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Figure 1.1 The interactions of NPs with the target antigen. Reproduced (adapted) from [19]. Copyright 2013, Elsevier.

      Nanotechnology has opened the way for developing novel vaccines based on nanomaterials, which have unique qualities and serve as antigenic delivery systems and immunomodulatory substances. Multiple research groups in the area are developing nanovaccines for a variety of diseases, and tremendous advantages from this nanotechnology are expected in the coming decades for both animal and human health.

1.2 Nanoparticles, an Alternative Approach to Conventional Vaccines

As an alternative to traditional vaccinations, the utilization of NPs exhibiting relevant antigenic moieties seems promising. These NPs might come from biological or synthetic sources. Inorganic NPs, polymer NPs, liposomes, virus-like particles (VLPs) NPs, and self-assembled protein NPs are among the antigen carriers currently being studied (Figure 1.2). Many biological systems, including infectious agents and biomolecules, are nanoscale in size [20]. NPs have been injected subcutaneously or intramuscularly, and they have the ability to pass through capillaries [21]. Recent advancements have made it possible to create NPs with distinct physical and chemical characteristics. The shape, size, solubility, surface composition, and so on can all be tuned and manipulated, allowing for the development of NPs with particular biological features. They can also be engineered to include a variety of compounds, including antigens, making them extremely valuable in vaccine development [22, 23]. Antigens can be incorporated into NPs through encapsulation or conjugation [24]. The native structure of antigens can be protected against proteolytic breakdown by NPs, and antigen transportation to antigen-presenting cells (APCs) can be improved [25]. Furthermore, NPs containing antigens have a local depot effect to ensure the presence of antigens in immune cells for a longer period of time [26]. Interestingly, carbon black (CB) NPs, carbon nanotubes (CNTs), poly lactic-co glycolic acid (PLGA) NPs, titanium dioxide (TiO₂) NPs, and silicon dioxide (SiO₂) NPs have all been shown to exhibit intrinsic immunomodulatory properties [27]. In point of fact, once ingested by APCs, these NPs transmit a signal that promote proteolytic degradation and the induction of oxidative stress, resulting in the release of lysosomal contents [28, 29]. These features indicate that NPs could be useful antigen transporters and innate immune stimulators in vaccinations.

Figure 1.2 Antigens are delivered to antigen-presenting cells (APCs) using surface-engineered NPs. Reproduced (adapted) from [30]. Copyright 2018, Frontiers.

1.3 Nanoparticles as Vaccine Delivery Vehicle

Liposomes, NPs having a phospholipid bilayer, have been used in the pharmaceutical industry since 1960 [31]. NPs have a diameter of 1 to 100 nm (Figure 1.3), whereas antigens or DNA segments have 1 to 10 nm [32]. Therefore, they are susceptible to being absorbed by biological systems, and it is the first step in eliciting immunogenicity. A nanovaccine is composed of NPs and can be composed of protein, lipid, metals, polymers, and other materials. The positive strategy of nanovaccines allows for not only antigen enhancement but also vaccine stability, immunogenicity, and effective target delivery. Due to advancements in nanovaccinology, many nanovaccines have already been licensed for clinical use, and others are in clinical trials [33–35]. Adjuvants and multiepitope antigens may be effectively co-delivered into target cells and APCs using nanovaccines, and the release of antigen in the cytoplasm and antigen cross-presentation can be precisely controlled.

Figure 1.3 The size comparison of various biological systems used in nanovaccinology. Reproduced (adapted) from [36]. Copyright 2016, IntechOpen.

1.4 Nanotechnology to Tackle the Challenges of Vaccine Delivery

       1.4.1 Polymeric Nanoparticles

Polymeric NPs can be made by polymerizing monomeric units or by using synthesized polymers. The customizable qualities, size, composition, and surface properties of these NPs make them appealing in the medical industry because they allow for regulated release and drug molecule protection [37]. Poly (lactic acid) (PLA), poly (glutamic acid) (PGA), PLGA, and chitosan are the most frequently used polymeric NPs for vaccine formulation [19]. The chemical structure of polymeric NPs is designed to behave differently in different environments to control the release rate. The function and uses of other polymeric NPs vary substantially depending on the inherent chemical and biological effects in distinct materials [38]. PLGA is a biodegradable and biocompatible polymer that can undergo in vivo hydrolysis [39] and encapsulate and release different biomolecules over time [40–42]. Under physiological conditions, these NPs may incorporate antigens and inhibit their destruction for one month, which is crucial for vaccination [43]. Furthermore, PLGA NPs promote antigen processing and display to susceptible cells by promoting antigen uptake by APCs [44]. Other NPs, chitosan, and N-(2-hydroxypropyl)methacrylamide/N-isopropyl acrylamide were also investigated as vaccines preparation [45–47]. Chitosan is a favorable and promising polymer for DNA vaccine formulation because of its cationic character, which allows attachment to the DNA, generating DNA–polymer interactions that protect DNA from enzyme hydrolysis. It is indeed extremely hydrophobic, inactive, and non-immunogenic, with bio adhesive properties that facilitate mucosal vaccination techniques. Chitosan has been further discovered to have natural adjuvant properties, stimulating dendritic cell maturation to enhance T helper 1 (Th1)

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