A subunit vaccine is a vaccine that contains purified parts of the pathogen that are antigenic, or necessary to elicit a protective immune response.
A "subunit" vaccine doesn't contain the whole pathogen, unlike live attenuated or inactivated vaccine, but contains only the antigenic parts such as protein subunit,
The first recombinant subunit vaccine was produced in the mid-1980s to protect people from Hepatitis B. Other recombinant subunit vaccines licensed include Engerix-B (hepatitis B), Gardasil 9
After injection, antigens trigger the production of antigen-specific antibody, which are responsible for recognising and neutralising foreign substances. Basic components of recombinant subunit vaccines include recombinant subunits, adjuvants and carriers. Additionally, recombinant subunit vaccines are popular candidates for the development of against infectious diseases (e.g. tuberculosis,
Recombinant subunit vaccines are considered to be safe for injection. The chances of vary depending on the specific type of vaccine being administered. Minor side effects include injection site pain, fever, and fatigue, and serious consist of anaphylaxis and potentially fatal allergic reaction. The are also vaccine-specific; they are generally not recommended for people with the previous history of anaphylaxis to any component of the vaccines. Advice from medical professionals should be sought before receiving any vaccination.
Discovery
The first certified subunit vaccine by clinical trials on humans is the hepatitis B vaccine, containing the surface antigens of the hepatitis B virus itself from infected patients and adjusted by newly developed technology aiming to enhance the vaccine safety and eliminate possible contamination through individuals plasma.
Mechanism
Subunit vaccines contain fragments of the pathogen, such as protein or polysaccharide, whose combinations are carefully selected to induce a strong and effective immune response. Because the immune system interacts with the pathogen in a limited way, the risk of
is minimal.
An effective vaccine would elicit the immune response to the antigens and form immunological memory that allows quick recognition of the pathogens and quick response to future infections.
A drawback is that the specific antigens used in a subunit vaccine may lack pathogen-associated molecular patterns which are common to a class of pathogen. These molecular structures may be used by for danger recognition, so without them, the immune response may be weaker. Another drawback is that the antigens do not infect cells, so the immune response to the subunit vaccines may only be Humoral immunity, not cell-mediated, and as a result, is weaker than those elicited by other types of vaccines.
To increase immune response, adjuvants may be used with the subunit vaccines, or booster doses may be required.
Types
>+Summary of subunit vaccine types |
!Types
!Description
!Examples
|
| >Protein subunit | Protein subunit]] from [[pathogen]]s ([[virus]] or [[bacteria]]) | hepatitis B, acellular pertussis vaccines
|
| >Polysaccharide | Bacterial wall]] | Serotype]] A, C, W-135, and Y
|
| >Conjugate | contains polysaccharide chains bound to carrier proteins, such as diphtheria and [[tetanus toxoid]], to boost the [[immune response]] | Hib vaccine]], meningococcal conjugate vaccine
|
Protein subunit
A
protein subunit is a polypeptide chain or
protein molecule that assembles (or "
coassembles") with other protein molecules to form a
protein complex.
Large assemblies of proteins such as
viruses often use a small number of types of protein subunits as building blocks.
A key step in creating a recombinant protein vaccine is the identification and isolation of a protein subunit from the pathogen which is likely to trigger a strong and effective immune response, without including the parts of the virus or bacterium that enable the pathogen to reproduce. Parts of the protein shell or
capsid of a virus are often suitable. The goal is for the protein subunit to prime the immune system response by mimicking the appearance but not the action of the pathogen.
Another protein-based approach involves self‐assembly of multiple protein subunits into a virus-like particle (VLP) or nanoparticle. The purpose of increasing the vaccine's surface similarity to a whole virus particle (but not its ability to spread) is to trigger a stronger immune response.
Protein subunit vaccines are generally made through protein production, manipulating the gene expression of an organism so that it expresses large amounts of a recombinant gene.
Protein-based vaccines are being used for hepatitis B and for human papillomavirus (HPV).
Polysaccharide subunit
Vi capsular polysaccharide vaccine (ViCPS) against
typhoid caused by the Typhi serotype of
Salmonella enterica.
Instead of being a protein, the Vi antigen is a bacterial capsule polysacchide, made up of a long sugar chain linked to a lipid.
Capsular vaccines like ViCPS tend to be weak at eliciting immune responses in children. Making a conjugate vaccine by linking the polysacchide with a
toxoid increases the efficacy.
Conjugate vaccine
A conjugate vaccine is a type of
vaccine which combines a weak
antigen with a strong antigen as a carrier so that the
immune system has a stronger response to the weak antigen.
Peptide subunit
A
peptide vaccine employs a
peptide instead of a full
protein.
Peptide-based subunit vaccine mostly used due to many reasons,such as, it is easy and affordable for massive production. Adding to that, its greatest stability, purity and exposed composition.
