Generation of antibody diversity Flashcards
immunoglobulin variation types
isotopic -> variations in heavy chain types + either kappa or lambda chain
allotypic -> smal variation in the constant region
idiotypic -> variability in the variable chain of an antibody = the variation that generates antibody specificity
what is germline DNA
information acquired from sex cells (egg and sperm)
somatic recombination?
somatic = alterations in DNA after conception –> somatic mutations can occur in any of the cells of the body except germ cells (sperm and egg)
“structure” of the immunoglobulin genes
germline genes are a large cluster of gene segment -> which are non functional (until somatic recombination)
what are the gene segments that code for immunoglobulins
variable region (V) segments
diversity (D) segments -> only Ig heavy and TCR beta chains
Joining (J) segments
constant (C) segments
RSS? where are they located
recombination signal sequence, and either right before or after a gene segment in the loci
where are the RSSs located (with specific segments)
each V segment has an RSS right after it
each J segment has an RSS immediately preceeding it
D segment has an RSS on each side
what are the two different types RSSs
RSS with a 23-base-pair-spacer and RSS with a 12-base-pair-spacer
structure of a RSS
an RSS contains a heptamer and a nonamer of a conserved sequence (a base sequence in a DNA molecule that has remained relatively unchange throughout evolution) + either the 23 or 12 bp spacer
RSS function
critical in somatic recombination -> bringing different segments of DNA together while losing the loop in between them (changing the DNA sequence of the Ig)
what is the “12-23 rule”
RSS with a 23-base-pair spacer can be only joined an RSS with a 12-base-pair sequence
RAG?
recombination activating gene enzymes
function of RAG enzymes
RAG-1/2 bind to the RSSs and bring them together = forming a synapsis. then RAG induces DNA cleavage exactly at the junction of the gene segment and the RSS. this leaves a hairpin of DNA at the end of the gene segments
Ku70:Ku80 ?
protein complex which holds the two hairpin DNAs together after RSSs are cleaved
the formation of the signal joint
after RSS cleavage, Ku70:Ku80 protein complex holds together the DNA ends. DNA ligase then binds the DNA ends together which generates a precise signal joint (RSSs are bound together)
how is the coding joint produced?
the hairpin DNA needs to be cut open so the two strands can be joined together -> facilitated by a comnination of proteins. DNA protein kinase and artemis open the haipin. then a bunch of enzymes process the DNA ends and facilitate their ligation
TdT ? function?
terminal deoxynucleotide transferase -> one of the enzymes that facilitate coding joint ligation
SCID?
severe combined immunodeficiency
gene rearrangement can result into additional nucleotides added to the joint, what are different types of additions?
- non-templated addition -> when the TdT adds extra random bases
- palindromic additions -> by DNA polymerase (dont know the mechanism)
what is the importance of nucleotide additions in the coding joint?
they alter the potential peptide reading frame –> basically, it is essential for producing the diversity of antibodies
allelic exclusion ?
a single T or B cell will only express the product of a single allele for each of its relevant antigen receptor genes = aka a single BCR will express only one heavy chain even though it has two receptors for it)
why is monospecificity important?
it ensures that a cell will only be able to interact with one antigen –> important because during clonal expansion, the clones produced will be identical as the primary B cell (meaning they will al have the same specificity)
location of somatic hypermutation
germinal centres of secondary lymphoid organs (where B cells are interacting with antigens)
what is the process of somatic hypermutation
the accummulation of mutations in the V region of immunoglobulins -> both heavy and light chains
what is the purpose of somatic hypermutation
some mutations might improve binding to the antigen -> those B cells will keep proliferating
another way of describing somatic hypermutation
affinity maturation -> the end result will improve the antibody affinity of the Ig to the specific antigen
which enzyme is key in facilitating somatic hypermutation and what is its function
activation induced cytidine deaminase (AID) -> AID induces point mutation during the replication process
what happens when a body has a lack of AID
antibodies are unable to class switch -> the body produces only IgM antibodies = hyper IgM syndrome type 2
what are the initial Ig isotypes produced and how are they generated
IgD and IgM and they are produced by RNA splicing
when do antibodies class switch?
after antigen stimulation (in the germinal centres) at the same time as somatic hypermutation
what are the 2 signals which induce class switching
- activation of B cells in the germinal centre
- CD40-CD40L signalling
mechanism of Ab class switching
looks similar to VDJ recombination
