B cell tolerance Flashcards
How are peripheral T cell numbers maintained in mature individuals?
division of mature T cells outside the central lymmphoid organs
Why is positive selection particularly important for a:b T cells?
ensures that an individuals T cells will be able to respond to peptides bound to one’s own MHC molecules
What is the default fate of developing lymphocytes, in the absence of any signal being received from the receptor?
death by apoptosis
Which cells in the bone marrow provide the signals for the differentiation of haematopoietic stem cells?
specialised nonlymphoid connective tissue stromal cells that are in intimate contact with the developing lymphocytes
What are the 2 functions of the stromal cells in the bone marrow?
form specific adhesive contacts with the developing lymphocytes by interactions between cell-adhesion molecules and their ligands; provide soluble and membrane cytokines and chemokines that control lymphocyte differentiation and prolfieration
What do HSCs first differentiate into?
multipotent progenitor cells
What is the difference between HSCs and MPCs?
aren’t self renewing stem cells
What cell surface receptor do MPPs have that binds to stromal cells?
FLT3
Give examples of transcription factors that MPPs possess that are required for the development of multiple haematopoietic lineages?
PU.1 and c-kit
What does differentation of the MPPs into the common lymphoid progenitor require?
signalling through the FLT3 receptor
What 2 subsets of progenitor cell does the MPP produce that gives rise to all lymphocytes?
one unnamed- produces the ILC subsets and the common lymphoid progenitor
What do B-cell committed CLPs give rise to?
pro-B cells
Expression of what receptor accompanies the production of lymphocyte progenitors from the multipotent progenitor cell?
IL-7 receptor
What induces the IL7 receptor expression?
FLT3 signalling and PU.1 activity
What 2 polypeptides make up the IL-7 receptor?
IL-7 receptor alpha chain and the common cytokine receptor y chain
What tyrosine kinase do all cytokine receptors with the common cytokine receptor y chain share?
Jak3
What membrane bound cytokine present on bone marrow stromal cells is required f or the growth of HSCs and earliest B-lineage cells?
stem cell factor
What receptor does stem cell factor interact with?
Kit
What chemokine is required fro the early stages of B cell devleopment?
CXCL12 (stromal cell-derived factor 1 SDF-1)
What is one of the roles of CXCL12?
reatin developing B cell precursors in the marrow microenvironment
What is hte pro-B cell defined by?
induction of the B lineage specific transcription factor E2A
What does E2A expression induce?
early B cell factor EBF
What is the function of E2A and EBF?
drive the expression of proteins that determine the pro-B cell state
Where do the earliest stem cells reside in the bone marrow?
endosteum
How do B cells migrate as they develop?
towards the central sinus of the marrow cavity
What are the stages of B cell development in order?
early pro-B cell; late pro-B cell; large pre-B cell; small pre-B cell; immature B cell; mature B cell
Where does development from an immature to mature B cell occur?
peripheral lymphoid organs
How is rearrangement of the heavy-chain locus initiated ?
E2A and EBF induce Rag-1 and Rag-2
When does D to J rearrangement occur?
mainly in the early pro-B cell stage
What is the function of Pax5?
targets the genes for CD19 and the gene for Iga ; induces expression of B-cell linker protein
How is Pax5 induced?
E2A and EBF
What indicates that Pax5 is required for commitment of the pro-B cell to the B cell lineage?
without Pax5 cells cannot develop further down the B cell lineage but can be induced to give rise to T cells and the myeloid lineage
What is the function of B-cell linker protein?
its a scaffold protein that is required for further devleopment of the pro-B cell and for signalling from the mature B cell antigen receptor
When does V-DJ rearrangement occur?
late pro-B cell
What defines the large pre-B cell?
expression of the complete immunoglobulin heavy chain
How does the large pre-B cell become a small pre-B cell?
by proliferation
What is seen in the small pre-B cell?
express the mu heavy chain alone in the cytoplasm as no longer express the surrogate light chains
What are the surrogate light chains?
lamda5 and VpreB
What happens once a B cell has successfully expressed a light chain gene?
immature B cell
What is seen on the surface of a mature B cell?
both mu and delta heavy chain expression
What are the earliest B-lineage surface markers?
CD19 and CD45R
What surface markers distinguish the pro-B cells?
CD43; Kit and IL-7R
What surface marker does the late pro-B cell start to express?
