1-28 Generation of Diversity of B and T cells Flashcards
naive b-cells express..
a membrane bound IgM B-Cell receptor
when naive b-cells encounter antigen, they
become activated and prolifterate into plasma cells that secrete IgM antibody in both secreted and membrane bound forms
How do activated b-cells co-express surface and membrane bound immunoglobulin?
occurs due to differential mRNA processing
starting at 5’ end, the variable coding region (VDJ) followed by IgM region and then IgD region
to produce membrane bound, hydrophilic section cut out and constant region is attached directly to hydrophobic region
to produce excreted form, hydrophobic section cut out, hydrophilic attached directly to constant region
co-expression of BCR Ig and secreted Ig does not require ____ ________
DNA rearrangement
describe immunoglobulin class switching
there are “Switch regions” between the various Ig motifs following the VDJ sequence. The enzyme AID nicks the DNA at these switch regions, and another enzyme brings these nicked regions together and repairs them, looping out intevening DNA. when transcription begins, one of the different Ig motifs will occur just after the variable region
after a b-cell encounters its specific antigen
its begins to tailor the antibodies it produces to the type of protection needed
IgE - allergies
IgA - secretory
IgG - various functions
class switch occurs at the _____ level
DNA
which mRNA of the immunoglobulin determines if it is secreted or membrane bound?
the Fc (constant) region of the heavy chain
how do you get so much B-Cell diversity?
Immunoglobulin gene consists of segments of DNA that can be mixed and matched
immunoglobulin genes have different segments, b-cell can pick one of each (lambda light chain locus, kappa light chain locus, heavy chain locus)
light chains have
V and J regions which are randomly chosen
HEavy Chains have
V, D, and J regions which are randomly chosen
The b cell combines
the light and heavy chains
the constant region - all start as
IgM
what flanks the V and J;s?
signal sequences that are recognized by enzymes that pull them together. these signal sequences work in such as way that they allow enzymes to work and cause recombination of the heavy chain or light chain loci
what enzymes are involved in this recombination?
RAG1 and RAG2.
what happnes if you are missing Rag1 or Rag 2?
severe combined immunodeficiency - cant rearrange the DNA in B or T cells.
describe the role of RAG
brings together sequences in a way that they are subject to splicing. Ligase comes and glues them together.
DNA is spliced, not the RNA, removing a “loop” of intervening DNA which is lost.
regions get added during
splicing. N and P regions
P regions are palendromic regions
N regions are truly variable
TDT
the two ends of clipped DNA get close, clip gets made, each end overhangs. TDT (terminal dexoydase transferase) adds random nucleotides to ends of DNA. Repair enzyme then does base matching to fill in
person lacking TDT?
makes antibody, makes t-cell receptors, but b-cells have low affinicty and t-cells dont react as well. Can’t create the random sequences that by chance have high affinity
more N regions in the heavy or light chains? why?
more in heavy chains because there is more TDT around during heavy chain rearrangment
most recombinations will….
fail to make a complete protein.
if you get out of reading frame, the contant region will not work.
2/3 rearrangements are out of reading frame.
describe chain rearrangement steps
- heavy - D-J rearrangements on both chromosomes of early pro-b cell
- heavy - VDJ rearrangements on first chromosome, if fails, try second chromosome, if fails, apoptosis
- light - if successful, kappa gene on first chromosome of light chain rearrangemnts, if fails, try second chromosome, if fails, lambda gene on first chromsoome, if fails, lambda gene on second chromsome, if fails, apoptosis
- light - if kapppa rearrangement successful - expresses mu and kappa(light chains) on (IgM)
- light - if lambda rearrangement successful and not kappa, expresses mu and lambda (light chains) on IgM
how does a cell determine if a heavy chain works?
create a “fake” or surrogate light chain, ssee if the heavy chain folds properly. If it can’t make ti to the surface or act properly with the light chain, apoptosis.
difference between light and heavy chain rearrangement?
each light chain chromosome gets 2 opportunitues (4 total) to rearrange. Each heavy chain chromosome gets 1 opportunity (2 total) to rearrange.
if the first time a light chain rearranges,…
great. but if not, it can keep taking different V and J sections from further down the chromosome until it runs out of time or runs out of dna OR successfully makes a V-J recombination.
how do rearrangements get selected on for increased affinity?
- cell commits to b cell lineage
- generates heavy chain diversity in pro-b population
- selectes for functional heavy chains, all other apoptosis
- generates light chain gene diversity in pre-b cell population
- selects for functional light chains
- b cells making functional IgM selected for.
only those that make the functional IgM survive.
prescence of RAD during rearrangment?
early on, burst of rad when rearranging heavy chain, then another burst later when rearranging light chain
when are surrogates made?
only after rearrangement of the heavy chains and UNTIL we are ready to rearrange the light chains. After heavy, before light.
when is TDT mostly present?
during the entire rearrangement of heavy chain, but fades as light chain rearranges.
Light chain shave less N regions made by TDT than heavy chains that have TDT present the whole time.
splicing can occur either way, creating ______ depending on
creating secreted or membrane bound, depending on cell signals
IgM and IgD splice type?
RNA splice
IgG, IgE, and IgA splice type?
DNA splcie
the process of t-cell and b-cell process is the same except for
mutations of b-cells occur during immune response across the whole variable region of the DNA. Single bases randomly mutated. localized to variable region.
purpose of somatic hypermutations?
happens to B-Cells when T-cells are stimulated, provides more diversity to fight infection
why are the mutations lined up in the hypervariable region?
Early on – mutations occur, but only few
As immune response matures and class switch, more and more mutations
Why lined up in hydrvaerribale region? If they make the antibody not bind to antigen anymore, cell doesn’t get stimulated – cell apoptosis
IF they make the antigen that binds to the antibody (bcell) better, will divide and get stimulated more.
AID is responsible for
both class switching AND somatic hypermutation
if a person has a dieases where t-cells dont stimulate b-cells properly
they would have no class switching and they would have low affinity antibodies becasue they didnt get antibody affinity maturation
there are antigens that are t-cell independant, describe.
stimulate b cells but not t-cells. (bacterial liposaccharides, because t-cells see protein) – you get IgM and no memory (wont get the Things that t-cells do)
without t-cell help..
Without t-cell help no AID no class switching or hypermutation
t-cells do not ____
class switch or mutate - T-cells are highly educated at self vs non-self. We do’t want a t-cell to start getting mutations in t-cell receptor after being selected for not attatcking self.
T-cell receptor look slike _____ and has two classes
looks like a b cell receptor, alpha-beta and gamma-delta
differences/similarities between b cell and t- cell diversity generation?
Is a mirror of the immunoglobulin molecule in terms of how it gets its varibalilty. Everything with RAg1 and 2, splicing, and addition of TDT to make N-regions which give more diversity to binding sties. Everything is the same in a t-cell as it is in a b-cell. Isntead of heavy then light, its beta then alpha chain
when is a b cell a b cell? when is a t-cell a t-cell?
what machinery makes this happen?
Until cell puts IgM on surface, not a b cell
Until a cell puts T cell receptor on surface, not a T-cell
Without RAG1 and RAG2, and protein kinases for splicing, you do not get B cells or T-Cells.
where does somatic hypermutation occur?
B-Cells only!
T-cells and b-cells use essentially the same mechanisms to
rearrange their antigen receptors
