B cells - adaptive immunity Flashcards
BCR
genetic composition
LC (kappa or lambda)
HC (IgM, IgE, IgG, IgA, IgD)
each chain: VR (V(D)J) and CR
soluble (first poly A splice site) or membrane bound (second poly A splice site
BCR
activation model
cross-linkage model questionabel
DISSOCIATION MODEL: BCR clusters in unstimulated state, ligand breaks clusters appart -> signalling
affinity
binding strengh of one receptor-ligand interaction
avidity
strength of protein-complex - protein interaction
even low affinity Ab can have high avidity when they are polyvalent (e.g. IgM)
Valency
numbr of binding elements -> Ig usually bivalent (two paratopes), IgM molecule can be polyvalent via complex formation
antigen-indipendent B cell development
multipotent HSC in fetal liver or bone marrow
production of CPL
B cell committment -> pro B cells, driving TF is Pax5 (inhibits Notch 1)
Early pro-B cell: initiation of D-J recomination in HC
Late pro-B cell: V-DJ recombination in HC
PRE-B CHECKPOINT: VDJ HC forms pre-B receptor by binding lambda5 -> VpreB inducs receptor dimerization -> signalling
Large pre-B cell: passes/passed checkpoint, signalling induces prolifration, shut down of preBCR expression and allelic excluison
Small pre-B cell: IgL recombination (V-J), first kappa than lymbda genes if unsuccessful
Immature B cell: VJ LC rearranged, IgM expression -> signal induction then receptor editing (autoreactive)
receptor editing
on LC of BCR
signalling after LC recombination -> receptor editing by combining new V and J segments
prevents autoreactivity
if still autoreactive when lambda and kappa segments are used -> deletion (ev. HC contributes significantly to autoreactivity)
Early pro-B cell
initiation of somatic recombination
D-J recombination
one chromosome!
Late pro-B cell
DJ recombined
V-DJ somatic recombination
Pre B Checkpoint
between/in late pro and large preB cell
VDJ recombined
binds lambda5
lambda binds VpreB and causes dimerization
induces signalling (positive selection)
Large preB cell
positive selection -> signalling
induces shutdown of preBCR expression
induces alelic inclusion
initiates IgL rearrangement
small pre B cell
V-J recombination of LC
first kappa gene rearrangement (later if necessary lambda)
immature B cell
VJ rearranged
if autoreactive then receptor editing
if autoreactivity persists (sgements of lambda and kappa used up, HC contributes to autoreactivity) than deletion
mechanism of somatic recombination in BCR
LC encoded in two loci (k and l), HC in one
segments contain a RSS (recombination signal sequence) -> 23 or 12 RSS
RAG1/2:
- alligns with RSS, captures the other (opposing!)
- Signal joint: ku70:80 binding, DNA ligase IV and XRCC4 form PRECISE JOINT
- Coding joint: ku70:80 binding, DNA-PK and Artemis open covalently closed hairpins, TdT randomly adds or deletes nts, pairing and ligation via DNA ligase IV and XRCC4 -> IMPRECISE JOINT
RAG in BCR recombination
RAG1/2:
- alligns with RSS, captures the other (opposing!)
- Signal joint: ku70:80 binding, DNA ligase V and XRCC4 form PRECISE JOINT
- Coding joint: ku70:80 binding, DNA-PK and Artemis open covalently closed hairpins, TdT randomly adds or deletes nts, pairing and ligation via DNA ligase IV and XRCC4 -> IMPRECISE JOINT
RSS
recombination signal sequence
23 and 12 RSS
only sequences with heterologoue RSS are paired
Signal joint BCR
- ku70:80 binding
- DNA ligase IV and XRCC4
- PRECISE JOINT
Coding joint BCR
- ku70:80 binding
- DNA-PK and Artemis open covalently closed hairpins
- TdT randomly adds or deletes nts
- pairing and ligation via DNA ligase IV and XRCC
-> IMPRECISE JOINT
Imprecise joint
coding joint
- ku70:80 binding
- DNA-PK and Artemis open covalently closed hairpins
- TdT randomly adds or deletes nts
- pairing and ligation via DNA ligase IV and XRCC
-> IMPRECISE JOINT
precise joint
signal joint
- ku70:80 binding
- DNA ligase IV and XRCC4
- PRECISE JOINT
RAG regulation
TRANSCRIPTIONAL: no RAG activity in mature cells
CHROMATIN & LOCUS ACCESSIBILITY: open chromatin structure and epigenetic modifications necessary for RAG binding -> RAG2 binds H3Kme3 -> required
3D LOCI ARCHITECTURE: segmnt genes loop around recombination center, formation of COHESIN LOOPS (CTCF mediated segment chosing)
BCR central tolerance mechanism
immature B cells causing signalling -> receptor editing
immediate effect of B cell activation
B cell activation by Ag binding with co-stimulatory signals
- upregulation of survival proteins and proliferation
- Ag and B7 expression elevated -> interaction with CD4 (CD40!)