CD24
What surface markers is a pre-B cell distinguished by?
enzyme BP-1, Kit is no longer expressed
When does V to DJ rearrangement occur on both chromosomes?
if V to DJ rearrangement does not produce a functional H chain on the first chromsome
What makes up the pre-B cell receptor?
H chain with VpreB and lambda5
What associates with the pre-B cell receptor?
Iga and Igb
What does association of the pre-B cell receptor with the Iga and Igb chains do?
tells the cell to stop H rearrangement and undergo proliferation
Which type of light chain rearrangement occurs first?
kappa light chain
What is the difference between D to J rearrangement and DJ to V rearrangemetn?
D to J occurs on both chromosomes whereas DJ to V rearrangement occurs first on only one
When is TdT expressed in B cell development?
pro-B cell stage
Why are there less N-nucleotides seen in light cahins?
expression of TdT declines at the pre-B cell stage
What is the checkpoint that mediates the transition betwen the pro-B cell and pre-B cell?
formation of the pre-B cell receptor and signalling through it
How are the signals required for transition through the chekpoint created by the pre-B cell receptor?
form dimers or oligomers that generate signals
What downstream signalling molecules are required for signalling through the pre-B cell receptor?
BLNK and Btk
What allows the pro-B cell to become sensitive to IL-7?
signallling generated by the pre-B cell receptor stops further rearrangement of the heavy chain locus—sensitivity induces prolfieration
How does the pre-B cell receptor prevent a B cell from producing 2 receptors of different antigen specificites?
enforces allelic exclusion
How does the pre-B cell receptor enforce allelic exclusion?
reduces the expression of Rag1 and Rag2; causes Rag2 to be targeted for degradation; reduces access of the heavy chain locus to the recombinase machinery
How are many cells with different antigen specificites created from one pre-B cell?
after proliferation, all the cells undergo light chain rearrangements
How can several attempts at recombination of a light chain gene be made on one chromosome?
repeated rearrangements of unused V and J segments can occur
How is the k:l ratio in mature lymphocyte populations useful in clinical diagnostics?
an aberrant ratio indicates the dominance of once clone and the presence of a lymphoproliferative disorder
What is release of immature B cells from the bone marrow dependent on?
expression of S1PR1
How do B cells leave the marrow?
through sinusoids
What happens if the BCR encounters a strongly cross-linking antigen in the bone marrow?
development is arrested
What are the 4 possible fates of a self-reactive BCR?
receptor editing; cell death by apoptosis restuling in clonal deletion; anergy; immmunological ignorance
What determines the fate of a self-reactive BCR?
the interaction of the BCR with the self-antigen
What causes a B cell to undergo receptor editing?
if recognises a multivalent self antigen
What happens in receptor editing?
replacement of the light chain until a nonself reactive receptor is produced or there are no additional light chain V and J segments avaialable
What happens to immature B cells that encounter more weakly cross-linking self antigens of low valence such as small soluble proteins?
anergy
Where are anergic B cells detained?
T cell areas of peripheral lymphoid tissues and cannot access lymphoid follicles
Why should immunologically ignorant B cells be considered inert?
they can be activated when there is lots of inflammation or when self-antigen reaches high concentration
What would happen is the elimination of self-reactive cells was too efficient?
receptor repertoire might become too limited
Why are not all autoreactive B cells purged in the bone marrow?
only lyphocytes specific for autoantigens that are expressed in or can reach the bone marrow are affected
Why do B cells only become fully mature in the periphery?
provides an opportunity for immature B cells to encounter peripheral self antigen and undergo tolerance
How does the expression of sIgM and sIgD change between immature and mature B cells ?
immature have more sIgM than sIgD whereas for mature it is the opposite
Which cells produce BAFF most abundantly?
follicular Dcs
What are follicular DCs?
non-haematopoieitc cells resident in B cell follicles specialised to capture antigens for recognition by B cell receptors
What are the 3 receptors for BAFF?
BAFF-R; BCMA; TACI
What is the difference between BAFF-R and BMCA/TACI?
BMCA and TACI also respond to APRIL
What is important about APRIL?
IgA production
What 2 transitional stages do immature B cells in the spleen proceed through?
T1 and T2
What are T1 and T2 defined by>
the absence/presence of CD21
What happens to mice lacking BAFF?
cannot acquire CD21 and progress to T2
What is required to promote the final stages of B cell maturation in the periphery?
weak BCR signals and BAFF
What disease is linked to overexpression of BAFF?
