Cell Biology and Monoclonal Antibodies

Stem Cells Revision and Ethical Considerations

  • Definition of Saviour Sibling: A "saviour sibling" is a child conceived primarily to provide a compatible stem cell, bone marrow, or umbilical cord blood transplant to an older brother or sister suffering from a life-threatening illness.
  • Resource References:     * Anand Varma TED Talk: "A thrilling look at the first 21 days of a bee's life."     * My Sister's Keeper (Film/Book reference regarding the ethics of saviour siblings).     * Ethical debate: Is it ethical to have a second child so that your first might live?

Stem Cells and Bioengineering in Tissue Engineering

  • Meniscus Tissue Engineering Components: Successful regeneration of functional meniscal tissue involves a combination of three key factors:     * Scaffold Materials: These include natural and synthetic polymers, as well as composite or "smart" materials.     * Stem Cell Applications: Primarily involves Mesenchymal Stem Cells (MSCs) and Induced Pluripotent Stem Cells (iPSCs).     * Signaling Factors: Crucial biochemical cues required for proper differentiation and regeneration.
  • Challenges in Bioengineering:     * Mechanical mismatch between the scaffold and native tissue.     * Phenotypic drift.     * Immune response from the host.     * Difficulties in clinical translation.
  • Proposed Solutions:     * Personalized treatments tailored to the patient.     * Implementation of 3D and 4D printing technologies.

Understanding Antibodies (Immunoglobulins)

  • Monoclonal Antibody (mAb or moAb) Definition: An antibody produced by a single clone of cells.
  • General Antibody Structure:     * Heavy (H) and Light (L) Chains: The molecule is composed of these two types of polypeptide chains.     * Variable Region: The segments of the heavy and light chains that form the F(ab) fragment; these are characterized by high affinity and determine antigen specificity.     * Constant Region: Determines the antibody's class and mediates effector functions (the Fc region).     * Antigen Binding: High-affinity interaction between the variable segments of the antibody and specific antigens.
  • Antibody Isotypes/Molecules:     * IgG: Standard Y-shaped molecule.     * IgM: Often exists as a pentameric structure joined by a J chain (prolongs efficacy/multiple binding sites).     * IgA: Often a dimeric structure joined by a J chain.     * IgE and IgD: Specialized single-unit antibodies.
  • Mechanism of Action:     * Block infection by sticking to germs/toxins.     * Label threats for cleanup by other immune cells.     * Spark broad immune system responses to protect the body.

Comparison: Polyclonal vs. Monoclonal Antibodies

  • Polyclonal Antibodies:     * Cost: Relatively cheap to produce.     * Population: A mixed population of different antibodies.     * Binding: May bind to different areas (epitopes) of the target molecule.     * Tolerance: More tolerant of small changes in protein structure (e.g., polymorphism, glycosylation, or denaturation).
  • Monoclonal Antibodies:     * Cost: Expensive to produce.     * Population: A single antibody species.     * Binding: Will only bind to a single specific site (epitope).     * Specificity: May recognize a particular form of a protein (e.g., specific phosphorylation state).

Therapeutic Applications of Monoclonal Antibodies

  • Cancer Therapy: Targeted destruction of malignant cells.
  • Autoimmune Diseases: Modulating the immune system in conditions like Rheumatoid Arthritis.
  • Infectious Diseases: Neutralizing viruses or bacteria.
  • Ophthalmology: Treating conditions like macular degeneration.
  • Cardiovascular Diseases: Managing chronic heart issues.
  • Neurological Disorders: Addressing conditions such as Alzheimer's or Multiple Sclerosis.
  • Bone Disorders: Managing osteoporosis or bone-density issues.
  • Transplantation: Preventing organ rejection via immunotherapy.
  • Allergic Conditions: Targeting IgE-mediated responses.

Hybridoma Technology and Somatic Cell Hybridization

  • The Problem: Primary B cells have a very short lifespan, making it impossible to produce identical, specific antibodies continuously in a lab.
  • The Solution: Hybridoma technology fusions B cells with immortal cells.
  • Definitions:     * Myeloma: Malignant plasma cells (B cells) that are capable of continuous (immortal) division.     * B Cell (B Lymphocyte): White blood cells of the adaptive immune system that produce specialized proteins (antibodies).     * Somatic Cell Hybridization: A technique fusing cells from different tissues or species (e.g., human and mouse) to create a single hybrid cell. Used for gene mapping and mAb production.
Step-by-Step Production of Monoclonal Antibodies
  1. Immunization: A mouse is immunized with a specific antigen.
  2. Isolation: B cells are isolated from the mouse's spleen.
  3. Cultivation: Myeloma cells are grown in culture.
  4. Fusion: Myeloma cells and B cells are fused using Polyethylene Glycol (PEG).     * PEG Mechanism: PEG binds a large number of water molecules, decreasing the chemical activity of water and dehydrating the lipid headgroups of the cell membranes, facilitating fusion.
  5. HAT Selection: Fused cells are grown in HAT medium to filter out unsuccessful fusions.
  6. Screening: Suitable cell lines that produce the desired antibody are identified.
  7. Multiplication: Cell lines are multiplied in vitro (culture) or in vivo (mouse ascites).
  8. Harvesting: The final monoclonal antibodies are collected.

