Introduction of Vaccine Adjuvant
Adjuvant, also known as immune modulator or Immunepotentiator, is an additive of vaccine. When injected into the body before or mixed with antigen, it can enhance the immune response to antigen or change the type of immune response. It is a non-specific immunepotentiator. And it has no antigenicity. The ideal adjuvant can not only enhance the immune response, but also make the body get the best protective immunity.
The concept of adjuvants emerged in the 1920s when an abscess developed at the site of a horse inoculated with diphtheria toxin, where specific antibody titers were high, and it was subsequently found that the abscess resulted from an injection of an unrelated substance that increased the immune response against the toxoid. In 1926, Glenny et al. proved that the substance with adjuvant activity of diphtheria toxin vaccine adsorbed on alum is aluminum compound. To this day, aluminum-based compounds remain the main human adjuvants.
The Development of Vaccine Adjuvants
In general, vaccine adjuvants are not approved as a single drug, but as a whole component of the vaccine, mostly aluminum adjuvants in the early days. However, the large-scale use of aluminum adjuvants has made its limitations, including potency, biodegradability, etc. The water-in-oil emulsion MF59 was licensed for use in seasonal influenza in 1997. In the following more than 20 years, with the maturity of subunit vaccine technology, it has greatly promoted the launch of more effective new adjuvants such as AS04, AS03 and CpG 1018. At present, the development of novel adjuvants has become one of the focuses of vaccine research. Although many adjuvants have shown high efficacy in preclinical models, most of them have not been approved due to the safety or tolerability of human vaccines.
The Functions of Vaccine Adjuvants
The functions of vaccine adjuvants mainly include: 1) regulating the type of immune response; 2) enhancing the immunogenicity, immune response speed and tolerance of the antigen; 3) reducing the dose of antigen or the vaccination dose required for immune protection; 4)Improve the immune efficacy of vaccines in infants, the elderly, or people with compromised immune systems.
The mechanism by which adjuvants enhance immune response has not been fully elucidated, and the effects of different adjuvants are also different. The most important role is to promote the body to produce antigen-specific humoral and cellular immunity, involving antigen presenting cells to antigen uptake, antigen processing and antigen presentation, and the multi-adjuvant effect can not be explained by a single mechanism.
Classification of Vaccine Adjuvants
From the discovery of aluminum adjuvants to now, various substances have been found to have adjuvant activity, which can be roughly divided into the following categories according to their chemical properties.
1. Inorganic Adjuvants
Aluminum salts mainly include aluminum phosphate, aluminum hydroxide and aluminum potassium sulfate. At present the commonly used aluminum adjuvants are aluminum hydroxide and aluminum phosphate. The aluminum hydroxide adjuvant is actually the incomplete dehydration product of Al(OH)3, that is, the fibrous crystalline aluminum hydroxide AlO(OH). Aluminum phosphate adjuvant is a complex of hydroxyl aluminum phosphate with amorphous structure. The binding of antigen to aluminum adjuvant mainly depends on electrostatic attraction hydrophobic interaction and ligand exchange.
The mechanism of action of aluminum adjuvant mainly includes depot effect and immune stimulation effect. Aluminum adjuvant can also stimulate innate immunity and induce monocytes to secrete IL-1β and IL-18 by activating the Nlrp3 inflammasome. In addition, aluminum adjuvants stimulate the release of damage-related pattern molecules (DAMPs) such as DNA, ATP and uric acid from necrotic cells at the injection site, and the released DAMPs contribute to the interaction between dendritic cells and CD4+T cells.
So far, aluminum adjuvants remain the only widely used adjuvants in human vaccines. However, aluminum adjuvants have great disadvantages: aluminum adjuvants are usually effective in increasing serum antibodies, but their weak ability to induce cellular immunity due to injection site reactions limits their application to a certain extent.
2. Emulsion Adjuvant – MF59
MF59 adjuvant is an oil-in-water emulsion containing 4-5% squalene, 0.5% Tween 80 and 0.5% polysorbide trioleate (Span 85). Span85 was dispersed in a buffer containing squalene and Tween80 and stirred at high speed until creamy. After that, the emulsion was repeatedly injected by microjet machine to form particles of O/W emulsion (160nm), and then filtered for sterilization. MF59 emulsion can maintain stability for more than 3 years after preparation. In 1997, the trivalent influenza vaccine Fluad, MF59 as an adjuvant, was approved in Europe. It was the first adjuvant to be included in a novel vaccine for human use after aluminum adjuvant, which was a major milestone in the history of vaccine adjuvants.
