B Cell/Antibody Function Flashcards
X-linked Agammaglobulinemia (XLA)
Defect in gene encoding Bruton’s tyrosine kinase (BTK)
Defective enzyme function
Defective enzyme level
Responsible for B cell maturation, survival, proliferation
X chromosome
Absence of B cells
Decreased IgG, IgA, IgM
Immune Deficiency (primary, secondary causes, estimations, live births)
Primary: Not caused by other factors; you’re born with it or develops for unclear reasons
Secondary: Caused by other factors
Diseases, medications, malnutrition could cause immune deficiency
200 primary immune deficiency diseases
1:1,200 live births
B cell deficiencies (what is absent, reduced, what is infectious consequence)
Absent or reduced follicles in germinal centers in lymphoid organs
Reduced serum Ig levels
Infectious consequence:
Pyogenic bacterial infections, enteric bacterial and viral infections
T cell deficiencies (reduced or defective, infectious)
May be reduced T cell zones in lymphoid organs
Reduced DTH reactions to common antigens
Defective T cell proliferative responses to mitogens in vitro
Infectious:
Viral and other intracellular microbial infections (Pneumoncystis jiroveci, other fungi, non-TB mycobacteria)
Virus associated malignancies
(EBV lymphomas)
Bad bacteria, opportunistic infections
Innate immune deficiencies
Variable abnormalities, depending on which component
Variable infections: pyogenic, bacterial, viral
B Cell maturation (and CD surface molecules)
Hematopoietic stem cell Lymphocyte Precursor Pro-B cell ~VDJ recombination~ Pre-B cell BTK checkpoint Immature B cell mature B cell
Surface molecules aka CD20 CD19 etc. change throughout development. If you want to give a drug that targets CD20, it would wipe out ever cell that has that marker, but not all (ex. plasma protein)
B lymphocyte (antigen recognition, effector functions)
Recognizes antigen
Neutralizes microbe
Phagocytosis
Complement activation
Helper T lymphocyte (antigen recognition, effector functions)
Microbial antigen presented by APC
Releases cytokines
- Activation of macrophages
- Inflammation
- Activation (proliferation and differentiation) of T and B lymphocytes
Cytotoxic T lymphocyte
Infected cell expressing microbial antigen
Killing of infected cell
B lymphocyte function (3)
- Produce antibodies
T dependent or T independent
Membrane bound (B cell receptor)
Secreted - Plasma cells: Long living mature B cells
Produce high affinity antibodies even after antigen removal
Stimulated by infections and vaccines - Memory cells: long living mature B cells
Do not produce antibody
Ready to rapidly respond upon antigen reintroduction
Antibody structure
2 identical heavy chains (Y) (1V, 3-4C domains)
2 light L chains (1V, 1C domain)
Variable V region
Variable
Diversity
Joining
Constant C region
Disulfide bonds
C terminus either anchored in plasma membrane (BCR) or terminate so it is secreted
Immunoglobulin domain (antibody)
Folded layers of a B pleated sheet held together by a disulfide bond connected by protruding loops
Fab (antibody)
Fragment antigen binding region
Hinge region (antibody)
flexibility for increased binding to the 2 Fab regions
Fc (antibody)
Fragment crystalline region
Crystalizes in solution
Responsible for the biologic activity of antibodies
Complementarity determining region (CDR) loops
In V regions of immunoglobulin domain (Vh and Vl chains)
3 on each
Responsible for antigen recognition
Single antigen that it can recognize
High affinity
CDR3 has the greatest variability and contributes the most to antigen binding
Light chain types: K and g
No functional difference
IgA
IgA1,2
3.5mg/mL serum
6 day half life
Mainly dimer, also monomer, trimer
Mucosal immunity (transport of IgA through epithelia) Major isotype in mucosal lymphoid tissues via TGF-B
IgD
Trace serum
3 day half life
Monomer
Naive B cell antigen receptor
IgE
0.05 mg/mL serum concentration
2 day half life
Monomer
Defense against helminthic parasites
Mast cell degranulation (Immediate hypersensitivity)
Allergic response
IgG
IG1-4
13.5 serum concentration
23 day half life
Monomer
- Fc receptor dependent phagocyte responses
- Opsonization
- Complement activation
- Antibody dependent cell-mediated cytotoxicity
- Neonatal immunity (can pass placenta; how newborns have immunity; their own immunity develops 4-6mo later)
- Feedback inhibition of B cells
- Memory antibody produced in response to vaccination and infection offering LT protection post exposure
most abundant Ab in the plasma
IgM
1.5 mg/mL serum
5 day half life
Pentamer
Naive B cell antigen receptor (monomeric form)
Complement activation
Produced rapidly after vaccination but degrades rapidly and doesn’t offer long term protection
Binding region of antibody (what type of interaction? affinity? avidity? cross-rxn?)
