-lived plasma cells. These foci are pronounced 5 days into the infection but are temporary.
Both types - Large volumes of antibody secretion. Up to 20% of protein synthesized by cell. Continued expression of surface IG + MHC-II (though less of both in plasma cells).
Then main difference seems to be plasmablasts continued ablity to class switch, proliferate + (potentially) continued somatic hypermutation.
Plasma cells also seem to negatively supress T_FH cells by decreasing Bcl-6 and IL-21 with cognate antigens on MHC-II.
The 2nd phase of B-cell response occurs when B-cells migrate into primary follicles and form germinal centers
- B cells become activated by both BCR and MHC:peptide recognition by helper T-cells
- These cell types first
- Major proteins:
- CD40L -> CD40.
- ICOS
- IL-21
- T-cell activation in the germinal center induces proliferation and then differentiation into either plasma cells or memory cells
- The "germinal center reaction" uses somatic hypermutation and class switching to . Somatic hypermutation through AID is unique to B-cells.
Diffussion / transport is a key property of antibody constant chains that determines their function. (Moving across organ barriers from site of synthesis.)
Polymeric immunoglobulin binds to the Fc regions of IgM and IgA and transports them across epithelial barriers
IgM are produced first
IgG is primary class in blood. IgA is primary in secretions / epithelial tissue. Neutralizing over opinization.
IgE binds avidly to mast cells, that release chemicals that cause vomiting, sneezing and coughing.
Generated right below epithelial layer and transported through epithelial cells to surface (eg. the gut or bronchi).
- Synthesized in lamina propria
- Internalized by epithelial cells along basolateral surface (both basal + lateral)
- Passes through cytoplasm in a process called transcytosis
- Extracellular domain of pIgR + antibody is cleaved
Cleaved EC domain is called Secretory Component.
- Bind to mucins in mucus (heavily glycosylated proteins)
- Protect antibodies from cleavage in gut
Where does this happen?
- Gut
- Respiratory epithelium
- Lactating Breast
- Other exocrine glands like tear + salivary
Gut
Bronchi
Toxins are effective at small concentrations. IgG need to diffuse rapidly and bind with high avidity. Usually a toxic domain and a cell binding domain..
- Clostridium tetani. Blocks inhibitory neuron activiy and leads to chronic contraction.
- Gas gangrene. Clostridium perfringens. Phospholipase
- Diphtheria. Corynebacterium diphtheria. Blocks protein synthesis
- Vibrio cholerae. cholera toxin. elevates cAMP and causes epithelial cells to lose water
- Anthrax. Bacillus anthracis.
- Botulism. Blocks release of acetylcholine, leading to paralysis. Clostridium botulinum
- Whooping cough. Bortedella pertussis. Pertussis toxin -
Recall complement has:
- antibody dependent - classical pathway - C1q
- antibody independent - lectin pathway
The structure of C1q requires that antibodies are bound to the surface of a cell and are not just floating around in serum.
Requires two or more Fc regions. One IgM molecule is enough. 2+ IgG needed. IgM is more efficient at activating complement.
IgM shitfs from planar to staple conformation upon binding antigen and new geometry is suitable for C1q binding.
- Fc receptors on phagocytes can clear complexes
- Erythrocytes can shuttle intact immune complexes to spleen and liver. There they are cleared by phagocytes with both Fc + CR1 receptors. Saturation of complex with complement proteins (C4b + C3b) seems to be involved in the recognition + shuttling of the complex.
Complexes that are not cleared deposit on the basement membranes of blood vessels, notably the glomerulus, where podocytes (the cells that filter blood) have CR1 and are responsible in some capacity for filtering
- IgM is the first isotype to express
Effector cell recruitment mediated by different Fc receptors is important to neutralize pathogens that are not already neutralized by binding - via phagocytosis or release of cytokines.
Two main components:
- \alpha chain - responsible for binding
- \gamma chain - responsible for signal transduction and transport to cell
CD64 - IgG - macrophage / neutrophil / eosinophil CD32 - IgG - MNE + B cell / mast cell (can be stimulatory or inhibitory depending on internal domain) CD89 - IgA - macrophage / neutrophil / eosinophil
Some receptors that bear the same name (CD32) are inhibitory and other are not.
Inhibitory Tyrosine associated Immune Motif
TRIM21 is a soluble Fc receptor in the cytoplasm with the highest affinity for IgG. Responsible for clearing ingested complexes by E3 ligation.
Some bacteria have capsular polysaccharide on membranes that prevent engulfment and are only susceptible when coated by complement.
These are TI-2 antigens (do not require T-cell help, which includes affinity maturation and memory B-cell). Induce secretion of IgM which recruit compelemnt.
Because immune cell response is inflammatory want to avoid false positives. Require multiple antibodies to be bound to pathogen to avoid this.
Fc receptors result in engulfment (acidic phagosome -> phygolysosome) or release of toxic chemicals.
Macrophages and neutrophils mainly engulf - bacteria / virus. Eosinophils mainly release chemicals - Helminth parasites / worms
Lymphoid lineage cells that have non-specific response:
- Recognize surface proteins directly
- Recognize antibodies bound to surface proteins. This has a name - ADCC.
Fc\epsilon bind with very low concentrations and bind without the presence of antigen. Most circulating IgE are already bound to mast cells in tissue and circulating basophils
After crosslinking of surface-bound IgE by multivalent antigens, fast response:
- release cytokines
- release lipids eg. prostglandin (restrict airway, cause inflammation, recruit immune cells)
- histomines to increase vascular permeability
Mast cells:
- recruit immune cells
- create blood flow to traffic antigens to lymph tissue
- muscle contraction expels antigens
Also involved in clearing parasites by clumping in the intenstine - mastocytosis.
Eosinophils release granules and are responsible for killing many types of parasites
- Antibodies are produced by B lymphocytes and neutralize antigens in different ways - binding, effector cells or complement.
- Antibody production requires a cognate T-cell with similar antigen recognition. B-cells move to T-cell boundary for this.
- Class switching and somatic hypermutation occur in germinal centers.
- B-cells can become plasmablasts?
- T_H produce cytokines that help with class switching