Cellular RNA Sensors Form First Line of Defense Against Neglected Mosquito-Borne Virus

A neglected mosquito-borne virus that can cause fever, brain inflammation, and fetal nervous system damage meets a surprisingly well-organized cellular defense when it infects human cells, according to new research published in PLOS Neglected Tropical Diseases. The study, led by Thomas Labadie and Karim Majzoub with colleagues in France, provides the first molecular characterization of how human cells detect Tonate virus (TONV), a member of the Venezuelan equine encephalitis antigenic complex that has remained largely understudied despite its potential to cause severe disease. The findings map the precise signaling pathway that recognizes the invader and mobilizes the body’s most potent antiviral weapon, the type I interferon system.

TONV belongs to the alphavirus genus, a group of enveloped RNA viruses transmitted primarily by mosquitoes and arthropods. While its close relatives, including Venezuelan equine encephalitis virus, chikungunya virus, and Mayaro virus, have received considerable attention from the research community, TONV has remained on the margins of virological research. Clinical reports associate the virus with febrile illness, encephalitis, and central nervous system abnormalities in fetuses, making it a pathogen of potential public health concern, particularly in regions where its mosquito vectors are established. Until now, however, almost nothing was known about which molecular sensors in human cells detect TONV upon infection and which signaling cascades are triggered to contain it.

The research team turned their attention to two well-established human cell lines, T98G glioblastoma cells and A549 lung epithelial cells, to dissect the interaction between TONV and the innate immune system. These cell types are commonly used to study antiviral responses because they mount robust and reproducible reactions to RNA virus infections. The investigators infected the cells with TONV and monitored the accumulation of viral genetic material and the activation of cellular defense pathways over time, using molecular techniques that allow visualization and quantification of viral components inside individual cells.

What they observed was a clear and rapid cellular alarm. TONV infection led to the accumulation of double-stranded RNA in the cytoplasm, the fluid interior of the cell where most metabolic activity takes place. Double-stranded RNA is a molecular signature that distinguishes many RNA viruses from the host cell’s own genetic material, which normally resides in the nucleus and does not form long double-stranded structures in the cytoplasm. This cytosolic double-stranded RNA acts as a danger signal, alerting the cell that something foreign is replicating inside it. The presence of this viral signature coincided with a robust activation of the type I interferon pathway, the body’s first-line antiviral defense mechanism that triggers the production of hundreds of interferon-stimulated genes capable of restricting viral replication.

To identify which cellular sensors were responsible for detecting TONV, the researchers systematically depleted key components of the RNA sensing machinery. They focused on RIG-I and MDA5, two cytoplasmic RNA helicases that function as pattern recognition receptors, meaning they patrol the cell interior looking for viral RNA signatures. When RIG-I or MDA5 detects viral RNA, it initiates a signaling cascade through an adaptor protein called MAVS, which resides on the outer membrane of mitochondria and other cellular organelles. MAVS, in turn, activates transcription factors, most notably IRF3, that travel to the cell nucleus and switch on the genes encoding type I interferons and other antiviral molecules. This pathway, known as the RLR-MAVS-IRF3 axis, is a central component of the cellular response to RNA viruses.

The experimental results were unambiguous. When the researchers depleted RIG-I, MDA5, MAVS, or IRF3 from the human cells, the TONV-induced interferon response was strongly reduced, demonstrating that each of these molecules is required for the cell to mount an effective alarm. More importantly, the loss of any of these components had direct consequences for viral replication. Cells lacking functional RIG-I, MDA5, MAVS, or IRF3 showed an increase in the proportion of cells containing detectable viral double-stranded RNA, indicating that more cells became productively infected. Viral RNA accumulation also rose significantly in these sensor-deficient cells, confirming that the RLR-MAVS-IRF3 axis actively restricts TONV replication rather than merely responding to it as a passive byproduct of infection.

These findings establish the RIG-I and MDA5 helicases as the primary sensors that detect TONV infection in human cells. The results also clarify the downstream signaling route: viral RNA recognition by RIG-I or MDA5 activates MAVS, which in turn triggers IRF3-dependent transcription of type I interferon genes. The interferon proteins released by infected cells then act in both autocrine and paracrine fashion, binding to receptors on the infected cell itself and on neighboring cells, priming them to express antiviral effector proteins before the virus can spread further. This coordinated response creates what immunologists call an antiviral state, a condition in which cells are metabolically and biochemically prepared to resist viral replication.

The researchers went one step further and asked whether exogenous type I interferon, supplied from outside the cells, could suppress TONV replication directly. The answer was emphatic. Treatment with recombinant type I interferon strongly inhibited TONV replication, reducing both the amount of viral RNA inside infected cells and the production of infectious viral particles released into the surrounding medium. This result confirms that the interferon pathway is not merely a marker of TONV infection but a functionally relevant antiviral mechanism capable of actively suppressing the virus. It also suggests that therapeutic strategies aimed at boosting type I interferon signaling could potentially be developed to limit TONV infection, although translating such findings from cell culture to clinical applications would require extensive further research.

The significance of this work extends beyond TONV itself. By identifying the RLR-MAVS-IRF3 axis as the major sensing pathway for this neglected alphavirus, the study fills a substantial gap in our understanding of how the alphavirus genus interacts with the human innate immune system. Different alphaviruses have evolved distinct strategies to evade or suppress host defenses, and knowing which pathways are engaged by TONV provides a foundation for comparative studies with its more notorious relatives. Such comparisons can reveal whether TONV lacks specific immune evasion mechanisms that make other alphaviruses more pathogenic, or whether it possesses unique strategies that could inform the design of broad-spectrum antiviral approaches.

Moreover, the study highlights the importance of investigating neglected pathogens even when they appear to be of limited immediate clinical relevance. The Venezuelan equine encephalitis complex includes viruses with the potential to cause outbreaks, and understanding the basic biology of how human cells respond to these agents is a prerequisite for developing vaccines, therapeutics, and diagnostic tools. The molecular characterization of TONV innate immune sensing presented by Labadie, Eloiflin, Denis, and their colleagues represents a foundational contribution to this effort, establishing a framework upon which future studies of alphavirus pathogenesis, host restriction factors, and antiviral immunity can be built. As climate change and urbanization continue to expand the geographic range of mosquito vectors, pathogens such as TONV may become increasingly relevant to global public health, making this type of fundamental research both timely and essential.

Subject of Research: Innate immune sensing of Tonate virus by the RIG-I, MDA5, MAVS, and IRF3 pathway in human cells

Article Title: The RLR-MAVS-IRF3 axis activates the IFN pathway to restrict Tonate virus (TONV) infection

Article References: Labadie, T., Eloiflin, R., Denis, Z., Motos, M., Re, J., Schussler, M., Chemarin, M., Moltini-Conclois, I., Courgnaud, V., Missé, D., Laguette, N., & Majzoub, K. (2026). The RLR-MAVS-IRF3 axis activates the IFN pathway to restrict Tonate virus (TONV) infection. PLOS Neglected Tropical Diseases, 20(10), e0014559. https://doi.org/10.1371/journal.pntd.0014559

Image Credits: AI Generated

DOI: 10.1371/journal.pntd.0014559

Keywords: Tonate virus, alphavirus, RIG-I, MDA5, MAVS, IRF3, type I interferon, innate immunity, antiviral response, Venezuelan equine encephalitis complex, double-stranded RNA, mosquito-borne virus