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Chapter 18
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Use of Nanotechnology in the Formulation of Vaccines
Yvonne Perrie and Cameron Webb
18.1 Immunity and Vaccination
Infectious diseases remain among the leading causes of death worldwide and vaccination offers one of the most effective strategies in global healthcare to address this. Vaccines harness the capacity of the human immune system to recognise and remember encounters with pathogen antigens. Despite rapid developments in vac­cine technology, there is an urgent need for more vaccines targeting challenging pathogens like Mycobacterium tuberculosis (the causative agent of tuberculosis). Moreover, it is essential to address antigenically diverse pathogens such as HIV, control outbreaks that pose a global health security threat (such as Ebola), and explore strategies to reinvigorate immune responses within the ageing immune system.
Nanoparticles have emerged as pivotal components in vaccine development, offering signicant contributions to various vaccine platforms, including mRNA vaccines. Nanoparticles offer unique properties that enable them to play a crucial role in enhancing the effectiveness and efcacy of various vaccine formulations. Given their versatility, nanoparticles can encapsulate and protect vaccine compo­nents, whether they are viral proteins, bacterial fragments, nucleic acids, or other immunogenic components. This protection shields antigens from degradation and facilitates appropriate cell delivery, thereby driving potency. Additionally, nanoparticles can be engineered to mimic the size, shape, and surface features of pathogens, thereby activating specic immune pathways and promoting potent immune responses. Surface modication of nanoparticles can also promote the
Y. Perrie () · C. Webb Strathclyde Institute of Pharmacy and Biomedical Sciences, Strathclyde Institute of Pharmacy and Biomedical Sciences, University of Strathclyde, Glasgow, UK e-mail: yvonne.perrie@strath.ac.uk
© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 I. F. Uchegbu et al. (eds.), Fundamentals of Pharmaceutical Nanoscience,
https://doi.org/10.1007/978-3-031-59478-6_18
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targeting of specic cells or tissues within the immune system, enhancing antigen uptake and presentation to immune cells, such as dendritic cells. Finally, nanoparticles can act as adjuvants in their own right, boosting the immune response and promoting long-lasting immunity. This combination of characteristics makes nanoparticles invaluable tools in the development of vaccines.
18.1.1 Developing Immune Responses
Regardless of vaccine innovation and development, the key and fundamental attri­bute that all vaccines commonly share is immunity against a pathogen without prior exposure to the specic disease in question. To achieve this, the vaccine aims to closely copy foreign pathogensmechanisms of action that induce the immune response without the pathogenicity and strong immune responses associated with the pathogens exposure. To achieve this, both innate and acquired immune responses must be activated (Fig. 18.1).
Innate immunity is immunity that an organism is born with. This type of immunity is part of our genetic prole and offers lifelong protection. The innate immune response is fast-acting and non-specic, so it responds the same irrespective of the pathogen it detects. In humans, the innate immune system encompasses physical and chemical barriers, that provide a rst line of defence with chemical
Fig. 18.1 Schematic of the three layers of immune defence. The rst layer consists of physical and physiological barriers, including the skin and mucous membranes (e.g. mouth, lung), body tem­perature, and differences in pH (e.g. in the stomach). If the pathogen crosses these barriers, it then faces innate (non-specic) and adaptive immune responses. (Created with
BioRender.com)
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(humoral) and cellular aspects as the second line of defence. Physical barriers that protect us from infection include our skin, eyelashes, tears mucous, and stomach acid. Chemical defences include complement activation and inammatory responses. Cellular defences within the innate immune response include phagocytes, natural killer cells, and mast cells. Their role is to identify non-self-pathogens, neutralise or destroy them, and activate the adaptive immune
The activation of the innate immune response characterises the initial stages of infection. The nature of the immune response is dependent on whether the threat is intracellular, such as a virus, or extracellular, like a bacterium. During this phase, the innate immune response identies potential invaders by detecting the absence of host MHC-class I molecules that signify selfor by recognising molecules commonly found in pathogens but not in the host. These recognised molecules are known as pathogen-associated molecular patterns (PAMPs) and their identication triggers processes such as phagocytosis or cytotoxic killing to eliminate the invaders. In the case of intracellular threats, specic receptors called pattern recognition receptors (PRRs) detect foreign components such as viral RNA, DNA, or intermediate products.
Consequently, the activation of PRRs leads to the production of pro-inammatory cytokines, the apoptosis of infected cells, or the labelling of infected cells for targeted destruction by other innate immune cells. Phagocytic cells, including dendritic cells, can internalise foreign substances and process them. Through this process, these cells break down the engulfed material and display fragments of it on their cell surfaces, which plays a crucial role in initiating the adaptive immune response. Due to this characteristic, they are classied as antigen-presenting cells (APCs).
