Lecture 16 - B Cell Activation and Antibody Production (part II) Flashcards
Alternating processing of a primary RNA transcript resutls in what?
The formation of mRNA for the membrane or secreted form of μ heavy chain
What does B cell differentiation results in an increasing fraction of what?
The μ protein produced as the secreted form
What are four exons of the Cμ gene?
Cμ1, Cμ2, Cμ3, Cμ4
In T-dependent responses, some of the progeny of activated IgM and IgD expressing B cells under what?
Heavy chaing isotype (class) switching and produce Abs with heavy chains of different classes
Where does isotype switching primarily occur? What are they driven by?
In the germinal centers
Driven by Tfh cells
What is another place were some isotype switching may occur? What are they driven by?
In B cells in extrafollicular foci, driven by extrafollicular Th cells
The capacity of B cells to produce different Ab isotypes provides what?
A remarkable plasticity in humoral immune responses
Abs of various classes perform distinct effector function and are involved in defense against:
Different types of pathogens
Class-switching occurs in response to changing what?
The constant regions of the heavy chains
What is the specificity of the Abs determined by ?
The variable regions
In class-switching, what remains unaltered?
The specificity of the Abs
Isotype switching in response to different types of microbes is regulated by what?
Cytokines produced by the Th cells that are activated by these microbes
What induces B cell switching to IgG?
IFN-γ produced by Th1
What induces B cell switching to IgE?
IL-4 produced by Th2
What does the response to viruses and intracellular bacteria involve? why?
IgG abs, because they block the entry of microbes into host cells and also promote phagocytosis by Mo
Intracellular pathogens activate what Th cells?
T helper cells of the Th1 subset to produce IFN-γ and IFN-γ+ Tfh cells to make more IFN-γ
The response to helminths is mainly driven by what Abs?
IgE, it particiipates in eosinophil and mast cell-mediated elimination of the helminths
What do helminths likely influence Tfh cells to differentiate into?
IL-4+ Tfh cells which produce Th2-type cytokines during the germinal center reaction
B cells in different anatomic sites switch to different ______.
isotypes
B cell in different anatomic sites switching to different isotypes is controlled by what?
The cytokines produced at these sites
What do B cells in mucosal tissues switch to ?
IgA
What antibody is most efficiently transported through epithelia into mucosal secretions, where it defends against microbes that try to enter through the epithelia?
IgA
What is the switching to IgA stimulated by?
TFG-β, which is produced by many cell types including Treg and Th cells
Also BAFF, a cytokine of the TNF family
CD40 signals work together with what to induce isotype switching?
Cytokines
What does CD40 engagement induce?
The enzyme activation-induced deaminase (AID)
What is AID crucial for?
Both isotype switching and affinity maturation
In immunodeficiencies related to the CD40-CD40L axis, what occurs?
Ab response to protein Ag is dominated by IgM Abs, whereas there is limited switching to other isotypes
What do mature B cells first produce?
IgM immunoglobulins
What is every C gene preceded by?
A switching (s) sequence that controls the rearrangement process
VDJ—-SCμ
What is CSR?
Is a “switch” of the immunoglobulin isotype from IgM/IgD to IgG, IgA, or IgE with similar Ag specificity but with different biological properties
Where does CSR take place?
In activated B cells between two switch regions (S) comprising repetitive sequences of palindrome-rich motifs
What changes occur in the Ch gene during CSR?
Expression of the Cμ region to one of the other Ch genes
What does CSR result in?
A looped-out deletion of the intervening DNA segments
What is CSR preceded by?
The expression of germline transcripts initiated from intronic promoters and regulated by specifically by various cytokines
What does the germline transcription open?
The chromatin structure of a specific S region and renders it accessible to the putative recombinase
What is a key enzyme required for isotype switching and affinity maturation?
AID
What is AID expression activated mainly by?
CD40 signals from Tfh cells
What does AID deaminates?
Cytosines in single-stranded DNA templates, converting C residues to uracil residues
What does Uracil N-glycosylase do?
UNG removes U residues to generate abasic sites where the APE1 endonuclease creates nicks that lead to a double-stranded break
What is affinity maturation?
The process that leads to increased affinity of Abs for a particular Ag
Affinity maturation is observed only in Ab responses to what?
T-dependent protein Ags
What is required for somatic mutation to be initiated?
CD40:CD40L interactions
During the germinal center reaction, what results in the production of Abs with high affinity for Ag?
Somatic hypermutations of Ig V genes and selection of B cells with high-affinity Ag receptors
In proliferating GC B cells in the dark zone, what do Ig V genes undergo?
Point mutations at an extremely high rate
What is the rate of point mutations in the Ig V regions of B cells in the dark zone?
