Researchers find a new way to supercharge the body’s natural antiviral defenses

A protein released by infected cells can help neighboring cells prepare their defenses against respiratory syncytial virus, or RSV. Adding more of that protein before infection reduced infectious virus levels roughly tenfold in laboratory experiments. The finding points toward a possible antiviral strategy that strengthens cells’ defenses instead of attacking the virus directly.

Washington State University researchers and collaborators describe the mechanism in Cell Communication and Signaling, which has published an early, peer-reviewed version of their study. Their work centers on ISG15, a protein already associated with antiviral immunity. The advance explains how ISG15 outside cells can activate a distinct protective response inside them.

The results come from cultured cells, rather than treatment trials in people. Importantly, the same study found that added ISG15 did not change SARS-CoV-2 replication in the tested cells. That difference makes the pathway a promising research target, while showing why broad protection against many viruses cannot yet be assumed.

ISG15 takes its name from interferon-stimulated gene 15. Interferons are immune signaling proteins that help cells respond to infection by increasing production of defensive molecules. Inside cells, enzymes attach ISG15 to other proteins through a process called ISGylation, which can contribute to antiviral defense.

Scientists also knew that cells release unattached ISG15 into their surroundings. Previous research had identified signaling roles for this extracellular protein, including effects on immune cells. The WSU-led team investigated whether it could also trigger antiviral ISGylation in epithelial cells, which form protective tissue linings.

“We found evidence this pathway was being activated during infection, but the activation was not very robust,” said Santanu Bose, a professor in WSU’s College of Veterinary Medicine and a senior author. The team then tested whether adding laboratory-produced, recombinant ISG15 could strengthen that response. This protein provided an experimental way to increase the signal beyond the levels generated during infection.

Graduate students Lindsay Grace Miller and Sudiksha Pandit, together with Fulbright postdoctoral scholar Kim Lam Chiok, led the work in Bose’s laboratory. The three share first authorship on the paper. Collaborators included researchers at the University of Idaho, Louisiana State University, the University of South Florida and the University of Wisconsin–La Crosse.

The researchers first infected human A549 lung epithelial cells with RSV and detected ISG15 in the surrounding culture fluid. After 48 hours, that fluid contained about 17 nanograms of ISG15 per milliliter. Tests showed little evidence of cells rupturing, suggesting the protein’s release did not primarily result from infected cells breaking apart.

Adding recombinant ISG15 to cells increased both intracellular ISG15 and ISGylation. Even the concentration measured during infection could produce that response, although subsequent experiments often used higher concentrations to investigate the mechanism. The findings connected the naturally released protein with an experimentally amplified cellular response.

To test whether this required conventional interferon signaling, the team used several complementary approaches. These included cells lacking key interferon signaling components, cells unable to produce type-I interferons, and chemical or antibody-based blockers. ISG15 still stimulated the response under those conditions, supporting a route that operates independently of the usual interferon pathway.

Microscopy also detected ISG15 responses in uninfected cells beside infected ones. Neutralizing extracellular ISG15 with an antibody reduced those responses, supporting its role as a signal between cells. The evidence fits a model in which infected cells help prepare neighboring cells before the virus reaches them. Exactly how ISG15 exits infected cells remains unresolved, however, and the authors identify that secretion process as another research question.

The outside protein did not simply enter cells to supply extra ISG15. Instead, it interacted with a cell-surface receptor called α5β1 integrin. That interaction activated focal adhesion kinase, or FAK, a protein involved in transmitting signals from the cell surface.

Blocking FAK or interfering with the integrin interaction weakened the ISGylation response. Reducing the receptor’s abundance also impaired the response. Related experiments in primary human bronchial epithelial cells supported the pathway’s signaling activity, extending that finding beyond the main laboratory cell lines.

Molecular modeling helped the team identify a region of ISG15 involved in receptor binding. Changing one amino acid in that region weakened its ability to bind the integrin and activate the response. The altered protein also provided less protection against RSV, connecting the proposed binding mechanism with antiviral activity.

Inside cells, the signal worked partly by preserving ISG15 already present. Experiments that blocked new protein production showed that extracellular ISG15 slowed the breakdown of the intracellular protein. This increased its availability for ISGylation without increasing activity of the gene encoding ISG15.

The researchers tested antiviral effects by treating cells with recombinant ISG15 before exposing them to RSV. Measurements of infectious virus showed an approximately tenfold reduction in both human A549 cells and interferon-production-deficient VeroE6 cells. Treated cultures also showed less virus-associated cell damage. Measuring infectious virus helped establish that the effect extended beyond changes in a fluorescent marker used to track infection.

Experiments that interfered with the natural signal produced the opposite result. Neutralizing extracellular ISG15 during RSV infection increased infectious virus levels approximately fifteenfold in VeroE6 cells. Reducing cells’ own ISG15 also removed the added protein’s protective effect, showing that the intracellular protein remained important.

SARS-CoV-2 provided a significant counterexample to those encouraging results. Added ISG15 did not alter its replication in either of the two cell types tested for that virus. The authors suggest that a viral enzyme capable of removing ISG15 modifications may help explain this resistance, although that explanation remains an interpretation.

Bose’s laboratory also reports antiviral effects against influenza A and parainfluenza viruses in additional, unpublished work. Those findings remain separate from the published RSV evidence. Independent evaluation of those results will be needed to establish how far the mechanism extends across respiratory viruses. A host-directed approach might reduce some opportunities for drug resistance, but the experiments do not establish protection against future viral mutations.

WSU reports patent and commercialization work, including a licensing option involving a biotechnology company. Such activity supports further development, without demonstrating that a treatment is safe or effective in patients. The reported antiviral experiments mainly tested preparation before infection, leaving effectiveness after an infection begins as an important question.

Possible applications in cancer or neurological disorders are more preliminary still. ISGylation participates in those conditions, but its effects depend on biological context, and the paper notes links with tumor development. The study therefore does not establish that increasing this pathway would benefit people with either type of disease.

Several mechanistic questions also remain unanswered. The researchers have not determined exactly which stages of RSV’s life cycle the response disrupts, or all the signaling steps connecting the receptor to antiviral protection. Resolving those gaps will help define where this cellular defense can be strengthened usefully and where viruses can evade it.

These resources explain ISG15’s extracellular signals, its wider immune functions and the ways viruses interfere with its activity.

ISG-15, beyond its functions in the cell: a mini review
This review examines how cells release ISG15 and how its extracellular functions vary across infections and species. (Cellular and Molecular Life Sciences, July 2025)

Papain-like protease regulates SARS-CoV-2 viral spread and innate immunity
This research investigates a coronavirus enzyme that removes ISG15 modifications and interferes with antiviral immune responses. (Nature, July 2020)

Modulation of Extracellular ISG15 Signaling by Pathogens and Viral Effector Proteins
Experiments explore how pathogens and viral proteins alter ISG15 secretion and extracellular signaling. (Cell Reports, June 2020)

ISG15 in antiviral immunity and beyond
This review describes ISG15’s diverse, pathogen-dependent activities, including antiviral effects and roles in immune regulation and tissue responses. (Nature Reviews Microbiology, May 2018)

Extracellular ISG15 Signals Cytokine Secretion through the LFA-1 Integrin Receptor
This study identifies an ISG15 receptor on immune cells, providing background for understanding how the protein signals outside cells. (Molecular Cell, November 2017)

Research findings are available online in the journal Cell Communication and Signaling.

The original story "Researchers find a new way to supercharge the body’s natural antiviral defenses" is published in The Brighter Side of News.

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