L12 B cell development II Flashcards
mature B cell tolerance control?
- Mature B cells are only activated if they present Antigen to
their cognate CD4+ T helper cell. - Interaction with Th cells activates B cells via “Two signal
hypothesis” (actually 3 in reality cuz can also be activated by cytokines) Circulate and move into secondary lymphoid organs where there are b cell and t cell zones. - B cells recognising antigen stop in the T cell zone of
peripheral lymphoid tissues. - If it is an exogenous antigen there will be T-B interaction
and activation. - If no T cells for self antigens exist so there will be no help
for a self reactive B cell – clonal deletion.
peripheral b cell activation?
B cells can NOT be activated by antigen alone! They
need a second signal.
The second signal comes from:
* Activated (CD4+ T helper cells)
* PRRs (sensing pathogen presence)
* Activated antigen presenting cells.
how b cells are activated:
T dependent pathway: classical b cell response to normal protein type antigens.
T independent type 1 and 2: absence of t cells. Different receptors in each.
T dependent and independent require bcr, both require cytokine stimulation, both require co stimulatory molecules from innate immune receptors e.g: toll like receptors.
T dependent and independent pathways in further detail?
In a T-dependent B cell response, two signals are required for B cell activation:
Signal 1: Interaction between the B cell receptor (BCR) and the antigen.
Signal 2: Provided by the interaction between CD40 on the B cell and CD40L (CD154) on the T cell.
CD40: A receptor on the surface of B cells.
CD40L (CD154): A ligand (binding partner) on the surface of activated T cells.
The binding (ligatory = interaction) of CD40 on the B cell with CD40L on the T cell delivers a crucial signal that ensures the B cell fully activates.
Initial reaction made through interaction of mhc molecule and tcr that upregulates expression of cf40 on the b cell, t cell already activated so expressing cd40 ligand. Strimulates b cell proliferation and differentiation. Stimulates isotype switching and somatic hypermutation.
T-idenpendentType I:
Type I antigens are specialized antigens that are strong signaling molecules, often referred to as polyclonal activators ( activate multiple B cells).
These antigens are recognized through Pattern Recognition Receptors (PRRs) (to recognise PAMPS) such as TLR-4 (Toll-like Receptor 4), which are part of the innate immune system giving second signal.
B cells receive two signals:
Signal 1: The antigen binds directly to the B cell receptor (BCR).
T-Independent Type II:
Triggered by large repeating molecules (eg
H. influenzae b).
These repetitive structures cluster B cell receptors (BCRs) together, generating a strong Signal 1 for activation. Bind multiple BCRs on the cell
surface causing cross linking
and activation.
Only a single signal is required to activate B cells, leading to the production of IgM-positive plasma cells.
Unlike T-dependent responses, this pathway does not support secondary diversification activities, such as:
Isotype switching (e.g., switching from IgM to IgG).
Somatic hypermutation (improving antibody quality).
Exception:
If the same antigen is also present on cancer cells, it can activate dendritic cells, which release a cytokine called BAFF (B cell Activating Factor).
BAFF acts as a second signal, allowing:
The production of IgM plasma cells.
Class switching to other antibody types (e.g., IgG or IgA).
Diversification activities, enhancing the antibody response.
germinal centre reactions?
Germinal centres are:
* sites of a massive burst of proliferation,
amplifying the B cell response.
* Sites of a further immunoglobulin diversification:
– Somatic hypermutation and class switching – both mediated by
AID
* Sites of B cell maturation into plasma cells and
memory cells.
* GCs formed in lymph nodes, spleen and GALT/MALT
* Transient structures: arise when b cells activated in immune response, disappear when response resolved.
Germinal centre reaction image: b cell and t cell zones where they reside and circulate. Kept apart until they are activated where they move and bind and interact to cognate th cell. Boundary zone: where germinal centre starts to form due to this rapid proliferation of b cells. Cluster of b cells forms the distinct structure?
Image 2: distinct structure diff areas- forms on boundary of t cell and b cell zone. Has dark zone and light zone. Cells are now germinal centre cells or central blasts/central sites. Within dark- central blasts as they are proliferating. Light zone- central sites as they are being selected. Class switching takes place in light zone. Stromal cells within lymphoid itself make up that area.
Germinal centre reactions- sites of proliferation,diversification,selection and differentiation: dark zone- b cells are proliferating and undergoing somatic hypermutation. Only b cells there. Light zone: here are the dendritic cells?? B cells are being selected for ability to bind to antigen. Here they decide to become either memory cells or plasma cells.
somatic hypermutation?
Somatic hypermutation: Occurs after B cells recognize an antigen and receive T-cell help in T-dependent responses.
It happens in the germinal centers of secondary lymphoid organs (e.g., lymph nodes, spleen). recognised pathogen antigen, b cells expand and diversify and produce antibodies.
some diversified cells are maintained in immune system and retain memory. Within the expanded pool of clones, each b cell has the same vdj recombination makeup?
During that proliferation process, undergo somatic hypermutation and can induce point mutations in the v d and j segments. This occurs due to the action of AID. ( As cells divide the Ig gene is mutated (Activation
induced Cytidine deaminase – AID)
* Mutation occurs in the DARK ZONE)
within that clone of cells that has expanded through proliferated, diff cells diff mutations at diff points in its variable region genes. Slightly diff capacity to recognise same antigen. Random process. Mutations can increase or decrease ability.