Sjogrens
How can marginal B cells be identified?
very high levels of CD21 expression
What is a major determinant of whether a B cell will be follicluar or marginal zone?
the specificity of the BCR
What is the function of marginal zone B cells?
poised to make rapid responses to antigens or pathogens filtered from the blood- early line of defense for blood borne pathogens
What is expressed by fDCs to attract B cells to the follicles?
CXCL13 (binds to CXCR5)
What happens to mice lacking Syk and what does this indicate about its function in maturation?
have immature B cells but not mature B cells therefore a Syk-transduced signal from the BCR must be required for full B cell maturation
What indicates that continuous BCR expression is required for B cell survival?
clonal deletion of BCRs in mature B cells results in loss of all mature B cells
What part of the immune system are B-1 B cells part of
innate immune system
Where are B-1 B cells found in high numbers?
peritoneal and pleural cavities
What is the function of B1 B cells?
major source of natural antibodies- those constitutively produced circulating antibodies secreted prior to infection
What do most antibodies made by B-1 B cells recognsie?
capsular polysaccharide
What is one important feature of B-1 B cells?
can produce IgM antibodies wihtou help from T cells; although response can be enhacned by cooperation
What are most B-1 B cells derived from?
progenitor cells found in the fetal liver
How are B-1 cells renewed?
self-renewal
What is the difference between the dependence on other signals in B-1 cells and B-2 cells ?
B-1 cells do not need BAFF or IL-7
What is a difference between immune tolerance and immunosuppression?
immune tolerance is antigen specific whereas immunosuppression is non-ag specific
What is immunosuppression?
when an appropriate immune response does not happen resulting in harm to the organsim- infection or tumour
What is the deletion in scurfy mice?
deletion in the forkhead domain of FOXP3
How long does it take scurfy mice to die?
3-5 weeks
What does FOXP3 stand for?
forkhead box protein 3 gene
What is the only treatment for IPEX
bone marrow transplantation
What is CLP Pax5 expressing cell?
committed B cell precursor
Which B cells undergo VJ joining of the light chain locus?
small pre-B cells
When do B cells start to express CD45?
early pro-B cell
What cytokine is required for a cell to become a large pre-B cell?
IL7
When do B cells start to express CD19?
late pro-B cell
What are marginal zone B cells?
non-class switched memory B cells that populate the marginal zone which can differentiate into short lived IgM secreting plasma cells
What happens once a B cell has encountered an antigen?
naive B cells move from the mantle zone to primary follicle to start a germinal reaction
What is the name for B cells in the dark zone?
centroblast
What are the B cells in the light zone called?
centrocyte
What are the features of plasma cells?
don’t express CD19; CD20; CD40; MHC-II or sIg, can no longer receive T cell help and are incapable of cell division
What happens to B cells that encounter self antigens in peripheral lymphoid tissue?
anergy if mature or Fas mediated apoptosis if transitional
What are the features of anergic B cells?
downregulate Cd21; express high levels of inhibitory receptors; downregulate lymph node homing CCR7 so leave follicle and die
What first identified the regultory potential of B cells?
transfer of splenocytes lacking B cells failed to prevent T cell induced delayed-type skin hypersensitivity, but if give splenocytes from a mouse already exposed to the antigen, don;t develop
Prpduction of what cytokine is linked to the regulatory function of B cells?
IL-10
What is the mouse model for Bregs?
no Breg deficient mouse model analogous to Scurfy
What is the phenotype for Bregs?
no single conclusive phenotype and no transcription factor analogous to FOXP3 foudn
What are other potential mechanisms by which B cells induce toelrance?
TGFb; IL-12; IL4; FasL expression
How can Breg function be induced?
with CD40L and CPG and IL-1b and IL-6
What do the factors that induce Breg function suggest about when they arise?
that they arise in the inflammatory site
What is the function of Breg?
suppress activated T cells and monocytes proliferation/cytokine secretion; induce Treg differentation
What are Tr1 cells?
IL-10+ regulatory T cells
How do Bregs carry out their functions?
IL-10 dependent; cellular contact is required
What diseases have been linked to decreased numbers and function of Bregs?
MS; ITP; SLe; RA
What are the risks with the therapeutic potential of immune regulation?
non-specific immune suppression- infection; tumours; autoimmunity; conversion to pathogenic cells
What diseases have ongoing trials into the use of Tregs as therapeticsc?
GvHD; transplantation and some autoimmune disorders
Why is it difficult to identify Bregs for studying them?
in order to stain for IL-10 you have to kill the cell