1 in 10^3 base pairs per cell division in V gene, which is about a 1000x higher rate than spontaneous rate mutation in other genes
Mutations in Ig V genes are also called what?
somatic hypermutation
The Vh and Vl genes of expressed heavy and light chains in each B cells contains how many nucleotides?
about 700
*this implies that mutations will accumulate in expressed V regions at an average rate of almost one per cell division
Ig V gene mutations continue to occur in the progeny of what?
individual B cells
Any B cell clone can accumulate what?
more and more mutations during its life int he germinal center
It is estimated that as a consequence of somatic mutations, the nucleotide sequences of IgG Abs derived from one clone of B cells can have how much of the germline sequence mutated?
5%
Usually translates up to 10 aa substitutions
Where are the mutations clustered in?
V regions, mostly in the Ag-binding complementarity-determining regions CDR
There are more mutations in IgM or IgG?
IgG
What does the presence of mutations correlate with?
Increasing affinities of the Abs for the Ag that induced the response
Mutations in V genes increase with time after what?
immunization and w repeated immunizations
The affinities of the Abs produced also tend to increase with what?
more mutations
somatic mutation of V genes generates Abs with what?
different affinities
What is necessary to rescue the B cells from apoptosis?
Binding to Ag displayed on follicular DCs or present Ag to germinal center Tfh cells
As the amount of available Ag decreases during an immune responses, what has a selective advantage? What does this lead to?
B cells with the highest affinity
This leads to an average increase in the affinity of Abs for Ag as the humoral immune response progresses
Because somatic mutation also generates many B cells that do not express high-affinity receptors for Ag and cannot be selected to survive, what are germinal centers sites for?
Tremendous apoptosis
B cells that have undergone somatic mutation migrate tow hat?
Into the FDC-rich ligh zone of the germinal center
B cells with high -affinity receptors for the Ag are best ablew to bind to Ag when it is present in what levels?
low concentrations by follicular DCs
What does Ag recognition by itself induce?
Expression of anti-apoptotic proteins of the Bcl-2 family
High-affinity B cells will preferentially endocytose and present the Ag for Tfh cells in the Gc which signal via what?
CD40L to promote the survival of the B cells
What do Tfh recognize on B cells to induce apoptosis?
Fas on B cells that are recognized by FasL on Tfh cells
How do B cells protect themselves from death involving Fas?
High-affinity B cells may activate endogenous inhibitors of Fas when their BCRs recognize Ag and thus protect themselves from death, while low affinity B cells are killed
Many tubors of B cells, B cell lymphomas, develop from what?
Germinal center B cells
What are B cell lymphomas caused by?
Chromosomal translocations of various oncogenes into Ig gene loci
DNA breaks associated with what facilitate incorporation of oncogenes?
With somatic hypermutation and isotpye swithcing
How can germinal centers also contribute to the pathogenesis of autoimmunity?
Somatic mutations may produce a self-reactive B cell clone in the germinal center
What are plasma B cells?
They are morphologically distinct, terminally differentiated B cells committed to abundant antibody production
How are Plasma B cells generated?
After the activation of B cells through signals from the BCR, CD40, TLRs, and other receptors including cytokine receptors
Short-lived plasma cells are generated during what?
T-independent responses
May as well be generated early during T-dependent responses in extrafollicular B cell foci
Where are short-lived plasma cells generally found in?
Secondary lymphoid organs and in peripheral non-lymphoid tissues
Long-lived plasma cells are generated where?
In T-dependent germinal center responses to protein Ag
What signals generate long-lived plasma cells?
Signals from BCR and IL-21 via a stage of their precursors called plasmablasts
Plasmablasts generated in germinal centers enter the circulation and home to what?
The bone marrow where they differentiate into long-lived plasma cells
Typically 2 to 3 weeks after immunization with a T cell-dependent Ag, the bone marrow becomes a major site of what?
Ab production
Long-lived plasma cells in the bone marrow are maintained by what?
BAFF allowing the cells to survive for long periods, often as long as the life span of the host
Long lived plasma cells may continue to secrete Abs for how long after the Ag is no longer present?
months or even years
Abs that are continually secreted for months are years after Ag is no longer present provide what?
Immediate protection if the Ag is encountered later
It is estimated that almost how much of the Ab in the blood of a healthy adult is produced by long-lived plasma cells and is specific for Ags that were encountered in the past?
50%
Memory B cells are generated mainly in what?
Germinal centers for T-dependent protein Ags
Memory B cells acquire the ability to do what without continuing Ag stimulation?
Survive for long periods of times
Memory B cells express high levels of what?
Anti-apoptotic protein Bcl-2
What do memory B cells typically express?
High-affinity(mutated) Ag receptors and Ig molecules of switched isotypes
Abs production is greatly accelerated after secondary exposure to Ags, and this can be attributed to what?
Activation of memory cells in germinal centers
Some memory B cells may remain in the lymphoid organ where they were generated, whereas others exit germinal centers and do what?
recirculate between the blood and lymphoid organs
Effective vaccines against microbes and microbial toxins must induce what? When will this only occur?
