Diversity vs. Specificity: Immunoglobulins Flashcards
immunoglobulin structure
two identical heavy and two identical light chains
immunoglobulin
aka antibody
can be membrane bound or secreted
all antibodies made by a single cell and have specificity
IgM
surface bound antibody
Immunoglobulin superfamily
IgM, T cell receptor, MHC (HLA) and Ig-alpha/Ig-beta
clonal selection
gene rearrangement events in the absence of antigen
-in bone marrow or thymus
clone has single specificity
10’7 - 10’11 different clones possible
primary immunoglobulin rearrangement
multiple germ line genes
-combinatorial diversification
junctional diversity
-addition of nucleotides during process of joining
secondary immunoglobulin rearrangement
somatic hypermutation
-point mutations occur in the fully assembled VJ and VDJ regions
somatic hypermutation
happens during the immune response
IL-3
secreted by T cells
IL-7
necessary for commitment to lymphoid lineages
location of heavy chain
chromosome 14
two different light chains?
kappa and lambda
3’ end of gene?
C (constant regions)
5’ end of gene
V (variable regions)
what contains D regions?
Ig heavy chain and TCR beta chain
how many V regions in heavy chain
about 100 V regions
different C regions of heavy chain determine what?
determine the isotype of the Ig
what genes in the heavy chain?
many C regions (9 classes)
-exon for domain and the hinge region
many V regions, J regions, and D regions
**Exons expressed / introns not expressed
what is in the kappa light chain?
only one C region
V regions have a leader exon and a V exon
several J regions between the V and C regions
lambda light chain
4 C regions (with interspersed J regions)
around 30 V regions
-each has a leader region and V region
allelic exclusion
have these genes on both mom and dad gene
express only mom/dad heavy and light chain
functional B cells never contain more than 1 heavy or light chain
-essential for specificity
expression of both alleles would render the B cell multispecific
can have Mom light and Dad heavy or vice versa
Pro-B cell
right after the stem cell
starts to secrete Rag and TdT enzymes which allow it to go from pro-B cell to pre-B cell
rearrangements of a heavy chain?
1 - D region joins with J region (forms DJ) - a random
2 - V region joined to DJ ( no C region yet) - forms VDJ
3 - VDJ is transcribed
4 - pre-mRNA has L-D-V-J and C regions and is spliced
- *C-mew and C-delta are the constant regions
- make IgM and IgD, respectively
5 - mRNA is translated in the cytoplasms and the leader is removed as protein transported into the ER
-heavy chain is assembled with light chain
mechanism of DNA rearrangement?
flanking the V, J, and D exons are RSS sequences
-each RSS has a nonamer and heptamer separated by 12 or 23 base pairs (1 or 2 turns)
**recombination only occurs between a 1 turn or 2 turn signal
recombination is catalyzed b Rag1 and Rag2 (VDJ recombinase)
RSS
recombination signal sequence
flank the V D and J exons during DNA rearrangements
Rag-1 and Rag-2
apart of the VDJ recombinase enzyme
catalyze the recombination of DNA segments
junctional diversity
at the junction of hairpin cleavage - have sticky ends
TdT catalyzes the random polymerization of nucleotides without the need for a template
have P-nucleotides added on sticky end
have N-nucleotides added in non-templated manner
TdT
catalyzes addition of nucleotides to the sticky ends of broken gene during rearrangement
P nucletides
added asymmetrically to the cleaved ends by TdT
N nucleotides
added without a template by TdT
pre-B receptor
complex of of the IgM without the surrogate light chains
-combined with the Ig-alpha and Ig-beta
**this is the first checkpoint - if heavy chain rearranged successfully
if not successful, cell dies by apoptosis (only 1 in 3 survive)
what light chain is made first?
always KAPPA
light chain rearrangement
V joins with J ( no D region in light chain)
otherwise, mechanism is the same as the heavy chain
receptor editing
nonproductive light chain rearrangements can be rescued
-check for self recognition by bone marrow
if light chain is again not productive (up to 5 for kappa)
-depends on which kappa is chosen
if all kappa is unsuccessful, will go to lambda light chain
**only in the light chains
immature B cell
has rearranged heavy and light chain successful
if all successful, will release a mature naive B cell
alternative splicing
of mew and delta C regions
get both IgG and IgM on the surface of B cell
components of the B cell receptor?
IgM heavy and light chain, Ig-alpha, and Ig-beta
checked by bone marrow stromal cells by presenting self (HLA)
- if strong interaction - negative selection - if low affinity - positive selection
combinatorial diversification
10’11 possible antibodies
not all gene segments are possible
pseudogene - accumulated mutations prevent encoding
several VDJ segments - no evolutionary pressure
-therefore they may be loss
during junctional diversity (N and P nucleotides)
-may add frameshifts, stop codons, and gobbledy gook
junctional diversity
can triple the diversity of genetic sequence alone
diversity is generated in the hypervariable region
secondary rearrangement diversity
is antigen specific
happens in the periphery
SCID
mutations in Rag-1 and Rag-2
also by mutation in Il-7