Three steps occur leading to creation of peptide subunit vaccine;
-
Epitope recognition
-
Epitope optimization
-
Peptide immunity improvement
Features
When compared with conventional attenuated vaccines and inactivated vaccines, recombinant subunit vaccines have the following special characteristics:
-
They contain clearly identified compositions which greatly reduces the possibility of presence of undesired materials within the vaccine.
-
Their pathogenicities are minimized as only fragments of the pathogen are present in the vaccine which cannot invade and multiply within the human body.
-
They have better
-
They are suitable for mass production due to the use of recombinant technologies.
-
They have high stability so they can withstand environmental changes and are more convenient to be used in community settings.
However, there are also some drawbacks regarding recombinant subunit vaccines:
-
Addition of adjuvants is necessary during manufacturing to increase the efficacy of these vaccines.
-
Patients will have to receive to maintain long-term immunity.
-
Selection of appropriate cell lines for the cultivation of subunits is time-consuming because microbial proteins can be incompatible to certain expression systems.
Pharmacology
Vaccination is a potent way to protect individuals against infectious diseases.
Active immunity can be acquired artificially by vaccination as a result of the body's own defense mechanism being triggered by the exposure of a small, controlled amount of substances to produce its own antibodies and memory cells without being infected by the real pathogen.
The processes involved in primary immune response are as follows:
-
Pre-exposure to the present in elicits a primary response. After injection, antigens will be Phagocytosis by antigen-presenting cells (APCs), such as and , via phagocytosis.
-
The APCs will travel to , where immature and are present.
-
Following antigen processes by APCs, antigens will bind to either MHC class I receptors or MHC class II receptors on the cell surface of the cells based on their compositional and structural features to form complexes.
-
Antigen presentation occurs, in which T-cell receptor attach to the antigen-MHC complexes, initiating clonal expansion and differentiation, and hence the conversion of naive T cells to cytotoxic T cells (CD8+) or helper T cells (CD4+).
-
Cytotoxic CD8+ cells can directly destroy the infection cells containing the that were presented to them by the APCs by releasing lytic molecules, while helper CD4+ cells are responsible for the secretion of that activates and cytotoxic T cells.
-
can undergo activation in the absence of via the B cell receptor signalling pathway.
-
After capture the immunogen present in the vaccine, they can present the substances to naive B cells, causing the proliferation of for antibody production.
-
Memory B cells and T cells are formed post-infection.
Under specific circumstances, low doses of are given initially, followed by additional doses named . Boosters can effectively maintain the level of memory cells in the human body, hence extending a person's immunity.
Manufacturing
The manufacturing process of recombinant subunit
are as follows:
-
Identification of Immunogenicity subunit
-
Subunit expression and synthesis
-
Extraction and purification
-
Addition of or incorporation to vectors
-
Formulation and delivery.
Identification of immunogenic subunit
Candidate subunits will be selected primarily by their
immunogenicity.
To be
Immunogenicity, they should be of foreign nature and of sufficient complexity for the reaction between different components of the
immune system and the candidates to occur.
Candidates are also selected based on size, nature of function (e.g.
Cell signaling) and cellular location (e.g. transmembrane).
Subunit expression and synthesis
Upon identifying the target subunit and its encoding
gene, the
gene will be isolated and transferred to a second, non-pathogenic organism, and cultured for
mass production.
The process is also known as heterologous expression.
A suitable expression system is selected based on the requirement of post-translational modifications, costs, ease of product extraction and production efficiency. Commonly used systems for both licensed and developing recombinant subunit include bacteria, yeast, cells, insect cells.
Bacterial cells
Bacteria are widely used for
Cloning vector, genetic modification and small-scale productions.
is widely utilised due to its highly explored
genetics, widely available genetic tools for
gene expression, accurate profiling and its ability to grow in inexpensive
Growth medium at high cell densities.
E. Coli is mostly appropriate for structurally simple proteins owing to its inability to carry out post-translational modifications, lack of protein secretary system and the potential for producing inclusion bodies that require additional solubilisation.
Yeast
Yeast matches
Bacteria cost-effectiveness, efficiency and technical feasibility.
Moreover,
yeast secretes
soluble and has the ability to perform post-translational modifications similar to
cells.
Notably, yeast incorporates more
mannose molecules during N-glycosylation when compared with other
,
which may trigger cellular conformational stress responses. Such responses may result in failure in reaching native protein conformation, implying potential reduction of
Half-life and
immunogenicity.
Regarding application, both the
HBsAg (
HBsAg) and the virus-like particles (
VLPs) of the major capsid protein L1 of human papillomavirus type 6, 11, 16, 18 are produced by
Saccharomyces cerevisiae.
