Age, genetics and lifestyle all influence how an individual responds to a viral infection, yet these factors do not fully explain why the same pathogen can produce dramatically different outcomes. Even genetically identical animals, living under carefully controlled conditions, may follow sharply divergent clinical trajectories after infection. New research from Hokkaido University suggests that one of the most important determinants may be hidden in the immune system’s response during the first day of infection—a narrow period in which antiviral defenses can either be organized effectively or fail to contain the virus.
The study, published in iScience, used mice infected with vesicular stomatitis virus (VSV), a pathogen that can cause lethal disease in susceptible animals. Although the mice were genetically identical and raised in the same environment, some survived while others died. This variation allowed the researchers to examine biological differences that are often dismissed as experimental noise. By comparing the animals at very early stages of infection, the team identified an immune checkpoint that appears to influence whether the host develops a protective response or progresses toward severe disease.
The decisive factor was the timing and intensity of type I interferon production, particularly interferon beta, or IFN-β. Type I interferons are antiviral signaling proteins released by infected cells and immune cells soon after viral detection. They bind to interferon receptors on neighboring cells, activating intracellular signaling pathways that stimulate the expression of hundreds of interferon-stimulated genes. These genes can inhibit viral replication, enhance antigen presentation and coordinate the activity of multiple immune cell populations. In the surviving mice, this response emerged rapidly, creating an early defensive environment before the infection could become systemically overwhelming.
The researchers found that mice capable of producing a strong burst of type I interferon were far more likely to survive lethal VSV infection. The result was not simply a matter of producing more interferon at any point during the disease. Instead, the timing of the response was critical. When the scientists blocked type I interferon signaling during the first 24 hours after infection, most of the mice died. Blocking the same pathway two days after infection had little effect on survival, indicating that the immune system’s protective decision is made during a sharply defined early window.
That early interferon signal also reshaped the behavior and composition of neutrophils, a class of innate immune cells best known for rapidly migrating to sites of infection and inflammation. Neutrophils can engulf pathogens, release antimicrobial molecules and generate inflammatory signals, but they are not a uniform population. The Hokkaido University team identified a distinct group of neutrophils expressing ICAM1, or intercellular adhesion molecule 1, after the early interferon response was activated. These ICAM1-positive neutrophils displayed heightened inflammatory signaling and a stronger antiviral profile than other neutrophil populations.
ICAM1 is a cell-surface adhesion protein that helps immune cells interact with blood vessels and other cells, enabling them to move into inflamed tissues and participate in coordinated immune responses. In this study, its presence marked a functionally distinct neutrophil state rather than merely a change in cell identity. The findings suggest that type I interferon does more than directly suppress viral replication: it also instructs the immune system to generate specialized innate immune cells capable of reinforcing antiviral defense. This places neutrophil heterogeneity at the center of the earliest phase of protection against severe viral disease.
The study’s experimental design was important because it focused on variation among animals that were otherwise expected to be nearly identical. In conventional experiments, differences between genetically identical mice may be treated as unwanted variability that obscures the average effect of infection or treatment. Associate Professor Tomohiko Okazaki, the study’s lead author, and his colleagues instead treated those differences as clues. By analyzing why some animals initiated an effective response while others did not, they were able to uncover a relationship between early IFN-β dynamics, ICAM1-positive neutrophils and survival.
The findings also illustrate why antiviral immunity must be carefully timed. An immune response that begins too late may be unable to prevent viral spread, while excessive or poorly regulated inflammation can itself damage tissues. Type I interferons have therefore been associated with both protection and pathology, depending on when and where they are produced. The new results indicate that the first 24 hours may represent a period when interferon signaling is especially beneficial because it establishes antiviral programs and mobilizes protective neutrophil populations before extensive tissue injury occurs. Once the infection has advanced, stimulating the same pathway may no longer restore the protective state.
The researchers say the work could eventually inform treatments for severe viral infections, although the findings remain limited to mice and cannot yet be directly applied to patients. Therapeutic strategies might seek to strengthen an inadequate early interferon response, deliver interferon-like signals or promote the formation and function of protective ICAM1-positive neutrophils. Such approaches would need to be precisely timed and carefully controlled, because indiscriminate activation of inflammatory pathways could increase tissue damage. Further studies will be needed to determine whether comparable interferon and neutrophil patterns occur in humans infected with clinically important viruses, and whether early biomarkers can identify patients at risk of deteriorating. By revealing an immune checkpoint that operates almost immediately after infection, the study offers a possible explanation for why individuals exposed to similar viral threats can experience profoundly different outcomes.
Subject of Research: Early type I interferon signaling, ICAM1-positive neutrophil heterogeneity and survival during lethal viral infection
Article Title: Type I IFN dynamics orchestrate protective ICAM1⁺ neutrophil heterogeneity as an early checkpoint for survival in lethal viral infection
News Publication Date: 18 August 2026
Web References: https://doi.org/10.1016/j.isci.2026.117189
References: iScience, DOI: 10.1016/j.isci.2026.117189
Image Credits: Riho Saito
Keywords
Viral infection, viral immunology, type I interferon, IFN-β, neutrophils, ICAM1, innate immunity, antiviral response, vesicular stomatitis virus, immune checkpoint, Hokkaido University, severe viral disease
