molecular basis of antigen recognition by b cells and antibodies
TRANSCRIPT
MOLECULAR BASIS OF ANTIGEN RECOGNITION BY B CELLS AND
ANTIBODIES
The immunoglobulin G (IgG) molecule
COMPLEMENT ACTIVATION
BINDING TO CELLS
DEGRADATION
TRANSPORT
ANTIGEN BINDING
IgG is built from twelve similar shaped immunoglobulin domains
The three-dimensional structure of immunoglobulin C and V domains
There are discrete regions of hypervariability in V domens
•Distinct regions of high variability and conservation led to the concept of a
FRAMEWORK (FR), on which hypervariable regions were
suspended.•Most hypervariable regions coincided with antigen contact points - the
COMPLEMENTARITY DETERMINING REGIONS (CDRs)
The hypervariable regions of antibody V domains lie in discrete loops at one end of the domain structure
CDR: complementarity-determining region
• Within the V domain, sequence variability is localized to
three complementarity-determining regions (CDRs).
• CDRs form three loops that are clustered at one end of the
domain.
• In the antibody molecule the CDRs of the heavy and light
chains form a variable surface that binds antigen.
Mechanisms of epitope recognition
• Linear and discontinuous epitopes
• Multivalent Antigens
• Polymeric Antibodies
• Epitope binding mechanisms
The nature of antigenic determinants
Two kinds of multivalent antigen
IgM is secreted as a pentamer of immunoglobulin monomers
IgA molecules can form dimers
Transcytosis of dimeric IgA antibody across epithelia is mediated by the poly-Ig receptor (pIgR)
IgG is a highly flexible molecule
Different hinge structures distinguish the four subclasses of IgG
Each human immunoglobulin isotype has specializedfunctions correlated with distinctive properties
Maternal IgG is transported by the neonatal Fc receptor (FcRn) across the placenta to the fetus
IgG half-life• FcRn is also present in the adult and involved in protecting IgG from
degradation• Accounts for the long (3 week) half-life of IgG compared to other Ig
isotypes
• Therapeutic agents that are fused to IgG Fc regions take advantage of this property e.g. Enbrel (TNFR-Fc)
Half-life in serumIgG - 21 daysIgA - 6 daysIgM - 5 daysIgE - 2.5 days
The receptor FcRn transports IgG from the bloodstream into the extracellular spaces of tissues
Epitopes can bind to pockets, grooves, extended surfaces, or knobs in antigen-binding sites
The noncovalent forces that hold together the Ag-antibody complex
• Pentameric IgM and dimeric IgA are synthesized in association with the same J chain.
• A poly-Ig receptor (pIgR) is responsible for the transcytosis of dimeric IgA across epithelia.
• FcRn receptors:- transport IgG across the placenta.
- transport IgG from the bloodstream into the extracellular spaces of tissues.
- protect IgG from degradation.
• Antigens are bound to antibodies through noncovalent bonds.
B cell antigen receptor complex
Cross-linking of B-cell receptors by antigens initiates a cascade of intracellular signals
ITAM: immunoreceptor tyrosine-based activation motif
Signal transduction by the BCR complex
Structure and function of the B-cell co-receptor
Signals generated from the B-cell receptor and co-receptor combine to activate B cells in response to surface and soluble antigens
• B cell activation requires
- cross-linking of surface immunoglobulin
- signals from the B-cell co-receptor
(or from PRR-s)
- helper T cells (in the case of protein antigens)
Production of a mouse monoclonal antibody
Possible use of monoclonal antibodies
- Identifying cell types
Immunohistochemistry
Characterization of lymphomas with CD (cluster of differentiation) markers
- Isolation of cells
Isolation of CD34+ stem cells for autologous/allogeneic transplantation (from
peripheral blood)
- Blood group determination (with anti-A, anti-B, and anti-D
monoclonals)
- Identification of cell surface and intracellular antigens
Cell activation state
- Therapeutic usage of monoclonal ABs
CD20+ anti-B-cell monoclonals in non-Hodgkin lymphoma
Prevention of organ rejection after transplantation
Anti-inflammatory antibodies (autoimmune diseases)
Monoclonals in tumor therapy
1. „Naked MAb”, unconjugated antibodyAnti-CD20 (rituximab – Mabthera/Rituxan, chimeric): B-cell Non-Hodgkin lymphomaAnti-CD52 (campath – Mabcampath, humanized): chronic lymphoid leukaemiaAnti-ErbB2 (trastuzumab – Herceptin, humanized): breast cancerAnti-VEGF (bevacizumab – Avastin, humanized): colorectalis tu. (+ Lucentis!)Anti-EGFR (cetuximab – Erbitux, chimeric): colorectalis tu. (+ Vectibix, rekomb. human)
2. Conjugated antibodyAnti-CD20 + yttrium-90 isotope (ibritumomab- Zevalin)Anti-CD20 + iodine-131 (tositumomab – Bexxar)
Monoclonal antibodies as treatments for disease
The antibodies made by a hybridoma cell line are all identical and are therefore called monoclonal antibodies.
Monoclonal antibodies are applied in:
- research
- diagnostic tests
- therapeutic treatments