Lecture 7 - Ab diversity and B cell development Flashcards
Describe B cell development
-B cells start out in bone marrow and fetal liver by rearranging their chromosomal DNA to create exon encoding the Ag binding pocket –> both light and heavy chain undero process
What is the only isotype generated at this stage of B cell development?
IgM (and a little IgD)
What happens after cells commit to being B cells?
Ab genes start to rearrange
B cells leave bone marrow with what bound?
IgM and maybe a little IgD
Describe 4 basic steps of B cell development described
1) B cell precursor rearranges its Ab genes –> generation of B cell receptors in bone marrow
2) immature B cell bound to self cell-surface Ag is removed –> negative selection in bone marrow
3) mature B cell bound to foreign Ag is activated –> migration of B cells to peripheral lymphoid organs and activation
4) activated B cells give rise to plasma cells and memory cells –> Ab secretion and memory cells in bone marrow and lymphoid tissue
What do the heavy and light chain genes consist of?
large numbers of variable (V) domain genes, joining (J) genes and (+/-) diversity (D) regions
How are variable region genes constructed?
by joining one of gene segments from each region (1 V seg + 1 D seg + 1 J seg) to form a continuous piece of DNA
What must occur to form a joined gene?
somatic rearrangement
What is somatic rearrangement?
random event which occurs only on one chromosome for each light or heavy chain genes (allelic exclusion)
What is the constant region encoded by?
separate exons
How is the constant and variable region exons joined?
by splicing of the primary RNA transcript
When can isotype switching occur?
following VDJ region recombination
What genes does the light chain region not have?
diversity region
True or False: each b cell develops receptors that can bind to a single epitope that is different than the epitope recognized by other B cells
True; does this during development in the bone marrow by splicing its chromosomal DNA to create new coding sequences for the Ag binding pocket exons
What part is most variable within Ab
AAs that contact the epitope
The immune system has the capacity to recognize what?
approx 10^10 different antigenic epitopes; each B cell produces Ab of a single specificity
The pool of B cells within an individual provides what?
universal recognition
What 5 processes aid in Ab diversity?
1) multiple germline gene exons
2) H and L chain combination
3) Imprecise joining of segments
4) Random insertion of bases
5) somatic hypermutation
During B cell development, what do stromal cells secrete?
B cell growth factors
As a pre-B cell develops, what happens?
Ab genes begin to rearrange –> heavy chain first, then light chain
Which cells survive and leave bone marrow?
those that successfully rearrange both heavy and light chain genes
What is “multiple germline genes”?
genomic organization of Ab genes has many different gene segments and permits rearrangement.
- using multiple gene segments, great diversity can be ahcieved
- B cells have different VDJ and VJ heavy and light chain rearrangements;
- B cells also have different combinations of VDJ and/or VJ segments
What is “H-L chain combinations”?
- heavy chain rearranges independently of light chain
- by combining different H and L chains, each with their own variable region rearrangements –> generate Ag-binding pockets of various specificty
How are the V, D or J regions selected?
randomly
What is “imprecise joining (or junctional flexibility)”
by arbitrarily splicing nucleic acids together within the region, different reading frames are generated leading to different AA structure at the Ag binding pocket
- occurs at all V-D-J junctions during recombination
- what is randomly spliced out changes the nucleic acids spliced together –> changes AA structure
What is “random base insertion (or P nucleotide addition)”?
- during recombination of V-D-J segments, nucleic acids can be randomly inserted into the junction
- generates a different AA structure at Ag binding pocket
Do most B cells make it out of bone marrow?
no
What is allelic exclusion?
rearrangement of Ab gene DNA occurring only on one chromosome for each light and heavy chain genes
What does allelic exclusion assure?
assures that each individual B cell generates only a single Ab mol rather than expressing genes from both chromosomes
-every Ab that B cell produces is identical
Describe an example
an animal with two different alleles for the heavy chain gene loci produces Abs using both chromosomes.
-for each individual B cell, only the blue or yellow chromosome is expressed, not both
What does allelic exclusion prevent?
B cells from having low avidity receptor interactions with Ag
-ensures each B cell produces identical Ab –> every Ab will bind to target Ag
What happens without allelic exclusion?
produces heterogeneous B cell receptors (Abs) with low avidity binding
In rearrangement of Ab variable region genes, heavy chain gene rearranges first (then light). what happens if first splice event fails?
chromosome may attempt to splice further down the gene cluster
Does choice of heavy chain VDJ regions influence which light chain V and J regions selected?
no, they are independent events
What are RAG proteins?
recombination activating gene
What do RAG proteins/enzymes do?
recognize conserved sequences flanking each gene segment, bend the DNA strand such that proteins interact and cleave out the intervening DNA sequences
What happens to animals who are deficient in RAG activity?
they are severely immunocompromised
B cells develop in bone marrow and are released with what?
rearranged Ig genes
What is found on surface of mature B cells released from marrow?
IgM +/- IgD
Rearrangement occurs when?
in absence of Ag
When does heavy chain VDJ region genes start rearranging?
Early pro-B cell –> large pre-B cell
When does light chain VJ region genes rearrange?
small pre-B cell –> immature B cell
At what stage of B cell development are surface Ig expressed?
Immature B cells
Why is IgM the first Ab produced by B cells?
it’s encoded immediately adjacent to VDJ genes
When is IgD also found on B cell surface?
on surface of newly formed B cells due to mRNA splicing artifact
After B cells are released from bone marrow, where do they go?
circulate through secondary lymphoid tissues - lymph nodes, tonsils, spleens and peyer’s patches
B cells that encounter Ag (and T cell help) form what?
foci called germinal centers that contain proliferating B cells
What happens to some of plasma cells?
some leave the germinal center and reenter circulation
-some take up residence in bone marrow and spleen
What happens in the periphery for B cells?
- maturation to an Ab secreting cell (plasma cell)
- isotype switching
- somatic hypermutation
- **these events occur following specific antigenic stimulation
If mature B cell fails to access secondary lymphoid tissue what happens?
cell dies
If B cell makes it to secondary lymphoid tissue, but doesn’t interact with Ag, what happens?
half life of 3-8 wks
What happens if B cell encounters Ag?
-longer lived
some mature into plasma cells –> Ab secretion (IgM)
-some form germinal centers
B cells start to mature in bone marrow, then move to peripheral tissues ONLY if what happens?
light and heavy chains rearrange successfully
What happens in periphery?
B cell encounters its Ag and receives T cell help (cytokines) –> convert to plasma cells that secrete IgM
-further activation results in isotype switching to secrete IgG or IgA
Exposure to Ag in periphery results in what?
- activation and cell division
- differentiation into plasma cells (Ab)
- class switching and somatic hypermutation (introduce AA changes)
- selection for receptor specificity with greater affinity (affinity maturation)
- differentiation into memory B cells
Which processes require exposure to Ag in periphery?
all activation of B cells and subsequent changes in DNA (somatic hypermutation and isotype switching) or RNA processing
What doesn’t require exposure to Ag?
gene conversion events
What are germinal centers?
specialized areas of secondary lymphoid organs
-if stimulated by antigen-specific T cells, B cells will multiply and start to secrete IgM –> undergo somatic hypermutation and selection for Ag binding in these sites
Where do B cells interact with T cells?
in secondary lymphoid organs
What is the first thing to happen following B cell activation?
proliferation and conversion to secretory IgM (plasma cell) –> then somatic hypermutation –> affects exon encoding the Ag binding site