Selection stage is important as immune system wants to select the b cell with improved affinity for antigen. Takes place in the light zone. Proliferared b cells move into light zone, have their bcr tested by competing for follicular Dendritic cells and a small no. follicular th cells. If b cell selection it can re enter dark zone and undergo further rounds of mutations. So cycle between them. So most successful cloness go back to dark zone. So get constant improvement of affinity. Together that process is called affinity maturation (increasing ab affinity).
So somatic hypermutation is the act of inducing mutations into the variable region genes. That are tested for affinity.
Driven by specific chemokines present in those 2 areas. Cxcl12 in dark zone cxcl13 in light zone.
what happens after somatic hypermutation?
Eventually cells stop cycling and differentiate
into Long lived “Memory” cells and Antibody
producing plasma cells
GC reactions take time to mount a
diverse, specific response
Plasma cells – Fight the infection
Memory B cells ensure a rapid (specific) response next time
antibody structure?
Variable region with two
binding sites which determine
the ANTIGEN SPECIFICITY
Fc region – constant region
which determines the
ISOTYPE of the antibody and
thereby the FUNCTION
Fc portion determines antibody structure and therefore function: CLASS SWITCHING
* Pentameric IgM
* Dimeric and secreted IgA
* Small IgG – very
* Fc portion determines complement activation
* IgM and IgG activate complement very effectively
* Different cells express different Fc receptors:
* Fc expressed on mast cells and basophils – granule release
* FcRI on DC, monocytes – effector cell activation and
phagocytosis
* FcRIII on macrophages, NK cells – effector cell activation and
ADCC
Cells use Fc receptors to recognise different
types of antibody
Diff receptors on cells recognise diff antibodies shape response of cell and so shape antibody effector functions.
SO: DIFF FC RECEPTORS ARE EXPRESSED ON DIFF CELLS AND ARE INDUCING DIFF ACTIVITIES WITHIN THOSE CELLS.
antibody effector functions?
Neutralises toxins and viruses
- blocks their interaction with cells
Opsonises pathogens
- promote phagocytosis (via Fc receptors)
Activates the complement cascade
- direct pathogen killing by MAC
Agglutinates particles (pathogen debris,
viruses etc)
Antibody-dependent cell-mediated
cytotoxicity (ADCC - via FcR)
- cytotoxicity by NK, Eo, mast cells and
Macrophages.
neutralisation?
viruses bind to receptor on cell and internalised and release genetic material to infect cell and undergo replication. Antibodies block molecules on virus surface that allow it to interact with receptors on target cell. In hiv: binds to gp120 and blocks interaction with cd4, blocks virus entering cell.
- Neutralising Ab against tetanus toxin (vaccination with inactive toxoid)
prevents binding to neurons. - Neutralising Ab against SARS-CoV-2 block Spike protein binding ACE2
receptors on cells
opsonisation?
all other activities require other cells from innate immune system. Here the antibody coats bacteria via ab portion variable region. Fc region is exposed and interacts with fc receptors on cells e.g: phagocytic cells and bind to antibody and in turn bind to the thing the antibody is bound to. Increases efficiency of pathogen clearance and stimulates efficiency of antigen presentation. So adaptive influences activity of innate immune system- cross talk.
* Antibody recognises pathogen coat proteins
* Binds phagocytic cells via Fc receptors
* Phagocytes engulf and destroy pathogen in vesicles
* Aids APC presentation of pathogen proteins – Innate/adaptive overlap.
ADCC?
Antibody Dependent Cell-mediated Cytotoxicity.
* Similar to opsonisation – Fc Receptor mediated on the surface of cells
* BUT causing release of cytotoxic mediators (perforin, granzymes,
Histamine) from cells (NK, Eosinophils and Macrophages. –
* Uses cells of innate immune system
complement activation by IgM and IgG
Initiates
complement
cascade to
produce MAC
Innate/adaptive
immune
Overlap
IgM isotype?
- The first immunoglobulin that B cells
produce (with IgD). First constant
regions downstream of VDJ. - The Fc region enables the molecules to
form pentamers (with J chain) and intra
chain disulphide bonds - Very good at forming immune
complexes. - BUT too large to diffuse into tissues or
cross the placenta. - Very efficient at activating complement
through the classical activation pathway.
IgM Immune complexes
Pentamer has 10 possible
binding sites – high Avidity
BUT low affinity (has not
undergone somatic
hypermutation and
selected)
Pentameric structure held by disulphide binds and j chain that stablises that structure. Has lots of binding sites so good at forming immune complexes. Downside- large
But have high ability to bind to antigens despite low affinity.
IgD?
Possible regulatory function but not sure. Only other isotype of igm that is independent of class switching, purely transcription driven event.
* Membrane-bound form of Immunoglobulin
on Naïve B cells (B cells early in
development before they see antigen)
* Function still not well understood.
* C lies immediately downstream of C.
IgD is formed by alternative splicing (NOT
class switching) of a transcript transcribed
from V region.
IgG?
ot dimerised or pentermarised, small so good at entering tissues. 4 constant regions encode diff igG isotypes
- IgG is the main antibody secreted in the blood
- ONLY produced after B cell activation & class
switching. - It is very good at opsonisation.
- Pathogens coated in IgG also become targets for
killing by Natural killer cells – ADCC. - Very good tissue entry & crosses placenta
- There are 4 subclasses: IgG1,IgG2 IgG3 & IgG4
(humans)
