Ab diversity/B cell development Flashcards
What is the primary effector function of B cells?
produce antigen specific Abs
Where are B cells produced? Where do they mature? From what are they derived from?
produced in bone marrow
mature in bone marrow
derived from common lymphoid progenitor
Describe mature naive B cells. What is the most important feature of the naive B cells?
they bear recprots on their surface that can potentially bind to a specific antigen
each Ig produced by any one B cell all have the same antigen speificity
What must occur for a B cell to be activated to proliferate and differentiate into either plasma or memory cells?
BCR encounter and bind to their specific antigen
What is a terminally differentiated B cell whose primary function is to produce antibodies?
plasma cells
Describe the difference in IgG and IgA plasma cells
IgG producing plasma cells migrate to bone marrow
IgA producers migrate to lamina propria of mucosal surfaces
What is a long lived antigen specific B cell that results from antigenic stimulationof a naive B cell during the primary immune response?
memory B cell
Upon subsequent exposure to specific antigen, memory B cells do what?
reactivated to differentiate into plasma cells as a component of a secondary immune response
Describe somatic recombination
immune system produces an enormous number of B cells daily, and most of them produce Ig that have unique specificity that gives rise to a repertoire of B cells
What is the antibody repertoire? How is it created?
complete collection of antibody specificites available within an individual
somatic recombination of Ig genes
What type of event is teh rearrangement of LC and HC genes?
antigen-independent developmental event
What region differs in Ab molecules?
V regions of LC and HC of Ab
T/F germline form of LC genes (kappa and lambda) and HC genes consist of the same genetic block
False, they consist of different genetic blocks
In the LC genes, what gene segments combine to create the variable region? The variable region is associated with what region that forms the complete LC gene?
variable (V) and joining (J) gene segments create the variable region
variable region is associated with constant (C) region forming the LC gene
HC germline genes consist of what gene segments that encode the variable region and combine with a constant(C) region gene?
variable (V), diversity (D), and joining (J)
Why is the leader sequence important in the organization of Ig LC and HC genes? What is the leader sequence for LC and HC?
imortant bc it directs the proteins into the cells secretory pathway following translation
lambda LC=> VJ, C
kappa LC => VJ, C
HC => VDJ, C
When and where does somatic recombination (variable region rearrangement) occur? Are these dependent or independent rxns?
rearrangements that take place are random and occur during B cell ontogeny in the bone marrow
totally independent of interaction of the developing B cell with antigen
Describe the steps of LC gene construction
- random rearrangement of single V w/ a single J to form a continuous piece of DNA that encodes entire variable region of LC
- C is adjacent to J but separated from VJ junction by non-coding (intronic) DNA sequence,
- RNA processing following transcription of rearragned DNA brings VJ jxn together with C to create complete coding region for LC
Describe the steps of HC gene construction
- random rearrangment of single D with single J to form DJ jxn
- random rearrangement of single V with DJ jxn to form complete DNA sequence that encodes the variable domain of HC
- C is adjacent to J but separated from VDJ jxn by non-coding (intronic) DNA sequence
- RNA processing following transcription of rearranged DNA brings VDJ jxn together with C gene segment to create complete coding region for HC
Describe how the joining of gene segments adds diversity to the variable region
joining of gene segments is not precise and consequently generates added didversity of recombined V region coding sequence
the gaps that are created are filled by random addition of nucletides so different codons result
T/F once the V region genes have rearranged then no further rearrangement of that chromosome can take place
true
Once somatic recombination occurs, all progeny of that B cell will express the same ___region genes. Where can the only change in the variable region genes of that B cell arise from?
V region genes
somatic hypermutation (affinity maturation)
***What are the 3 main processes (assoc. w/ gene segment recombination) that help to generate Ab diversity?*** When do each of these mechanisms occur?
