The ISG15 Story: Uncovering a Versatile Immune Defender Against Listeria
Every year, foodborne pathogens cause millions of illnesses worldwide, posing particular danger to pregnant women, young children, the elderly, and those with compromised immune systems. Among these microscopic threats lurks Listeria monocytogenes, a potentially deadly bacterium that can breach our intestinal barriers and invade our cells. But our bodies are not defenseless—they're equipped with an elaborate security system that detects and eliminates such invaders. Recent scientific discoveries have revealed an unexpected hero in this cellular defense network: a tiny protein called ISG15.
Listeria monocytogenes is one of the most virulent foodborne pathogens, with a mortality rate of 20-30% in infected individuals, particularly affecting immunocompromised populations.
Once considered primarily as a soldier in the war against viruses, ISG15 has now been unmasked as a crucial defender against bacterial infections. This article explores the fascinating story of how scientists discovered ISG15's remarkable ability to counteract Listeria infection, revealing new dimensions of our innate immune system and opening potential avenues for future therapies.
Interferon-Stimulated Gene 15 (ISG15) is one of the first and most powerfully induced proteins when our cells detect an invasion. As its name suggests, it's activated by interferons—signaling proteins that cells release when under attack. ISG15 belongs to the ubiquitin-like protein family, which means it structurally resembles ubiquitin, the famous cellular tag that marks proteins for destruction.
Inside cells, ISG15 can be covalently attached to hundreds of target proteins through a process called ISGylation, modifying their function in the midst of infection.
When secreted, free ISG15 acts as a signaling molecule that stimulates immune cells like lymphocytes and natural killer cells.
| Feature | Description |
|---|---|
| Full Name | Interferon-Stimulated Gene 15 |
| Protein Type | Ubiquitin-like protein |
| Discovery Year | 1979 |
| Primary Inducers | Interferons, pathogens, LPS |
| Key Functions | Protein modification, immune signaling |
| Known Roles | Antiviral defense, antibacterial defense |
For decades, researchers focused on ISG15's antiviral properties. Studies showed that mice lacking ISG15 became more susceptible to influenza, Sindbis, and herpes viruses. Many viruses even evolved specific mechanisms to dismantle ISG15, clear evidence that it posed a significant threat to their survival 3 . Its role in bacterial defense, however, remained mysterious until recently.
To appreciate ISG15's bacterial combat skills, we must first understand its adversary. Listeria monocytogenes is a remarkable and dangerous pathogen. As a foodborne bacterium, it survives in refrigerated conditions that stop most other bacteria. When ingested, it employs sophisticated tactics to invade our cells, escape from the protective phagosome, and replicate freely in the cytoplasm—the cell's internal environment.
Listeria enters host cells through phagocytosis, exploiting the host's own cellular machinery.
The bacterium produces listeriolysin O (LLO) to break out of the phagosome and enter the cytoplasm.
Once in the cytoplasm, Listeria replicates rapidly, using host nutrients and machinery.
Listeria uses host actin to propel itself to the cell membrane and spread to neighboring cells.
Once in the cytoplasm, Listeria appropriates the host's cellular machinery to propel itself through the cell and into neighboring cells, spreading infection while largely avoiding detection by the immune system. Yet, our cells have evolved countermeasures—and ISG15 represents one of the most intriguing.
In 2015, a groundbreaking study revealed that ISG15 production skyrockets when Listeria invades nonphagocytic cells 1 . The research team made several surprising observations:
Perhaps most surprisingly, the early induction of ISG15 occurred through an interferon-independent pathway—a previously unknown route for ISG15 activation during bacterial infection.
Typically, ISG15 activation follows a well-established script: pathogen detection → interferon release → ISG15 production. But Listeria short-circuits this conventional signaling by activating ISG15 through the cytosolic surveillance pathway 1 .
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When Listeria's genetic material leaks into the host cell's cytoplasm, it's detected by cytosolic sensors.
STING (Stimulator of Interferon Genes) is activated by the detected bacterial DNA.
TBK1 protein relays the detection signal to downstream effectors.
