Asubunit vaccine is avaccine that contains purified parts of thepathogen that areantigenic, or necessary to elicit a protectiveimmune response.[1][2] Subunit vaccine can be made from dissembled viral particles in cell culture orrecombinant DNA expression,[3] in which case it is arecombinant subunit vaccine.
A "subunit" vaccine doesn't contain the whole pathogen, unlikelive attenuated orinactivated vaccine, but contains only the antigenic parts such asproteins,polysaccharides[1][2] orpeptides.[4] Because the vaccine doesn't contain "live" components of the pathogen, there is no risk of introducing the disease, and is safer and more stable than vaccines containing whole pathogens.[1]Other advantages include being well-established technology and being suitable forimmunocompromised individuals.[2] Disadvantages include being relatively complex to manufacture compared to some vaccines, possibly requiringadjuvants andbooster shots, and requiring time to examine which antigenic combinations may work best.[2]
The first recombinant subunit vaccine was produced in the mid-1980s to protect people fromHepatitis B. Other recombinant subunit vaccines licensed includeEngerix-B (hepatitis B),Gardasil 9[5] (Human Papillomavirus),Flublok[6] (influenza),Shingrix[7] (Herpes zoster) andNuvaxovid[8] (Coronavirus disease 2019).
Afterinjection, antigens trigger the production of antigen-specificantibodies, 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 ofvaccines againstinfectious diseases (e.g.tuberculosis,[9]dengue[10]).
Recombinant subunit vaccines are considered to be safe for injection. The chances ofadverse effects vary depending on the specific type ofvaccine being administered. Minor side effects include injection site pain, fever, andfatigue, and seriousadverse effects consist ofanaphylaxis and potentially fatalallergic reaction. Thecontraindications are also vaccine-specific; they are generally not recommended for people with the previous history ofanaphylaxis to any component of the vaccines. Advice from medical professionals should be sought before receiving any vaccination.
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.[11]
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 ofside effects is minimal.[2]An effective vaccine would elicit the immune response to the antigens and formimmunological memory that allows quick recognition of the pathogens and quick response to future infections.[1]
A drawback is that the specific antigens used in a subunit vaccine may lackpathogen-associated molecular patterns which are common to a class of pathogen. Thesemolecular structures may be used byimmune cells for danger recognition, so without them, the immune response may be weaker. Another drawback is that the antigens do not infectcells, so the immune response to the subunit vaccines may only beantibody-mediated, notcell-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.[2]
| Types | Description | Examples |
|---|---|---|
| Protein subunit | containsisolated proteins frompathogens (virus orbacteria) | hepatitis B,acellular pertussis vaccines |
| Polysaccharide | contains chains ofpolysaccharides (sugar molecules) found in the pathogen's capsule such ascell walls of some bacteria | pneumococcal polysaccharide vaccine,meningococcal vaccine preventing diseases fromNeisseria meningitidisgroup A, C, W-135, and Y |
| Conjugate | contains polysaccharide chains bound tocarrier proteins, such asdiphtheria andtetanus toxoid, to boost theimmune response | pneumococcal conjugate vaccine,haemophilus influenzae type b conjugate vaccine,meningococcal conjugate vaccine |
Aprotein subunit is apolypeptide chain orprotein molecule that assembles (or "coassembles") with other protein molecules to form aprotein complex.[12][13][14] Large assemblies of proteins such asviruses often use a small number of types of protein subunits as building blocks.[15] 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 orcapsid 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.[16] Another protein-based approach involves self‐assembly of multiple protein subunits into avirus-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.[17][16][18]
Protein subunit vaccines are generally made throughprotein production, manipulating thegene expression of an organism so that itexpresses large amounts of arecombinant gene.[16][19] A variety of approaches can be used for development depending on the vaccine involved.[17]Yeast,baculovirus, ormammalian cell cultures can be used to produce large amounts of proteins in vitro.[16][19][20]
Protein-based vaccines are being used forhepatitis B and forhuman papillomavirus (HPV).[17][16] The approach is being used to try to develop vaccines for difficult-to-vaccinate-against viruses such asebolavirus andHIV.[21] Protein-based vaccines for COVID-19 tend to target either its spike protein or its receptor binding domain.[17] As of 2021, the most researched vaccine platform for COVID-19 worldwide was reported to be recombinant protein subunit vaccines.[16][22]
