Application of Antibodies

Antibody Structure and Composition

Composition of Antibodies

• Antibodies consist of two light chains and two heavy chains that are linked together with covalent disulfide (S-S) bridges.

• The structure includes:

  • Variable Regions (V): Responsible for the antigen-binding activity.

  • Constant Regions (C): Responsible for the effector function activity.

Antibody Specificity

Determination of Specificity

• Antibody specificity is determined by the variable region of both heavy and light chains.

• The Complementarity-Determining Regions (CDR) are highlighted as the most variable parts of the antibody, which are crucial for specificity.

Immunoglobulin (Ig) Classes and Subclasses

Classification of Light and Heavy Chains

• Light Chains:

  • VL (Variable Light) and CL (Constant Light), e.g., Cκ or Cλ.

• Heavy Chains:

  • VH (Variable Heavy) and CH (Constant Heavy), e.g., Cμ, Cδ, Cγ, Cα, Cε.

Table of Antibody Isotypes and Functions

Functions of Antibodies

Activities of Antibodies

• Neutralization: Blocking the interactions with cellular receptors.

• Opsonization: Enhancing pathogen ingestion by phagocytes and activating the complement system leading to pathogen lysis.

• NK Cell Activation: Triggered by binding to Fc receptors.

• Mast Cell Activation: Occurs via interaction with IgE receptors.

Immune Response Against Pathogens

Overview of Immune Response

1. Pathogen Neutralization: Antibodies bind and neutralize pathogens.

2. Recognition by Macrophages: Recognizing pathogens for phagocytosis.

3. Phagocytosis by Macrophages: Breakdown of microbial cells, enhancing the immune response.

4. Activation of T and B Cells: Includes a polyclonal response where naive T cells activate B cells, leading to plasma cells that produce antibodies specific to the pathogen.

Polyclonal vs. Monoclonal Antibodies

Characteristics of Monoclonal and Polyclonal Antibodies

Monoclonal Antibodies:

• Derived from a single clone of B cells.

• Bind to a single epitope, displaying high specificity.

• Higher production times and costs due to complex manufacturing processes.

• Utilized in precision-focused research applications such as diagnostic assays (ELISA, Western blot) and therapeutics (e.g., targeted cancer therapies).

Polyclonal Antibodies:

• Derived from multiple clones of B cells.

• Recognize multiple epitopes, providing broader specificity.

• Versatile and widely applicable in various research applications.

• Quicker and more cost-effective to produce.

• Common uses include immunohistochemistry (IHC), immunofluorescence (IF), Western blot, and early diagnostic tests.

Monoclonal Antibody Production Process

Steps to Generate Monoclonal Antibodies

1. Immunization of mice with cancer-specific antigens to stimulate antibody production.

2. Isolation of antibody-secreting plasma cells.

3. Fusion of plasma cells with myeloma cells to generate hybridomas.

4. Selection in HAT medium followed by ELISA screening to identify successful hybridomas.

5. Expansion of selected hybridomas to produce monoclonal antibodies.

Applications of Antibodies

Basic Research and Diagnostics

Diagnostic Applications:

• Monoclonal antibodies are extensively used in medical diagnostic tests due to their high affinity and selectivity.

• Common diagnostic applications include:

  • Enzyme-Linked Immunosorbent Assay (ELISA): Techniques used to detect proteins in fluid samples (e.g., blood), characterized by color change in the presence of the target.

  • Western Immunoblotting: Technique for detecting and quantifying proteins in complex mixtures.

  • Immunohistochemistry: Staining of tissue sections for distribution mapping of cells.

  • Flow Cytometry: Detecting cell surface or intracellular markers using fluorescently labeled antibodies.

Immunohistochemistry/Fluorescence

Technique Overview:

• Used to identify proteins in cells or tissues which requires:

  • Fixed cells or tissues

  • A directly labeled antibody or indirect second antibody.

• Utilizes microscopy for detection.

Example Image Analysis:

• 7-color cross-section of a Rhesus Macaque lymph node showing various immune cell markers visualized via confocal fluorescence imaging.

DAB Immunohistochemistry Procedure

Steps:

1. Sample collection and preparation.

2. Conducting the immunohistochemistry assay.

3. Microscopy and data analysis to detect stained cells, which appear brown under the microscope as a result of DAB precipitate formation.

Enzyme-Linked Immunosorbent Assay (ELISA)

Overview of ELISA Procedure:

• Used in viral diagnostics or cytokine detection requiring:

  • Pure preparation of a known antigen (standard) and two antibodies for binding.

