Visualization of early events in mRNA vaccine delivery in non-human primates via PET–CT and near-infrared imaging

Visualization of the spatio–temporal trafficking of vaccines after their delivery would help evaluate the efficacy of candidate formulations and aid their rational design for preclinical and translational studies. Here, we show that a dual radionuclide–near-infrared probe allows for quantitative, longitudinal and non-invasive monitoring, via positron emission tomography–computed tomography and near-infrared imaging of cynomolgus macaques, of the trafficking dynamics to draining lymph nodes of a model messenger RNA vaccine labelled with the probe. After intramuscular administration of the vaccine to the monkeys, we observed the dynamics of the mRNA vaccine at the injection site and in the draining lymph nodes, performed cellular analyses of the involved tissues using flow cytometry and identified through immunofluorescence that professional antigen-presenting cells are the primary cells containing the injected mRNA and encoding the antigen. This approach may reveal spatio–temporal determinants of vaccine efficacy in preclinical and translational studies employing large mammals.