Background Lipid nanoparticle (LNP)-encapsulated mRNAs have emerged as effective vaccination tools to stimulate protective immunity. The most common application of this technology is to deliver mRNAs that encode antigenic proteins which promotes antigen presentation by dendritic cells (DCs) to stimulate antigen-specific lymphocyte responses. It is unclear if additional immunostimulatory signals that are necessary for durable vaccine efficacy can be induced via mRNA-encoded proteins delivered via LNPs.

Methods Here we use LNPs to deliver mRNAs encoding a Pattern Recognition Receptor (PRR), which acts as a constitutively active innate immune enzyme.

Results The mRNA-encoded innate immune enzyme induced the expression of T cell costimulatory molecules, major histocompatibility complex proteins, and several cytokines, including type I interferons from murine and human DCs. These activities far exceeded the immuno-stimulatory activities of LNPs encoding antigen mRNAs. Consequently, the immunization of mice with antigen-LNPs in combination with innate immune enzyme-LNPs lead to robust production of antigen-specific IFNγ-producing T cells. These T cell responses were durable, and circulated through the lymphatics and blood, reaching tissues such as the lung. Immunizations with antigen-LNPs alone, akin to what are used in the clinic, stimulated weak and transient T cell responses. Antibody responses specific to antigen-LNPs were biased towards type I isotypes when co-injected with the innate immune enzyme-LNPs, as compared to immunizations with antigen-LNPs alone.

Conclusions Herein, we report that mRNAs encoding a constitutively active innate immune enzyme strongly enhance DC activities. These findings establish the use of mRNAs encoding an innate PRR as catalytic adjuvants, which may prove useful in enhancing the immunogenicity of nucleic acid-based vaccines.

Ethics Approval All procedures involving animals were reviewed and approved by the Institutional Animal Care and Use Committee of Explora BioLabs (IACUC protocol number: EB17–010-300). Procedures involving human materials were reviewed and approved by the Institutional Biosafety Committee of Corner Therapeutics (Project Number 2021–01). Collection protocols and donor-informed consent are approved by an Institutional Review Board (IRB) at donor collection sites, and donors gave informed consent before collection.