Vi capsular polysaccharide vaccine (ViCPS) againsttyphoid caused by the Typhi serotype ofSalmonella enterica.[23] Instead of being a protein, the Vi antigen is abacterial capsule polysacchide, made up of a long sugar chain linked to a lipid.[24] Capsular vaccines like ViCPS tend to be weak at eliciting immune responses in children. Making aconjugate vaccine by linking the polysacchide with atoxoid increases the efficacy.[25]
Aconjugate vaccine is a type ofvaccine which combines a weakantigen with a strong antigen as a carrier so that theimmune system has a stronger response to the weak antigen.[26]
Apeptide-based subunit vaccine employs apeptide instead of a fullprotein.[27] 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.[28] Three steps occur leading to creation of peptide subunit vaccine;[29]
When compared with conventionalattenuated vaccines andinactivated vaccines, recombinant subunit vaccines have the following special characteristics:
However, there are also some drawbacks regarding recombinant subunit vaccines:
Vaccination is a potent way to protect individuals againstinfectious diseases.[34]
Active immunity can be acquired artificially byvaccination as a result of the body's own defense mechanism being triggered by the exposure of a small, controlled amount ofpathogenic substances to produce its own antibodies and memory cells without being infected by the real pathogen.[35]
The processes involved in primary immune response are as follows:
Under specific circumstances, low doses ofvaccines are given initially, followed by additional doses namedbooster doses. Boosters can effectively maintain the level ofmemory cells in the human body, hence extending a person'simmunity.[32][33][42]
The manufacturing process of recombinant subunitvaccines are as follows:[citation needed]
Candidate subunits will be selected primarily by theirimmunogenicity.[43] To beimmunogenic, they should be of foreign nature and of sufficient complexity for the reaction between different components of theimmune system and the candidates to occur.[44] Candidates are also selected based on size, nature of function (e.g.signalling) and cellular location (e.g.transmembrane).[43]
Upon identifying the target subunit and its encodinggene, thegene will be isolated and transferred to a second, non-pathogenic organism, and cultured formass production.[45] The process is also known asheterologous expression.[citation needed]
A suitableexpression system is selected based on the requirement ofpost-translational modifications, costs, ease of product extraction and production efficiency. Commonly used systems for both licensed and developing recombinant subunitvaccines includebacteria,yeast,mammalian cells,insect cells.[46]
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Bacterial cells are widely used forcloning processes,genetic modification and small-scale productions.[47]Escherichia coli (E. Coli) is widely utilised due to its highly exploredgenetics, widely available genetic tools forgene expression, accurate profiling and its ability to grow in inexpensivemedia at high cell densities.[48]
E. Coli is mostly appropriate for structurally simple proteins owing to its inability to carry outpost-translational modifications, lack ofprotein secretary system and the potential for producinginclusion bodies that require additional solubilisation.[47][48][49] Regarding application,E.Coli is being utilised as theexpression system of thedengue vaccine.[10]
Yeast matchesbacterial cells' cost-effectiveness, efficiency and technical feasibility.[47] Moreover,yeast secretessolubleproteins and has the ability to performpost-translational modifications similar tomammalian cells.[49]
Notably, yeast incorporates moremannose molecules duringN-glycosylation when compared with othereukaryotes,[50] which may triggercellular conformational stress responses. Such responses may result in failure in reaching native protein conformation, implying potential reduction ofserum half-life andimmunogenicity.[47] Regarding application, both thehepatitis B virus surface antigen (HBsAg) and thevirus-like particles (VLPs) of the major capsid protein L1 ofhuman papillomavirus type 6, 11, 16, 18 are produced bySaccharomyces cerevisiae.[citation needed]
Mammalian cells are well known for their ability to perform therapeutically essentialpost-translational modifications and express properly folded,glycosylated and functionally active proteins.[48][51][52] However, efficacy of mammalian cells may be limited byepigeneticgene silencing andaggresome formation (recombinant protein aggregation).[48] For mammalian cells, synthesised proteins were reported to be secreted into chemically defined media, potentially simplifying protein extraction and purification.[47]
