Ab diversity/B cell development (class) Flashcards
T/F each Ig molecule produced by any one B cell is identical
true
What is the complete collection of antibody (BCR) specificities generated by somatic recombination?
antibody repertoire
T/F each B cell displays many copies of the same BCR
on its surface
true
incredible repertoire of B cell receptors is generated via a unique genetic mechanism known as what?
somatic recombination
Describe the characteristics of somatic recombination
“totally” random process that is antigen-independent
mediated by two enzymes (RAG-1 and RAG-2; the
recombination activation genes)
In the lambda light chain locus, which parts are constant? describe how they are arranged…VJC
(L) V –V–V ——J–C—-J—C—–J—–C
C regions are constant, V and J are variable
In every B cell, which chain occurs first?
heavy chain always occurs first
Every B cell Ab is identical for what reason?
allelic exclusion
only 1 chromsosome is used at a particular time
pairing of V and J gene segments gives rise to many different what?
λ or κ light chain variable regions
pairing of V, D, and J gene segments gives rise to many different what?
heavy chain variable regions
Joining segments provide some diversity. But what gives the light more diversity to the coding region?
recombination of light chain when the joining of gene segments occurs because it is not precise
this gives it more generated
Why is Somatic Recombination of Ig Genes Such an Important Mechanism?
- the human genome contains approximately 40,000 genes
- the immunoglobulin genes code for up to 10^11 different antibody molecules (B cell receptors)
Give a general description of class switching
Change-Out of Constant Domains of the Heavy Chain
What is mutation that occurs at high frequency in the rearranged variable-region DNA of Ig genes in activated B cells, resulting in the production of variant antibodies, some of which have higher affinity for the antigen?
somatic hypermutation(affinity maturation)
Where does affinity maturation occur? and how it coincides with B cells?
- occurs in follicles in secondary lymphoid tissues
- B cells that express higher affinity Abs are positively selected
Cassette system of gene segments recombination occurs via what? describe it
somatic recombination
- antigen independent
- imprecise joining of gene segments
- pairing of rearranged light and heavy chains
What is ability of B cells to change the isotype of the Ab it produces (without changing the antigen specificity)? describe it
class switching
- antigen dependent
- this enables the immune system to select the type of Ab that is needed to best deal with the invading pathogen
Affinity maturation is synonymous with what?
somatic hypermutation
Is somatic hypermutation(affinity maturation) antigen dependent or independent?
antigen dependent
What enzyme is involved in signal transduction from cell-surface receptors during B cell development? What is this used for?
Bruton’s thymidine kinase (Btk)
B cell development
What genes make double- stranded breaks in DNA during somatic recombination? What is the reason for these?
recombination activation genes (RAG-1 and RAG-2)
B cell development
What enzyme catalyzes addition of N nucleotides at the junctions between rearranging gene segments? Why is this important
terminal deoxynucleotidyl transferase (TdT)
necessary for B cell development
What enzyme catalyzes switch recombination (class switching); also required for somatic hypermutation (affinity maturation)? Why is this important?
Activation-induced cytidine deaminase (AID)
B cell development
Describe the class switching WRT steps involved
- isotype switching signal received
- looping-out of DNA occurs
- switch region recombination occurs (intervening DNA is removed)
Without expression of IL-7, what will not occur? What is this molecule?
B cell repertoire
B cell growth factor
BM stromal cells are critical for B cell development. Describe this path to an immature B cell
- Lymphoid progenitor => CAMs, VLA-4, VCAM-1 (help bind to BM stromal cell)
- early pro-B cell=> Kit, SCF (help bind to BM stromal cell)
- late pro-B cell => once bound to BM stromal cell, IL-7 produced
- pre-B cell => CAMs provide binding to BM stromal cell
- immature B cell that is always 1st an IgM
Give the 7 stages of B cell development
- stem cell
- early pro-B cell
- late pro-B cell
- large pre-B cell
- small pre-B cell
- immature B cell
- mature B cell
Describe the HC rearrangement of the 7 steps of B cell development
- stem cell => germline
- early pro-B cell => DJ rearranging
- late pro-B cell => V-DJ rearranging
- large pre-B cell => VDJ rearranged
- small pre-B cell => VDJ rearranged
- immature B cell => VDJ rearranged
- mature B cell => VDJ rearranged
Describe the LC rearrangement of the 7 steps of B cell development
- stem cell => germline
- early pro-B cell => germline
- late pro-B cell => germline
- large pre-B cell => germline
- small pre-B cell => V-J rearranging
- immature B cell => VJ rearranged
- mature B cell => VJ rearranged
Describe the surface Ig of the 7 steps of B cell development
- stem cell => absent
- early pro-B cell => absent
- late pro-B cell => absent
- large pre-B cell => u chain transiently at surface as part of pre-BCR - mainly intracellular
- small pre-B cell => intracellular u chain
- immature B cell => IgM expressed on cell surface
- mature B cell => IgD, IgM made from alternatively spliced HC transcripts
Describe the time value wrt to somatic hypermutation(affinity maturation)
affinity maturation increases along with the total number of responses which is generally at 7, 14, 21 days
Describe the selection of self-tolerant B cells (4)
- multivalent self molecule => clonal deletion or receptor editing => apoptosis
- soluble self molecule => migrates to periphery => anergic B cell
- Low affinity noncross-linking self molecule => migrates to periphery => mature B cell
- No self rxn => migrates to periphery => mature B cell
Name the 3 steps of a non-positive selection for B cell tolerance (location/action for first 2). How long do these cells live?
- bone marrow => open repertoire of mature B cells => tolerance induction
- Blood & 2nd lymph tissues => additional tolerance induction => self-tolerant immature B cells and anergized B cells
If no positive selection occurs then B cells fail to enter lymphoid follicles => B cells have a half-life of about 3 days
Name the 4 steps of a positive selection for B cell tolerance (location/action). How long do these cells live?
- bone marrow => open repertoire of mature B cells => tolerance induction
- Blood & 2nd lymph tissues => additional tolerance induction => self-tolerant immature B cells and anergized B cells
- Positive selection => B cells enter lymphoid follicles => recirculate naive B cells for 3-8 weeks
- Stimulation by antigen => longer lived mature recirculating memory B cells => express high affinity IgG, IgA, IgE
What is the order of class switching? Why is this important?
- IgM
- IgD
- IgG
- IgE
- IgA
Once class switching occurs, it cannot switch back to another class that is upstream.
Describe how the LC is arranged wrt to DNA, RNA and the actions involved
germline DNA: separate V J C gene segments ** somatic recombination **
rearranged DNA: V-J join but C is separate segment transcription
primary RNA transcript: VJ joined but C is separate splicing
mRNA: VJ joined and C joining translation
polypeptide chain: VJC joined to form LC
Describe how the LC is arranged wrt to DNA, RNA and the actions involved
germline DNA: separate V D J C gene segments
- somatic recombination => V separate, D-J join, C separate
rearranged DNA: V - D-J join C is separate segment transcription
primary RNA transcript: VDJ joined but C is separate splicing
mRNA: VDJ joined and C joining translation
polypeptide chain: VDJC joined to form HC
Describe the steps of B cell development along with its specific location for each step
- B cell precursor rearranges its Ig genes (attached to BM stromal cell) => generation of BCR in BM
- Immature B cell bound to self cell-surface antigen removed from repertoire => Neg selection in BM
- Mature B cell bound to foreign antigen activated => B cells migrate to peripheral lymphoid organs
- activated B cells give rise to plasma and memory cells => Ab secretion and memory cells in BM and lymphoid tissue
