The response to mRNA vaccines needs to be sufficient for immune cell activation and
recruitment, but moderate enough to ensure efficacious antigen expression. The choice
of the cap structure and use of N1-methylpseudouridine (m1Ψ) instead of uridine, which
have been shown to reduce RNA sensing by the cellular innate immune system, has led
to improved efficacy of mRNA vaccine platforms. Understanding how RNA modifications
influence the cell intrinsic immune response may help in the development of more effective
mRNA vaccines. In the current study, we compared mRNA vaccines in mice against influenza
virus using three different mRNA formats: uridine-containing mRNA (D1-uRNA), m1Ψ-modified
mRNA (D1-modRNA), and D1-modRNA with a cap1 structure (cC1-modRNA). D1-uRNA vaccine
induced a significantly different gene expression profile to the modified mRNA vaccines,
with an up-regulation of Stat1 and RnaseL, and increased systemic inflammation. This
result correlated with significantly reduced antigen-specific antibody responses and
reduced protection against influenza virus infection compared with D1-modRNA and cC1-modRNA.
Incorporation of m1Ψ alone without cap1 improved antibodies, but both modifications
were required for the optimum response. Therefore, the incorporation of m1Ψ and cap1
alters protective immunity from mRNA vaccines by altering the innate immune response
to the vaccine material.