Both affinity maturation and memory B cell formation
It will occur only if the vaccines are able to activate helper T cells
The concept of vaccines activating helper T cells has been applied to the design of what type of vaccine?
Vaccines for capsular polysaccharides, which is incapable of stimulating T cells.
Polysaccharide acts as a carrier for the foreign protein, they are covalently linked to form hapten-carrier conjugate
Such vaccines, called conjugate vaccines, more readily induce high-affinity Abs and memory cells
What Ags are termed thymus independent or T independent?
Many non-protein Ags, such as polysaccharides and lipids that stimulate Ab production in the absence of Th cells
The Abs produced in the absence of Th cells have what general characteristics?
Low affinity and consists mainly of IgM, with limited isotype switching to some IgG subtypes and also to IgA
Most T-independent Ags are composed of repeated identical Ag epitopes termed:
multivalent
Multivalent Ags may induce what that leads to activation without a requirement for cognate T cell help?
Induce maximal cross-linking of the BCR complex on specific B cells
TI Ab responses may be initiated where?
In the spleen, bone marrow, peritoneal cavity, and mucosal sites
What B cell subsets are especially important for Ab responses to TI antigens?
Marginal zone and B-1 subsets of B cells
What are a distinct population of B cells that mainly respond to polysaccharides?
Marginal zone B cells
After activation of marginal zone B cells, what do the cells differentiate into?
short-lived plasma cells that produce mainly IgM
Where may TI Ags persist for prolonged periods where they are recognized by specific B cells?
On the surfaces of marginal zone Mo in the spleen
What lineage of B cells respond readily to TI Ags mainly in the peritoneum and in the mucosal sites?
B-1 cells
What are the most important TI Ags?
polysaccharides, glycolipids, and nucleic acids
TI Ags cannot be processed and presented with what?
MHC molecules- therefore they cant be recognized by CD4 helper cells
Many TI Ags of polysaccharide nature activate the complement system by which pathway?
The alternatThe alternativew pathway, generating C3d, which binds to the Ag and is recognized by CR2 on B cells
TI responses may also be facilitated by additional signals derived from microbial products that activate what on B cells?
TLRs
Some TI non-protein Ags can induce Ig isotypes other than IgM. In humans, the dominant Ab class induced by pneumococcal capsular polysaccharide is what?
IgG2
Cytokines produced by non-T cells may stimulate ______ in TI responses
Isotype switching
BAFF produced by DCs and Mo can induce the synthesis of what? This may be also be further facilitated by the activation of TLRs on these B cells
AID in Ag-activated B cells
What is secreted by many non-lymphoid cells at mucosal sites that help mediate the IgA switch?
TGF-β
Many bacterial polysaccharides of encapsulated bacteria belong to what Ags?
TI
What is the major mechanism of host defense against infections by encapsulated bacteria?
Humoral immunity
Individuals with immunodeficiencies of humoral immunity are especially susceptible to life-threatening infection by what pathogens?
Pneumoccocus, menigococcus, and Haemphilus
TI Ags also contribute to the generation of natural antibodies, which are present in the circulation of normal individuals and are apparently produced without what?
overt exposure to pathogens
What are most natural Abs? What are they postulated to be produced by?
Low-affinity anti-carbohydrate Abs
Produced by peritoneal B-1 cells stimulated by bacteria that colonize the gastrointestinal tract and by marginal zone B cells in the spleen
Many polysaccharide vaccines, such as the penumoccocal vaccine, induce what type of protective immunity?
long-lived
Rapid and large secondary responses typical of memory (but without much isotype switching or affinity maturation) may also occur one secondary exposure to what Ags?
polysaccharide (carbohydrate) Ags
What are natural Abs that recognize certain glycolipids (blood group Ags) expressed on the surface of many cell types, including blood cells?
Anti-ABO Abs
Blood group Ags and Abs are not important for host defense but for what instead?
For blood transfusions and transplantation
Ag-Ab complexes can simultaneously bind to what receptors on B cells?
Membrane Ig (through Ag) and the FCγRIIB receptor through the Fc portion of the antibody
Ag-Ab complexes binding to membrane Ig and FcγRIIB simultaneously causes what?
phosphatases associated with the cytoplasmic tail of the FcγRIIB inhibit signaling by the BCR complex and block B cell activation
FcR-mediated Ab feedback is what type of mechanism in humoral immune responses?
Physiologic control mechanism
FcR-mediated Ab feedback is triggered by what?
Secreted Ab and blocks further Ab production as apposing the activation via CR2
It is not clear under which circumstances secreted Abs provide complement mediated amplification or what?
Fc receptor - mediated inhibition
A likely scenaria is that iGm which activates complement but does not bind to the FcγR are involved in ________. Whereas increasing production of IgG leads to _________,
Amplification
Feedback inhibition
A ________ in the FcγRIBB gene has been linked to susceptibility to the autoimmune disease systemic lupus erythematosus in humans.
polymorphism