Mechanisms of HAT Selection and HGPRT

  • HGPRT (Hypoxanthine-guanine phosphoribosyltransferase): A crucial enzyme in the Purine Salvage Pathway.     * It recycles purines (hypoxanthine and guanine) by attaching a chemical group (PRPP) to convert them into useful nucleotides (IMP and GMP).     * This allows cells to conserve energy instead of building purines from scratch (de novo pathway).
  • How HAT Media Works:     * Components: Hypoxanthine, Aminopterin, and Thymidine.     * Aminopterin: Blocks the de novo pathway for DNA synthesis.     * The Survival Scenarios in HAT Medium:         1. Unfused Myeloma Cells: Die. They are HGPRT-deficient (HGPRT-) and cannot use the salvage pathway when the de novo pathway is blocked.         2. Unfused B-Cells: Die. Although they are HGPRT-positive (HGPRT+), they have a naturally short lifespan and die off in culture.         3. Hybridoma Cells: Survive. They inherit immortality from the myeloma cell and the HGPRT enzyme from the B-cell, allowing them to use the salvage pathway to synthesize DNA while being immortal.

Modes of mAb Action in Therapy

  • Direct Action:     * Induce Apoptosis: Signaling the cell to undergo programmed death.     * Blocking Receptors: Binding to growth factor receptors to prevent growth signals.
  • Indirect Action:     * Recruitment: Recruiting cytotoxic immune cells to the target.     * Delivery: Carrying a toxic drug or radioactive isotope directly to the cell.
  • Types of Cancer mAbs:     1. Naked mAb: Unconjugated antibodies (e.g., ADCC - Antibody-Dependent Cellular Cytotoxicity).     2. Immunoconjugates: Antibodies linked to toxins (Immunotoxins) or radioisotopes (Radioimmunoconjugates).     3. Multistep Targeting: Using Biotinylated ligands and Streptavidin to localize treatment.     4. Bispecific mAbs: Antibodies that can bind to two different antigens simultaneously (e.g., a tumor cell and a killer T-cell).

Challenges and Engineering Solutions (HAMA)

  • The HAMA Problem: Human patients often see mouse-derived antibodies as foreign, mounting an immune response called HAMA (Human Anti-Mouse Antibodies).     * Consequences: Rapid elimination of the drug and potential kidney damage due to immune complexes.
  • Genetic Engineering Solutions:     * Chimeric Antibodies: Combine the mouse variable regions (antigen-binding) with human constant regions (effector parts).     * Humanized Antibodies: Replace only the mouse hypervariable regions (amino acids in the binding site) into a human antibody structure.     * Fully Human Antibodies: Produced using transgenic mice or phage display.

Diagnostic Applications

Pregnancy Tests (HCG Detection)
  1. Action: Urine containing hCG (Human Chorionic Gonadotropin) is applied to the wick.
  2. Mobile Antibodies: hCG binds to mobile antibodies tagged with an enzyme to form an hCG/antibody complex.
  3. Test Line: These complexes move up and bind to immobilized antibodies in the test window. The enzyme triggers a color change (positive result).
  4. Control Line: Excess mobile antibodies (not bound to hCG) move further up and bind to different immobilized antibodies in the control window. This confirms the test is functional regardless of pregnancy status.
Other Diagnostic Uses
  • Rapid Tests: COVID-19, Strep A, and Gonorrhea (GNH) tests utilize similar lateral flow mAb technology.
Specific Therapeutic Examples
  • CR6261: A monoclonal antibody that binds to a broad range of influenza viruses, including the 1918 "Spanish Flu" and H5N1 avian flu.
  • Herceptin (Trastuzumab):     * Target: Used for HER2-positive breast and stomach cancers.     * Mechanism: Binds to HER2 receptors to block growth signals, stop blood vessel recruitment (starving the tumor), and flag cells for destruction by immune cells.     * Administration: Often used alongside chemotherapy for up to one year.