7/8: Antibody diversity and B cell development

Question 1

how is the antigen binding pocket formed

Answer 1

• junction of the heavy and light chain variable regions

- rearrangement of chromosomal DNA to create the exon encoding it

Question 2

how does each b cell make a unique receptor

Answer 2

rearrangement and mutation of the germline DNA

Question 3

what is the only isotype generated at first

Answer 3

IgM

Question 4

what is tolerance

Answer 4

absence of immune response to own antigens

Question 5

steps in B cell development

Answer 5

• generation of b cell receptors in bone marrow
• negative selection in the bone marrow
• migration of b cells to peripheral lymphoid organs and activation
• antibody secretion and memory cells in bone marrow and lymphoid tissues

Question 6

how does each b cell develop receptors (antibodies) that can bind to a single epitope different than the epitope recognized by other b cells

Answer 6

during development in the bone marrow –> splicing its chromosomal DNA to create new coding sequences for the antigen binding pocket exon

Question 7

how is the region of the antibody gene that binds the epitope generated

Answer 7

by a semi-random rearrangement and combining of gene segments –> different exons for the antibody chains within each b cell

Question 8

multiple germline gene exons

Answer 8

both heavy and light chains may be encoded by multiple variable region DNA segments that are spliced together to form the variable region exon

Question 9

H and L chain combinations

Answer 9

the antigen binding site is formed by the interaction of both chains, and any given H chain can dimerize with any given L chain

Question 10

imprecise joining of segments

Answer 10

during somatic rearrangement, the gene segments for a variable region can be spliced together using any of several nucleotides

Question 11

random insertion of bases

Answer 11

during splicing, nucleotide bases can be randomly inserted into the splice junction, thereby changing the codon frame

Question 12

somatic hypermutation

Answer 12

areas within the variable regions which are subject to germline base substitutions during cell division –> the only diversity mechanism that occurs after a b cell encounters its specific antigen

Question 13

what has to happen for a b cell to survive and leave bone marrow

Answer 13

successful rearrangement of both heavy and light chain genes

Question 14

multiple germline genes

Answer 14

• many different gene segments and permits rearrangement

- great diversity can be achieved

Question 15

H-L chain combinations

Answer 15

• heavy chain rearranges independently of the light chain
• by combining different H and L chains, each with own variable region rearrangements, can generate antigen binding pockets of various specificity

Question 16

3 regions in antibody genes

Answer 16

V, D, and J segments (plus constant region exons)

Question 17

VDJ gene rearrangement

Answer 17

bringing segments together through splicing

Question 18

imprecise joining (junctional flexibility)

Answer 18

• arbitrarily splicing nucleic acids together within a region –> different reading frames generated –> different amino acid structure at the antigen binding pocket
• occurs at all VDJ junctions during recombination

Question 19

random base insertion (or P nucleotide addition)

Answer 19

• during recombination of VDJ segments, nucleic acids can be randomly inserted into the junction
• generates a different amino acid structure at the antigen binding pocket

Question 20

what is allelic exclusion

Answer 20

-rearrangement of the antibody gene DNA is a random event that occurs only on one chromosome for each heavy or light chain genes

Question 21

what does allelic exclusion assure

Answer 21

that each individual b cell generates only a single antibody molecule rather than expressing the genes from both chromosomes (each b cell produces identical antibodies)

Question 22

does the heavy or light chain rearrange first

Answer 22

heavy chain, then light chain

Question 23

does the choice of heavy chain VDJ region influence which light chain V and J regions will be selected

Answer 23

no

Question 24

what does RAG stand for

Answer 24

recombination activating genes

Question 25

what do RAG proteins/enzymes do

Answer 25

• recognize conserved sequences flanking each gene segment
• bend the DNA strand such that the proteins interact
• cleave out the intervening DNA sequences

Question 26

what happens if an animal is missing RAG

Answer 26

severely immunocompromised

Question 27

why is IgM the first antibody produced by b cells

Answer 27

it is encoded immediately adjacent to the VDJ genes

Question 28

how are IgM and IgD formed (mechanism)

Answer 28

via differential splicing

Question 29

what are germinal centers

Answer 29

contain proliferating b cells

Question 30

steps in b cell development

Answer 30

• mature in the bone marrow
• circulate through the secondary lymphoid tissues
• form germinal centers
• plasma cells leave and reenter circulation
• maturation, isotype switching, somatic hypermutation

Question 31

when do b cells convert to plasma cells

Answer 31

if it encounters its antigen and receives t cell help (cytokines)

Question 32

what does activation after converting b cells to plasma cells result in

Answer 32

isotype switching to secrete IgG or IgA

Question 33

what does exposure to antigen in the periphery result in for a b cell

Answer 33

• activation and cell division
• differentiation into plasma cells
• class switching and somatic hypermutation
• selection for receptor specificity with greater affinity
• differentiation into memory b cells

Question 34

what does all activation of b cells and subsequent changes in DNA or RNA processing in the periphery require

Answer 34

exposure to antigen

Question 35

what needs to happen for b cells to multiply and secrete IgM

Answer 35

stimulation by antigen-specific t cells

Question 36

what does alternative splicing yield

Answer 36

membrane IgM (BCR) or secretory IgM