Lipid-formulated RNA vaccines induce the systemic release of pro-inflammatory cytokines to cause dose-dependent reactions like fever and chills. Such effects are less pronounced in C57BL/6 and Balb/c mice, which tolerate much higher vaccine doses than human patients. This discrepancy makes predicting human inflammatory responses from in vivo data difficult and highlights the importance of developing more relevant preclinical models. In a recent Nature Immunology publication, researchers implicate the interleukin-1 (IL-1) pathway in triggering RNA vaccine-associated innate signaling. Their work reveals that mice, unlike humans, preferentially upregulate anti-inflammatory IL-1 receptor antagonist (IL-1ra) relative to IL-1. These findings suggest IL-1ra-deficient mice to be a viable model for predicting patient responses to lipid-formulated RNA vaccines and other innate immune challenges.
Human and murine immune cells respond differently to RNA vaccines
To identify the factors responsible for human sensitivity to RNA vaccines, Tahtinen et al. challenged human peripheral blood mononuclear cells (PBMCs) with a liposomal vaccine containing unmodified RNA. They then monitored the production of various cytokines via a multiplex immunoassay. At lower dose levels, both IL-1β (a pro-inflammatory cytokine) and IL-1ra (an anti-inflammatory cytokine) were released in similar concentrations. However, at higher doses, the release of IL-1β increased approximately 10-fold and was associated with elevated levels of other pro-inflammatory cytokines, whereas IL-1ra release remained constant.
In contrast, a similar experiment performed in murine leukocytes showed IL-1ra to be highly released at baseline and further increased upon vaccine treatment. At the same time, induction of IL-1β was only observed at high dose levels. This indicated that, in humans, the buffering properties of IL-1ra are likely overcome as the level of innate stimulation increases. When the same vaccine was subsequently administered to wildtype and IL-1ra-deficient mice, the latter developed a cytokine release syndrome (CRS) phenotype characterized by hypothermia and weight loss, mirroring the CRS symptoms exhibited by human patients.
Strategies for reducing reactogenicity
To investigate the underlying causes of vaccine reactogenicity, including whether RNA modification induced the release of different IL-1 family members, Tahtinen et al. generated a range of different formulations to treat human PBMCs. These included both unmodified RNA and RNA featuring 1-methyl-pseudouridine instead of uridine to reduce activation of Toll-like receptors 7 and 8 (using TriLink’s CleanCap® AG for co-transcriptional capping), which were formulated in liposomes or lipid nanoparticles. The resultant data support context-dependent reactogenicity for modified RNA, with the degree of immunostimulation varying based on formulation.
With RNA vaccines seeing increased use following successes in protecting against COVID-19, elucidating the underlying mechanisms of action is essential to maximize their potential. By demonstrating that vaccine-induced systemic inflammatory responses are driven by IL-1 and antagonized by IL-1ra, and that optimizing formulation can reduce side effects linked to modified RNA, Tahtinen et al. have helped extend the reach of these important products.
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Article Reference: Tahtinen S, Tong AJ, Himmels P, et al., IL-1 and IL-1ra are key regulators of the inflammatory response to RNA vaccines, Nat Immunol. 2022;23(4):532-542.