Antigen transfer from non-APCs to APCs impacts the efficacy and safety of protein- and mRNA- based vaccines

• Compared with professional APCs, most non-APCs demonstrate higher competence in mRNA-transfection and protein-uptake; • Vaccinated non-APCs activate specific immunoresponse by communicating antigenic information with nearby APCs; • Such antigenic communication is influenced by different determinants for mRNA vaccine and protein-based vaccine; • Such “off-target” immunostimulation is closely related to the efficacy and safety of vaccines. In brief : We here reveal that compared with professional antigen-presenting cells (APCs), most somatic cells (non-APCs) prevailing in different vaccination sites demonstrate higher competence in mRNA-transfection and protein-uptake. And it’s ubiquitous for these non-APCs to activate specific immunoresponse by communicating antigenic information with nearby APCs, which is closely related to the efficacy and safety of vaccines. Vaccine is a powerful weapon against the invasion of pathogenic substances. Nowadays, mRNA vaccine and protein subunit vaccine are the representatives of newer vaccine platforms and traditional immune approaches respectively, but their detailed in vivo mode of action is still full of mysteries. Contrary to the stereotype that professional antigen-presenting cells (APCs), especially dendritic cells (DCs), are the only destination of antigenic vaccine component, increasing attention has been given to the somatic cells (non-APCs) predominate in different vaccination sites, which might also be positively vaccinated. However, it is unclear whether and how non-APCs receiving such “off-target” immunostimulation would translate into differences in the magnitude or quality (i.e., efficiency and safety) of vaccine response. Here, mRNA- or protein- loaded cationic nanoemulsions (CNEs) was prepared and used to investigate the immunization by non-APCs abundant at different routes of administration. Results revealed a superior ability of most non-APCs to transfect mRNA or uptake exogenous protein, which served as indirect immune elicitors by subsequently transferring antigen to the nearby APCs. For mRNA vaccine, the competence and mode of such antigenic communication was determined by the secretory characteristics of mRNA-encoded protein and the intrinsic secreting-ability of antigen donors. Specifically, the intracellular residency/extracellular secretion of transfected protein by non-APCs might greatly determine vaccine efficacy and safety. While for protein subunit vaccine, the overall quantity of antigen internalized by non-APCs may be a key parameter. This study systematically identified the action mode of vaccines from the perspective of non-APCs, which may provide clinical guidance for the design and application of protein- or mRNA- based therapeutics.