Unit 04: Influenza A - Biology, Evolution, and Immune Response

Characteristics and Classification of Viruses

• Definition and Nature of Viruses

  • Viruses are categorized as intracellular parasites that depend entirely on a host cell to complete their life cycle.

  • The fundamental objective of a virus is to infiltrate a host cell and commandeer its metabolic pathways to synthesize new viral particles.

  • Viruses exhibit significant diversity in morphology and host specificity, targeting specific cell types based on surface interactions.

• Essential Viral Components

  • Genetic Material: All viruses contain a genome consisting of either DNA or RNA. This genome is relatively small, encoding only a limited number of genes compared to cellular organisms.

  • Capsid: This is a protective protein shell that encases the genetic material. Capsids often feature specialized proteins, such as spikes or tail fibers, which facilitate attachment to the membrane of a target host cell.

  • Envelope: Certain viruses, including influenza, possess an outer envelope derived from the plasma membrane of the previous host cell. While it provides an additional layer of protection and aids in cell entry, it does not function as a regulatory membrane for material transport like a true plasma membrane.

• The Biological Status of Viruses

  • Traditional biology utilizes five criteria to define life: energy and material processing, internal environmental maintenance (homeostasis), response to stimuli, reproduction, and evolutionary adaptation.

  • Viruses fail to meet the criteria for independent energy processing and material synthesis, as they require host epithelial cells (such as those in the respiratory tract) to reproduce.

  • Giant Viruses: The discovery of "giant viruses," such as the Mimivirus (approximately $400\,nm$) and Pithovirus (approximately $1.5\,\mu m$), has challenged historical classifications. These viruses approach the size of small bacteria (approx. $2\,\mu m$) and possess more complex genomes than typical viruses.

The Structure and Naming of the Influenza Virus

• Influenza Core Strains

  • There are four primary types of influenza: A, B, C, and D.

  • Influenza A and B are the most significant threats to human health.

• Nomenclature System

  • The World Health Organization (WHO) utilizes a standardized naming convention: Host (if non-human), Type ($A$ or $B$), Strain, and Year of Isolation.

  • Influenza A Classification: Further identified by the specific glycoproteins on their surface, known as Hemagglutinin ($H$) and Neuraminidase ($N$) spikes. These spikes are essential for identifying and entering host cells.

  • Influenza B Classification: Identified by lineage designations, specifically the $B/Yamagata$ or $B/Victoria$ lineages.

• Seasonal Vaccine Strains ($2023$–$2024$)

  • Modern flu vaccines are quadrivalent, meaning they target four specific viral versions.

  • Egg-based vaccine components for the $2023-2024$ season include:

    • $A/Victoria/4897/2022$ ($H1N1$) $pdm09$-like virus.

    • $A/Darwin/9/2021$ ($H3N2$)-like virus.

    • $B/Austria/1359417/2021$-like virus ($B/Victoria$ lineage).

    • $B/Phuket/3073/2013$-like virus ($B/Yamagata$ lineage).

  • Cell-culture or recombinant-based vaccines utilize similar strains, such as the $A/Wisconsin/67/2022$ ($H1N1$) $pdm09$-like virus.

Viral Reproduction and Life Cycles

• Viral Specificity

  • Capsid proteins have complex three-dimensional shapes that allow them to bind to specific receptors or molecules on the surface of target host cells.

• The Lytic Cycle

  • This cycle involves the immediate hijacking of the host cell for rapid viral production:

    1. Attachment: The virus identifies and binds to the target cell.

    2. Penetration: Genetic material is introduced into the cell via direct injection or endocytosis (where the cell engulfs the virus).

    3. Biosynthesis: The host's machinery is forced to manufacture viral proteins and replicate the viral genome.

    4. Maturation: Viral components are assembled into new virions.

    5. Release: New viruses exit the cell either via lysis (bursting) or exocytosis (budding). Enveloped viruses often acquire their envelope from the host's plasma membrane during this phase.

• The Lysogenic Cycle

  • In this cycle, the virus remains latent. The viral DNA integrates into the host's genome. As the host cell divides, it replicates the viral DNA alongside its own. The virus may eventually be triggered by environmental factors to enter the lytic cycle.

  • Approximately $8\%$ of the human genome is estimated to be viral in origin due to historical lysogenic activity.

• RNA Virus Processing

  • Because host cells use DNA as a template for protein synthesis, RNA viruses like influenza must provide their own mechanisms.

  • Influenza uses an enzyme called RNA polymerase to convert its RNA genome into mRNA for translation by host ribosomes.

Impact of Influenza on the Human Body

• Primary Infection Site

  • Influenza primarily targets the epithelial cells that line the respiratory tract (pharynx, trachea, and lungs).

