Human defensin peptide HNP1 binds to the SARS-CoV-2 Spike protein with more than 20-fold greater affinity than to blood proteins, destabilizing Spike and blocking viral infection in cell culture.
>20-fold binding preference for SpikeHNP1 defensin bound to the SARS-CoV-2 Spike protein with submicromolar affinity, more than 20 times stronger than its binding to serum albumin, enabling targeted viral neutralization.
What the researchers found
HNP1 bound to the SARS-CoV-2 Spike protein with submicromolar affinity — more than 20-fold stronger than its binding to serum albumin. Both HNP1 (an α-defensin) and retrocyclin RC-101 (a θ-defensin) interfered with Spike-mediated membrane fusion, Spike-pseudotyped lentivirus infection, and authentic SARS-CoV-2 infection in cell culture.
The mechanism involves defensins destabilizing and precipitating the Spike protein and inhibiting its interaction with the ACE2 receptor — the cellular entry point for SARS-CoV-2. This correlates with the known ability of defensins to unfold proteins with high conformational plasticity.
Serum reduced the antiviral activity of HNP1, likely due to competitive binding with serum proteins. However, at high concentrations, HNP1 still inactivated the virus even in serum, suggesting physiologically relevant antiviral potential.
Why it matters
Understanding how the body's own antimicrobial peptides fight SARS-CoV-2 illuminates natural defense mechanisms that work regardless of viral variants. Unlike vaccines and antibodies that target specific Spike protein sequences, defensins work by physically destabilizing the protein structure — a mechanism that could remain effective against future variants. This knowledge could inform the development of defensin-based antiviral therapies or nasal sprays for infection prevention.
How the study worked
The researchers used multiple in vitro assays: binding affinity measurements between defensins and Spike protein, Spike-mediated membrane fusion assays, Spike-pseudotyped lentivirus infection assays, and authentic SARS-CoV-2 infection experiments in cell culture. They tested the effects with and without serum to assess physiological relevance. Protein destabilization and precipitation assays characterized the mechanism of Spike inactivation.
What this study cannot tell us
All experiments were conducted in cell culture — in vivo antiviral efficacy in animals or humans has not been tested. Serum significantly reduced HNP1 activity, raising questions about whether physiological concentrations in the respiratory tract are sufficient for meaningful antiviral effects. The study does not address defensin stability in the airways or potential toxicity at the high concentrations needed to overcome serum inhibition. Only two defensins were tested against one virus strain.
How to read the evidence
This is an in vitro laboratory study using multiple complementary assays including authentic virus. The mechanistic evidence is strong and consistent across assay types, but no animal or human data exist to confirm in vivo relevance.
When this study was published
Published in 2022 during the COVID-19 pandemic, this study addressed a timely question about innate immune defense. The findings remain relevant as SARS-CoV-2 continues to circulate and new variants emerge.
The bigger picture
Defensins represent an ancient arm of immunity that predates adaptive immune responses. This study adds SARS-CoV-2 to the growing list of viruses that defensins can neutralize through protein destabilization. As concerns about emerging variants and pandemic preparedness persist, variant-agnostic antiviral approaches based on innate immune peptides could complement vaccines and monoclonal antibodies, which can be rendered less effective by mutations in the Spike protein.
Questions still open
- Are naturally occurring defensin levels in the respiratory tract high enough to provide meaningful protection against SARS-CoV-2 infection?
- Could defensin-based nasal sprays or inhalers be developed as a variant-agnostic COVID-19 prevention strategy?
- Do individuals with higher natural defensin levels experience milder COVID-19 outcomes?
Common questions
What are defensins and how do they fight viruses?
Could defensins replace vaccines for COVID-19 prevention?
Read the original research
Inhibition of SARS-CoV-2 Infection by Human Defensin HNP1 and Retrocyclin RC-101.
Journal of molecular biology, 434(6), 167225
Citation
Kudryashova, Elena; Zani, Ashley; Vilmen, Geraldine; Sharma, Amit; Lu, Wuyuan; Yount, Jacob S; Kudryashov, Dmitri S. (2022). Inhibition of SARS-CoV-2 Infection by Human Defensin HNP1 and Retrocyclin RC-101.. Journal of molecular biology, 434(6), 167225. https://doi.org/10.1016/j.jmb.2021.167225