Source: New type of host defence against Zika, dengue viruses revealed (The Hindu, April 24, 2024)
The presence of a virus in saliva or semen doesn’t necessarily mean it transmits that way. Scientists recently unraveled this mystery. Our bodies produce decoy sacs called extracellular vesicles, brimming with proteins viruses use for entry. These abundant sacs block the virus, preventing infection and unconventional transmission.
This discovery explains why some viruses like Zika linger in unexpected fluids and opens doors for future research on viral defense and evolution.
Understanding Viral Transmission
Presence of Virus in Bodily Fluids:
- Zika, dengue, and chikungunya viruses found in saliva and semen.
- Not spread orally or sexually.
- Research team explains the puzzle.
Virus Behavior Inside the Body:
- Viruses present in bodily fluids contacting the environment.
- Viruses need to be in the right location to infect new cells.
- Viruses are selective in the cells they infect due to tropism.
Tropism and Cell Receptors:
- Viruses have proteins on their surface that interact with host cell receptors.
- HIV infects cells making CD4 protein.
- SARS-CoV-2 uses ACE2 receptor.
- Different viruses target different cell types.
Viral Transmission Dynamics:
- Viral transmission involves a race between virus replication and the immune system.
- Viruses must transmit before being destroyed by the immune system or the host dies.
- Some viruses have proteins with receptors on multiple cell types, facilitating faster transmission.
The PS Receptor
What is the PS receptor?
- The PS receptor is for a protein called phosphatidyl serine (PS).
- PS protein is expressed by dying cells as a signal for immune cells to destroy them.
- Immune cells fuse with dying cells to quietly destroy them.
Viral Hijacking:
- Viruses use apoptotic mimicry to hijack the PS pathway.
- They express PS protein on their surfaces.
- This allows them to infect cells targeted for destruction.
Challenges in Studying Viral Transmission:
- Complexity of PS Receptor
- Viruses using the PS receptor for entry pose difficulties.
- PS receptor is expressed by many cells, not just immune cells.
- Virus presence in compartments doesn’t guarantee transmission.
- Example: Zika Virus
- Zika virus detected in semen, saliva, and breast milk.
- Rarely spreads through these means despite target cells’ presence.
- Zika mainly transmits via mosquitoes.
Body’s Defense Mechanism against Viral Infection
Extracellular Vesicles as Defense Mechanism
- Identified by scientists at the Institute of Molecular Virology.
- Found in saliva and semen.
- Contain PS proteins that hinder viral infection.
Function of Extracellular Vesicles
- Transport substances within cells.
- When secreted, they’re called extracellular vesicles.
- Abundant in bodily fluids.
Competition with Viruses
- PS-containing vesicles compete with viruses for entry.
- They crowd out viruses from receptors.
- Prevent viral infection.
Implications of the Discovery
Impact on Different Viruses
- PS-coated vesicles inhibit viruses using PS receptors.
- Doesn’t affect viruses using other receptors.
- Dengue, chikungunya, West Nile, ebola, and vesicular stomatitis viruses inhibited.
Therapeutic Potential
- Novel host defense mechanism.
- Opens avenues for further research.
- Potential therapeutic applications yet to be explored.
Evolutionary Implications
- PS-containing vesicles may have influenced virus evolution.
- Viruses may have adapted alternative transmission routes.
- Speculative but intriguing possibility.