Differentiation and activation of B cells Flashcards
Antibodies in saliva correlated with dental caries
SIgA and IgG
Antibody which is the main defense mechanism in saliva
- SIgA
- plasma cells produce IgA in salivary glands
- IgA present in saliva inhibit attachment of oral streptococci species to epithelial cells
- bacteria in saliva are coated with IgA
what antibodies in the oral cavity do bacterial deposits contain
IgG and IgA
where does differentiation of B cells occur?
bone marrow
Immunoglobulins also act as ___ because they bind to antigens on the surface of extracellular microbes
opsonin
Which immunoglobulin is produced in the highest concentration in the mammary glands, parotid glands, and submandibular glands by plasma cells?
IgA
B cells express ___ that recognize antigens
membrane-bound antibodies (immunoglobulins)
BCR transmit signals via…
associated invariant membrane proteins: Ig-alpha and Ig-beta
BCR on each B cell clone is specific for…
a specific antigen
Collection of what makes up the immune repertoire?
distinct clones (>10^11 specificities)
describe the structure of an immunoglobulin (Ig)
- Y shape molecules formed by 2 heavy chains (a, d, e, g, m) and 2 light chains (k or l)
- 60% of human antibodies have 2 k chains (never a mixture of k and l)
describe the structure of the light chain
- k or l
- 1 variable (V) region on N terminal and 1 constant (C) domain
describe the structure of the heavy chain
- a, d, e, g, m
- 1 variable (V) region on N terminal and 3 or 4 constant (C) domains
- longest
Which region does the antigen bind to in an Ig?
variable region - this is what changes and is what is specific
called Fab region
which region of the immunoglobulin mediates effector function?
Constant region, called Fc region
*when B cells do isotope switching, they switch the heavy chain constant region
what two forms do immunoglobulins exist in?
Membrane-bound or secreted
describe the hyper variable regions of immunoglobulins
- there are 3 complementarity determining regions (CDR) that form the antigen-binding site
- they are located in both heavy and light chain variable domains
- CDR3 exhibits the greatest variability and most antigen binding
describe the binding of an antigen by antibody
- binding site = variable domains of light and heavy chains
- 2 binding sites per Y structure
- binds a wide variety of antigens (proteins, lipids, polysaccharides, nucleic acids)
- native antigen is recognized (doesn’t need to be processed)
*T cell receptor can only bind processed peptides
___ can bind in pockets or grooves, or on extended surfaces in the binding sites of antibodies
antigens
epitope
Small, accessible portion of an antigen that can be recognized; antigen part recognized by antibody
Affinity vs. Avidity
Affinity = strength of binding between the groove and antigen
Avidity = total strength of binding including number of epitopes bound (monomeric immunoglobulin has lower avidity than dimeric immunoglobulin because has lower number of binding sites)
Describe the 5 isotypes, which are based on the heavy chain
- IgD, IgE, and IgG are monomeric: 2 binding sites
- IgM are pentamers: 10 binding sites
- IgA are mainly dimers: 4 binding sites
The function of isotypes is conferred by…
the heavy chain
IgA isotype function
Mucosal immunity
IgD isotype function
Naive B cell antigen receptor; never gets into the circulation
IgE isotype function
Defense against helminthic parasites, immediate hypersensitivity (allergy)
IgG isotype function
Opsonization, complement activation, antibody-dependent cell-mediated cytotoxicity from NK cells, neonatal immunity, feedback inhibition of B cells
IgM isotype function
Naive B cell antigen receptor (monomeric form), complement activation
How is the enormous diversity of BCR generated? (overview)
Recombination of Ig gene segments and junctional diversity
Describe the germline organization of heavy and light chains
- heavy chain locus contains V, D, J, and C genes
- light chain locus contains V, J, and C genes
- heavy/light chain loci contains 30-45 V gene segments and few C genes
- several short diversity (D) and joining (J) gene segments
- Cm is the first region cluster, thus IgM is the first antibody produced
describe the recombination of immunoglobulin genes
- random selection of segments
1. Recombination of D and J segments
2. Recombination of V segments with fused D-J element
what mediates recombination of immunoglobulin genes?
A lymphocyte-specific VDJ recombinase: brings two segments close together
What diversity is limited in recombination of immunoglobulin genes?
Combinatorial: there can only be 3x10^6 possible combinations
What diversity is unlimited in recombination of immunoglobulin genes?
Junctional diversity: due to exonuclease (removing nucleotides) and terminal deoxyribonucleotidyl transferase (TdT) (adding nucleotides)
How do B cells mature and get selected (overview)
selection is based on expression of functional antigen receptors
Where does B cell maturation and selection occur?
Bone marrow in an antigen independent manner
What are the two checkpoints for maturation of B cells?
- Expression of Igm protein promotes survival/proliferation signals (preB)
- Pre-BCR triggers recombination of the light chain locus and IgM expression
*signals from pre-BCR and IgM promote survival of the cells
Steps of B-cell maturation
- Stem cell is induced to proliferate by IL7
- Pro-B cells formed: do not express any Ig
- Recombined heavy chain gene (VDJ) in pre-B cells: expression of pre-BCR (only 55% pass this checkpoint)
- Proliferation to immature B cell: second checkpoint is expression of membrane IgM
- mature B cells express IgM and IgD
Describe negative selection of B cells
Eliminate B cells that binds with high affinity ubiquitous self-antigens
Describe receptor editing of B cells
- binding of an antigen in BM with high affinity can reactivate the VDJ recombinase and trigger light chain recombination
- results in change of the Ig specificity
- advantage as B cells are not eliminated
How are B cells activated (overview)?
Antigen recognition results in clonal expansion and differentiation
What are the phases of the humoral response?
