Excess IL-18 Rewires Antiviral Immunity Toward HLH/MAS in Mouse Models
Interleukin-18 sits at the center of a lethal immune misfire in a new mouse study that links cytokine excess to hemophagocytic lymphohistiocytosis and macrophage activation syndrome. In the model, chronic IL-18 overproduction did not simply amplify inflammation in a uniform way. It altered natural killer cell biology, distorted cytotoxic T-cell behavior, and changed how the host responded to different viral challenges.
The work matters because HLH and its macrophage activation syndrome subtype are not driven by a single defect. They emerge from a dysregulated immune network in which infection, innate immune activation, and defective cytotoxic control can feed each other. This study places IL-18 in that web as more than a biomarker. It acts as a functional driver of disease, but the downstream effect depends on the pathogen context and on which immune compartment is being stressed.
Baseline NK Cell Abnormalities Mirrored MAS-Susceptible States
Mice with chronic IL-18 overproduction, referred to as Il18 tg mice, already carried an immune phenotype resembling MAS-susceptible patients before infection began. Baseline natural killer cell numbers were reduced, while cytotoxic T lymphocytes showed activation. Those two features alone point toward a system already shifted away from balanced antiviral surveillance.
Transcriptomic analysis added more detail. NK cells showed signatures consistent with IL-18/IFNγ-axis activation and cell-cycle upregulation. In practical biological terms, the cells were not simply absent or inert. They appeared to be caught in a reprogrammed state shaped by persistent cytokine pressure, with evidence of activation but not necessarily effective antiviral readiness.
Different Viruses Produced Different Immune Consequences
The most revealing part of the study came from comparing two viral challenges. During lymphocytic choriomeningitis virus infection, Il18 tg mice developed HLH/MAS without losing viral control. In that setting, the infection triggered disease, but the NK-cell compartment was not the decisive bottleneck. The phenotype still fit a hyperinflammatory syndrome, yet the host retained enough control over the virus to avoid failure at that specific level.
Mousepox produced a very different sequence. Because mousepox depends heavily on NK-cell responses early in infection, it served as a more stringent test of whether IL-18 excess could distort innate antiviral defense. Early after infection, Il18 tg NK cells failed to activate appropriately. At the same time, cytotoxic T cells rapidly hyperactivated, and initial mousepox control remained intact.
That balance did not last.
As infection progressed, the mice developed signs of HLH/MAS, including hepatosplenic necrosis. Viral control later worsened, likely because the animals failed to expand mousepox-specific cytotoxic T cells. The sequence is clinically important even in a preclinical setting: immune overactivation in one compartment did not compensate for dysfunction in another. Instead, the early mismatch appeared to unravel pathogen-specific control over time.
Excess IL-18 Can Coexist With NK Cell Immunodeficiency
One of the clearest conclusions is that excess IL-18 does not produce a single stereotyped immune effect. It can drive a form of context-specific NK immunodeficiency. In the mousepox setting, the NK-cell defect was not a matter of total absence. The cells were present but failed to activate normally when the infection required them.
The authors also examined mice lacking IL-18 binding protein, or Il18bp KO mice. Despite having more normal resting lymphocyte populations, these mice succumbed in a similar way to viremic HLH/MAS. The finding argues against the idea that a single baseline lymphocyte abnormality explains the syndrome. Persistent IL-18 excess can be pathogenic even when resting immune cell profiles look less abnormal than in the transgenic model.
Adoptive NK Cell Transfer Partially Restored Host Defense
A rescue experiment gave the study its most direct mechanistic support. Transfer of pre-activated NK cells into Il18 tg mice partially improved virus-specific cytotoxic T-cell responses, viral control, and survival. The effect was incomplete, but it was directional and biologically coherent.
This is the key translational signal in the paper. Restoring NK-cell function altered more than one downstream outcome. It improved the antiviral response and helped blunt the fatal trajectory. The result suggests that NK cells are not passive victims of the cytokine storm; they are part of the machinery that can determine whether the host exits infection with controlled inflammation or progresses into uncontrolled hyperinflammation.
Why the Findings Matter for HLH and MAS Biology
HLH has long been understood as a syndrome where infection and immune dysregulation reinforce one another. This study sharpens that model by showing that excess IL-18 can push the system in two different directions at once: nonspecific T-cell hyperactivation and defective NK-cell function. Those are not redundant abnormalities. They can coexist, and together they can create a host response that looks activated on the surface but functions poorly against the specific pathogen.
The bedside implication is conceptual rather than immediately therapeutic. IL-18 excess may not map to one uniform treatment pathway across all infection settings. A cytokine profile that appears inflammatory may hide a deeper defect in antiviral coordination, especially when the triggering pathogen depends on intact NK-cell responses. In that context, the same cytokine axis can sustain inflammation while weakening the host’s capacity to contain the infection that provoked it.
The work also helps explain why HLH and MAS can be so unstable clinically. The syndrome is not defined only by high cytokine levels. It is defined by the failure of the immune system to self-correct after activation begins. In these mouse models, IL-18 pushed the immune response into that failure state through a combination of NK-cell dysfunction, abnormal T-cell activation, and altered pathogen-specific control.
The study remains preclinical, and the models are not a substitute for human disease. Even so, the logic is compelling: excess IL-18 can be dangerous in more than one way, and the path to fatal hyperinflammation may depend on which antiviral arm of immunity is called upon first. In mousepox, that difference proved decisive.