B cell activation (bowden 9/5) Flashcards
Clonal selection
The process of gene rearrangement of the heavy and light chains and the combinatorial association of these chains occurs during B cell development and is independent of antigen.
antigen simply selects those clones which have the appropriate receptor
the selected clones are then activated, proliferate, and differentiate into antibody secreting plasma cells
IL-3
acts on immature progenitors
CD20 and CD19
on surface of mature B cell
these are pan B cell markers
if you do an analysis of someones lymphocytes in their blood, use antibodies against CD20 or CD19 to look for those
Mature naive B cells exit marrow and enter what?
secondary lymphoid organs…
Lymph nodes and spleen
enter via HEV
if not activated by antigen they die
what do B cells that develop from fetal liver-derived stem cells differentiate into?
B-1 cells
these response to non-protein Ag in the mucosa
found in GALT/MALT/BALT
have an extra marker on their surface (CD5+)
make predominantly shortlived IgM responses
what do B cells that develop from BM progenitors after birth differentiate into?
B-2
Two subsets of B-2 include:
Follicular B cells
Marginal B cells
Follicular B cells
of the B-2 subset
re-circulating B cells (Majority)
give rise to long-lived plasma cells (IgG, IgE, IgA)
see protein antigens and require helper T cells
spleen and other lymphoid organs
Marginal B cells
of the B-2 subset
reside in the spleen DON”T recirculate
respond to blood-borne polysaccharide antigens
make predominantly IgM
Membrane bound Ig does not have…
cytoplasmic tail… so it cannot signal
all of signalling from binding of mIg comes from the Ig-alpha and Ig-beta and B cell co-receptors.
Antigen dependent phase
Response initiated by recognition of antigen (its epitope) by B cell specific for that antigen (idiotope)
Antigens binds to membrane Ig on naive cells and activates these cells
activation can be T dependent or T independent
T cell dependent
some T cells stimulate, or help, B lymphocytes to produce antibodies
In the absence of T cell help, protein antigens elicit weak or no antibody responses
Protein antigens are processed in antigen-presenting cells (APCs) and recognized by helper T lymphocytes, which play an important role in B cell activation and induce heavy-chain isotype switching and affinity maturation.
T independent
Polysaccharides, lipids, and other nonprotein antigens stimulate antibody production without the involvement of helper T cells
The antibodies produced in response to T-independent antigens show relatively little heavy-chain isotype switching and affinity maturation.
Naive B cells in circulation
Travel through secondary lymphoid tissue
enter from blood if going to spleen
in lymph if going into nodes
migrate to primary lymphoid follicles which contain FDC’s
enter through HEV
if they don’t encounter antigen they migrate to primary follicle
receive signal to survive from FDC’s
exit through efferent lymphatic vessels
competition of B cells for survival signals
too many B cells, not enough FDC’s to provide signals for survival
naive b cells die within weeks in absence of antigen
L selectin
expressed on naive b cells
binds to peripheral node addresin (PNAd)
what does CCR7 bind and where is it expressed
expressed on Naive B cells
binds to CXCL19 or CXCL21 on lymph node HEV’s
What do naive b cells express
L selectin
CCR7
LFA1
CXCR4
what does LFA1 bind?
ICAM-1 on lymph node HEV’s
what does CXCR4 bind to ?
CXC12 on lymph node HEV’s
B cell homing
Cytokines produced by HEV and stromal cells activates integrins
B cell migrates to follicular zone by CXCL13
Dendritic cells migrate in from periphery and bring in antigen (attracted to CCL21)
Dendritc cells secrete CCL18 and CCL19 which attract T cells to the lymph node
B cells respond to CXCL19 and once in the lymph node they migrate to where the FDC’s are secreting CXCL13
CXCL13
Mediates follicle migration
is secreted by Follicular dendritic cells (FDC’s) in the lymph node which attracts more B cells
B cell activation first signal (one type)
Ag recognition by membrane bound Ig’s
Must crosslink 2 or more BCR
Signaling occurs through Igalpha and Igbeta cytoplasmic tails
prepares cell for interaction with 2nd signal
at this point it is minimally activated
B cell activation first signal (with complement)
Ag bound with C3d
C3d binds CR2 (the BCR co-receptor complex)
signaling occurs through the Igalpha and Igbeta, CR2 and CD19 cytoplasmic tails
prepares cell for second signal
you do not need cross linking for signaling of two or more Ig’s if CD3 is involved ***
100 fold more immunogenic
what is the BCR co-receptor complex
CR2 and CD19
after B cells are activated they upregulate and downregulate what?