- different pairing of many gene segments to form the variable domains of both the LC and HC
- imprecise joining of gene segments during somatic recombination process
- different pairing of many possible LC with many possible HC that can be produced
each of these occur during initial development of the B cell in the bone marrow
The 2 copies of a gene on a pair of chromosomes are called what?
alleles
What is a process by which the protein from only one allele is expressed while other allele(s) are silenced? When is this observed mostly?
allelic exclusion
observed in genes for cell surface receptors such as B cells
Describe allelic exclusion wrt to B cells
in B cells, successful HC gene rearrangement on 1 chromosome results in shutting down rearrangement on 2nd chromosome
If no successful rearrangement occurs, then rearrangement takes place on the 2nd chromosome
If no successful rearrangement occurs on either chromosome then the cell dies
What is the result of allelic exclusion?
all the antigen receptors on an individual lymphocyte will have the same AA sequence in the variable domains of LC and HC proteins
What is the term for a single nucleotide substitutions (point mutations) that occur at very high frequency in the rearragned variable region DNA of Ig genes?
Somatic hypermutation
What process occurs in activated B cells during a germinal center rxn and results in production of variant Abs, some of whcih have higher affinity for the antigen?
somatic hypermutation
What process results in Ab populations that have increasing affinity for specific antigen?
affinity maturation
Where are most of the point mutations confined to?
complementarity determining regions (CDR) also called the hypervariable region (HV)
T/F somatic hypermutation and affinity maturation are both an antigen-dependent process
true
T/F Isotype switching (or class switching) is an antigen dependent process
During isotype switching, V-region exons produced by any given B cell are determined during what?
pro- and pre- B cell stage of maturation
Except for somatic hypermutation, a B cell and all of its progeny will express the type of what genes? Which genes can change as the response matures?
both V genes of LC and HC
C(hc) genes expressed by a B cell line can change as the response matures
What Ig does each B cell initially produce?
IgM
Describe the B cell as it matures from the initial production of IgM
same V region exons can be expressed with different C(hc) gene resulting in either IgG, IgE, or IgA Abs with same specificity as the initial IgM (w/o affinity maturation)
T/F isotype switching to IgD does not occur
true
Isotype switch can only occur after B cells have been what?
after B cells are stimulated by antigen and hav received T cell help in form of cytokine signals
Cytokine signals produced by what type of cells (be specific) promote class switching to a particular Ab isotype? How does it affect the B cell?
T helper cells (Th1 or Th2)
causes the B cell to switch to the most appropriate HC
***Although class switching does not affect the antigen-combining region of the Ab molecule, it leads to something else. What is it and how is it done?***
It leads to functional diveristy of hte antibody repertoire by producing different isotypes of Ab that have the same specificity as the parental B cell
As isotype swtiching occurs, the recombination event permanently removes what?
the intervening DNA sequence so further isotype switching to a C(hc) gene that has been removed is impossible
Where does the switch region of the B cell lie?
in introns between V region exon and Cu gene
and at equivalent sites upstream of C genes that encode other HC isotypes and catalyzed by switch recombinase
Once switching from the IgM of the C region to the IgA-1 C region then further switching can only occur where?
switching can only occur to C(hc) genes downstream to the igA-1 C region can occur so IgG-3 C region cannot be switched to
B cell development can be divided into 6 broad phases. what are they?
- expression of surface receptor molecules (Igs); rearrangement of LC and HC must occur
- (-) selection in bone marrow to remove B cells bearing receptors that bind “self”
- (+) selection of developing B cells continue to develop into mature B cells
- search for infection as mature B cells recirculate bw lymphoid tissue until activated
- activation and proliferation of B cells upon encounter with cognate antigen
- proliferating B cells differentiate givign rise to plasma and memory cells
What are the 2 stages B cell maturation?
antigen independent and antigen dependent
Describe the antigen independent development of B cells in its 4 phases
- Pro-B cell=>earliest B cell w/ rearrangement of HC Ig gene (in bone marrow)
- Pre-B cell=> productive VDJ is comple and IgM is express on B cell surface; LC rearrangement begins (in bone marrow)
- immature B cell=> LC rearrangment complete & IgM expressed then undergoes self tolerance selection 1. Clonal deletion, 2. Anergy
- mature b cell =>survive self tolerance differentiate to become mature B cells so IgM and IgD expressed on surface and are recirculated through peripheral lymphoid tissues
What is the term when immature B cells specifically interact with multivalent “self” antigens in the bone marrow undergo apoptosis and thus depleted from B cell population. when this occurs, what is the term for this?
clonal deletion
depleted from B cell population is negative selection
What is the term when immature B cells specifically interact with soluble antigen in bone marrow and signaled to down-regulate cell surface expression of IgM thus leaving the B cell unable to respond to specific antigen?
