Monitoring mRNA vaccine antigen expression in vivo using PET/CT

Noninvasive visualization of the distribution and persistence of mRNA vaccine antigen expression in mammalian systems has implications for the development and evaluation of future mRNA vaccines. Here, we genetically fuse E. coli dihydrofolate reductase (eDHFR) to the delta furin diproline modified SARS-CoV-2 spike glycoprotein (\({\small \mathsf{S2P^{\Delta f}}}\)) mRNA vaccine and image its expression in female mice and male non-human primates using \({\small \mathsf{[^{18}F]}}\)fluoropropyl-trimethoprim \({\small ([^{\mathsf{18}}\mathsf{F}]\mathsf{FP}}\)-\({\small \mathsf{TMP})}\). Whole body positron emission tomography (PET) imaging revealed transient expression of the vaccine antigen in the injection site and draining lymph nodes (dLNs). Fusion of eDHFR did not impact S2P immunogenicity and no humoral or cellular immune response was detected against eDHFR in either species. In this work, we show that eDHFR can be used as an mRNA-encoded PET reporter gene to monitor the spatiotemporal dynamics of mRNA vaccine antigen expression in vivo. This technique could be applied in clinical translation of future mRNA vaccines or therapeutics.