Mammalian cells
cells are well known for their ability to perform therapeutically essential post-translational modifications and express properly folded,
glycosylated and functionally active proteins.
However, efficacy of mammalian cells may be limited by
Epigenetics gene silencing and
aggresome formation (recombinant protein aggregation).
For mammalian cells, synthesised proteins were reported to be secreted into chemically defined media, potentially simplifying protein extraction and purification.
The most prominent example under this class is Chinese Hamster Ovary (CHO) cells utilised for the synthesis of recombinant varicella zoster virus surface glycoprotein (gE) antigen for Shingrix.
Baculovirus (insect) cells
The
Baculoviridae-
insect cell expression system has the ability to express a variety of recombinant proteins at high levels and provide significant eukaryotic protein processing capabilities, including
phosphorylation,
glycosylation,
myristoylation and
palmitoylation.
Similar to
cells, proteins expressed are mostly
soluble, accurately folded, and biologically active.
However, it has slower growth rate and requires higher cost of
growth medium than
bacteria and
yeast, and confers
toxicological risks.
A notable feature is the existence of elements of control that allow for the expression of secreted and
membrane-bound proteins in Baculovirus-insect cells.
Licensed recombinant subunit that utilises Baculoviridae-insect cells include Cervarix (papillomavirus C-terminal truncated major capsid protein L1 types 16 and 18)
Extraction and purification
Throughout history, extraction and purification methods have evolved from standard
chromatography to the utilisation of
affinity tags.
However, the final extraction and purification process undertaken highly depends on the chosen expression system. Please refer to subunit expression and synthesis for more insights.
Addition of adjuvants
Adjuvants are materials added to improve
immunogenicity of recombinant subunit
.
Adjuvants increase the magnitude of adaptive response to the vaccine and guide the activation of the most effective forms of immunity for each specific pathogen (e.g. increasing generation of T cell memory).
Appropriate adjuvants are chosen based on safety, tolerance, compatibility of antigen and manufacturing considerations.
Formulation and delivery
Drug delivery are primarily divided into polymer-based
Drug delivery (
microspheres and
) and live
Drug delivery (gram-positive bacteria, gram-negative bacteria and
)
Polymer-based delivery systems
Antigen are often encapsulated within
microparticle or
. Common microspheres made using
Polylactic acid and
PLGA allow for controlled
antigen release by degrading in vivo while
including multilamellar or unilamellar vesicles allow for prolonged release.
Polymer-based drug delivery confer advantages such as increased resistance to degradation in Gi tract, controlled antigen release, raised particle uptake by immune cells and enhanced ability to induce cytotoxic T cell responses.
Live delivery systems
Live
drug delivery, also known as vectors, are cells modified with ligands or
to improve the
immunogenicity of recombinant subunits via altering antigen presentation,
biodistribution and trafficking.
Subunits may either be inserted within the carrier or genetically engineered to be expressed on the surface of the vectors for efficient presentation to the mucosal immune system.
Advantages and disadvantages
Advantages
-
Cannot revert to virulence meaning they cannot cause the disease they aim to protect against
-
Safe for immunocompromised patients
-
Can withstand changes in conditions (e.g. temperature, light exposure, humidity)
Disadvantages
-
Reduced immunogenicity compared to attenuated vaccines
-
Require adjuvants to improve immunogenicity
-
Often require multiple doses (Booster dose) to provide long-term immunity
-
Can be difficult to isolate the specific Antigen which will invoke the necessary immune response
-
It is not easy to supervise conjugation chemistry which leads to noncontinuous variation
Adverse effects and contraindications
Recombinant subunit
are safe for administration.
However, mild local reactions, including
induration and swelling of the injection site, along with
fever,
fatigue and
headache may be encountered after vaccination.
Occurrence of severe
hypersensitivity reactions and
anaphylaxis is rare,
but can possibly lead to
of individuals.
can vary among populations depending on their
physical health condition, age,
gender and
Genetics predisposition.
Recombinant subunit are contraindication to people who have experienced allergic reactions and anaphylaxis to or other components of the previously.
Licensed vaccines
Hepatitis B
Engerix-B (produced by GSK) and
Recombivax HB (produced by merck) are two recombinant subunit
licensed for the protection against
hepatitis B. Both contain
HBsAg harvested and purified from
Saccharomyces cerevisiae and are formulated as a suspension of the
antigen adjuvanted with
alum.
Antibody concentration ≥10mIU/mL against HBsAg are recognized as conferring protection against hepatitis B infection.
It has been shown that primary 3-dose vaccination of healthy individuals is associated with ≥90% seroprotection rates for Engerix-B, despite decreasing with older age. Lower seroprotection rates are also associated with presence of underlying chronic diseases and immunodeficiency. Yet, GSK HepB still has a clinically acceptable safety profile in all studied populations.
