Fig. 1: Overview of COVID-19 NP vaccine components, immunomodulatory properties and intracellular fate.

a, Schematic representation of different NPs used for mRNA, DNA and protein-based COVID-19 vaccines. For mRNA, the lipid NCs contain ionizable lipid, PEG–lipid, structural lipid (distearoylphosphatidylcholine, DSPC) and cholesterol. DNA cargos are, instead, protected by either proteolipid NPs, formulated with neutral lipid and the proprietary fusion-associated small transmembrane (FAST) proteins or with polymeric poly(lactic-co-glycolic acid (PLGA) NPs, whereas protein-based NVs use nanoemulsion formulations, such as AS03, Matrix-M and MF59 (oil in water emulsion of α-tocopherol, squalene or saponin), or virus-like particles (chimeric multimers of proteins and phospholipids self-assembled to mimic the viral structure). b, NV immune modulation overview. (1) Inductive phase: IM NV administration induces an inflammatory cytokines release and immune cell recruitment (for example, neutrophils, macrophages and APCs). APCs (for example, DCs) internalise, process and load the antigen on MHC molecules. (2) Antigen-loaded APCs and free NPs (<250 nm) move through the lymphatic vessels to the draining lymph nodes, where subcapsular sinus macrophages uptake the free NPs and present them to B cells. (3) Migratory DC present antigens to the T cells, which differentiate into effector T helper or T cytotoxic cells. (4) Activated T helper cells move to the T/B zone border and interact with antigen-specific B lymphocytes. T cells drive the formation of a germinal centre for B-cell maturation. Some of the B cells differentiate into plasma blasts, whereas others differentiate into long-lived plasma cells and memory B cells. (5) Activated immune cells leave the lymph node and enter the bloodstream. (6) Effector phase: activated immune cells exit the bloodstream and exert their effector function. Plasma cells secrete antibodies and activated cytotoxic and helper T cells carry out cell-mediated responses. Antigen-specific memory B and T cells can be reactivated upon pathogen encounter, leading respectively to antigen-specific antibodies production and killing of the infected cells. c, Schematic overview of NV-processing and antigen-presentation pathways by APCs. mRNA cargo embedded within ionizable lipid NPs (upper right) enters the cell via the endosome and as the endosome maturates, pH reduction induces the protonation of the lipid (lipid changes colour from light to dark pink). The interaction of the positively charged lipids with the endosomal anionic lipids alters the membrane stability and induces fusion and the release of the mRNA payload into the cytoplasm (endosomal escape). Ribosomes translate the mRNA into proteins, which undergo proteosomal degradation and loading onto MHC-I and/or MHC-II for antigen presentation. Some of the mRNA molecules can be sensed by innate immune sensors (Toll-like receptors and retinoic acid-inducible gene I (RIG-I)), which leads to IFN-1 secretion. Proteolipid and polymeric NPs (upper left) deliver the DNA payload through the fusogenic FAST protein or by means of electroporation, respectively. DNA enters the nucleus and undergoes nuclear transcription; the newly synthetized mRNA exits the nucleus and is translated into protein. Nanoemulsions and virus like particles (left) enter the cell, and proteins are directly processed within the endosome and exposed onto MHC-II. NF-κB, nuclear factor-κB. ER, endoplasmic reticulum; TAP, Transporter associated with antigen processing protein complex.