- cytokine receptor expression changes
- upregulation of CCR7 -> migration to T cell area
antigen-dependent B cell development
Ag binding in LN
migration to T cell zone (CCR7 upregulation)
interaction with CD4 T cells (CD40!)
T and B cell migration to follicle
d4 early GC formation in follicle
GC for diversification (SHM), selection and class switch initiation
not all B cells enter GC -> some proliferate into short-lived plasma cells for faster response
memory B cell formation
GC cells
T-FH cells: produce BCl6, essential for GC formation, mediate expansion via competition of B cells for T cell help
FDCs: long term Ag storage, concentration and presentation, involved in selection process (affinity to FDC)
diversification of B cells
location and mechanism
somatic hypermutation
“faulty” repair mechanisms
in dark zone of GC
AID expression induced by CD40:CD40L
AID = deaminase, ususally repair via base-excision (UNG) or mismatch repair (MSH2/6)
in SHM translation over apyrimidinic site or repair with mistake-prone low-fidelity DNA polymerase
Selection of higher affinity B cells
models
in light zone of GC
2 models: either apoptosis when affinity is low or proliferation when affinity is high
affinity measured towards Ag presented by FDC
cells enter aapoptosis, reenter the dark zone for SHM or proliferate
proliferation/expansion dependend on T cell help T-FH
class switch
cytokine-mediated activation of switch region promotor
induces transcription
AID active
UNG removes generated U and causes apyrimidinic residue
APE1 removes ribose and induces ssB
in repair mechanisms dsB is initated
DSBR machinery joins the switch regions and excises intervening sequences (Cm)
AID
deaminase
induces SHM and CSR
IgA
protects epithelial barriers
neutralizes pathogens and their toxins
IgG
neonatal immunity
opsonization
ADCC (bind FcgRIII on NK cells)
feedback inhibition of B cells
activates complement (C1q binds)
IgM
pentamer in soluble form
naive B cell receptor
complement activation (binds C1q)
Magnitude of neutralizing Ab response
massive in cytopathic viruses -> fast and effective, prevents excessive host damage, CTL response later and reduced
minimal in non-cytopathic viruses -> strong CTL response responsible for clearance, Ig mor opsonizing and less neutralizing
IgD
naive B cell receptor
IgE
defence against helmiths
immediate hypersensitvity
bind FceRI on mast cells -> sensitizing
B1 cells
compared to B2, types and activation
restricted BCR reservoir (no SHM) compared to B2 cells (follicular B cells)
only IgM (no CSR)
MARGIAL ZONE B cells: T indinpendent short lived plasma cells
INNATE-LIKE B1 cells: T independent, short lived plasma cells
ACTIVATION: BCR signalling and TLR signalling
immunogenicity of microbes
dependend on several factors
acessibility of important sites for e.g. entry
organisation -> e.g. abundancy of epitopes
B cell repertoire -> e.g. corresponding paratope not often encoded
Ab function
neutralizing (surface coating)
complement activation
opsonization (ADCC)
Immunecomplexes
effect
formation of Ag:Ab complex, can have several effects
ANTIVIRAL: complex mediates macrophage recognition (FcgR), phagocytosis and degradation, cytokine release promoting clearing immune response
ADE: antibody-dependent enhancement, complex mediates phagocytosis but viral entry strategy, cytokine release more immunopathological
COMPLEMENT ACTIVATION: Fc of IgG binds C1q -> triggers cascade, phagocytosis, cell lysis and immune recruitment
PHAGOCYTOSIS: macrophages or DC
ADCC
T CELL ACTIVATION: phagocytosis, processing -> Ag presentation on APCs
viral persistance strategies
HIGH TITER REPLICATION: requires non-cytopathic virus, or target cell with high replenishing potential
LATENCY: non-replicative mode until re-activation
LOW TITER REPLICATION: continous low level replication and immune response
High titer replication
ineffective immune clearance due to tolerance, immune complex formation, viral variations etc
virus must be non-cytopathic or target cell with high replenishing potential
TOLERANCE: absence of viral-specific immunity
- deletion of naive T cell clones
- exhaustion of CTLs (low pathology despite high viral load)
- absence of specific Ab response
