B Cells 5 Flashcards
what are the 4 proteins involved in BCR diversity and their roles?
- RAG1/2 –> brings specific segments of DNA together
- Artemis –> cuts DNA at hairpin
- Exonuclease –> removes some nt for joining
- TdT –> excess trimming at D junction of heavy chain
what are the 2 chains of TCR and their regions?
ALPHA –> multiple V and J, 1 C
BETA –> multiple V, D, J, 1 C
what is the recombination of TCR called?
somatic recombination
where does somatic recombination occur? reversible or irreversible?
occurs in thymus
IRREVERSIBLE
what are the 4 similarities with TCR and BCR recombination?
- V, D, and J are flanked by RSS
- RAG1/2 recognizes these sequences
- artemis cuts DNA hairpins
- TdT adds non-coded nucleotides in joining regions
what is the difference btwn TCR and BCR recombination?
in Ig heavy chain, the D segment is surrounded by 2 RSS, both with 12 bp –> 12(D)12
In TCR beta chain, the D segment is surrounded by 12 and 23 RSS –> 12(D)23
do TCRs have CDRs?
yes
describe the diversity of TCR
CDRs are the sites in TCR with the most diversity
which CDR is the most important for diversity of TCR?
CDR3
where are CDR1 and CDR2 encoded?
CDR1 and CDR2 encoded within the V segment of alpha and beta chains
where is CDR3 encoded?
CDR3 encoded within the D and J segments of TCR chain
what is allelic exclusion?
ensures that each B cell synthesizes only 1 allele for the heavy chain and 1 allele for the light chain
how does allelic exclusion work?
both alleles from each parent is PRESENT but only 1 is expressed and silences the gene for the other allele
how does the non-expressed allele become silenced by the expressed allele?
the expressed allele sends a signal to silence the part of the gene that codes for the other chromosome
the chromosome becomes methylated and inaccessible to transcription machinery
why does allelic exclusion occur?
to ensure B cells only express copies of BCRs with the same specificity
which 2 types of Ig are part of the FIRST WAVE of secreted antibodies?
IgM and IgD
what do mature naive B cells express initially?
initially, newly formed B cells express IgM as their primary B cell receptors
what do mature naive B cells express after IgM?
after expressing IgM, B cells will switch to expressing IgD
how are IgM and IgD expressed on B cell surface?
IgM and IgD co-expressed on B cell surface
what happens to mature naive B cells once they are activated on by signal 1 and 2?
upon activation by signal 1 and 2, some can form primary focus and become plasmablasts that secrete IgM or IgD
what allows the B cell to start secreting antibodies and how does it change the production of Ig subtypes?
ALTERNATIVE RNA SPLICING
what does alternative RNA splicing do to make specific Ig subtypes?
all DNA in B cells is transcribed into long primary mRNA transcripts which are spliced into 2 types of mRNA molecules i.e. splice out exons for the Ig you don’t want
what happens during alternative RNA splicing when we want IgD?
if we want IgD, the IgM RNA exons will be spliced out (vice versa if we want IgM)
what does alternative RNA splicing do to allow antibody secretion?
certain carboxy terminus for transmembrane Ig vs secreted Ig so you splice out the terminus you don’t want
what are the 5 isotypes of heavy chains?
M (mu)
D (delta)
G (gamma)
A (alpha)
E (epsilon)
how do we switch from producing different subtypes of Ig?
different heavy chain gives different Ig class
which Ig classes have varianets?
IgG and IgA
how many possible constant regions are there for all Ig?
9
are the types of constant regions the same or different for diff species?
the constant regions are different in different species
what is another name for secondary lymphoid follicle?
germinal centre
what happens in the secondary lymphoid follicle/germinal centre?
activated B cells become plasma cells and secrete antibodies with DIFFERENT ISOTYPES and HIGHER AFFINITY than antibodies initially produced by plasmablasts
what are the 3 processes for B cells to make antibodies that are more effective when they are plasma cells?
- somatic hypermutation
- affinity maturation
- class switching
what occurs during secondary diversification?
- somatic hypermutation
- affinity maturation
- class switching
where does secondary diversification occur?
in germinal center
when does secondary diversification occur?
once B cell has received signal 1 and 2
what occurs during somatic hypermutation?
gains a higher affinity for its antigen but its specificity remains the same
what occurs during class switching?
replaces 1 heavy chain constant region with one of a different isotype
what do somatic hypermutation and class switching act on?
act on Ig genes that have already been rearranged –> VDJ recombination has already occurred
what does somatic hypermutation act on and where do it occur?
acts on activated B cells in germinal centers peripheral lymphoid organs (where B cells activate)
how does somatic hypermutation occur? (2)
- high rate of random point mutations in V gene sequences that improve or decrease Ag binding
- mutations that increase affinity for Ag are selected
what is affinity maturation?
the selection of mutations with increased Ag affinity in somatic hypermutation
why does somatic hypermutation occur during secondary and tertiary responses?
to get antibodies with increasingly higher affinity so you get a better response each time
what causes class switching?
cytokines secreted by follicular helper T cells in germinal center
when does class switching occur?
after B cell activation
is class switching reversible or irreversible?
irreversible
what guides class switch recombination?
class switch recombination is guided by SWITCH REGIONS that are upstream of each constant gene
describe the process of class switching (3)
- enzyme makes double strand break in switch region
- this cuts out a portion of DNA with some constant regions, leaving some constant regions remaining
- the constant region that is upstream of VDJ will be expressed
