Panhandle UTR Design Confirms Protection in Mice with Low-Dose Influenza mRNA Vaccines
Designing influenza vaccines to reduce the dose
A mouse study reports that engineering untranslated regions (UTRs) derived from influenza A virus improved the protection provided by low-dose mRNA vaccines. The researchers confirmed protection with virus-matched vaccines administered twice at 0.1 μg per dose. Protective efficacy in people has not yet been established.
Influenza viruses undergo continuing antigenic changes, creating a need for approaches that improve the breadth of vaccine protection. mRNA vaccines offer rapid development and flexible design, but achieving sufficient antigen expression and immune responses at low doses also requires attention to regulatory elements beyond the antigen sequence. This study focused on UTRs, which regulate mRNA stability and protein production.
Low-dose protection using the viral panhandle structure
The researchers took inspiration from the M segment of influenza A virus. They designed UTRs so that complementary sequences at the two ends of the RNA would pair to form a panhandle structure, then introduced mutations into the 3′ UTR to strengthen base pairing. Among these designs, M1+2 produced higher protein expression in cultured cells and mice than unmutated M-derived UTRs and an α-globin UTR control. However, predicted structural stability did not consistently correlate with expression efficiency, making it difficult to explain the effect solely by increased stability.
The vaccines contained mRNA encoding influenza hemagglutinin (HA) antigen, packaged in lipid nanoparticles (LNPs). Mice received two intramuscular injections of 0.1 μg each, 14 days apart, and underwent viral challenge on day 28 after the first dose. In separate experiments using HA vaccines matched to H1N1, H3N2, or influenza B, all mice in the M1+2 groups survived without weight loss during the 14 days after challenge. Each group in each experiment contained six mice. This does not mean that a single vaccine provided cross-protection against all three viruses.
The comparison dose also matters. At 2 μg per dose, all vaccine groups survived regardless of UTR type, and hemagglutination inhibition antibody titers in the M1+2 group did not differ significantly from those of other leading candidates. The protective advantage of the M1+2 design was more apparent at the low dose. A separate immune analysis detected antigen-specific interferon gamma (IFN-γ) secretion 24 hours after a booster, although the researchers described the magnitude of the T-cell response as modest. This analysis examined responses shortly after antigen re-exposure and should not be interpreted as direct evidence of strong T-cell immunity in the 0.1 μg protection experiment.
Applications to other vaccines and clinical efficacy require further evaluation
These results suggest that UTR optimization could help reduce the dose of influenza mRNA vaccines. However, the comparator was the α-globin UTR formulation used in this study; this was not a direct comparison with a finished commercial vaccine.
Whether the same design can be applied to vaccines targeting other viruses or cancer antigens remains to be studied. If dose reduction is maintained during further development, it could offer benefits for production costs and supply, but this study did not establish economic benefits or reduced side effects in people. The durability of long-term protection and the effective dose and safety in people require separate evaluation. The study conditions and results are presented in the original NPJ Vaccines paper.
Influenza viruses pose a persistent threat to global public health, causing widespread respiratory illness and significant morbidity. Vaccination remains the most effective strategy to reduce the burden of both seasonal and pandemic influenza. mRNA vaccines represent a promising alternative to conventional vaccine platforms due to their rapid development, flexibility, and high efficacy. Nonetheless, optimizing non-coding regulatory elements such as untranslated regions (UTRs) remains crucial for enhancing mRNA vaccine performance. In this study, we designed a novel UTR derived from the influenza A virus M segment, engineered to form optimal panhandle structures through selective base-pair enhancing mutations (M1 + 2), aiming to improve mRNA translation efficiency and immunogenicity. Using both reporter and HA antigen-encoding mRNAs, we demonstrated that the M1 + 2 UTR significantly enhanced protein expression in vitro and in vivo compared to unmodified UTRs and the canonical α-globin UTR control. In a murine model, low-dose (0.1 μg) vaccination with HA mRNA-lipid nanoparticles (LNPs) incorporating the M1 + 2 UTR elicited robust innate, humoral, and T cell-mediated immune responses, and conferred complete protection against lethal challenge with seasonal influenza strains, including H1N1, H3N2, and IBV. Our findings underscore the potential of rational UTR design in developing more efficacious and dose-sparing mRNA vaccines.
The findings give vaccine developers a basis for comparing UTRs, alongside antigen sequences, when designing low-dose candidates. For example, developers could vary the UTR while keeping the HA antigen and delivery conditions the same, then test whether antibody responses and protection against viral challenge are maintained at lower doses. The potential demonstrated in this mouse study is limited to two doses of 0.1 μg each. Further development must assess whether increased expression translates into long-term protection and whether the balance between immune activation and safety is maintained. Increased production output and lower costs are potential benefits if these evaluations progress; the findings do not establish a lower, safe dose for people.