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Viral interferon mode pf action 5 marker

Interferons are cytokines released by virus-infected cells that signal neighboring cells to activate antiviral defenses. They work through JAK-STAT signaling to upregulate antiviral proteins like PKR, OAS, and Mx proteins.
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Here is a complete 5-marker answer on the Mode of Action of Viral Interferons:

Viral Interferon - Mode of Action

(Medical Microbiology 9e; Sherris & Ryan's Medical Microbiology, 8e)

The 5-Step Mechanism (Exam Answer)

Step 1 - Induction & Release

A virus-infected cell (or plasmacytoid dendritic cell) detects double-stranded RNA (dsRNA) produced during viral replication. This is the major trigger for synthesis and release of IFN-α and IFN-β (Type I interferons) within hours of infection. A single dsRNA molecule per cell is sufficient to trigger this response.

Step 2 - Binding to Neighbouring Cells

Released interferon does not protect the already-infected cell. Instead, it binds to specific cell-surface receptors (IFNAR) on uninfected neighbouring cells via JAK/STAT signalling. This is the "early warning system" for surrounding cells.

Step 3 - Establishment of the "Antiviral State"

Via JAK-STAT signalling, interferon upregulates the synthesis of three key antiviral proteins inside the neighbouring cell:
  • Protein Kinase R (PKR)
  • 2',5'-Oligoadenylate Synthetase (OAS)
  • Mx protein (especially important for influenza)
These proteins are produced but remain inactive until dsRNA is detected inside that cell (i.e., until that cell itself becomes infected).

Step 4 - Activation by Viral dsRNA & Enzymatic Cascades

When that cell is subsequently infected, viral dsRNA activates the antiviral enzymes through two parallel pathways:
PathwayEnzymeMechanism
Pathway 1PKRPhosphorylates eIF-2α (elongation initiation factor) → blocks ribosome assembly → inhibits protein synthesis
Pathway 2OAS → RNase L2',5'-OAS synthesizes 2',5'-oligoadenylate → activates RNase L → degrades viral mRNA
Pathway 3Mx proteinBlocks influenza by inhibiting viral transcription directly

Step 5 - Immune Activation & Systemic Effects

Interferons also amplify immune defenses:
  • Activate NK cells for early killing of infected cells
  • Upregulate MHC class I on cell surfaces → better antigen presentation → enhanced killing by cytotoxic T lymphocytes (CTLs)
  • Trigger systemic flu-like symptoms (fever, myalgia, malaise) via hypothalamic effects

Diagrams

Fig 1 - Induction of antiviral state (Medical Microbiology 9e):
Interferon antiviral state induction diagram
Fig 2 - Dual pathway cascade (Sherris & Ryan's Medical Microbiology 8e):
Virus-induced interferon pathways flowchart

Key Points for 5 Markers

  1. Interferon does NOT directly block virus - it induces a protective state in uninfected cells
  2. dsRNA is the key trigger for both interferon production AND activation of antiviral enzymes
  3. Two main enzymatic pathways: PKR (protein synthesis inhibition) + OAS/RNase L (mRNA degradation)
  4. Mx protein is specifically relevant to influenza
  5. The antiviral enzymes remain latent in uninfected cells - dsRNA from active infection is required to switch them on (prevents damage to healthy uninfected cells)
Sources: Medical Microbiology 9e (Sherris & Ryan), Ch. 10; Sherris & Ryan's Medical Microbiology 8e, Ch. 7
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