ICL 2.1: B-Cell Development Flashcards
where do immunoglobulins come from?
immunoglobulin protein is either membrane expressed or secreted
what are the 4 phases of B cell development?
- maturation = generation of B cells in BM
- selection = elimination of self-reactive B cells in BM
- migration/activation = activation of B cells by foreign antigen in secondary lymphoid tissues
- proliferation = differentiation to antibody-secreting plasma cells and memory B cells in secondary lymphoid tissues
where do B cells acquire functional B cell receptors?
BM
aka primary lymphoid tissue
what is B cell selection?
test for autoreactive B cell lineages
where does B cell maturation occur?
BM
where does B cell selection occur?
BM
few anergia cells can be seen in circulation
where does B cell activation usually occur?
secondary lymphoid tissue
what happens during B cell development?
- B cell precursor rearranges its immunoglobulin genes
- immature B cell bound to self cell-surface antigen is removed from the repertoire = negative selection in BM
- mature B cell bound to foreign antigen is activated
- activated B cells give rise to plasma cells and memory cells
there’s Ab secretion in BM and lymphoid tissue and memory cells in lymphoid tissue
what are the stages of rearrangement and expression of H chain genes during B cell development?
- stem cell = germline
- early pro-B cell = D-J rearrangement
- late pro-B cell = V-DJ rearrangement
- large pre-B cell = VDJ rearranged
- small pre-B cell = VDJ rearranged
- immature B cell = CDJ rearranged
- mature B cell = CDJ rearranged
what are the stages of rearrangement and expression of L chain genes during B cell development?
- stem cell = germline
- early pro-B cell = germline
- late pro-B cell = germline
- large pre-B cell = germline
- small pre-B cell = V-J rearranging
- immature B cell = VJ rearranged
- mature B cell = VJ rearranged
what are the stages of rearrangement and expression of Ig during B cell development?
- stem cell = none
- early pro-B cell = none
- late pro-B cell = none
- large pre-B cell = μ heavy chain. surrogate light chain. pre b-cell receptor on cell surface
- small pre-B cell = μ chain in ER
- immature B cell = μ heavy chain. λ or ϰ light chain. IgM on surface.
- mature B cell = IgD and IgM on surface
which cell stage is the earliest identifiable cell of the B cell lineage?
pro-B cells
what are the 7 stages of B cell development?
- stem cell
- early pro-B cell
- late pro-B cell
- large pre-B cell
- small pre-B cell
- immature B cell
- mature B cell
when does heavy chain D and J joining occur?
early pro-B cell stage
when does heavy chain V to DJ joining occur?
late pro-B cell stage
this results in a function μ heavy chain but it’s not surface expressed
in what stage of B cell development are μ chains first expressed?
large pre-B cell
there’s transient expression of μ heavy chain on the surface = pre-B cell receptor on surface; possesses some signaling capacity
μ chain combines with two proteins that mimic Ig light chains = Vpreb and λ5
what does binding of the pre-B cell receptor cause?
binding of the pre-B cell receptor induces cell division and formation of the small pre-B Cell
a majority of pre-B cell receptors are internalized but sometimes there’s a few still on the surface
surrogate light chain synthesis stops and Ig heavy chain rearrangement stops
rearrangement of variable light chain starts
what happens during the immature B cell phase?
expression of a fully functional IgM receptor on the surface signifies formation of an immature B cell
light chain rearrangement is complete
light chains have assembled with m chains to form IgM
fully functional IgM is present on the surface
surrogate light chain synthesis ceases
so far, development has been independent of specific antigens
what happens during the mature B cell phase?
mature B cells express both fully functional IgM and IgD receptors on the surface which posses the SAME antigen specificity
mature b cells have NOT been exposed to specific antigens = naive B cells
what does B cell development in the BM depend on?
non-lymphoid stromal cells
what do non-lymphoid stromal cells do?
- make specific cell-surface contacts with developing B cells through
- adhesion molecules on stroma
- ligands on B cells - provide growth factors that stimulate lymphocyte differentiation and proliferation
- SCF promotes pro-B cells
- IL-7 is necessary for proliferation and development of pre-B cells
- VCAM
B cell development in the BM is dependent on non-lymphoid stromal cells! direct contact is required!!
immature b cells in later stages can develop without direct stomal cell influences but they still require growth factors and cytokines
where are pro-B cells found?
early pro-B cells are concentrated near the endosteum (inner bone surface)
as they mature, B cells move toward the center of the marrow cavity and they lose TdT = pro-B marker
what does the survival of a developing B cell depend on?
the productive rearrangement of a heavy- and light-chain gene
not all DNA rearrangements result in a suitable reading frame to be translated into an immunoglobulin chain
what happens during junctional diversity?
extra nucleotides are added during heptamer-nonamer joining which increases diversity
hairpin formation at the ends of D and J segments are cleaved to produce P-nucleotide sequence
TdT then adds N-nucleotides to cut the DNA strand
the strands are then paired and DNA is repaired!!
there are productive rearrangements that result in a functional Ig chain and others produce non-functional Ig which leads to apoptosis
does the heavy chain locus or light chain locus rearrange first?
heavy-chain locus rearranges before the light-chain locus
heavy chain has 2 alleles
D-J rearrangements can occur in both heavy alleles
V-DJ rearrangements can only occur one allele at a time
how does light chain locus rearrangement happen?
it takes place one locus at a time
ϰ locus usually rearranges before λ locus
rearrangement continues until a successful light-chain is produced
which enzymes are responsible for Ig gene rearrangement?
RAG1 and RAG2
they recognize recombination signal sequences (RSS) and make double stranded breaks in DNA
what does TdT do?
adds random nucleotides to cut ends of DNA
it’s important for junctional diversity but not essential for recombination
it’s on in pro-B cells then off in pre-B cells, adding most N nucleotides in heavy chain
what is Btk?
Btw = bruton’s tyrosine kinase
it’s encoded on the X chromosome and is essential for B cell maturation
what do Btk mutations cause?
B cell deficiency
you get recurrent infections from common extracellular bacteria
what is Bruton’s x-linked aggamaglobulinemia?
a hereditary inability to make antibodies
the responsible mutation is located on Btk gene on X chromosome encoding tyrosina kinase essential for B cell maturation in bone marrow
the diagnosis is possible when blood show complete lack of CD19 or CD20 cells (B cell) and low levels of antibodies (IgM, IgG, IgA, IgE)
when is CD19 and CD45R expressed during B cell maturation?
early pro-B cell through immature B cell stages
aka everything except stem cell phase
when is CD43 expressed during B cell maturation?
early pro-B cell through large pre-B cell
when is CD24 expressed during B cell maturation?
late pro-B cell through immature B cell phase
when is c-Kit expressed during B cell maturation?
stem cell through early pro-B cell
when is IL-7 receptor expressed during B cell maturation?
stem cell through large pre-B cell