Anergy (unable to respond to specific antigen
What are the 2 ways in which negative selection occurs to further mature the B cell?
- B cells that undergo apoptosis due to interaction with self antigen
- B cells that interact with soluble antigen then signaled to down regulate cell surface expression of IgM so they do not survive very long
B cell development in the bone marrow is dependent on what?
non-lymphoidal stromal cells which provide specialized micro-environments for B cells at various stages of development
What are the 2 functions of stromal cells?
- make specific cell-surface contact with B cells through interactions between adhesion molecules
- produce growth factors that are essential survival signals for B cells (stem cell factor and IL-7)
What happens if a B cell precursor does not productively rearrange both its HC and LC and express IgM on its surface?
it will die
Somatic recombination is dependent on activity of 2 enzymes encoded by recombination activation genes (RAG1, RAG2). What are these proteins responsible for?
responsible for making double stranded breaks in DNA during somatic recombination
deficiency will cause profound immunodeficiency
During the maturation of B cells, what enzyme catalyzes the addition of N nucleotides at the jxns between rearranging gene segments? (critical process)
terminal deoxynucleotidyl transferase (TdT)
What enzyme is involved in the transduction of signals from cell-surface receptors during B cell development?
Bruton’s thymidine kinase (Btk)
What happens to individuals that do not have a functional Btk gene?
little or no circulating Ab bc B cell development halts at the pre-B cell stage
Btk gene is encoded on what chromosome and a deficiency is linked to what disease?
encoded on X chromosome
deficiency is known as X linked agammaglobulinemia
How does negative selection of self reactive B cells work?
once HC and LC are expressed and IgM is displayed on surface of cells, negative selection occurs
- B cells binding to self cell surface molecules undergo clonal deletion
- B cells binding to soluble self antigens are anergic and die
- B cells that do not bind to self then migrate to periphery to continue maturation process and become mature naive B cells
What process is taking place when the germinal center B cells proliferate vigorously and their variable region genes undergo somatic mutations
affinity maturation
How do B cells receive signals from helper T cells?
helper T cells encounter the peptide:MHC complex to produce cytokines and CD40 ligand which together trigger B cells to proliferate and differentiate into either memory or plasma cells
The BCR has 2 functions. What are they and describe them?
- Serves as receptor for specific antigen=> delivers 1st activation signal to B cell following interaction iwth specific antigen
- receptor mediated endocytosis=> surface Ig delivers specific antigen to interior of B cell where it’s degraded and then displayed on B cell in association with MHC class II which is termed antigen presentation
What are proteins made by cells that affect the behavior of other cells which bind to specific receptor moleucles on their target cells? What produces these proteins?
lymphokines or cytokines are produced by lymphocytes
What is required for class switching? What influences class switching?
CD40L is required so class switching does not occur in individuals not expressing CD40L
cytokines produced by helper T cells influence class switching
T/F somatic hypermuttion only occurs following activatino of an antigen-specific B cell
true
Describe positive selection of mature B cells
results: higher affinity Ab production leads to immune response that becomes increasingly more effective over the course of immune response (affinity maturation)
somatic mutation result in production of B cells bear BCR that have higher affinity for cognate antigen and stimulated to continue proliferating and ultimately become predominant clone of B cells specific for antigen
How are Ab molecules directed to insert into membrane or be secreted?
Large quantities of IgM are produced then they can class switch to produce other isotypes of Ab on their surface
transmembrane domains anchors the Ab molecule into the membrane
secreted form of Ab lack transmembrane domain and has secretory tail at carboxyterminal end of each HC
Production of membrane bound and secreted forms of Abs is achieved by what?
alternative RNA processing