The most prominent example under this class isChinese Hamster Ovary (CHO) cells utilised for the synthesis of recombinantvaricella zoster virus surface glycoprotein (gE) antigen forSHINGRIX.[7]CHO cells are recognised for rapid growth and their ability to offer process versatility. They can also be cultured in suspension-adapted culture in protein-free medium, hence reducing risk ofprion-induced contamination.[47][48]
Thebaculovirus-insect cellexpression system has the ability to express a variety ofrecombinant proteins at high levels and provide significant eukaryotic protein processing capabilities, includingphosphorylation,glycosylation,myristoylation andpalmitoylation.[53] Similar tomammalian cells, proteins expressed are mostlysoluble, accurately folded, and biologically active.[54] However, it has slower growth rate and requires higher cost ofgrowth medium thanbacteria andyeast, and conferstoxicological risks.[47] A notable feature is the existence of elements of control that allow for the expression of secreted andmembrane-bound proteins in Baculovirus-insect cells.[47][53]
Licensed recombinant subunitvaccines that utilisesbaculovirus-insect cells includeCervarix (papillomavirus C-terminal truncated major capsid protein L1 types 16 and 18)[47][55] andFlublok Quadrivalent (hemagglutinin (HA) proteins from four strains ofinfluenza viruses).[6]
Throughout history, extraction andpurification methods have evolved from standardchromatographic methods to the utilisation ofaffinity tags.[56] However, the final extraction and purification process undertaken highly depends on the chosenexpression system. Please refer to subunit expression and synthesis for more insights.[citation needed]
Adjuvants are materials added to improveimmunogenicity of recombinant subunitvaccines.[57]
Adjuvants increase the magnitude ofadaptive response to thevaccine and guide the activation of the most effective forms ofimmunity for each specificpathogen (e.g. increasing generation of T cell memory).[57][58][59][60] Addition of adjuvants may confer benefits including dose sparing and stabilisation of final vaccine formulation.[57][60]
Appropriate adjuvants are chosen based on safety, tolerance, compatibility of antigen andmanufacturing considerations.[57] Commonly used adjuvants for recombinant subunitvaccines areAlum adjuvants (e.g.aluminium hydroxide),Emulsions (e.g.MF59) andLiposomes combined withimmunostimulatory molecules (e.g. AS01B).[57][59]
Delivery systems are primarily divided into polymer-baseddelivery systems (microspheres andliposomes) and livedelivery systems (gram-positive bacteria,gram-negative bacteria andviruses)[citation needed]
Vaccine antigens are often encapsulated withinmicrospheres orliposomes. Common microspheres made usingPoly-lactic acid (PLA)[61] andpoly-lactic-co-glycolic acid (PLGA)[61] allow for controlledantigen release by degrading in vivo whileliposomes including multilamellar or unilamellar vesicles allow for prolonged release.[59]
Polymer-baseddelivery systems confer advantages such as increased resistance to degradation inGI tract, controlledantigen release, raised particle uptake byimmune cells and enhanced ability to inducecytotoxic T cell responses.[59] An example of licensed recombinant vaccine utilisingliposomal delivery isShringrix.
Livedelivery systems, also known asvectors, are cells modified withligands orantigens to improve theimmunogenicity of recombinant subunits via alteringantigen presentation,biodistribution and trafficking.[62] Subunits may either be inserted within the carrier orgenetically engineered to be expressed on the surface of thevectors for efficient presentation to themucosal immune system.[45]
Recombinant subunitvaccines are safe for administration.[65][66] However, mild local reactions, includinginduration andswelling of the injection site, along withfever,fatigue andheadache may be encountered after vaccination.[65][67][68] Occurrence of severehypersensitivity reactions andanaphylaxis is rare,[69] but can possibly lead todeaths of individuals.Adverse effects can vary among populations depending on theirphysical health condition, age,gender andgenetic predisposition.[70][71]
Recombinant subunitvaccines arecontraindicated to people who have experiencedallergic reactions andanaphylaxis toantigens or other components of thevaccines previously.[72][73] Furthermore, precautions should be taken when administeringvaccines to people who are indiseased state and duringpregnancy,[72] in which their injections should be delayed until their conditions become stable and after childbirth respectively.
