B-cell Maturation and Activation (Part 2) Flashcards
The Life of a B-cell
Mature Follicular B cells Reside in the Follicle
The Life of a B cell - Peripheral B cell
- Mature B cells (“Naive”)
- Plasma cells (Short-lived)
Rapidly produce low-affinity antibodies - Germinal center (GC) B cells
Massive proliferation
Antibody selection/maturation - Memory cells
Circulate & tissue-resident
Re-enter germinal center - Plasma cells (Long-lived)
Produced higher-affinity antibodies
Terminally differentiated
Reside in bone marrow
Antibody Response
B cell activation
Naive B cells express IgM and IgD B cell receptor (BCR)
Crosslinking of BCR induces B cell activation
BCR associates with
* Signaling chains (Ig alpha/Ig beta)
* Co-receptors (CD19, CR2/CD21, CD81)
B-cell can also be activated by:
* Toll-like receptor stimulation (LPS)
* CD40 (B-cell) - CD40L (T-cell)
Two types of B cell immune responses
T-dependent:
* Requires T-cells (CD40 & cytokines)
* Induces germinal center response
* Isotypes swtich and affinity maturation
* Slower
T-independent
* Does not require T-cells
* Multivalent antigents (e.g., bacterial polysaccharides)
* Usually with Toll-like receptor stimulation (e.g., LPS)
* Mostly IgM and lack affinity maturation
* Faster
Overview of T-dependent Response
Overview of T-dependent Response
Two Signals Hypothesis
(T-dependent B cell activation)
Signal 1
1. Antigen binds BCR –> triggering signals and endocytosis
2. Antigen –> peptides –> loading MHC class II (pMHC-II)
Signal 2
Activated CD4 Th cell with a TCR specific for the pMHC-II will “help” the B cells. T cell will:
1. Provide CD40L to CD40 on B cells
2. Secret cytokines (IL-21, IL-4, IFN-gamma) to B cells.
Outcome- Activated B cells will proliferate and differentiate into:
* Short-lived plasma cells
* Germinal Center B cells
* Memory B cells
CD40L mutation –> X-linked Hyper IgM Syndrome.
B cells fail to switch from IgM to other isotypes
B-cell antigen presentation
- The B-cell epitope (antibody binds) and T cell epitope (T cell receptor binds) arelikely to be different. E.g., Different position on the same protein or even on a different protein (as long as they are physically linked).
- B-cells can recognize proteins and non-proteins (sugars, small molecules, DNA, etc.)
- Most T-cell only recognize peptides on MHC
Outcome of T-dependent responses
- Short-lived plasma cells
- Isotype switching (e.g., IgM -> IgG)
Class swithc recombination (pre-germinal center) - Germinal center reaction
Germinal Center (GC)
Part I
Specialized microstructure in secondary lymphoid tissues (lymph node/spleen/tonsil)
B cell will undergo massive proliferation
Clonal selection of B cells (B cells with higher affinity will survive)
Antibody undergoes affinity maturation (increase affinity to antigen)
KEY: Germinal center is essential for B cell antibody response
Germinal Center (GC)
Part II
Two distinct structure in the germinal center
* Light zone (selection)
* Dark zone (higher cell density - more proliferation)
* Cyclic reentry (going back and forth between zones)
Follicular dendritic cells (FDC)
* Light zone
* Display protein antigen (immune complexes) to B cells
* Survival signal (BAFF)
Somatic Hypermutation (SHM)
Creates B cell clones with varying degrees of affinity to antiges for “clonal selection”
Random mutations at the V regions of IgH and IgL (antigen binding)
Clonal selection and affinity maturation - Step 1
Clonal selection and affinity maturation - Step 2
Clonal selection and affinity maturation - Step 3
Clonal selection and affinity maturation
As B cells repeatedly mutate their DNA (point mutations, breaks), B cell lymphomas are frequently derived from GC B cells
Long-term Protection Generated from GC
Memory B cells
* Lower affinity to antigen
* Fewer SHM
* Circulate
* Can re-engage GC response rapidly
Long-lived plasma cells
* Higher affinity to antigen
* Reside in bone-marrow
* Constantly secret antibody (e.g. measles vaccine)
Increase antibody affinity
Recap for Peripheral B Cell Differentation
Antibody Diversification
Class switch recombination (CSR; isotype switching)
* Switch from one isotype to another (IgM -> IgA)
* Deleting intervening DNA
Somatic hypermutation
* Mutating the variable regions (IgH, IgL)
Both processes only happen in B-cells
Tightly regulated processes (DNA mutations)
Both require: Activation Induced Cytidine Deaminase (AID)
KEY: B cells use CSR to change out the Ig constant regions (isotype switching) and SHM to change affinity.
Activation-induced cytidine deaminase (AID)
- Required for class switch recombination (isotype switching) and somatic hypermutation
- Expressed in activated B cells and GCB
- Deaminate C to U on ssDNA
- Causing various mutations and DNA breaks
AID (AICDA) mutation causes Immunodeficiency with Hyper-IgM, Type 2 (HIGM2) - Susceptibility to bacterial infections
KEY: AID is one of the most essential enzymes in B cells required for CSR and SHM
Class switch recombination
(isotype switching)
S regions (switch regions)
* Repetitive sequences
* Targeted by AID (R-loop)
* dsDNA breaks (random)
Germline Transcript (GLT)
* Non-coding RNA at S regions
* Induced by cytokines (e.g.IL-4)
* Different cytokines induce different GLT (e.g., TGF-beta -> IgA)
Can switch successively
IgM->IgG1
IgG1-> IgE
Can fail to swithc (break at Su joins back)
AID and CSR
- AID is recruited to R-loops at S regions (Su and Sa)
- AID deaminates ssDNA (C->U) and generate multiple single-stranded (ss) breaks
- Multiple ss breaks become double-strand breaks
- Breaks at two S regions may join (e.g. IgM->IgA)
KEY: Cytokines -> GLT -> R-loop -> AID recruitment -> dsDNA breaks -> looping and joining
Somatic Hypermutation
(SHM)
High mutation rate:
* 1 in 10^3 base per cell division (vs 1 in 10^10 at other loci)
* AID recruitment mechanism is similar to CSR
* Mutations also by error-prone DNA polymerase
* How AID actually preferentially recruited to Ig variable genes is still unclear (some off-target)
Transcriptional Regulation of Early B cell Development
Key Transcription factores:
PAX5, EBF1: B cell identity
PAX5-KO: ProB cells can de-differentiate inot T-cells
KEY: PAX5 is the B cell lineage trasncription factor
Transcriptional Regulation of Peripherial B-cell Differentiation
Key transcription factors:
PAX5, EBF1: B-cell identity
BCL6: Germinal center (GC) B-cell
IRF4: GC B and Plasma Cell
BLIMP1 (PRDM1): Plasma cell
KEY: Transcription factores BLC6 and BLIMP1 are important for GC B cells and plasma cells, respectively.
Epigentic Regulation of B-cell DNA demethylation
Summary
Summary I
Naive B cells express IgM and IgD
B cells can be activated by:
* Crosslinking BCR
* TLR and CD40 signals
Two types of B cell immune responses (T-dependent and T-independent)
The germinal center is an essential location for B cell immune responses
* Increase antibody affinity (SHM)
* Differentiation of plasma cells and memory cells
Summary II
Antibody diversification includes:
* Class switch recombination (isotype switching; e.g., IgM-> IgG1)
* Somatic hypermutation
Both processe require AID (activation inducecyrtinde deaminase)
AID deaminates cytosine to uracil
Summary III
Epigentic Regulations
BIG PICTURE SUMMARY
