Final Exam (New Content) Flashcards
What is the point of the germinal center
It is a site for improving B cell receptors
What do FDCs do inside the germinal center *
- Secrete BAFF for survival of nearby B cells
- FDCs hold some antigen that the B cell receptors bind
- (only one or two) Cognate pair proliferates (b cells that enter the GC are referred to as centrocytes)
What follicle is the GC known as?
The secondary follicle
- Clonal Expansion
- FDCs secrete IL-6, Il-15, and BAFF for B cells to help them proliferate
- Cognate B cells (the ones connected to the few cognate pairs that went into the Germinal Center) become centroblasts and divide into hundreds of daughter cells (more centroblast) and stop expressing B cell receptors on their surface
- They ‘open’ the rearranged V regions of the heavy AND light chain DNA
- Cognate Tfh divide, too
- this is occuring in the dark zone of the GC
Centroblasts
Germinal center B cells that are dividing and mutating heavy and light chain genes that encode the B cell receptor. Centroblasts arise due to cytokines secreted by FDCs
*dark zone
*You are a centroblasts from the moment you recieve signals from FDC to expand all the way until you actually have a mutated BCR (after AID)
Centrocytes
Germinal center B cells that are competing with mutated BCRs for limited antigen from FDCs and co-stimulation from cognate Tfh. They switch the class of BCR that they express under the guidance of cognate Tfh cytokines
- Somatic hypermutation during expansion
*All FDCs do is they stop the B cell from expressing their BCR and opens up their variable regions. To actually create mutations, they need Tfh cell help
– Tfh use CD40L to stimulate centroblasts to express AID enzyme
- AID mutates variable regions of rearranged Ig DNA
- AID mutates both the light and heavy chain and each centroblast mutates differently
AID
Activation-induced cytidine deaminase
Changes cytidines to uracils in ssDNA
AID makes point mutations at a rate of 1 per 1000 base pairs (hypermutation)
Common DNA repair enzymes randomly replace U with A,C,T or G which makes those mutations
(this is because the body knows DNA shouldn’t have uracil, the DNA repair enzymes just see this as damage so they will randomly add a nucleotide that is supposed to be in DNA. this is what creates the hypermutation)
Affinity maturation
Must select the B cells with beneficial mutations. The ones with mutated BCR that have the highest affinity for original antigen
- Somatic hypermutation (2)
After a few days, centroblast become centrocytes that express a mutated BCR, mutations appear as changes to antigen-binding sites
So when the B cell is expressing a mutated BCR after a few days, what kind of cell is it?
- Centrocyte
- Affinity Maturation
- There are 100s of centrocytes expressing differently mutated BCRs. Not all are useful
- Must select the ones that bind antigen with the highest affinity.
- The mutated centrocytes rush to the FDC to bind the limited antigen on FDCs. They need an antigen signal and T cell co-stimulation to survive
- Centrocytes will die if they don’t receive signals within days.
- Dead cells are phagocytosed and cleared by macrophages
What do successful centrocytes express?
- Bcl-Xl thta prevents programmed cell death and induces clonal proliferation
*Affinity maturation is a form of positive selection for useful B cells AFTER they encounter antigen… the adaptive immune system evolves to keep up with rapidly evolving pathogens
*even after hypermutation, a centrocyte can go back into the dark zone and becomes a centroblast again, adn mutates some more
Is proliferations and somatic hypermutation in the dark zone or light zone?
Dark
(competing is in the light)
Centrocytes switch ___ using AID too
Centrocytes switch C regions using AID too
Tfh cytokines instruct centrocytes to switch to various isotypes
- Class (isotype) switching *also occurs in the GC
Cognate Tfh provide signals to guide the isotype of BCR that a centrocyte expresses
- CD40L to induce AID first
- Class-specifying cytokines for different isotypes
What is Class (isotype switching) switching
AID enzyme driven recombination of constant gene segments of the HEAVY CHAIN DNA that leads to switching from IgM or IgD to another isotype (IgG, IgA, or IgE) without changing the specificity for antigen
How does a Pre-Tfh know which cytokines to secrete for isotype switching?
- Pre-Tfh were programmed to secrete class switch cytokines during their earlier activation. (Like after it got the signal to become a Tfh, it got a lot of other cytokines too, especially at the band between T and B cell areas)
- The Tfh secretes those switch cytokines as a cognate pair in the germinal center
When are these cytokines secreted by Pre-Tfh?
When the T cell is in cognate pairs in the germinal center
How does the switching of regions actually work?
- The combination of CD40L and whatever fate-specified cytokines are released will induce NFkB and a specific STAT TF (depending on the cytokine). This will cause the TFs to land upstream of particular constant regions (like the 3 g, e, or a region)
- In front of each region except for D is a switch region
- The RNA polymerase will open up the DNA to transcribe, giving AID the opportunity to come in (because of the repeated nucleotide sequences) and remove all the C’s to replace them with Uracil
- Certain enzymes will see this and try to fix it by cuttign out any regiosn that have the U, creatign nicks in the DNA.
- This process will occur for the Cmu region as well.
- A DNA repair enzyme will come in and put the regions that got nicked together, the Cmu and whichever region was specified.
- This will cut out everything in the middle
- Now you VDJ segment should be in front of the constant region you want to encode and the RNA polymerase will make the mRNA so you can make teh heavy chain protein.
Is hyper-IgM syndrome casued by defective B or T cells?
T cells because they are not giving the fate-specifying cytokines to create certain STAT TFs that will bind to certain parts of the DNA
*Could technically be both since it would constitute a combination of a lot of things
When B cells emerge from the germinal center, what types of cells are they?
- Plasma cells or Memory B cells
Where plasma cells migrate dictates how long they live
Shortest life - tissues like the infected site
Short life - secondary lymphoid tissues
Long life - bone marrow
How long do memB cells live?
They forego this infection and patrol for the next encounter, they live for years !