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 in vitro. 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.