8 apr. 2026
Morrissette et al. (BioRxiv)
Keywords
● Autoimmunity
● Neurodegeneration
● Neuroimmunology
● Autoimmune experimental encephalomyelitis
● Multiple sclerosis
Main Findings
IL-18 is an inflammasome-activated cytokine that is associated with increased T-cell activation and autoinflammatory conditions. Il-18 is a potent inducer of lymphocyte IFNγ production, which has been shown to have complex roles in autoimmune diseases such as Multiple sclerosis (MS), exhibiting both pro-inflammatory and pro-regenerative roles in animal models of neurodegeneration. Previous work has shown that IL-18 depletion in a murine model of MS, autoimmune experimental encephalomyelitis (EAE), shows either no effect or modest protection. This study sought to explore how excess IL-18 impacts autoreactivity and susceptibility to neurological degeneration in EAE.
This preprint induces EAE in two mouse models with excess IL-18 availability either through overexpression of Il18 (Il18tg) or deletion of the IL-18 binding protein (Il18bp-/-). They find that mice with excess IL-18 production are unexpectedly protected from the development and progression of neurologic defects in comparison to wildtype controls, and that excess IL-18 does not induce hyperinflammation, indicated by no effect on splenomegaly, anemia, thrombocytopenia, and leukopenia. Excess IL-18 led to modestly increased levels of serum IFNγ and related cytokines/chemokines (CXCL9, MCP-1) at the peak of T-cell priming (12 days post-induction) and at the peak of CNS involvement (18 days post-induction). This was associated with a decrease in activated autoreactive CD4 T-cells in the periphery and spinal cords of mice with excess IL-18 production as seen by flow cytometry of splenocytes and spinal cords of Il18bp-/- and wildtype mice, as well as UMAP expression of CNS CD4 T-cell surface marker expression. This study finds that the protective effects of excess IL-18 is mediated by CD8 T-cells which adopt an effector phenotype rather than a central memory phenotype that is lost following tamoxifen-inducible deletion of Il18r1 specifically on mature CD8 T-cells (E8icre/creIl18r1fl/flIl18tg). Moreover, administration of an IL-18 agonist engaged CD8 effector T-cells to diminish autoreactive CD4 T-cell infiltration to the CNS. Notably, the neuroprotective effects of excess IL-18 production were also dependent on IFNγproduction and activity.
Overall, this study finds that excess IL-18 drives an effector response in CD8 T-cells and subsequent IFNγ-dependent suppression of autoreactive CD4 T-cells. Thus, Il-18 has a potential neuroprotective role by preventing the accumulation of CNS-targeting CD4 T-cells in EAE.
Limitations
• Authors utilize two mouse models with excess IL18 production within this preprint (Il18tg vs Il18bp-/-). Although interesting, it is important to be consistent and present a comparison of results across both reported models to strengthen the authors conclusions.
• While the authors assessed changes in hyperinflammation with excess IL18 production in Il18bp-/- mice, their preprint is missing data on decreased histological inflammation and effects on anemia, leukopenia, and thrombocytopenia from Il18tg mice. This may be relevant due to differences in baseline IFNγ, even without stimulation, between the models. Providing representative data will increase the strength of their report.
• It may be worth assessing myelin-specific responsiveness of activated CD8 effector cells to assess autoreactivity which could also contribute to secondary demyelination. Is there an increase in EAE-associated cytokines in CD8 effector cells (IL-17, GM-CSF)?
• The authors could address the potential question of cellular threshold limitations on CD8 T-cell neuroprotection. 2D2;Il18bp-/-developed more severe EAE than 2D2 controls. What is the threshold of CD4:CD8 ratio required for CD8 T-cells to remain protective in EAE? An experiment to assess this could include the injection of IL18-treated CD8 T-cells into 2D2;Il18bp-/- mice and controls. This would strengthen the physiological relevance of the described system.
• The authors provide evidence for a novel protective role of CD8 effector T-cells and subsequent IFNγ-related pathways in EAE. However, it remains unclear whether the IL-18R on CD8 T-cells mediates the protective effect. The authors should include validation of decreased IFNγ production after CD8-specific IL18 receptor knockout in Il18tg mice. This will make a stronger connection between IL18, effector CD8 T-cells and IFNγ.
• This preprint highlights a protective role of IFNγ but falls short providing a mechanism for IFNγ in mediating its CNS-specific effects. It would be fantastic to see how IFNγ protects from EAE. Could a potential involvement of CNS-resident glial cells explain the authors observations?
This preprint assesses the protective role of IL-18 in EAE clinical score but does not address the impacts of excess IL-18 in remyelination. To address whether IL-18 promotes remyelination, the authors could investigate immunofluorescence staining of myelin basic protein (MBP), myelin-associated glycoprotein (MAG), and 2’,3’-Cyclic nucleotide 3’-phosphodiesterase (CNP) expression, as well as electron microscopy for changes in myelin ultra-structural analysis. Given that remyelination is not required for resolution of symptoms in an acute relapse, and that the resolution of inflammation and adaptive responses are sufficient (Franklin et al., Nat Rev Neurosci, 2008, PMID: 18931697), this preprint may be addressing the role of excess IL-18 in inflammatory responses in EAE and may not address specific protective roles on myelin maintenance or remyelination. To address how excess IL-18 may influence remyelination, the authors’ report would benefit from administering a cuprizone diet to Il18tg or Il18bp-/- mice and assess changes in remyelination.
• While the authors extensively assess changes in T-cell populations throughout EAE, it is important to characterize CD8 T-cells in relation to Type 1 regulatory cells. This can be achieved by measuring CD39 and IL-10 production.
Significance/Novelty
This preprint identifies that protective CD8 T-cells exposed to excess IL-18 adopt an effector CD8 phenotype whereas previous studies identified protective CD8 T-cells to have a more central-memory phenotype. They find that CD38 and Helios are phenotypic markers of regulatory CD8 T-cells with excess IL-18, which assists in the identification of CD8 T-cell subsets in future neurodegenerative models. This paper contradicts previous findings that IL-18 is pro-inflammatory and contributes to failed myelin repair in a context-dependent manner. Nonetheless, it highlights the need for future studies assessing the role of IL-18 on CD8 T-cell heterogeneity and subsequent effects on remyelination.
The CD8 effector phenotype found in this paper connects to the current field of remyelination which is assessing the role of infiltrating CD8 T-cells to remyelinating lesions wherein IFN-responsive glia have been identified to enhance white matter repair (Dolan et al, BioRxiv). This paper is clinically relevant as it may identify a novel factor to enhance remyelination in MS, for which we currently lack approved therapies.
Credit
Reviewed by Bianca Hill as part of a cross-institutional journal club between the Icahn School of Medicine at Mount Sinai, the University of Oxford, the Karolinska Institute and the University of Toronto.
The author declares no conflict of interests in relation to their involvement in the review.