Human Papillomavirus (HPV)
Cervarix,
Gardasil and
Gardasil are three recombinant subunit
licensed for the protection against HPV infection. They differ in the strains which they protect the patients from as
Cervarix confers protection against type 16 and 18,
confers protection against type 6, 11, 16 and 18,
and Gardasil 9 confers protection against type 6, 11, 16, 18, 31, 33, 45, 52, 58
respectively. The
contain purified
VLPs of the major capsid L1 protein produced by recombinant
Saccharomyces cerevisiae.
It has been shown in a 2014 systematic quantitative review that the bivalent HPV vaccine (Cervarix) is associated with pain (OR 3.29; 95% CI: 3.00–3.60), Allergy (OR 3.14; 95% CI: 2.79–3.53) and redness (OR 2.41; 95% CI: 2.17–2.68) being the most frequently reported adverse effects. For Gardasil, the most frequently reported events were pain (OR 2.88; 95% CI: 2.42–3.43) and Allergy (OR 2.65; 95% CI: 2.0–3.44).
Gardasil was discontinued in the U.S. on May 8, 2017, after the introduction of Gardasil 9
Influenza
Flublok Quadrivalent is a licensed recombinant subunit
vaccine for active
immunization against
influenza. It contains HA proteins of four strains of
influenza virus purified and extracted using the
Baculoviridae-
insect expression system. The four viral strains are standardised annually according to United States Public Health Services (USPHS) requirements.
Flublok Quadrivalent has a comparable safety profile to traditional trivalent and quadrivalent vaccine equivalents. Flublok is also associated with less local reactions (RR = 0.94, 95% CI 0.90–0.98, three RCTs, FEM, I2 = 0%, low‐ certainty evidence) and higher risk of chills (RR = 1.33, 95% CI 1.03–1.72, three RCTs, FEM, I2 = 14%, low‐certainty evidence).
Herpes Zoster
Shingrix is a licensed recombinant subunit
vaccine for protection against
Herpes zoster, whose risk of developing increases with decline of varicella zoster virus (VZV) specific immunity. The vaccine contains
VZV gE antigen component extracted from CHO cells, which is to be reconstituted with
adjuvant suspension AS01
B.
Systematic reviews and meta-analysis have been conducted on the efficacy, effectiveness and safety of Shingrix in immunocompromised 18–49 year old patients and healthy adults aged 50 and over. These studies reported Humoral immunity and cell-mediated immunity rate ranged between 65.4 and 96.2% and 50.0–93.0% while efficacy in patients (18–49 yo) with haematological malignancies was estimated at 87.2% (95%CI, 44.3–98.6%) up to 13 months post-vaccination with an acceptable safety profile.
COVID-19
Nuvaxovid is a recombinant subunit vaccine licensed for the prevention of SARS-CoV-2 infection. Market authorization was issued on 20 December 2021.
The vaccine contains the SARS-CoV-2 spike protein produced using the
Baculoviridae expression system, which is eventually adjuvanted with the
Matrix-M adjuvant.
History
While the practice of
immunization can be traced back to the 12th century, in which ancient
Chinese people at that time employed the technique of
variolation to confer immunity to
smallpox infection, the modern era of vaccination has a short history of around 200 years. It began with the
invention of a vaccine by Edward Jenner in 1798 to eradicate
smallpox by injecting relatively weaker
cowpox virus into the human body.
The middle of the 20th century marked the golden age of vaccine science. Rapid technological advancements during this period of time enabled scientists to cultivate cell culture under controlled environments in laboratories,
Emergence of genetic engineering techniques revolutionised the creation of vaccines. By the end of the 20th century, researchers had the ability to create recombinant vaccines apart from traditional whole-cell vaccine, for instance Hepatitis B vaccine, which uses the viral to initiate .
As the manufacturing methods continue to evolve, vaccines with more complex constitutions will inevitably be generated in the future to extend their therapeutic applications to both infectious and non-infectious diseases, in order to safeguard the health of more people.
Future directions
Recombinant subunit
are used in development for
tuberculosis,
,
soil-transmitted helminths,
feline leukaemia
and COVID-19.
Subunit vaccines are not only considered effective for SARS-COV-2, but also as candidates for evolving immunizations against malaria, tetanus, salmonella enterica, and other diseases.
COVID-19
Research has been conducted to explore the possibility of developing a heterologous
SARS-CoV receptor-binding domain (RBD) recombinant protein as a human
vaccine against COVID-19. The theory is supported by evidence that
convalescence serum from
SARS-CoV patients have the ability to neutralise SARS-CoV-2 (corresponding virus for COVID-19) and that amino acid similarity between
SARS-CoV and SARS-CoV-2 spike and RBD protein is high (82%).