Vh and Vl chain CDRs
Form clefts where epitope of molecules bind
Epitopes may be AA (linear) or shape related (conformational)
Reversible, non-covalent interactions
H bonding
Hydrophobic
Charged based
Affinity: strength of one Ag-Ab bond
Increases with repeated exposure (why we repeat vaccination)
Avidity: total strength of multiple bonds
Cross reactivity: Abs may bind other structurally related Ags with similar epitopes
T-dependent B cell Responses
APCs present to T cells that activate B cells to induce class switching and affinity maturation
Called:
Isotype-switched high affinity antibodies
Memory B cells, long-lived plasma cells
T cell helps B cell produce antibodies long term
Otherwise, PROTEIN antigens cause weak or absent antibody responses without T cell help
T-independent B cell responses
Polysaccharides, lipids, other non-protein antigens elicit antibody production without involvement of T cells
Simple process to T dependent antigens
Less isotype switching
Less affinity maturation
Mainly IgM, low-affinity antibodies, short-lived plasma cells
Common Variable Immune Deficiency
Recurrent bacterial sinopulmonary infections, GI infections, systemic complications
Because you don’t make antibody
Low IgG <400
Low IgA and/or low IgM
Insufficient responses to vaccinations
Unknown and likely multiple etiologies
Onset at any age
BCR Signaling (B cells and TLRs)
B cells express complement receptor for activation when exposed to microbial antigen
- Microbial bound C3d (released from C3 during complement activation) binds to complement receptor 2 (CR2) on the B cell surface to activate B cells
- C3d-CR2 binding enhances B cell activation as part of the innate immune response
TLR activates B cells by promoting B cell proliferation, differentiation, and antibody secretion
example of adaptive + innate immunity working together
BCR Signaling - mechanism
- Ab bound to antigen
- Tyrosine phosphorylation occurs
- Activation of downstream enzymes via biochemical intermediates
- Production of transcription factors leading to the immune response
(Myc, NFAT, NFkB, AP-1)
What happens when an antigen is recognized by both the T and B cell?
- Naive T cell activated by Ag presented by DCs differentiates into Th cell
- Naive B cell activated by Ag binding
Migrate toward each other to cause the antibody response
Some migrate back to form germinal centers producing further antibody response
What interaction must occur to allow B cells to produce other isotypes?
Th cells express CD40L that binds to CD40 on the B cell
Binding must happen in order to get isotype switching
(B cells normally just make IgM/IgD)
In order to make IgG, IgA, and IgE, this interaction must occur with cytokine mediators
> > > B cell differentiation and proliferation
Isotype switching (VDJ)
VDJ exon encoding the V domain of an IgM heavy chain and moves it adjacent to a downstream C region
Activation-induced deaminase (AID) removes uracil to create DNA nicks (induced by CD40)
dsDNA breaks into 2 switch regions and brought back together and repaired (removal of a portion of DNA)
New heavy chain isotype (IgA, G, E)
same specificity as before
Isotype switching (cytokine dependent)
IL-10 from Tfh cells stimulate isotype switching to IgG via IFN-g
>opsonizes microbes
>promotes microbial phagocytosis and intracellular killing
IL-4 from Tfh cells stimulate isotype switching to IgE
>response to helminthic infections with IL-5 stimulated eosinophils
>generate the allergic response
Affinity maturation
Affinity of antibodies changes with prolonged or repeated Ag exposures
Th cell-dependent protein antigens
Somatic hypermutation of immunoglobulin genes
Selection of high-affinity B cell survival (others will die)
Hyper IgM Syndrome
X linked
Defective CD40-CD40L binding due to defective CD40L gene
Autosomal recessive form associated with defect in AID
IgM is elevated
IgG and IgA (helpful for fighting infections) is absent due to absence of class switching
Normal B cell numbers
Selective IgA Deficiency
IgA is the secretory antibody
Located on mucosal surfaces
Prone to sinopulmonary bacterial infections, otitis, and conjunctivitis
Prone to GI infections
Prone to allergies and autoimmune disease
Monoclonal Antibodies
- Isolate spleen cells from mouse immunized with antigen X
- Fuse spleen cells (that produce x-antibody) with mutant immortal myeloma line (unable to grow in selection medium
- Only fused cells (hybridomas) grow
- Screen supernatants of each clone for anti-x antibody and expand positive clones
Hybridomas producing monoclonal anti-X antibody
Select antibody of desired specificity for targeted therapy for disease
IgG supplementation Therapy
Pooled from plasma donors (from people who have been sick or vaccinated)
Intravenous or subcutaneous infusions
Given on a weekly to monthly basis for whole life
Does not work in patients who have selective IgA deficiency
Give to people who don’t make enough IgG to replenish immune system
CD19, CD20, CD21
A surface marker on B cells
