Virus Cultivation and Assays

Virus Cultivation & Assays

Introduction to Virus Cultivation

  • Historical Context: For many years, viruses could not be routinely propagated in cultured cells. Most human viruses were grown in laboratory animals or plants, leading to limitations in research and vaccine development.

    • Examples of early cultivation methods include:

    • TMV (Tobacco Mosaic Virus) studied in tobacco plants.

    • Poliovirus, which was propagated by infecting non-human primates, paralyzing them, and then extracting brain tissue for virus stock production.

Virus Propagation Methods

  • Embryonated Chicken Eggs (1930s)

    • An important discovery allowing the cultivation of viruses. Many viruses can be propagated in embryonated chicken eggs.

    • Process involves drilling a hole in the egg shell 5 to 14 days after fertilization and injecting the virus into a specific replication site.

    • Different viruses are grown at distinct locations within the egg.

Egg Cultivation Process

  • Fluid Injection: 10ml of fluid is injected into the egg, providing a cushion for the embryo and lining cells for virus propagation. This method is primarily used for the influenza virus today.

  • Flu Vaccine Production: The majority of influenza virus vaccines are produced using this method, achieved through automated processes in factories. On average, 100-150 million eggs are used in the US to prepare the flu vaccine.

Discoveries in Cell Culture

  • John Enders, Thomas Weller, and Frederick Robbins (1949)

    • Key discovery made at Harvard Medical School that poliovirus could multiply in human cell cultures.

    • Sparked a revolution in virus study and vaccine preparation (polio vaccine introduced in 1954, followed by measles and rubella).

    • Primary cultures derived from embryonic tissues were crucial in this advancement, earning the Nobel Prize in 1954.

Types of Cell Culture

  • Primary Cultures

    • Prepared directly from animal tissues with a limited lifespan of 5 to 20 cell divisions.

    • Examples include:

    • Primary human foreskin fibroblasts

    • Mouse fibroblast cell line (3T3)

    • Human epithelial cell line (HeLa)

    • Continually used as a method for virus propagation.

  • Continuous Cell Lines

    • Consist of one cell type that can be propagated indefinitely in culture.

    • Commonly derived from tumor tissues or treated with mutagenic chemicals.

    • Not suitable for vaccine production due to aneuploidy and potential tumorigenicity.

Cell Culture Techniques

  • Adhering Cells: Cells adhere to culture dish bottoms; viruses are added to these cells for infection.

  • Suspension Cultures: Cells may be maintained in suspension, utilizing spinning magnets for agitation, allowing for the production of large quantities of virus suitable for large-scale applications like X-ray crystallography.

HeLa Cells

  • Originated in the 1950s from Henrietta Lacks, a young woman whose tumor cells divided indefinitely in culture—marking a significant breakthrough in cell cultivation, albeit surrounded by ethical controversy.

Advances Toward Organoid Cultures

  • Traditional cell cultures are often significantly different from the actual biological tissues. Recent techniques focus on:

    • Blastocyst-Derived Cultures: Utilizing the inner cell mass.

    • Somatic Cell Cultures: Further developing closer representations of organ structures, such as lung epithelium through air-liquid interface cultures that differentiate into a variety of cell types, including ciliated and mucus-producing cells.

Cytopathic Effects (CPE)

  • Identification: Cytopathic effects can be observed under light microscopes, where infected HeLa cells show visible changes upon exposure to poliovirus.

  • Observations include:

    • Rounding up and detachment of cells from culture dishes

    • Swelling of nuclei and eventual cell lysis.

Specific Examples of CPE

  • SARS-CoV-2 Effects: Phase-contrast microscopy revealing effects on Vero cell monolayers from nasopharyngeal specimens showcasing syncytia (giant cells with multiple nuclei) indicating viral infection and fusion processes induced by the virus.

Measurement of Infectious Units

  • Methods to assess viral infectivity include:

    • Plaque Assays: Developed in the 1930s, initially for bacteriophages, it allows rapid measurement of viral titers. Each plaque represents a clear zone formed by lysis of cells infected by a single virus.

    • Dulbecco's Modification: Adaptation for animal viruses in 1952, which involved using a monolayer of cells instead of agar plates.

    • Calculation of titer is typically in plaque-forming units per milliliter (PFU/ml).

Advanced Plaque Assay Techniques

  • Serial Dilutions: Virus stocks are diluted and plated on cell monolayers, allowing for the calculation of titer based on the resulting plaques.

  • Visual impacts of viral infections on cell cultures are confirmed through staining methods that highlight dead or damaged cells.

Infection Kinetics and Growth Cycles

  • One-Step Growth Cycle: Method originated by Ellis & Delbruck in 1939 to analyze bacteriophages. This approach studies viral reproduction in cells by synchronizing infections.

  • Multiplicity of Infection (MOI): The ratio of the number of infectious virus particles added per cell (calculated as PFU/cell), influencing the infection outcomes and number of plaques formed.

  • Graphs: Illustrate either one-hit or two-hit kinetics, providing insight into viral replication mechanisms and the relationship between infection dose and plaque formation.

Measurement Techniques for Virus Detection

  • Hemagglutination Assay: Utilizes the hemagglutinin glycoprotein on influenza viruses that binds to red blood cells. Visible agglutination signals the presence of the virus and is commonly used for quick assessments.

  • Enzyme-linked Immunosorbent Assay (ELISA): Tests detect viral proteins or antibodies via direct and indirect immunoassays used for a range of viral diseases including influenza, HIV, and SARS-CoV-2.

  • Deep High-Throughput Sequencing: Techniques employed in metagenomics to sequence viral samples directly, allowing for rapid pathogen identification and genomic studies of emerging viruses such as SARS-CoV-2.

Phylogenetic Analysis and Viral Evolution

  • Phylogenetic Trees: Maps genetic relationships among organisms. Primarily used to assess evolutionary distances and track viral lineage variations.

    • Each division indicates a node (common ancestor) providing insight into genetic changes over time.

Conclusion

  • The constant advancement in virology research, including cultivation methods, genetic analysis, and infectious disease management, shapes modern understanding and approach to viral pathogenesis and treatment strategies.

Note: All procedures, measurements, and methodologies regarding virus cultivation, CPE evaluation, and assays have immense practical implications for vaccine development and public health responses to viral outbreaks.