Traditional aluminum adjuvants can trigger strong humoral immunity but weak cellular immunity, limiting their application in some vaccines. Currently, various immunomodulators and delivery carriers are used as adjuvants, and the mechanisms of action of some of these adjuvants are clear. However, customizing targets of adjuvant action (cellular or humoral immunity) and action intensity (enhancement or inhibition) according to different antigens selected is time-consuming. Here, we review the adjuvant effects of some delivery systems and immune stimulants. In addition, to improve the safety, effectiveness, and accessibility of adjuvants, new trends in adjuvant development and their modification strategies are discussed.

Recently, numerous attempts have been made to evaluate the potential of chitosan as an adjuvant; however, few have explored the mechanism underlying the adjuvant activity of chitosan.

OSW-1, isolated from the bulbs of Ornithogalum saundersiae Baker, is a steroidal saponin endowed with considerable antitumor properties. Biosynthesis of the 4-methoxybenzoyl group on the disaccharide moiety of OSW-1 is known to take place biochemically via the phenylpropanoid biosynthetic pathway, but molecular biological characterization of the related genes has been insufficient. Cinnamic acid 4-hydroxylase (C4H, EC 1.14.13.11), catalyzing the hydroxylation of trans-cinnamic acid to p-coumaric acid, plays a key role in the ability of phenylpropanoid metabolism to channel carbon to produce the 4-methoxybenzoyl group on the disaccharide moiety of OSW-1. Molecular isolation and functional characterization of the C4H genes, therefore, is an important step for pathway characterization of 4-methoxybenzoyl group biosynthesis. In this study, a gene coding for C4H, designated as OsaC4H, was isolated according to the transcriptome sequencing results of Ornithogalum saundersiae. The full-length OsaC4H cDNA is 1,608-bp long, with a 1,518-bp open reading frame encoding a protein of 505 amino acids, a 55-bp 5′ non-coding region and a 35-bp 3'-untranslated region. OsaC4H was functionally characterized by expression in Saccharomyces cerevisiae and shown to catalyze the oxidation of trans-cinnamic acid to p-coumaric acid, which was identified by high performance liquid chromatography with diode array detection (HPLC-DAD), HPLC-MS and nuclear magnetic resonance (NMR) analysis. The identification of the OsaC4H gene was expected to open the way to clarification of the biosynthetic pathway of OSW-1.

Adjuvants can regulate the immune response triggered by vaccines. Traditional aluminum adjuvants can induce humoral immunity, but they lack the ability to effectively induce Th1 cellular immunity, which is not conducive to the development of vaccines with improved protective effects. Aluminum adjuvants from different sources may have different physicochemical properties, and therefore, completely different immune responses can be triggered. This suggests that adjuvant recognition by the immune system and its responses are closely associated with the physicochemical properties of the adjuvant itself. To test this hypothesis, in this study, we developed a new method for preparing an aluminum adjuvant. This aluminum adjuvant has a pseudoboehmite structure, strong protein adsorption capacity, and excellent suspension stability. The adjuvant was tested using the hepatitis B virus surface antigen (HBsAg) as a model antigen for immunization; the results showed that this aluminum adjuvant effectively induced not only humoral immunity but also an outstanding cellular immune response. These results provide a reference for improving the efficacy of adjuvants.

Lipid droplets (LDs) are organelles with good natural biocompatibility that are abundant in eukaryotic cells and are considered good candidates for use in biological delivery inside cells. Importantly, numerous studies have reported that LDs also participate in immune processes. Here, we investigated whether LDs can also be used as vaccine adjuvants. We designed and prepared a variety of artificial nano-LDs with a nanometer size (200–300 nm) to test this possibility. Artificial nano-LDs have the same monolayer lecithin membrane as natural LDs; in contrast to natural LDs, artificial nano-LDs do not have proteins on their surfaces, which is beneficial to ensure better biocompatibility. Compared with other oil-in-water emulsion adjuvants (MF59 [AddaVax] and AS03 [AddaS03] used in licensed vaccines), vitamin E was used instead of squalene as the oil phase. The artificial nano-LDs (V3, V11, and V12) prepared in this study have structures similar to those of MF59 (AddaVax) and AS03­(AddaS03) under scanning electron microscopy and transmission electron microscopy and have better stability than MF59 (AddaVax) and AS03 (AddaS03); they can even withstand high-temperature sterilization at 120 °C for 30 min. In vivo test results showed that the artificial nano-LDs had good biocompatibility. Recombinant hepatitis B surface antigen and recombinant varicella-zoster virus glycoprotein E were used as model antigens to evaluate the adjuvant effect. We found that our artificial nano-LDs induced excellent humoral and cellular immunity (comparable to that of MF59 [AddaVax]). In addition, apart from the initial increase in the particle size upon high-temperature sterilization, the adjuvant effect of artificial nano-LDs did not decrease.