A recent discovery is rewriting textbooks and revealing an overlooked defender in the immune systems of jawed vertebrates.
For decades, scientists have believed the interferon family was composed of just three types. This long-standing classification has now been upended by the groundbreaking discovery of a fourth type, Interferon-υ (IFN-υ).
This newfound cytokine is not a mere variant; it represents a distinct and ancient branch of the immune system, with unique functions that are reshaping our understanding of how vertebrates, from fish to primitive mammals, have fought viruses for millions of years.
Before diving into the new discovery, it's essential to understand the old guard. Interferons (IFNs) are signaling proteins released by host cells in response to pathogens like viruses. They are the body's alarm system, triggering nearby cells to heighten their antiviral defenses.
(e.g., IFN-α, IFN-β) - The classic antiviral interferons. They are first responders, produced by almost any infected cell, signaling through a receptor complex called IFNAR to rapidly establish a widespread antiviral state in the body.
(IFN-γ) - Known as the "immune interferon," this type is less about direct antiviral activity and more about regulating immunity. It is produced by immune cells like T cells and natural killer (NK) cells.
(IFN-λ) - These interferons share antiviral functions with Type I IFNs but signal through a different receptor (IFNLR1/IL-10R2). Their action is often more targeted, providing frontline defense at mucosal surfaces.2
(IFN-υ) - This newfound cytokine represents a distinct and ancient branch of the immune system, with unique functions that are reshaping our understanding of vertebrate immunity.
For years, this three-type model was considered complete. However, clues in the genomes of non-mammalian vertebrates suggested a missing piece.
The revolution began with a 2022 study published in Nature Communications. Researchers were investigating the complex immune systems of teleost fish, like zebrafish, which possess a rich diversity of class II cytokines and their receptors. Some of these receptors had no known partner molecule.2 3
The initial characterization of zebrafish IFN-υ provides a perfect case study of how this discovery was made and verified.
Researchers first identified the putative ifnυ gene in the zebrafish genome and cloned its full-length cDNA sequence.2 3
Using co-immunoprecipitation (Co-IP), they expressed the IFN-υ protein along with various candidate receptor proteins.
They tested whether IFN-υ binding to its receptors could trigger immune response pathways and expression of interferon-stimulated genes (ISGs).
The results were clear and decisive:
| Type | Examples | Primary Receptor Complex | Main Functions |
|---|---|---|---|
| Type I | IFN-α, IFN-β | IFNAR1 / IFNAR2 | Broad, rapid antiviral response; activation of innate immunity |
| Type II | IFN-γ | IFNGR1 / IFNGR2 | Immune regulation; activation of macrophages and adaptive immunity |
| Type III | IFN-λ (IL-28/29) | IFNLR1 / IL-10R2 | Targeted antiviral defense at mucosal surfaces |
| Type IV | IFN-υ | IFN-υR1 / IL-10R2 | Antiviral and antibacterial activity; distinct induction kinetics |
Subsequent research has shown that the functions of Type IV interferon are even broader than initially thought. A 2024 study on grass carp IFN-υ (CiIFN-υ) revealed a surprising twist: it possesses direct antibacterial activity, particularly against gram-negative bacteria.6
The study showed that CiIFN-υ could aggregate and kill bacteria directly outside the cell, a function more akin to antimicrobial peptides than classical interferons. In live fish, CiIFN-υ infection reduced tissue damage and bacterial load, improving survival rates. This dual antiviral and antibacterial function makes IFN-υ a uniquely versatile defender in the vertebrate immune arsenal.6
A unique feature of Type IV IFN not found in other interferon types
While all interferons activate defense genes, the devil is in the details. Research in the fish Carassius gibelio highlights critical functional differences between Type IV IFN (CaIFNυ) and a typical Type I IFN (CaIFNa1).4
Type I IFN (CaIFNa1) induces a faster and stronger initial wave of ISG expression. In contrast, Type IV IFN (CaIFNυ) triggers a slower, more sustained response, eventually reaching similar levels.4
The promoters of the two IFN types respond differently to immune triggers. The Type IV IFN promoter is most effectively stimulated by NF-κB, while the Type I IFN promoter is most responsive to IRF3.4
| Interferon Type | Induction of mx1 (6h) | Induction of viperin (6h) | Kinetic Profile |
|---|---|---|---|
| Type I (CaIFNa1) | ~10.3-fold increase | ~11.5-fold increase | Rapid, strong early peak |
| Type IV (CaIFNυ) | Lower initial increase | ~4.8-fold increase | Slower, sustained response |
The discovery of IFN-υ has profound evolutionary implications. This interferon type has been identified in a wide range of jawed vertebrates, including fish, amphibians, reptiles, birds, and primitive mammals.2 5 Its presence across these species indicates it is an ancient component of the vertebrate immune system, likely dating back hundreds of millions of years.
Intriguingly, the genes for IFN-υ and its specific receptor, IFN-υR1, appear to be absent in higher mammals, including humans and mice.5 The evolutionary reasons for this loss are still a mystery. One hypothesis is that the roles of IFN-υ were gradually taken over by the expanding and specializing families of Type I and Type III interferons in these mammalian lineages.
| Research Tool | Function in Research | Example from Studies |
|---|---|---|
| RACE PCR | A technique to clone the full-length cDNA sequence of an unknown gene. | Used to obtain the complete coding sequence of zebrafish ifnυ.2 |
| Co-immunoprecipitation (Co-IP) | Determines physical protein-protein interactions. | Confirmed the binding between IFN-υ and the receptor subunits IFN-υR1 and IL-10R2.2 |
| Recombinant Protein | Lab-produced version of a protein for functional studies. | Recombinant IFN-υ protein was produced and used to treat cells, demonstrating its ability to induce ISGs and inhibit viruses.4 5 |
| Gene Expression Knockdown/Knockout | Reduces or eliminates the expression of a specific gene to study its function. | Used in zebrafish models to confirm that loss of ifnυ leads to reduced antiviral immunity.2 |
| Plaque Assay | Measures the quantity of infectious virus particles produced. | Quantified the reduction in viral titers (e.g., SVCV) in cells pre-treated with IFN-υ.4 |
The identification of the Type IV interferon system is more than just an addition to a list; it's a paradigm shift. It reveals a previously hidden layer of complexity in the immune defense of jawed vertebrates.
For researchers in aquaculture, understanding IFN-υ could lead to new strategies for protecting economically important fish species from devastating viral and bacterial diseases.5 6
On a broader scale, this discovery forces us to look at the immune system not as a static set of components but as a dynamic, evolving structure. The story of IFN-υ—ancient, versatile, and lost in some lineages—provides a fascinating window into the evolutionary pressures that have shaped our defenses over millennia. It opens up a new frontier, reminding us that even in well-studied biological systems, there are still fundamental discoveries waiting to be made.