  • An enzyme linked chemically to the second antibody.

• Detection occurs via a color change reaction.

Detailed Steps of ELISA:

1. Wells are pre-coated with the capture antibody, and the sample is added.

2. The capture antibody binds to the antigen with high specificity.

3. The primary antibody binds the immobilized analyte.

4. Biotin-labeled detection antibody binds to the analyte.

5. Streptavidin-HRPO binds to biotin and catalyzes an enzymatic color reaction with TMB substrate, yielding a colored product.

Western Blotting Technique

Overview and Purpose:

• A method for identifying proteins in a cell lysate.

• Cells are lysed with detergent, and proteins are separated by SDS-PAGE, followed by detection using enzyme-labeled antibodies.

• Applications are found in both basic research and clinical diagnostics.

Steps of Western Blotting:

1. Lysing of cells to release protein content.

2. Protein suspension preparation and SDS-PAGE for separation.

3. Electrotransfer of proteins to a PVDF membrane.

4. Antibody probing for specific proteins.

5. Chemi-imaging to visualize the protein of interest.

Flow Cytometry Technique

Overview:

• Flow cytometry detects individual cells passing through a laser beam, allowing for analysis of cell surface and intracellular proteins.

• Requires:

  • A cell suspension.

  • Fluorescently labeled antibodies.

Procedure Steps:

1. Preparation of Cell Pellet for analysis.

2. Staining for specific surface or intracellular antigens.

3. Sorting of cells as they pass through the laser, creating droplets containing individual cells.

4. Analysis of data for population of interest based on scatter and fluorescence.

Therapeutic Applications of Antibodies

Therapeutic Uses of Monoclonal Antibodies:

• Antibody therapy primarily relies on antibodies produced through biotechnology methods, specifically monoclonal antibodies from hybridoma technology developed by Köhler and Milstein in the 1970s.

• The production of chimeric, humanized, and human antibodies through antibody engineering has allowed for broader therapeutic applications, particularly for conditions such as autoimmune diseases or cancer.

Process of Therapeutic Monoclonal Antibody Production:

• Steps similar to those for diagnostic applications, emphasizing the need for antibody screening and selection for therapeutic efficacy.

Evolution of Antibody Products

Development Timeline:

1. From mouse hybridoma technology to more advanced in vitro antibody libraries.

2. The introduction of transgenic mouse models and the commercialization of human hybridomas.

3. Development of chimeric, humanized, and fully human antibodies for therapeutic applications.

Application of Monoclonal Antibodies in Cancer Therapy

Impact of Monoclonal Antibodies:

• Monoclonal antibodies have significantly impacted cancer therapy, achieving targeted therapies that recognize proteins on cancer cells. This induces activation of immune effector cells through processes like antibody-dependent cell-mediated cytotoxicity (ADCC) and antibody-dependent phagocytosis (ADPD).

• Example Treatments:

  • Rituximab: Used for treatment of B cell leukemia, showcasing the mechanism of action via interaction with CD20 antigens on B cells.

Mechanism of Action for Rituximab:

• Illustrated interactions include:

  • Binding of Rituximab to B-cell CD20, which aids in immune cell engagement (e.g., macrophages and NK cells).

Herceptin (Trastuzumab) Mechanism of Action:

• Works by binding to HER2 receptors on breast cancer cells, preventing HER2 shedding and correlating with improved therapeutic outcomes.

Keytruda Mechanism of Action:

• Functions as a checkpoint inhibitor by blocking PD-1 receptors on T cells, thereby enhancing immune responses against tumors, reducing tumor progression by approximately 40% in melanoma patients compared to standard chemotherapy.

Monoclonal Antibodies Against Amyloid Proteins

Alzheimer’s Disease Therapy:

• Clinical trials are investigating whether monoclonal antibodies can prvent the toxic clumping of amyloid-β proteins implicated in Alzheimer’s disease.

• Each drug targets different phases of amyloid aggregation, including:

  • Donanemab: Binds to plaques.

  • Lecanemab: Binds to protofibrils.

  • Solanezumab: Stops initial aggregation phase.

  • Gantenerumab: Blocks fibril elongation.

  • Aducanumab: Blocks second phase of aggregation.

Summary of Key Points

• The structure and function of antibodies are closely interconnected, with diverse applications ranging across basic research, diagnostics, and therapy.

• Monoclonal antibodies are illustrated as valuable tools for immunologists, and advancements in antibody engineering have led to highly effective treatments for a wide array of diseases, from cancer to neurological conditions such as dementia.