An innovative and stable mRNA-LNP microneedle vaccine elicits humoral and multifunctional cellular immune responses

During the COVID-19 pandemic, the use of lipid nanoparticles (LNPs) augmented the development of mRNA vaccines. However, their ultralow-temperature storage and transportation requirements, as well as their heavy reliance on injection by professional medical staff, have limited large-scale vaccination in many developing countries. Herein, we developed a simple and widely deployable microneedle (MN) vaccine delivery system (mLNP-man-MN) for mannose-modified LNPs (mLNP-man) loaded with mRNA encoding the SARS-CoV-2 spike receptor-binding domain by utilizing three-dimensional printing and polydimethylsiloxane micro molding methods. This delivery system is composed of a dissolvable polymer mixture that was optimized for high bioactivity by screening formulations <i>in vitro</i>. We have demonstrated that this MN system can maintain the physicochemical properties and bioactivity of the mRNA-LNP complex even when stored at 4 °C for at least one month or at 25 °C for two weeks. Moreover, mLNP-man-MNs target the epidermis and dermis, which are rich in antigen-presenting cells, thereby eliciting effective innate immune responses and inducing robust systemic humoral responses, as well as multifunctional cellular immunity in the spleen. Importantly, the MN system induced a certain level of pulmonary T-cell responses compared to those induced by intramuscular injections, thereby providing some protection against lung invasion by the SARS-CoV-2 pseudovirus in mice.