Vaccine adjuvant

Vaccine immunology has been well developed since last century. To combat against a specific disease by vaccine, an innocuous form of disease agent such as killed or weakened bacteria or viruses was usually employed to stimulate antibody production and activate cellular immunology. Many vaccines such as smallpox, measles, and polio in most cases have been proven very effective in human protection.^33-34 For example, reported cases of wild polio virus was reduced to less than 100 cases globally in 2015.³⁵ However, vaccines often come with risks of adverse events and large-scale production of vaccine is very challenging.³⁶ Therefore, there is a growing need for a potentiator, so called adjuvants, which incorporate in therapeutic vaccine formation to enhance, modulate and prolong the immune response.

An adjuvant is a substance that added to vaccines to increase the immunogenicity and protection against infection. The word “adjuvant” means “to aid” which comes from the Latin word adjuvare. Adjuvants help activating the immune system by regulating the humoral or cellular immunity according to the objective of vaccination. Combining with adjuvants, human vaccines based on weakened or inactivated pathogens could elicit robust protective immune responses. For example, adjuvanted H5N1 pandemic influenza vaccines showed improved immune responses compared to un-adjuvanted vaccines in animal models.³⁷ Therefore, adjuvants have several important benefits in vaccine

immunology, especially reducing the amount of required antigen and reducing the number of vaccine doses.

1.2.1.

Table 5 lists series of adjuvants that have been licensed in use with human vaccines to date.

Various of adjuvants have been used in human vaccines, such as mineral salts, oil in water, liposome, and TLR agonist. The development of adjuvants has been very slow.

Until the early 1990’s, aluminum salts were the only licensed adjuvants used in human vaccines in the United States.³⁸ It is also known as Alum, generally considered as a stimulator of Th2 type immune response, and is the most widely used adjuvants in practical human vaccination.^39-40 After decades, oil-in-water MF59 and AS03 adjuvants were used in pandemic influenza and avian influenza vaccines to improve the humoral and cell-mediated immunity.⁴¹ Both MF59 and AS03 contain squalene, but they have different compositions. AS03 has a component of α-tocopherol to modulate innate immune response.⁴² HBV vaccines were registered with TLR agonist class adjuvants AS04 and CpG ODN which were licensed in 2005 and 2018, respectively. Recently, liposome class adjuvant AS01 with the components of MPL and QS-21 were approved to be used in Herpes zoster vaccines. It is worthy mentioning that QS-21 is the second

non-endogenous substrate that was approved by FDA as an adjuvant.

Table 5. Licensed vaccine adjuvants, their class, components and registered vaccine.

Adjuvants

MPL; aluminum hydroxide Hepatitis B virus;

human

Liposome MPL; QS-21 Herpes zoster

CpG ODN (2018)

TLR9 agonist 1018ISS Hepatitis B virus

1.2.2. QS-21 adjuvanted vaccine

QS-21 is one of the most potential adjuvant with favorable immune responses by promoting high antigen-specific antibody responses.^43-44 Unlike aluminum hydroxide, QS-21 promotes a balanced of both IgG1 and IgG2 production.^{27, 45-46} Further studies showed that QS-21 induces IL-2, IFN-γ, and Th1 bias immunity in vaccine responses.^43,

47-48

Besides of being used solely as an adjuvant , the amphiphilic property of QS-21 allowed it to be formulated into a liposome form (AS01) or an oil-in-water emulsion (AS02) to induce humoral and cellular immunity against cancers and pathogens (Table 6).⁴⁹ Some of them had been proceeded to clinical trial, SHIGRIX^®, a combination of varicella-zoster virus glycoprotein gE and AS01, is the only approved vaccine to prevent herpes in 2017. Both Maria vaccine Mosqurix^® and OBI-822/QS-21 are being evaluated in phase III clinical trial for the intervention of malaria and triple negative breast cancer (NCT03562637, NCT03608878) respectively.^50-51 The phase III trial of GM2/QS-21 was terminated due to low efficacy.⁵² In addition, QS-21 has also been applied with ACC-001 in phase II trial of early Alzheimer’s disease.^53-54

Table 6. QS-21 adjuvanted vaccines.

Combination (brand name) Intervention Status

VSV gE/AS01 (SHIGRIX^®) Herpes zoster Approved (Oct. 2017) RTS, S/AS01 (Mosqurix^®) Malaria Phase III

GM2-KLH/QS-21 Stage II melanoma Phase III (terminated) OBI-822/QS-21 Triple negative breast cancer Phase III

ACC-001/QS-21 Early Alzheimer’s disease Phase II (completed)

1.2.3. Plausible mechanism of QS-21

Although the potency of QS-21 had been investigated in over 100 clinical trials, the

exact mechanism of action of QS-21 in vaccination still remains unknown. It is generally agreed that QS-21 does not directly interact with Toll-like receptors (TLR), and does not operate by depot effect.⁵⁵

Since QS-21 has the potential ability to increase T-cell response, it was proposed that the C4-aldehyde moiety could form a Schiff base with amino groups on T-cell surface receptors to activate T-cell.⁵⁶ To investigate its importance in activating immune responses Fernández-Tejada et al. synthesized QS-21 analogues by removing the C4-aldehyde group. However, the results revealed that either reducing the C4-C4-aldehyde to alcohol or alkyl group, saponins demonstrated the similar immunoresponses in mouse vaccination with GD3-KLH, MUC1-KLH, and OVA.⁵⁷ Thus, the hypothesis of the Schiff base mechanistic hypothesis remains as question.

DC-SIGN, a receptor on DC, usually binds to fucopyranosyl residues and biases its response toward Th2 immunity.⁵⁸ Since QS-21 induce both Th1 and Th2 immunity, The fucose residue of QS-21 was proposed to take responsibility to be an internal moiety binding to DC-SIGN to induce both Th2 immunity.⁵⁹

The amphiphilic property of QS-21 has been postulated that is important since the deacylated saponin (DS-1, Figure 6) lost the capacity to stimulate Th1 immunity.⁶⁰ The

lipophilic acyl chain might facilitate the delivery of exogenous antigens into the APC and further inducing immunity in T-cells.⁶¹ In addition, triterpene’s high affinity for cholesterol could make QS-21 facilitates the endosomal escape of antigens, leading to DC activation and antigen cross-presentation by collecting the endosomes-lysosomes and destabilizing them.^62-63