Immune Responses to Viruses and Vaccination: Study Notes

Immune Responses to Viruses

Innate Immune Response to Intracellular Viruses

The innate immune system provides the first line of defense against viral infections, particularly those that infect cells (intracellular viruses). Key components include interferons (IFNs), natural killer (NK) cells, and inflammation.

• Interferons (IFNs): These are cytokines produced by virus-infected cells. They induce an antiviral state in neighboring cells, upregulate antigen presentation, and activate immune cells.

• Natural Killer (NK) Cells: NK cells are cytotoxic lymphocytes that recognize and kill virus-infected cells, especially those with reduced MHC class I expression.

• Inflammation: Viral infection triggers the release of cytokines and chemokines, leading to inflammation. This recruits immune cells to the site of infection and enhances the immune response.

Example: During influenza infection, infected respiratory epithelial cells release IFN-α and IFN-β, which help limit viral spread and activate NK cells.

NK Cells: Cytotoxic Lymphocytes of the Innate Immune Response

NK cells play a crucial role in the early defense against viruses by targeting infected cells that have downregulated MHC class I molecules.

• Activation: NK cells are activated by the absence or reduced expression of MHC class I on target cells.

• Functions: NK cells release perforin and granzymes to induce apoptosis in virus-infected cells.

• Mechanism: Perforin forms pores in the target cell membrane, allowing granzymes to enter and trigger programmed cell death (apoptosis).

Example: NK cells are important in controlling herpesvirus infections during the early stages before adaptive immunity is fully activated.

Inflammation and Cytokine Release

Inflammation is a hallmark of the innate immune response and is triggered by the release of cytokines from infected or damaged cells.

• Cytokines: Small proteins such as interleukins (ILs), tumor necrosis factor (TNF), and interferons (IFNs) that mediate and regulate immunity and inflammation.

• Role: Cytokines recruit immune cells to the site of infection, increase vascular permeability, and promote the elimination of pathogens.

Example: IL-1 and TNF-α are key cytokines that promote fever and inflammation during viral infections.

Activation and Function of CD8+ T Cells (Cytotoxic T Lymphocytes)

CD8+ T cells, also known as cytotoxic T lymphocytes (CTLs), are essential for the adaptive immune response to viruses. They recognize and kill virus-infected cells.

• Activation: CD8+ T cells are activated when their T cell receptor (TCR) recognizes viral peptides presented by MHC class I molecules on infected cells, along with co-stimulatory signals from antigen-presenting cells (APCs).

• Effector Function: Once activated, CD8+ T cells kill infected cells by releasing perforin and granzymes, similar to NK cells.

• Detection: CD8+ T cells survey cells for the presence of foreign (viral) peptides bound to MHC class I molecules.

Example: CD8+ T cells are critical for clearing cytomegalovirus (CMV) and other persistent viral infections.

Adaptive Immunity: B Cells and Antibodies

T-Dependent B Cell Activation and Antibody Function

B cells are activated by antigens with the help of T helper (Th) cells, leading to the production of antibodies that neutralize pathogens and facilitate their clearance.

• T-Dependent Activation: B cells present antigen to Th cells via MHC class II. Th cells provide co-stimulatory signals (e.g., CD40L-CD40 interaction) and cytokines, fully activating the B cell.

• Antibody Functions:

  • Neutralization of pathogens by binding to viral particles and preventing entry into host cells.

  • Opsonization, enhancing phagocytosis by macrophages and neutrophils.

  • Activation of the complement system, leading to lysis of pathogens.

Example: Antibodies against influenza virus hemagglutinin prevent viral entry into respiratory epithelial cells.

Vaccines and Immunization

Current Vaccine Example: mRNA COVID-19 Vaccine

Modern vaccines use various technologies to induce protective immunity. The mRNA COVID-19 vaccines (e.g., Pfizer-BioNTech, Moderna) are a recent advancement.

• Type of Vaccine: mRNA vaccine (contains messenger RNA encoding the SARS-CoV-2 spike protein).

• Administration Schedule: Typically two doses given 3-4 weeks apart, with possible booster doses.

• Immunity Generated: Induces both humoral (antibody-mediated) and cellular (T cell-mediated) immunity against the spike protein.

• Effectiveness: High efficacy in preventing symptomatic COVID-19 and severe disease.

Example: The Pfizer-BioNTech COVID-19 vaccine demonstrated over 90% efficacy in clinical trials.

Vaccine Misinformation and the Importance of High Vaccination Rates

Vaccine misinformation can undermine public health efforts. Understanding and addressing common myths is essential for maintaining high vaccination coverage and preventing disease outbreaks.

• Common Anti-Vaccine Myths:

  • "mRNA vaccines alter your DNA" – Myth: mRNA does not enter the cell nucleus or integrate into DNA; it is degraded after protein translation.

  • "Vaccines cause autism" – Myth: Extensive studies have found no link between vaccines and autism.

  • "Natural immunity is better than vaccine-induced immunity" – Myth: Vaccines provide safe, effective immunity without the risks of natural infection.

  • "Vaccines contain harmful toxins" – Myth: Vaccine ingredients are present in safe amounts and are rigorously tested for safety.

  • "COVID-19 vaccines were rushed and are unsafe" – Myth: mRNA vaccine technology has been in development for years, and vaccines underwent thorough clinical testing.

• Why High Vaccination Rates Are Important: High coverage creates herd immunity, protecting those who cannot be vaccinated and reducing the spread of infectious diseases.

Example: Measles outbreaks occur when vaccination rates drop below the herd immunity threshold.

Summary Table: Key Immune Components Against Viruses

Component	Type	Main Function	Example
Interferons (IFNs)	Innate	Induce antiviral state, activate immune cells	IFN-α/β in influenza
NK Cells	Innate	Kill virus-infected cells lacking MHC I	Control of herpesvirus
CD8+ T Cells	Adaptive	Kill infected cells presenting viral peptides	Clearance of CMV
B Cells/Antibodies	Adaptive	Neutralize virus, opsonization, complement activation	Antibodies to influenza