Lecture 15: B Cell Activation and Antibody Production II Flashcards
What results in the formation of mRNA for the membrane or secreted form of the u (mu) heavy chain
Alternative processing of primary RNA transcript
What results in an increasing fraction of the u protein produced as a secreted form
Ag activation of B cell
What are required for somatic hypermutation of Ig V genes to occur
Tfh cells and CD40:CD40L interaction
Where does affinity maturation occur
In the germinal center
Affinity maturation is a process that occurs through
A repeated process of somatic hypermutation of B cell receptors and subsequent clonal selection
Hypermutation can occur where (on gene)
Anywhere along the VDJ region of Ig genes
Can be heavy or light chains or both
What is expressed by B cells that initiates somatic hypermutation in the GC, and how does it work
AID
Induces cytosine to uracil deamination
-This is a mismatch that can be repaired by base excision
Where do Ig V genes undergo point mutations at a very high rate
In the dark zone of the GC
Can occur at an average rate of almost one per cell division
Can Ig V mutations continue to occur in progeny of B cells
Yes, any B cell clone can accumulate more and more mutations during its life in the GC
IgG vs IgM # of mutations
IgG has many more mutations
Mutations correlate with
Increased Ab affinity for Ag that induced response
Why do B cells with highest affinity for Ag have selective advantage
Because they are more likely to bind Ag on FDC which will rescue them from apoptosis
Where is a site of tremendous apoptosis
Germinal center, because somatic mutation also generates many B cells that do not express high affinity Ag receptors and die
Where do B cells migrate after somatic hypermuation
FDC-rich light zone of the GC
Reasons that high affinity B cells survive
- Binding to Ag induces increased expression of anti-apoptotic protein Bcl2
- High affinity B cells will endocytose and present Ag for Tfh cells in GC and induce CD40:CD40L signaling
- Expression of endogenous inhibitors of Fas may occur in high affinity B cells that have bound Ag on BCR
What is a primary cause of B cell lymphomas
Translocation of various oncogenes into Ig gene loci
-DNA breaks associated with somatic hypermutation and isotype switching facilitate these translocations
Isotype switching primarily occurs where and is facilitated by who
Germinal centers facilitated by Tfh cells
Some isotype switching my also occur where and facilitated by
Extrafollicular foci, driven by extrafollicular Th cells
Antibody specificity is determined by what region of the Ig chains
Variable regions
Intracellular pathogen leading to IgG switching- how does this happen
Intracellular pathogen activate Th1 cells which make IFN-y, which also likely induces IFN-y+ Tfh cells to make more IFN-y
IFN-y induces IgG switching
Helminth leading to IgE switching
Helminths likely influence Tfh cell differentiation in to IL-4+ Tfh cells which produce Th2 type cytokines like IL-4 to induce IgE switching
B cells in mucosal areas would likely switch to what Ab class- and what would cause this
IgA switching, caused by TGF-b produced by many cells including Treg cells and macrophages
What co-stimulates the switch to IgA
BAFF produced by DCs and macrophages
Isotype switching and affinity maturation are critically dependent upon ___ which is induced by
AID, which is induced by CD40 engagement from Tfh cells
Immunodeficiencies related to CD40-CD40L might cause what Ab to dominate
IgM, because class switching wont occur without CD40/CD40L and AID
Every C gene is preceded by a ______ that controls rearrangement process
Switching sequence
AID switches C to U, and then what happens
UNG removes U residue, then endonuclease APE1 creates nicks that lead to DS breaks- probably fixed by NHEJ
CSR is preceded by
Expression of germline transcripts which open the chromatin structure of a specific S region and make it accessible to the recombinase
Switch regions are comprised of
Repetitive sequences of palindrome rich motifs and result in looped out deletions
Short lived plasma cells are generated in
Extrafollicular compartments during T-independent responses
-may also be generated early during T-dependent responses in extrafollicular B cell foci
Short lived plasma cells typically found where
In secondary lymphoid organs or peripheral non-lymphoid tissues
Long lived plasma cells are generated
In T-dependent responses to protein Ag in the GC
-Signals from BCR and IL-21 are involved via stage of their precursors called plasmablasts
Plasmablast to long lived plasma cell route
Generated in GC and home to one marrow where they differentiate into long lived plasma cells
Long lived plasma cells are maintained by
BAFF
% of Ab in blood produced by long lived plasma cells
50%
B cells generated in the bone marrow exit as _____ which are ____ and express ____
Exit as pre- B cells which are immature and express IgM
-They further mature into naïve B cell and then follicular or MZ B cells in the spleen
Activated (T-independent) MZ and Follicular B cells turn into
Plasmablasts and short lived plasma cells
Activated (T-dependent) MZ and F B cells turn into
Memory B cells, and express Abs of switched class/higher affinity
When is a long lived plasma cell formed
When memory B cells are reactivated by Ag
Long lived plasma cells survive where
In the bone marrow mostly, but also secondary lymph organs
Do memory B cells still require Tfh cell-mediated regulation following Ag recall?
Yes, Ag specific memory Tfh cells likely still regulate memory B cells responses
What may stimulate Ab production in the absence of Th cells (TI responses)
Non-protein Ags such as polysaccharides or lipids
What Abs are produced in the absence of Th cells
Low affinity IgM mainly, also some IgG and IgA
What makes TI Ags able to stimulate B cell activation without T cell help
Many TI Ags are multivalent, and induce maximal cross-linking of the BCR complex on B cells, leading to activation without T cell help
Which B cells are especially important for response to TI Ags
MZ B cells and B-1 B cells
MZ B cells are a distinct population of B cells in the ___ that mainly respond to _____ and differentiate into
In the spleen, respond to polysaccharides and differentiate into short lived plasma cells that produce mainly IgM
B-1 B cells respond to TI Ags mainly where
Peritoneum and mucosal sites
TI Ags cannot be processed and presented by
MHC molecules, and therefor cannot be recognized by CD4+ helper T cells
Polysaccharides activate ____ system by the _____ pathway, leading to
Activate compliment system by the alternative pathway, generating C3b and then C3d which is recognized by CR2 on B cell
Two ways B cells can be activated without T cell help
- Compliment coated Ags can ligate both BCR and CR2
- Simultaneous PAMP recognition and TLR signaling
Cytokines produced by ____ may stimulate isotype switching in TI responses. Examples
Non-T cells
- TGF-b secreted at many mucosal sites causes IgA
- BAFF produced by DCs/MOs induces AID
What contributes to the generation of natural antibodies, what are they and who generates them
TI Ags
They are low affinity anti-carbohydrate Abs
Produced by B-1 cells in GI tract and MZ B cells in spleen
Memory occurs only if vaccines are able to activate
T helper cells
How are vaccines to capsular polysaccharides produced (as they do not stimulate T cells)
Polysaccharide is linked to a foreign Ag
What happens when Ab-Ag complex simultaneously bind BCR and the FcyRIIB receptor
FcyRIIB associated phosphatases inhibit signaling by the BCR complex and block B cell activation
IgG vs IgM and FcyRIIB activation
IgM activates complement but do not bind to FcyR are involved in amplification, while IgG leads to feedback
FcyRIIB knockout mice show
Uncontrolled Ab production
