Polymeric nanoparticle vaccines to combat emerging and pandemic threats

Abstract

Subunit vaccines are more advantageous than live attenuated vaccines in terms of safety and scale-up manufacture. However, this often comes as a trade-off to their efficacy. Over the years, polymeric nanoparticles have been developed to improve vaccine potency, by engineering their physicochemical properties to incorporate multiple immunological cues to mimic pathogenic microbes and viruses. This review covers recent advances in polymeric nanostructures developed toward particulate vaccines. It focuses on the impact of microbe mimicry (e.g. size, charge, hydrophobicity, and surface chemistry) on modulation of the nanoparticles' delivery, trafficking, and targeting antigen-presenting cells to elicit potent humoral and cellular immune responses. This review also provides up-to-date progresses on rational designs of a wide variety of polymeric nanostructures that are loaded with antigens and immunostimulatory molecules, ranging from particles, micelles, nanogels, and polymersomes to advanced core-shell structures where polymeric particles are coated with lipids, cell membranes, or proteins.

Keywords: Cellular immunity; Humoral immunity; Nanoparticle vaccine; Polymer nanostructure; Self-assembly; Subunit vaccine.

Graphical abstract

Fig. 1

Engineering polymeric nanostructures for vaccine development. The physicochemical properties of the particulate polymers can be engineered to enhance desirable immune responses, including size, surface chemistry, controlled release, charges, and hydrophobicity.

Fig. 2

Antigen processing and presentation in a dendritic cell (DC) that lead to activation of CD4⁺ T-cells (left) and CD8⁺ T-cells (right). (1) Antigen-loaded particles are internalized by DC through cell uptake pathways such as phagocytosis or endocytosis. To activate CD4⁺ T cells, (2a) particulate antigens are processed to peptide fragments by proteases in endosomes, (3a) subsequently loaded onto MHC II molecules, (4a) and the formed MHC II–peptide complex is then trafficked to the cell surface where it presents the antigen to CD4⁺ T cells bearing cognate T cell receptors (TCRs). To activate CD8⁺ T cells (following the cytosolic pathway as depicted), (2b) the antigens are translocated to cytosol and (3b) subsequently processed by proteasome. (4b) The resulting peptide fragments are transported to endoplasmic reticulum (ER), (5b) then loaded onto MHC I molecules in ER, followed by (6b) trafficking of the formed MHC I–peptide complex to cell surface where they can interact with CD8⁺ T cells bearing cognate TCRs. Activated CD4⁺ T cells can differentiate into T helper type 1 (Th1), Th2, Th17 and T follicular helper (Tfh) cells producing various cytokines for signaling B cell activation, whereas activated CD8⁺ T cells can kill infected/mutated cells.

Fig. 3

(A) Modes of presentation of pattern-recognition receptor (PRR) agonists (depicted as red-filled hexagon) to polymeric nanoparticles: (a) by encapsulation and (b) by surface display, achieved through physical and chemical interactions. (B) Polymeric nanoparticles incorporated with PRR agonists target PRRs that are located at different cellular domains to shape adaptive immune responses. The polymeric nanoparticles can target TLR1, TLR2, TLR4, and TLR6 at cell membrane. Cellular uptake of the nanoparticles leads to signalling endosomal TLRs including TLR3, TLR7, TLR8, and TLR9. Escaping the endosomes is required to target RIG-I, NOD1 and NOD2 at cytosol, as well as STING at endoplasmic reticulum. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

(A) Different structural features of particulate polymers used for vaccine development. (B) Assembly/loading of antigens into the particulate polymers. (C) The size range of the polymeric nanostructures used in vaccine development.

Fig. 5

Polysaccharide nanoparticles for vaccine development against human immunodeficiency virus (HIV). (A–C) Three nanoparticle vaccine formulations formed based on complexation between oppositely charged polysaccharides containing HIV protease cleavage site peptide antigen (PCS5) and/or molecular adjuvant poly(I:C): (A) complexes between chitosan (CS) and dextran sulfate (DS) entrapping PCS5 (diameter, ~119 nm); (B) complexes between PCS5-conjugated CS and DS entrapping poly(I:C) (diameter, ~141 nm); and (C) complexes between CS and PCS5-conjugated hyaluronic acid (HA) entrapping poly(I:C) (diameter, ~211 nm). (D) Humoral immune responses in mice following intramuscular injection with each of the nanoparticle groups (50 mice per group) as compared to that in nontreated naïve mice. Arrows indicate the time of vaccination. Reproduced with permission from Ref. [146]. Copyright (2019) American Chemical Society.

