Assignment 6 Flashcards
what is the role of B lymphocytes?
express antigen recognizing receptors (antibodies) on their cell surface - the receptors mediate the interaction between the B cell and the intact antigen
- Interact with antigen and process/present it to CD4+ T cells
- Antibody factory following differentiation of plasma cells
- Some differentiation to memory B cells
- Express antigen receptors (mIg) on cell surface
- BCR complex
at what developmental phase does progenitor B cell acquire its antigen specific receptor (immunoglobulin)?
hematopoeisis
membrane immunoglobulin (mIg) are expressed in associated with heterodimer, ______
CD79a/CD79b = Ig alpha/Ig beta
what is the role of CD79a/CD79b = Ig alpha/Ig beta?
couples mIg to intracellular signaling pathways
in naive mature B cells, they heavy chain constant regions are ____ and ____, while the light chain constant regions are either _____ or _____
mu
delta
kappa
lambda
what segments make up the light chain variable region of the mIg?
V and J
variabel and joining
what segments make up the heavy chain variable region mIg?
V, D and J
variable, diversity and joining
the variable regions of the light and heavy chain are encoded in the genome by segments that are _______ during B cell development
randomly!
the constant regions, light and heavy chain differ from ______ but are the ____ in all antibodies
one another
same
why do light and heavy chains have more potential diversity for the B repertoire?
because they are not restricted in their association
where is the antigen binding site contain within?
combined variable regions of the light and heavy chain = hypervariable regions = complementary regions = paratope
what types of forces are between the antigen and antigen binding site on the antibody?
non-covalent - this means antigens can come on and off!
what does the attraction between the antigen binding site and an epitope determine?
affinity
the overall attraction, including all Fab regions is referred to as ?
avidity
the construction of different mIg on B cell clones is the result of?
- multiple copies of germline V,D and J gene segments
- random selection and combination of V,D, and J gene segments
- junctional diversity generated by the addition or deletion of bases
- random assortment of light and heavy chains
the random selection of V,D, and J gene segments generated B cell receptors that may be_____
autoreactive => those B cells must be deleted or inactivated!
define somatic recombination
the process in which DNA in the loci that encode the variable region is cut and recombined to make an intact gene for the variable regions of the light and heavy chains
the variable region gene and constant region genes are transcriibed into hnRNA which is spliced to mRNA which is translated to a light chain and a heavy chain
the variable region gene and constant region genes are transcriibed into hnRNA which is spliced to mRNA which is translated to a light chain and a heavy chain
where does transcription and translation of light and heavy chain genes occur?
bone marrow during B cell development
how many B cell clones can an individual generate? aka total diversity of BCR
10^9 to 10^11 - each having mIg whose variable regions will differ from those present on other clones
what do the B cell clones collectively represent?
B cell repertoire
how is the diversity among B cell clones possible?
because of V,D, and J segments which encode the variable regions and are present as multiple germline genes
how is recombination triggered?
activation of recombinases, nuelceoprotein products of the RAG-1 and RAG-2 genes (recombination activating genes)
gene rearrangement of the variable light chain involves the selection and ligation of what?
1 J and 1 V to = VJ segment
gene rearrangement of the variable heavy chain involves the selection and ligation of what?
1 D and 1 J to = DJ gene segment which combines with a randomly selected V gene segment to = VDJ of a heavy chain!
what is the random selection of V (D) and J referred to as?
combinatorial diversity
what happens to the intervening Vs, Ds or Js during recombination of either light or heavy chains?
they are deleted!
how is junctional diversity generated?
additional DNA nucleotides may be deleted or added at V, D or J gene segment junctions in order to maintain (downstream) an open reading frame
what template independent DNA polymerase mediates the incorporation of nucleotides at junctions?
terminal deoxynucleotidyl transferase (Tdt)
describe allelic exclusion
the successful rearrangement of a heavy chain variable region from 1 chromosome (maybe from mom) inhibits the somatic recombination of the heavy chain variable region of the other membrane of the chromosome pair (maybe from dad)
what is the net effect of allelic exclusion?
all mIg present on the cell surface of any B cell will have the same heavy chain variable region
B cell differentiation in the bone marrow occurs before any exposure to what?
foreign antigen
what is B cell differentiation characterized by
expression and silencing of distinct sets of genes at discrete stages of development
what is the ultimate goal of B cell maturation?
to generate a B cell repertoire that is non-autoreactive!
what is the Pro-B cell stage (transcription) characterized by?
transcription of multiple genes => RAG-1 and RAG-2, CD19, Tdt and CD79
what is the Pre-B cell stage (Expression of Pre-BCR) characterized by?
expression of a pre-B cell receptor (pre-BCR) in association with the CD79a/CD79b heterodimer.
what makes up the pre-BCR
2 mu heavy chains with variable region that has been rearranged by somatic recombination
2 heavy chains are paired with 2 pseudo light chains!
what directs proliferation and further differentiation of pre-B cells?
signaling via the pre-BCR complex
what genes are expressed in the Pre-B cell stage?
pre-BCR
CD79a/CD79b
CD19 (continues)
CD20 (introduced!)
at what stage does recombination of the light chain variable region occur?
Pre-B cell stage
at what stage does recombination of the heavy chain variable region occur?
Immature B cell stage (Tolerance induction)
what characterized the Immature B cell stage?
shifts in receptor expression from Pre-BCR to BCR
what does the shifts in receptor expression from Pre-BCR to BCR require?
assembly of the somatically rearranged light and heavy chains with CD79 to form BCR and translocate to the cell surface - pre-BCR is down regulated
how is specific inactivation of B cells expressing autoreactive mIg (tolerance induction) is accomplished by..
apoptosis or induction of anergy (non responsiveness)
why is tolerance induction only possible in the Immature B cell stage?
because novel BCR is expressed on the Cell surface
what is the mature B cell stage characterized by?
co-expression of cell surface IgM and IgD => consequence of alternative splicing!
mature B cells leave the bone marrow, enter blood stream and migrate to peripheral lymphoid tissues. They recirculate if they do not encounter antigen in secondary lymphoid tissue
what level of alternative splicing occur at and what is the result?
- occurs at the hnRNA level
- leads to expression of both IgM and IgD on cell surface
what mediates the exit of B cells from blood into peripheral lymph nodes?
adhesive molecules (Lselectin) on B cells
what does Lselectin on B cells interact with
vascular addressin molecules present on high endothelial HEVs
what is the role of addressins?
control sites of “homing” for B cells
different addressins are expressed in peripheral lymph nodes vs. intestinal mucosal HEVs
what are mutation in Btk kinase associated with
X-linked agammaglobulinemia
what is the role of Btk
plays a critical role in B cell activation, differentiation and proliferation
where does the initial block in B cell development occur as a consequence of Btk defect?
pro-B to pre-B cell transition => pre B cells are defected and maturation is stopped at this point = cannot enter circulation
in XLA pts, what numbers of B cells in the peripheral blood is seen and what are serum Ig levels? What does this suggest
low
low
- suggests that presence of a compensatory pathway that can at least in part, take over the role of Btk in human B cells…