When the innate immune system alone proves insufcient for controlling a foreign threat, the adaptive immune system is triggered. In contrast, acquired immunity is not inherent in organisms and is not genetically predet ermined, like innate immunity. Adaptive (or acquired) immunity is acquired throughout our lifetime due to exposure to specic antigens, either through natural means such as infection or via vaccination. It is characterised by its specicity, memory, and ability to mount a faster and stronger response upon re-exposure to the same antigen. Unlike the rapid and generalised response of innate immuni ty, the adaptive immune response is slower, taking days or even weeks to develop after initial encounter (known as the primary immune response). However, it exhibits specicity towards the particular antigen(s) present and can retain long-term immunological memory. This memory allows for a quicker and more efcient response if the same antigen is re-encountered in the future. However, adaptive immunity can wane, particularly if there is no regular re-exposure to the antigen.
The adaptive immune system is triggered through the action of signalling mole­cules and/or the presentation of antigens by antigen-presenting cells. Among these antigen-presenting cells, dendritic cells play a crucial role by displaying major histocompatibility complex class II (MHC class II) molecules on their surface, facilitating the presentation of foreign peptides and ensuring appropriate immune activation. The adaptive immune response encompasses two main components: the
response.
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cell-mediated immune response and the humoral immune response. The cell­mediated response is driven by T cells, which can directly eliminate infected cells or help stimulate B cells towards antibody production. Upon activation, naïve B cells differentiate into memory cells and plasma cells. Memory cells and plasma cells are integral to long-term immunity. Plasma cells secrete large quantities of antigen­specic antibodies, effectively neutralising and elimina throughout their lifespan. Memory cells, on the other hand, can persist for extended periods, reactivating upon re-exposure to their specic targe t antigen. This reactivation prompts the production of antibodies, resulting in a swifter and more robust response to repeated exposures.
ting the target pathogen
18.1.2 How Do Vaccines Promote Immune Responses?
As outlined, our immune system is developed to offer a wide range of protection; however, one issue is that during an infection the body must be able to respond quickly and vigorously enough to provide the appropriate protection without the individual suffering the potentially lethal consequences of the infection. To address this, vaccines have been developed. A vaccine can be dened as a biological product that can safely induce an immune response that promotes protection against infection and/or disease on subsequent exposure to a pathogen (Pollard and Bijker 2021). The goal of a vaccine is to develop long-lived immunological protection, whereby the rst encounter with a pathogen is remembered and recognised by the immune system. Therefore, the immune system can generate a rapid, protective response against the infection. To do this, the vaccine should contain antigens that represent components of the pathogen. These antigens are normally protein-based and can be derived directly from the pathogen or produced synthetically. However, polysac­charide antigens can also be used to induce immune responses. Nanoparticles can support vaccine efcacy by protecting and delivering such antigens to immune cells, thereby supporting the priming of the immune system so that subsequent exposure to the antigen results in a rapid immune response (Fig.
18.2).
18.2 Current Vaccines Options
In their traditional organisation, vaccines are classied as live and non-live. This classication allows us to distinguish between vaccines that contain attenuated replicating strains of the pathogenic organism and those that contain only compo­nents of the pathogen. More recently, several other platforms have been developed and many of these exploit nanoparticles to promote immune responses.
Live-attenuated vaccines are composed of microbial agents that have undergone specic mutations, rendering them less capable of growing in human cells and no longer pathogenic to humans. Although these microorganisms can still infect their
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Fig. 18.2 (a ) Examples of nanoparticles that can be used to improve the delivery of vaccines by (b) promoting a primary response and acquired immunity such that when the immune system encoun­ters the pathogen, it can quickly respond to infection. (Created with
BioRender.com)
target cells, the resulting infection is mild, and the replication of the microorganisms is limited. Prominent examples of vaccines produced using this approach include the Bacillus Calmette–Guérin (BCG) vaccine and the combined measles, mumps, rubella, and yellow fever vaccine (MMR). Such vaccines are generally effective in eliciting both humoral and cell-mediated immune responses. However, it is impor­tant to note that live-attenuated vaccines carry a potent
ial risk of reverting to virulence. Therefore, their administration is not considered safe for immunocom­promised individuals. Despite this limitation, these vaccines have proven to be valuable tools for promoting immune protection against targeted diseases.
Killed whole or inactivated vaccines are composed of microorganisms or viruses that have been rendered non-infectious through processes such as heat treatment or chemical treatment with substances like formaldehyde. These methods effectively eliminate their ability to cause infection while preserving their immunogenicity. While these vaccines provide safety advantages, they generally exhibit lower ef­cacy compared to live-attenuated vaccines. They typically induce primarily humoral immunity and often necessitate booster doses to maintain protection. Several exam­ples of killed whole or inactivated vaccines include trivalent-inactivated inuenza vaccines, cholera vaccines, and hepatitis A vaccines.
