Adjuvant System 04 (AS04)

Adjuvant System 04 (AS04) is a proprietary vaccine adjuvant platform composed of aluminum hydroxide (alum) and 3-O-desacyl-4’-monophosphoryl lipid A (MPL), a non-toxic derivative of the lipopolysaccharide from Salmonella minnesota. This system is designed to enhance the immune response by combining the depot-forming properties of aluminum salts with the potent immunostimulatory effects of MPL, which acts as a Toll-like receptor 4 (TLR4) agonist (Didierlaurent et al., 2009). When vaccine antigens are co-administered at the aluminum oxide particle surface, the alum component facilitates antigen uptake by antigen-presenting cells (APCs) and triggers the NLRP3 inflammasome, while the MPL component induces the transient production of pro-inflammatory cytokines and the migration of activated APCs to the draining lymph nodes (Marrack et al., 2009; Ghimire, 2015). This coordinated action results in a robust and sustained antibody response and the induction of antigen-specific memory B and T cells, which is particularly effective for vaccines targeting viral infections like Human Papillomavirus (HPV) and Hepatitis B (Garçon et al., 2007). AS04 is the primary adjuvant used in the HPV vaccine Cervarix and the Hepatitis B vaccine Fendrix (EMA, 2007). Safety profiles for AS04-adjuvanted vaccines generally show higher rates of local reactogenicity, such as injection site pain and swelling, compared to alum-only vaccines, but they remain well-tolerated in large clinical populations (Exley et al., 2010).

Other names

AS04 adjuvantAlum-MPLAluminum hydroxide and Monophosphoryl Lipid AAluminium oxide-adsorbed vaccine systemAS04 adjuvant system

AS04 enhances the immune response through a dual mechanism: the aluminum hydroxide component provides a depot effect for sustained antigen release and activates the NLRP3 inflammasome, while the Monophosphoryl Lipid A (MPL) component acts as a Toll-like receptor 4 (TLR4) agonist to stimulate the production of pro-inflammatory cytokines and the maturation of antigen-presenting cells.