Effects of virus infection on cells, cancer and viruses

Viruses are obligate intracellular parasites, meaning they can multiply only inside living cells.
When a virus infects a host cell, it can cause a wide range of effects — from no visible damage (latent infection) to cell destruction or transformation into a cancer cell.

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The interaction between a virus and its host cell is therefore central to understanding viral pathogenesis, immunity, and oncogenesis (cancer formation).

2. Stages of Virus–Cell Interaction

1. Attachment (Adsorption):
   The virus binds to specific receptors on the host cell membrane.
   Example: HIV binds to CD4 receptors on T-helper cells.
2. Penetration:
   The virus or its genetic material enters the cell via endocytosis or membrane fusion.
3. Uncoating:
   The viral capsid is removed, releasing the nucleic acid into the cytoplasm.
4. Replication and Synthesis:
   The virus uses host machinery to synthesize its nucleic acids and proteins.
5. Assembly (Maturation):
   Newly synthesized viral components assemble into complete virions.
6. Release:
   Mature viruses exit the cell by lysis (cell destruction) or budding (without immediate cell death).

Each of these stages may alter cellular structure or function, producing the effects of viral infection described below.

3. Effects of Virus Infection on Host Cells

The consequences of viral infection depend on:

• The type of virus
• The host cell type
• The immune response
• The number of infecting virions

A. Cytopathic Effects (CPE)

These are visible morphological and biochemical changes in infected cells, observable under the microscope.

B. Physiological and Biochemical Changes

1. Inhibition of host DNA, RNA, and protein synthesis
   (The virus uses the cell’s machinery for its own replication.)
2. Alteration of membrane permeability
   Causes ion imbalance and cell swelling.
3. Cytoskeletal disruption
   Leads to abnormal cell morphology.
4. Interference with cell cycle regulation
   Some viruses push the cell into continuous division.
5. Release of lysosomal enzymes
   Contributes to cell death.

C. Types of Viral Infections in Cells

4. Cancer and Viruses (Oncogenic Viruses)

Oncogenic viruses are viruses capable of inducing cancer (tumors) in animals or humans.
They alter the normal regulatory mechanisms of cell growth, causing uncontrolled cell division.

B. Mechanisms of Viral Oncogenesis

1. Insertional Mutagenesis (Integration of Viral DNA)
   1. Viral genome integrates into host DNA and disrupts normal gene regulation.
   1. Example: Hepatitis B virus (HBV) and Human papillomavirus (HPV).
2. Activation of Oncogenes (proto-oncogenes → oncogenes)
   1. Viral genes activate cellular proto-oncogenes that regulate growth and differentiation.
3. Inactivation of Tumor Suppressor Genes
   1. Viral proteins inactivate tumor suppressor genes such as p53 and Rb.
   1. Example: HPV E6 and E7 proteins block p53 and Rb.
4. Chronic Inflammation and Regeneration
   1. Long-term viral infection leads to cell damage, inflammation, and increased mutation rate.
   1. Example: Hepatitis C virus (HCV) → liver cirrhosis → hepatocellular carcinoma.
5. Expression of Viral Oncogenes (v-onc genes)
   1. Some viruses carry their own oncogenes that directly transform cells.
   1. Example: Rous sarcoma virus (RSV) carries src gene.

C. Classification of Oncogenic Viruses

1. DNA Oncogenic Viruses

2. RNA Oncogenic Viruses

D. Viral Genes Involved in Oncogenesis

E. Examples of Virus-Induced Cancers

5. Outcomes of Virus–Cell Interaction

6. Detection of Viral Transformation

Laboratory indicators of viral transformation include:

• Loss of contact inhibition (cells pile up)
• Altered cell morphology (rounded, refractile)
• Chromosomal abnormalities
• Tumor formation in experimental animals
• Expression of viral antigens on cell surface

7. Prevention and Control

1. Vaccination – HPV and HBV vaccines reduce cancer risk.
2. Screening programs – For early detection (Pap smears, liver screening).
3. Antiviral therapies – Suppress chronic viral replication (e.g., interferon, antivirals).
4. Public health measures – Safe blood transfusions, needle safety, sexual hygiene.