MF59 adjuvant induces a stronger humoral immune response than aluminum adjuvant. The results of animal experiments on influenza vaccine showed that MF59 adjuvant induced antibody immune response 100 times higher than aluminum adjuvant, and also induced certain Th1 type cellular immune response. GLP toxicology studies on MF59 and its vaccine showed no potential systemic toxicity.
The mechanism of action of the MF59 adjuvant is to first activate macrophages and DC cells at the injection site, which produce chemokines (such as CCL2, CXCL8, CCL4, and CCL5), and then recruit immune cells such as neutrophils, eosinophils, monocytes and DC cells to the injection site, increasing the efficiency of antigen uptake and delivery. This further increases the number of antigen-presenting cells entering the lymph nodes, which are better able to activate T and B cells to produce an immune response.
3. Water-soluble adjuvants – saponins
Saponins have been used for a long time as vaccine adjuvants, and the component QS-21 is a triterpene glycoside purified from the bark extract of the Quillaja saponaria Molina, the mechanism of which is currently unknown. In human and animal experiments, QS-21 can promote the response of antigen-specific antibodies CD4+ and CD8+ helper T cells.
4. Microparticle Adjuvants – AS01
AS01 is a liposomal adjuvant containing the immunostimulant monophosphate acyl lipid A (MPL) and saponin QS-21. Ingredients Contains Saponin QS-21 50μg, 3-O-Deacyl-4′-Monophosphoryl Lipid A (MPL, Detoxifying Derivative of Salmonella Endotoxin Lipopolysaccharide) 50μg, Dioleoyl Phosphatidyl Choline (DOPC) 1mg, cholesterol 0.25mg. MPL and QS-21 were co-formulated in liposomes in the presence of cholesterol.
Its mechanism of action is derived from two immune stimulants, MPL and QS-21. MPL activates the natural immune system through Toll-like receptor 4 (TLR4), which can directly activate the antigen-presenting cells expressing TLR4 and stimulate the secretion of cytokines and the expression of synergistically stimulating molecules. QS-21 can activate Caspase 1 in subcapsular lymphoid sinusial macrophages (SSMs). Both components act simultaneously on the innate immune system and synergistically activate new pathways. However, QS-21 is hemolytic and needs to be used in combination with cholesterol and liposome forms to eliminate side effects.
5. Adjuvants targeting pattern recognition receptors
The immune system needs to recognize and distinguish foreign antigens from its own antigens in order to fight against the invasion of foreign pathogens. This recognition is done by innate immune cells, known as dendritic cells or macrophages, through pattern recognition receptors (PRR). PRR has multiple receptor families, including Toll-like receptors (TLRs), Nod-like receptors (NLRs), C-type lectin receptors (CLRs), RIG-I like receptors (RLRs) and cytoplasmic DNA/RNA receptors. These receptors recognize lipopolysaccharides (LPS), lipoproteins, flagellin, and some specific nucleotide sequences. Once the pattern recognition receptors are stimulated by the above substances, the expression of inflammation-related genes is activated, activating the immune system to respond to the infection.
6. Cytokine adjuvants – IL
Cytokines are a class of immunomodulatory substances that produce responsiveness after the body’s immune system is activated by antigens and various immune adjuvants, and have obvious immune adjuvant effects. Cytokines can be divided into interleukins (IL), interferon (IFN-γ), tumor necrosis factor superfamily, colony stimulating factors, chemokines, growth factors, etc.
IL is a cytokine secreted by leukocytes. The most obvious adjuvant effects are IL-2 and IL-12, which are mainly used in pre-clinical studies of DNA vaccines. IL-2 is a cytokine secreted by DC, which can act on T, B and NK cells, induce Th1 type response and activate effector T lymphocytes, selectively enhance the differentiation and proliferation of Th1 type cells, induce and release various cytokines including IFN-γ, and produce specific cellular immune response. Relevant studies have shown that the use of high doses of IL-2 has considerable toxicity and is easy to cause vascular leakage. Therefore, the safety of IL-2 as an adjuvant needs further study. Il-12 can activate the killing function of NK cells, stimulate the secretion of IFN-γ, differentiate Th0 cells into Th1 cells, and enhance the lysis activity of vaccine. Il-12 is an important pro-inflammatory factor. Due to the rapid clearance of IL12, its application is also limited to a certain extent, and it is more likely to act in combination with other factors.