Toxoid vaccines are produced by some microorganisms that produce toxic com­pounds responsible for causing the disease (i.e. tetanus toxin and diphtheria toxin). Toxoids are inactivated forms of these toxic compounds. In addition to being successful vaccines, toxoids may also be used to increase the immunogenicity of some other vaccines, such as Haemophilus inuenzae type B (Hib), which contains a polysaccharide unit from the virus conjugated to diphtheria or tetanus toxins.
Subunit vaccines, which include puried proteins, recombinant proteins, poly- saccharides, and synthetic peptides, can initiate strong immune responses when combined with adjuvants. The main benets of subunit vaccines are their inability to revert to a pathogenic form, decreased toxicity, reproducible production, and improved antigen specicity; however, the immune response induced by such
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vaccines is short-lived, and thus several boosts are required to achieve protection. For the Hepatitis B virus, for example, only the surface protein of the virus is used to generate the subunit vaccine.
Virus-like vesicles mimic the characteristics of viruses while lacking their genetic material. They are derived from the outer membrane of cells, typically from bacteria or mammalian cells, through various techniques such as cell disruption or membrane blebbing. These vesicles can be engineered to display specic antigens on their surface, triggering an immune response upon administration. The Group B menin­gococcal vaccine is an example of this type of vaccine.
Proteinpolysaccharide conjugates represent a class of vaccines where poly­saccharides from pathogenic microorganisms are chemically linked to carrier pro­teins. This conjugation process enhances the immunogenicity of the polysaccharides, as they alone may not elicit a robust immune response. The carrier protein serves as a scaffold, enabling the immune system to recognise, and mount a stronger immune response against the attached polysaccharide. Examples of these types of vaccines include Streptococcus pneumoniae and Haemophilus inuenzae type B vaccines.
Nucleic acid vaccines use either DNA or messenger RNA (mRNA) to deliver genetic instructions to cells within the body. DNA vaccines contain a small, circular piece of DNA encoding the antigen of interest, while mRNA vaccines carry a synthesised mRNA sequence encoding the antigen. Once administered, the genetic material is taken up by cells, which use it as a blueprint to produce the antigen internally. This antigen production triggers an immune response, stimulating the production of specic antibodies, and activating immune cells. Examples of mRNA vaccines include the Pzer-BioNTech and Moderna COVID-19 vaccines.
Recombinant viral vector vaccines use the replication machinery of viruses to deliver and express specic antigens of interest. These vaccines employ harmless viral vectors, engineered to carry and deliver genetic material encoding the desired antigen into the cells of the vaccinated individual. The viral vectors, such as adenoviruses or modied vaccinia viruses, have been modied to remove their ability to cause disease while retaining their ability to efciently infect cells and produce the desired antigens. Once inside the host cells, the viral vec tors instruct the cells to produce the antigen, triggering an immune response. Examples of these types of vaccines include the Oxford-AstraZeneca SARS-CoV-2 vaccine.
Not all these options may be applicable for a given pathogen and Fig. an overview of options for vaccine develo pment against a viral infection. In general, live vaccines produce strong immune responses but may induce some mild disease and may have the potential to replicate in an uncontrolled manner in immunocom­promised individuals. In contrast, non-live vaccines do not present this risk; however they often require an adjuvant to improve their ability to induce an immune response.
On several of these vaccine platforms, delivery systems and/or adjuvants are required; DNA and RNA vaccines require a delivery system to protect the nucleic acid payload, while peptide and sub-unit vaccine formulations need adjuvants to improve immune response, and their mechanisms of inducing immune responses can differ (Fig.
18.4). The mRNA vaccines are delivered to bystander cells inside lipid
18.3 gives
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Fig. 18.3 An overview of different platforms for viral vaccine development. Strategies include live attenuated, inactivated, and use of synthetic peptides and recombinant approaches. (a) Live attenuated, (b) whole inactivated, (c) split inactivated, (d) synthetic peptides, (e) virus-like particles, (f) DNA or RNA, (g) recombinant subunits, (h) recombinant bacterial vectors, (i) recombinant viral vectors. (Created with BioRender.com)
Fig. 18.4 Immune responses induced by nanoparticle-based vaccines. mRNA vaccines use lipid nanoparticles to deliver modied antigen-coding mRNA to bystander cells at the injection site. The translated antigen is then secreted, taken up, and processed by antigen-presenting cells (APCs). In sub-unit protein-based vaccines, the protein antigen can be entrapped or associated with liposomes, allowing direct processing by APCs. Virosomes, containing protein antigens on their surface nanoparticles, are also directly processed by APCs. (Created with BioRender.com)