Transcription factors IRF3 and IRF7 are activated and directly trigger ISG15 gene expression.
| Component | Function in ISG15 Induction |
|---|---|
| Bacterial DNA | Trigger molecule detected in host cytoplasm |
| STING | Sensor protein that detects bacterial DNA |
| TBK1 | Signaling protein that relays the detection signal |
| IRF3/IRF7 | Transcription factors that activate ISG15 gene |
This clever bypass allows cells to mount a rapid defense without waiting for the slower interferon intermediary, providing a critical advantage in the arms race between host and pathogen.
To establish ISG15's protective function against Listeria, researchers designed an elegant series of experiments 1 :
They compared ISG15 production in human cell lines infected with either pathogenic L. monocytogenes or non-pathogenic L. innocua.
Using a viral protein called B18R that blocks interferon receptors, they confirmed the interferon-independent nature of early ISG15 induction.
They validated findings in U5A cells—mutant cells lacking a functional interferon receptor—demonstrating normal ISG15 induction despite impaired interferon signaling.
They infected mice with Listeria and observed robust ISG15 induction in liver tissue after 72 hours.
The experimental results consistently demonstrated that ISG15 provides genuine protection against Listeria:
This represented the first clear evidence that ISG15 functions as a bacterial restriction factor—a cellular component that specifically inhibits bacterial growth and survival.
ISG15's power lies in its ability to modify diverse cellular proteins through ISGylation. Using stable isotope labeling in tissue culture (SILAC)—a sophisticated proteomic technique—researchers identified the specific proteins that ISG15 targets during Listeria infection 1 6 .
The findings were striking: ISG15 predominantly modifies integral membrane proteins of the endoplasmic reticulum and Golgi apparatus 1 . These cellular compartments are essential for processing and transporting proteins, including immune signaling molecules.
The ISGylation of ER and Golgi proteins correlates with increased secretion of cytokines—immune signaling proteins that recruit and activate other immune cells to combat infection 1 . By tagging these specific proteins, ISG15 essentially reprograms the cell's secretory machinery to amplify distress signals and coordinate a more effective immune response.
Later research published in 2019 expanded our understanding by mapping the complete "ISGylome"—the collection of all ISGylated proteins—in livers of Listeria-infected mice 2 . This comprehensive analysis identified:
The study found that ISG15 modifies four key regulators of autophagy (mTOR, WIPI2, AMBRA1, and RAB7), enhancing this antibacterial pathway during infection 2 .
| Target Category | Example Proteins | Functional Consequences |
|---|---|---|
| ER/Golgi Proteins | Unspecified integral membrane proteins | Increased cytokine secretion |
| Metabolic Enzymes | Various enzymes in metabolic pathways | Metabolic reprogramming |
| Autophagy Regulators | mTOR, WIPI2, AMBRA1, RAB7 | Enhanced antibacterial autophagy |
Studying ISG15's role in infection requires specialized tools and techniques. Here are some essential components of the ISG15 research toolkit:
| Tool/Method | Function in ISG15 Research |
|---|---|
| SILAC (Stable Isotope Labeling) | Quantitative proteomics to identify ISGylated proteins |
| B18R Protein | Blocks interferon receptor to test interferon-independent pathways |
| USP18C61A/C61A mice | Genetically altered mice with enhanced ISGylation due to defective deconjugating enzyme |
| Isg15-deficient mice | Genetic model to study ISG15 absence |
| Diglycine remnant proteomics | Mass spectrometry approach to identify ISGylation sites |
The discovery of ISG15's antibacterial function represents a significant expansion of our understanding of innate immunity. This versatile protein provides a dual defense system—protecting against both viral and bacterial pathogens through mechanisms that are both interferon-dependent and independent.
The implications of this research extend beyond fundamental knowledge. Understanding how ISG15 restricts bacterial growth could inform new therapeutic strategies against intracellular pathogens. The recent development of a Listeria-based vaccine targeting ISG15 for renal cell carcinoma demonstrates how this knowledge might be applied in unexpected ways 5 .
As research continues, scientists hope to answer remaining questions: How exactly does ISGylation of specific target proteins enhance their function? Can we harness ISG15's protective power therapeutically? How do other pathogens evade or counteract ISG15?
What's clear is that this tiny cellular warrior, once known only for its antiviral prowess, has revealed itself as a versatile defender in our immune arsenal—a reminder that even in the microscopic battles within our cells, there are still surprising stories to be told.
This article was based on scientific publications from eLife, Nature Communications, and other peer-reviewed journals.