Fig. 6

Polymeric nanoparticle vaccines against H1N1 influenza virus. The nanoparticles are composed of a complex between linear, cationic polymer poly(N,N-cystaminebis(acrylamide)-co-4-amino-1-butanol (pABOL) and self-amplifying RNA (saRNA) encoding hemagglutinin (HA) antigen from the H1N1 A/California/07/2009 strain. (A) Synthesis of pABOL through aza-Michael polyaddition of 4-amino-1-butanol to N,N-cystaminebis(acrylamide) catalyzed by triethylamine (a), and its subsequent ionic complexation with saRNA, and high transfection efficiency of the formed nanoparticles as compared to poly(ethyleneimine) (PEI) based nanoparticles (b). (B) Typical TEM image of pABOL-saRNA nanoparticles stained with 2% uranyl acetate (scale bar: 100 nm). (C) Immunogenicity of the nanoparticles at different pABOL molecular weights and saRNA doses after intramuscular vaccination of mice: HA-specific IgG titer (a), HA inhibition (HAI) titer of Cal/09 flu virus (b), and neutralization IC50 against Cal/09 flu virus (c). Reproduced with permission from Refs. [127]. Copyright (2020) American Chemical Society.

Fig. 7

Viromimetic nanoparticle vaccines against Middle East Respiratory Syndrome Coronavirus (MERS-CoV). (A) The nanoparticles are composed of lipid coated PLGA nanoshells that are loaded with the molecular adjuvant cdGMP at the core and conjugated with receptor binding domain (RBD) of the MERS-CoV spike antigen at the surface (a) and the representative cryo-TEM image of the nanoparticles (b). (B) Immunogenicity and protective efficacy of the nanoparticles after subcutaneous vaccination of human DPP4-transgenic mice on days 0 and 21: Titers of 100% neutralizing serum antibody after four weeks of the last administration (a), viral loads in the lungs of immunized mice following intranasal challenge with MERS-CoV EMC/2012 strain quantified using a Vero E6 cell-based assay (b) and quantitative PCR (c), and the survival curve for the challenged mice (d). Reproduced with permission from Ref. [96]. Copyright (2019) John Wiley and Sons.

Fig. 8

Multiantigenic nanotoxoids against Pseudomonas aeruginosa. (A) Synthesis and use of the nanotoxoids: (i) PLGA nanoparticles were coated with macrophages to enable capture, retention and hence neutralization of P. aeruginosa secretions (PaS) within the macrophage; (ii) subcutaneous or intranasal immunization of these nanotoxoids in mice elicited potent humoral immune responses for antibacterial protection against P. aeruginosa. (B) Immunogenicity and protective efficacy of the nanotoxoids as compared to blank solutions (10% sucrose) after subcutaneous vaccination in mice on days 0, 7, and 14: IgG titers in mice sera on day 21 (a), IgG titers in the lungs of immunized mice following intratracheal challenge with P. aeruginosa on day 35 (b), and bacterial loads in the lungs of immunized mice after the challenge (c). Reproduced with permission from Ref. [138]. Copyright (2019) American Chemical Society.

Fig. 9

Overview of the particulate vaccine platform technology based on a core-shell protein-coated poly(3-hydroxybutyric acid) (PHB) particle against viral and bacterial pathogens, and their robust immune responses. The particles coated with protein antigens can be formed in vivo through self-assembly in engineered, endotoxin-free E. coli., and are composed of (i) PHB and (ii) dimeric PhaC protein synthase that bridges the PHB at the core (through covalent bonds) with the protein antigens at the shells. Capsular-polysaccharide (CPS) antigens can also be chemically conjugated in vitro on the particle surface, e.g. 19F (CPS from S. pneumoniae serotype 19F) and MenC (CPS from N. meningtidis serogroup C).

Fig. 10

Bacterial polyester particles for the development of tuberculosis vaccines. The particles are composed of hydrophobic poly(3-hydroxybutyric acid) (PHB) displaying PhaC protein linked to the fusion protein antigens Ag85B-TB10.4-Rv2660c (H28) from Mycobacterium tuberculosis, called particle-H28. (A) Representative TEM images of Escherichia coli containing particle-H28 (left) and the purified particle-H28 (right). (B) Schematic overview of particle-H28 displaying multiple protein antigens. (C) Cytokine responses after subcutaneous injection of particle-H28 adjuvanted with dimethyl dioctadecyl ammonium bromide (DDA) micelles in mice as compared to placebo (DDA only), particle (displaying PhaC only), and soluble His₆-H28 peptides. Reproduced with permission from Ref. [131]. Copyright (2019) John Wiley and Sons.