• Role of Cytokines

  • Symptoms are often the result of the body's immune response, specifically the release of chemical signals called cytokines.

  • Digestive System: Cytokines reduce appetite centers in the brain, leading to nausea or vomiting.

  • Muscular System: Breakdown of fibers and fluid accumulation cause joint and muscle aches.

  • Nervous System: Cytokines signal the hypothalamus to increase body temperature, resulting in fever and headaches.

  • Integumentary System: To conserve heat during a fever, blood vessels in the skin restrict, causing chills.

Composition and Production of Flu Vaccines

• Vaccine Ingredients

  • Aluminum salts: Enhance the immune system's detection of the virus.

  • Antibiotics: Prevent bacterial contamination during storage.

  • Egg protein: Residual traces from the production process.

  • Formaldehyde: Inactivates toxins from the virus or bacteria.

  • Stabilizers (Gelatin/MSG): Protect the vaccine from temperature shifts.

  • Thiomersal: A preservative used in multi-dose vials.

• Production Methods

  • Egg-based Method: The most common approach; viruses are grown in fertilized chicken eggs, then killed and purified. It has the longest development time.

  • Recombinant Method: The fastest method, involving the production of only the $H$ spike protein using insect cell cultures. This creates egg-free vaccines and does not require the whole virus.

Principles of Evolution and Natural Selection

• Evolution in Biology

  • Evolution is a unifying theory describing how populations change over time through heritable genetic modifications.

  • Influenza demonstrates rapid evolution by moving between animal hosts (birds, pigs) and humans, exchanging genetic material.

• Natural Selection (Darwin and Wallace)

  • Variation: Heritable differences exist within a population.

  • Struggle for Existence: Populations produce more offspring than resources can support, leading to competition.

  • Fitness: Individuals with advantageous traits survive and reproduce at higher rates.

  • Adaptation: Over generations, favorable traits increase in frequency.

• Forms of Selection

  • Stabilizing: Favors average phenotypes; selects against extremes.

  • Directional: Shifts the population toward one extreme phenotype.

  • Disruptive: Favors both extremes; selects against the average.

Mechanisms of Influenza Virus Evolution

• Antigenic Drift

  • Small, gradual mutations in the RNA genome lead to minor changes in the surface spikes ($H$ and $N$). This can reduce the effectiveness of previous immunity or vaccines.

• Antigenic Shift

  • A more drastic change occurring when two different influenza strains infect the same host cell simultaneously. The strains exchange genetic segments, resulting in a novel virus with a new combination of spikes. This often leads to global pandemics because the human population has no prior immunity.

The Human Immune System

• Immune System Organs

  • Red Bone Marrow: Site of white blood cell production and $B$ cell maturation.

  • Thymus: Site where $T$ cells mature.

  • Lymph Nodes: Filter interstitial fluid (lymph).

  • Spleen: Filters the blood.

• Immune Recognition (Self vs. Non-Self)

  • The body uses Major Histocompatibility Complexes ($MHC$) as identification markers.

  • MHC Class I: Found on all nucleated body cells.

  • MHC Class II: Specifically on immune cells like macrophages and dendritic cells.

  • Antigen: Any foreign substance (usually a protein or carbohydrate) that triggers an immune response.

The Immune Response to Infection

• Innate (Nonspecific) Defenses

  • Physical Barriers: Skin, mucous membranes, stomach acid ($low\,pH$), and cilia.

  • Inflammatory Response: Release of histamine causes capillary leakage (slowing pathogens) and attracts phagocytes (neutrophils and macrophages) to destroy the invaders.

  • Cytokines: Including Interferons (viral alerts) and Interleukins (immune regulation).

• Adaptive (Specific) Defenses

  • Cell-Mediated Immunity: Cytotoxic $T$ cells target and kill infected body cells by inducing apoptosis through the release of perforins and granzymes.

  • Antibody-Mediated Immunity: $B$ cells differentiate into plasma cells that secrete antibodies. Antibodies bind to free pathogens, marking them for destruction.

  • Memory Cells: Both $B$ and $T$ memory cells stay in the body, ensuring a faster and stronger secondary response upon re-exposure to the same pathogen.

Future Directions in Influenza Research

• Antiviral Medications

  • These drugs target specific stages of the viral life cycle: Attachment, Uncoating (removal of capsid), or Release (escaping the host cell).

  • Tamiflu: Specifically inhibits the release phase of the influenza virus.

• Universal Flu Vaccine

  • Researchers are focusing on "core proteins" within the viral capsid. Unlike surface spikes, these proteins do not undergo rapid mutation, potentially allowing for a single vaccine that provides long-term protection against many different strains.