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ENGERIX-B (produced by GSK) andRECOMBIVAX HB (produced by merck) are two recombinant subunitvaccines licensed for the protection againsthepatitis B. Both containHBsAg harvested and purified fromSaccharomyces cerevisiae and are formulated as a suspension of theantigen adjuvanted withalum.[74][75]
Antibody concentration ≥10mIU/mL againstHBsAg are recognized as conferring protection against hepatitis B infection.[74][75]
It has been shown that primary 3-dosevaccination of healthy individuals is associated with ≥90% seroprotection rates forENGERIX-B, despite decreasing with older age. Lower seroprotection rates are also associated with presence of underlyingchronic diseases andimmunodeficiency. Yet, GSK HepB still has a clinically acceptablesafety profile in all studied populations.[76]
Cervarix,GARDASIL andGARDASIL9 are three recombinant subunitvaccines licensed for the protection againstHPV infection. They differ in thestrains which they protect the patients from asCervarix confers protection against type 16 and 18,[55]Gardasil confers protection against type 6, 11, 16 and 18,[77] and Gardasil 9 confers protection against type 6, 11, 16, 18, 31, 33, 45, 52, 58[5] respectively. Thevaccines contain purifiedVLP of the major capsid L1 protein produced by recombinantSaccharomyces cerevisiae.[citation needed]
It has been shown in a 2014 systematic quantitative review that the bivalent HPV vaccine (Cervarix) is associated withpain (OR 3.29; 95% CI: 3.00–3.60),swelling (OR 3.14; 95% CI: 2.79–3.53) andredness (OR 2.41; 95% CI: 2.17–2.68) being the most frequently reported adverse effects. For Gardasil, the most frequently reported events werepain (OR 2.88; 95% CI: 2.42–3.43) andswelling (OR 2.65; 95% CI: 2.0–3.44).[78]
Gardasil was discontinued in the U.S. on May 8, 2017, after the introduction of Gardasil 9[79] and Cervarix was also voluntarily withdrawn in the U.S. on August 8, 2016.[80]
Flublok Quadrivalent is a licensed recombinant subunitvaccine for activeimmunisation againstinfluenza. It containsHA proteins of fourstrains ofinfluenza virus purified and extracted using theBaculovirus-insectexpression system. The four viral strains are standardised annually according toUnited States Public Health Services (USPHS) requirements.[6]
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 ofchills (RR = 1.33, 95% CI 1.03–1.72, three RCTs, FEM, I2 = 14%, low‐certainty evidence).[81]
SHINGRIX is a licensed recombinant subunitvaccine for protection againstHerpes Zoster, whose risk of developing increases with decline ofvaricella zoster virus (VZV) specificimmunity. The vaccine containsVZV gE antigen component extracted fromCHO cells, which is to be reconstituted withadjuvant suspension AS01B.[7]
Systematic reviews andmeta-analyses have been conducted on the efficacy, effectiveness and safety ofSHINGRIX inimmunocompromised 18–49 year old patients and healthy adults aged 50 and over. These studies reportedhumoral andcell-mediated immunity rate ranged between 65.4 and 96.2% and 50.0–93.0% while efficacy in patients (18–49 yo) withhaematological malignancies was estimated at 87.2% (95%CI, 44.3–98.6%) up to 13 months post-vaccination with an acceptablesafety profile.[82][83]
NUVAXOVID is a recombinant subunit vaccine licensed for the prevention ofSARS-CoV-2 infection. Market authorization was issued on 20 December 2021.[84] The vaccine contains theSARS-CoV-2 spike protein produced using thebaculovirusexpression system, which is eventually adjuvanted with theMatrix M adjuvant.[8]
While the practice ofimmunisation can be traced back to the12th century, in which ancientChinese at that time employed the technique ofvariolation to conferimmunity tosmallpox infection,[citation needed] the modern era of vaccination has a short history of around 200 years. It began with theinvention of a vaccine by Edward Jenner in 1798 to eradicatesmallpox by injecting relatively weakercowpox virus into the human body.[citation needed]
The middle of the 20th century marked the golden age of vaccine science.[citation needed] Rapid technological advancements during this period of time enabled scientists to cultivatecell culture under controlled environments in laboratories,[85] subsequently giving rise to the production of vaccines againstpoliomyelitis,measles and variouscommunicable diseases.[citation needed] Conjugated vaccines were also developed usingimmunologic markers including capsularpolysaccharide andproteins.[85] Creation of products targeting common illnesses successfully lowered infection-relatedmortality and reducedpublic healthcare burden.
Emergence ofgenetic engineering techniques revolutionised the creation of vaccines. By the end of the 20th century, researchers had the ability to createrecombinant vaccines apart from traditionalwhole-cell vaccine, for instanceHepatitis B vaccine, which uses the viralantigens to initiateimmune responses.[85]
As themanufacturing methods continue to evolve, vaccines with more complex constitutions will inevitably be generated in the future to extend their therapeutic applications to both infectious andnon-infectious diseases,[citation needed] in order to safeguard the health of more people.
Recombinant subunitvaccines are used in development fortuberculosis,[9]dengue fever,[10]soil-transmitted helminths,[86]feline leukaemia[87] andCOVID-19.[88]
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.[11]
Research has been conducted to explore the possibility of developing a heterologousSARS-CoV receptor-binding domain (RBD) recombinant protein as a humanvaccine againstCOVID-19. The theory is supported by evidence thatconvalescentserum fromSARS-CoV patients have the ability to neutraliseSARS-CoV-2 (corresponding virus forCOVID-19) and that amino acid similarity betweenSARS-CoV andSARS-CoV-2 spike and RBD protein is high (82%).[88]