upregulate CCR7
downregulate CXCR5
after T cells are activated they upregulate and downregulate what?
downregulate CCR7
upregulate CXCR5
done in order to migrate to
CCL21
secreted by HEV and stromal cells
what expresses a receptor for CCL21
Dendritic cells
when it binds to CCL21 it migrates into the lymph node
CCL18 and CCL19
secreted by Dendritic cells
attracts T and B cells to developing lymph node
Second signal
Stimulation of b cells by antigen induces expression of B7
this allows for interaction with CD28 on Th cells
Th cells are now activated and can perform effector function of providing cytokine help to B cells
T activation causes expression of CD40L that interacts with CD40 on B cells
The interaction of CD40 with ligand on the T cell is the second signal that causes activation and B cell differentiation
what is B7 expressed on and what does it bind?
B cells
binds CD28
what is CD28 expressed on?
T cells
constitutively expressed
CD40L
expressed on T cells after T cells are activated due to the interaction of CD28/B7
How does activation of B and T cells maintain specificity of the immune response?
B/c the expression of B7 and CD40L are dependent on antigen stimulation, only lymphocytes specifically interacting with antigen (lymphocytes specific for that antigen epitope) are activated
CD40
constitutively active on B cells
Function of cytokines released by Th cells
Class switching
Augment B cells differentiation and proliferation (Clonal expansion)
IL-4
promotes class switching to IgE
IFN-gamma
induces switch to IgG2a
TGF-beta and IL-5
lead to switch to IgA
Switch recombination in cross switching
occurs in germinal center
CD40/CD40L and cytokines trigger isotype switching
Rearranged VDJ gene segment on heavy chain recombines with a downstream C region gene (not being used)
Class switching does not change the specificity of an antibody molecule, only its isotope and therefore potentially its biological function
AID
activation-induced deaminase
key enzyme in class switching
expressed by CD40 signals
Somatic Hypermutation: Affinity Maturation
Introduction of point mutations in the Variable regions of Ig genes, resulting in an expansion of the antibody repertoire to generate high-affinity antigen-specifc antibodies =hypermutation
AID converts Cs to Us
after somatic hypermutation occurs B cells still have to be selected by FDC’s
takes place in the germinal center
often occurs simultaneously with class switching
after somatic hypermutation, what is the fate of B cells…
only B cells with high affinity antigen receptors encounter antigen on the FDC’s and present antigen to T fh cells
these are the B cells that survive
Plasma cells
Terminally differentiated Ab secreting B cell
surface markers
CD27
loses CD19 and CD20 surface markers
Memory cells
survive for long periods of time without additional Ag stimulation
capable of mounting a rapid response to subsequent exposure
responsible for secondary immune response
why don’t IgM’s undergo affinity maturation…
b/c you always are expressing IgM and if you don’t have the CD40 L CD40 interaction you don’t get class switching and you also don’t get somatic hypermutaiton of IgM
no signals from helper T cells –> no switching
CD40: CD40L required for isotype switching: T dependent antigen only
Memory cells are only induced by…
protein antigens
b/c in order to go through isotype switching and somatic hypermuation you need T cell help, which t-dependent antigens require…
so…. no memory cells are created for t-independent antigens b/c they don’t lead to these final steps
Primary response is induced by?
all immunogens
Secondary response is induced by
only protein antigens
T-dependent antigens
soluble proteins
can do isotype switching
can do affinity maturation (somatic hypermutation)
can have immunological memory
No polyclonal activation
T independent antigens
Chemical nature:
- bacterial cell-wall components (LPS)
- capsular polysaccharides
- polymeric protein antigens
usually no isotype switching
no affinity maturation
no immunologic memory
no polyclonal activation
Control mechanism/Antibody feedback
secreted antibody binds to antigen, forms Ag/Ig complex
Antigen/antibody complex binds to B cell IgG and Fc receptor
causes a block in b cell receptor signaling
Natural antibodies
IgM only
Produced by B-1 (sit in the mucosa) and mariginal zone b cells
specific for bacteria in the area
cross react with blood anlloantigens
Primary response
time lag after immunization/infection 5-10 days
smaller antibody production
antibody istotype IgM>IgG
Antibody affinity is lower, more variable
relatively high doses of antigens for required immunizations
Secondary response
time lag after immunization/infection 1-3 days
larger peak response (antibody production)
relative increase in IgG and IgA or IgE
higher average affinity
only induced by protein antigens
required immunization: low doses of antigens
