SARS-CoV-2-encoded ORF8 protein possesses complement inhibitory properties

Area of Science:

• Immunology
• Virology
• Biochemistry

Background:

• Hyperactivation of the complement system is a hallmark of severe COVID-19.
• Mechanisms by which SARS-CoV-2 evades complement-mediated elimination are not fully understood.

Purpose of the Study:

• To investigate the interaction between SARS-CoV-2 proteins and the human complement system.
• To elucidate the role of SARS-CoV-2 ORF8 protein in complement regulation.

Main Methods:

• Protein-protein interaction assays using human complement C3/C3b and SARS-CoV-2 ORF8.
• Functional assays to assess complement pathway activity (factor I-mediated proteolysis, factor B activation, hemolysis assay).
• In-silico protein-protein docking analysis.

Main Results:

• SARS-CoV-2 ORF8 protein directly binds to human complement C3/C3b.
• ORF8 inhibits the alternative complement pathway by blocking factor I-mediated proteolysis and factor B activation.
• ORF8 acts as a complement inhibitor with an IC50 of 2.3 μM in a hemolysis assay.
• In-silico analysis suggests ORF8 sterically hinders cofactor binding to C3b.

Conclusions:

• SARS-CoV-2 ORF8 protein is a novel inhibitor of the complement alternative pathway.
• ORF8 binding to C3b disrupts complement amplification, contributing to viral immune evasion in COVID-19.