- mature B cells expressing IgM and IgD
- stimulus (antigen) binds to BCR (signal 1)
- signal 2 coming from Helper T cells cytokines or other stimuli bind
- Proliferation and differentiation occurs
- Results in antibody secretion (IgM only), isotype switching, affinity maturation, or Memory B cell
Where does isotype switching occur?
germinal centers of lymph nodes - after this can secrete different isotypes like IgG or IgA
what is affinity maturation?
Process that selects for B cells producing antibodies of highest affinity to an antigen of interest through successive exposure to that antigen in the periphery. Leads to an increase in the affinity between the antibody and an antigen following repeated exposures with the antigen
What happens to plasma cells producing high affinity Ig?
They migrate to BM or mucosal tissues and survive for years
lifetime of memory B cells
Memory B cells last a lifetime in mucosal tissues and blood with no restimulation
Primary vs secondary antibody responses
Primary: usually 5-10 day lag after immunization; smaller peak response; usually IgM>IgG; lower average affinity, more variable
*plasma cells in bone marrow become memory B cells
Secondary response: quicker, larger, IgG more abundant, high affinity
describe antígen-induced signaling
- cross linking of BCR leads to activation of transcription factors involved in B cell proliferation and differentiation (signal transmitted by invariable Ig-alpha and Ig-beta)
- requires cluster of at least 2 BCR (this is to control the immune response: want to make sure there’s a real antigen problem before reacting)
what are the innate immune signals other than antigen recognition?
- simultaneous engagement of antigen receptor and CR2 or TLR enhances B cell activation
- CD19 and CD81 deliver the CR2 activating signal
describe antigen receptor-mediated B cell activation
- initiates proliferation and differentiation
- enhances their ability to interact with T cells, responsiveness to cytokines, migration from follicle to T cell zone, antibody secretion
what are the two types of antibody response?
- T-dependent response
- T-independent response
T-dependent response
B-cell needs the help of a T-helper cell to make antibodies
- to protein antigen
- isotype switching
- high affinity Ig
- follicular B cells
T-indepedent response
- to nonprotein antigen (antigens which T cells cannot recognize)
- IgM
- low affinity Ig (need T cell interaction for high-affinity)
- marginal zone and B1 cells (especially in peritoneum)
Sequence of events of the T and B cell interaction
- simultaneous activation of T (dendritic cell in T cell zone) and B cells (antigen recognition in follicle)
- migration towards each other where short-lived plasma cells are induced
- migration of short-lived plasma cells back to the follicles where germinal center reaction occurs (with help of follicular helper T cells to generate long-lived plasma cells)
What causes T cells to be found in the T cell zone?
Expression of CCR7
*to meet with B cells, they thus must down-regulate CCR7 and up-regulate CXCR5 to end up at border of T and B cell zone
What causes B cells to be found in the B cell zone?
Expression of CXCR5
*must downregulate to be able to meet T cells, while up regulating CCR7. Will up-regulate again to get back to follicle.
Describe the antigen presentation by B cells to Thelper cells
- B cells process antigen
- B cells and T cells recognize different epitopes of the same protein: native confirmational epitopes for B cells; peptide fragment for T cells
describe the cross-talk between B and T cells
- recognition of peptides presented by B cells induces CD40L up regulation in T cells and production of cytokines (IL21)
- engagement of both CD40 and cytokine receptor on B cells lead to their proliferation and differentiation in plasma cells
How is the cross-talk between B and T cells a “two-way street”?
Initial interaction of B cells presenting the antigen to T cells results in up regulation of CD40L and specific cytokines, which ultimately helps to lead to B cell proliferation and differentiation of the B cells
What happens in the germinal center? (overview)
- Isotype Switching
- Affinity maturation of immunoglobulins
- Selection of high-affinity B cells
describe the germinal center of the lymph node
- has a dark zone and light zone
- dark zone is where B cell proliferation occurs after activation and migration
- light zone contains follicular dendritic cells and Tfh cells, which interact with B cells to induce affinity maturation and isotype switching
describe immunoglobulin isotype switching
- switching requires CD40L signal from Tfh cells
- type of heavy chain isotype is determined by cytokines produced by Tfh cells (IgG and IgE) or tissues (IgA)
What happens to B cells when IL-10 or IFNgamma is made by Tfh?
Switch from IgM to IgG
What happens to B cells when IL-4 is made by Tfh?
Switch from IgM to IgE
What happens to B cells when TGF-beta or BAFF is made by Tfh?
Switch from IgM to IgA
describe the process of switch recombination
- CD40 and cytokine receptor signals stimulate transcription through one of the C regions downstream of Cm
- CD40 signal induces activation of cytidine deaminase (AID) which results in the deletion of the Cm gene
- recombination of S region brings VDJ exon next to a different C gene
- specificity of the Ig is the same, while the C region is different and reflects the function of the Ig
describe the process of affinity maturation
- ability of antibodies to bind protein antigen increases as antigen persists or recurs
- point mutation in the V regions
- occurs in the germinal center
- results in selection of B cells with high-affinity antigen receptors
- Ig gene mutation = one in 10^3 base pairs per cell division
- interaction with follicular dendritic cells that display antigen and Tfh promote their survival
- B cells compete for antigen and those with high affinity for antigen survive
follicular dendritic cells (FDCs)
- not dendritic cells
- antigens are bound to their surface, they are not being processed for presentation
- antigens on surface can be bound by BCR
step-by-step process of affinity maturation
- B cell activation by protein antigen and helper T cells
- migration into germinal center
- B cells with somatically mutated Ig V genes and Igs with varying affinities for antigen
- B cells with high-affinity membrane Ig bind antigen on follicular dendritic cells and present antigen to helper T cells
- B cells that recognize antigen on FDCs or interact with helper T cells are selected to survive; other B cells die
