SARS-CoV-2 Uses Nonstructural Protein 16 to Evade Restriction by IFIT1 and IFIT3

Area of Science:

• Virology
• Immunology
• Drug Discovery

Background:

• Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) employs mechanisms to evade the host innate immune system, a critical factor in disease pathogenesis and therapeutic development.
• The nonstructural protein 16 (NSP16) of SARS-CoV-2 functions as a ribonucleoside 2'- O methyltransferase (MTase), essential for mRNA capping and potentially protecting the virus from host restriction factors.

Approach:

• Engineered a SARS-CoV-2 mutant with a disrupted NSP16 active site to investigate its role in viral infection and pathogenesis.
• Utilized an *in vitro* system and a hamster model to assess the *in vivo* and *in vitro* attenuation of the NSP16 mutant.
• Investigated the role of interferon-stimulated genes (ISGs), specifically IFIT1 and IFIT3, in mediating the antiviral response against the NSP16 mutant.

Key Points:

• The NSP16-mutant SARS-CoV-2 exhibited significant attenuation *in vitro* and *in vivo*, with reduced viral replication and disease in hamsters.
• Mechanistically, the NSP16 mutant demonstrated increased sensitivity to type I interferon (IFN-I) *in vitro*.
• Silencing IFIT1 or IFIT3 partially restored viral fitness to the NSP16 mutant, confirming their role in sensing the lack of 2'- O methylation.

Conclusions:

• SARS-CoV-2 NSP16 plays a critical role in evading host innate immunity, particularly the type I interferon response.
• Targeting NSP16 activity with methyltransferase inhibitors, such as sinefungin, in combination with type I interferon, represents a promising strategy for developing novel antiviral therapies against SARS-CoV-2.