The path forward.

For many decades the only adjuvants accepted in human licensed vaccines have been particulate substances such as alum and emulsions. These compounds have been identified empirically, based on their ability to enhance immune responses to vaccination in animals, without understanding their mechanism of action. Thanks to the increased knowledge of the innate immune system, many new adjuvants, designed around known Pattern Recognition Receptors (PRRs) including Toll-like receptors (TLRs) have been identified. A TLR4 agonist is part of a licensed vaccine and TLR9 ligands are in late stage clinical testing. Adjuvants targeting alternative PRRs have been validated in preclinical models. In the future we have to expect more sophisticated adjuvant formulations, including multiple PPR ligands combined with novel antigen delivery systems. In addition to traditional adjuvants, other innovative strategies improving vaccine immunity are emerging. Among them combinations of vaccines with cytokines, inhibitors of metabolic pathways, modulators of baseline inflammation levels, monoclonal antibodies targeting checkpoint inhibitors and compounds depleting of regulatory cells. The introduction of novel technologies has the potential to support the development of vaccines with increased efficacy targeting infections as well as non-communicable diseases. However, the full potential of any novel vaccine strategy can be only captured if vaccination programs are implemented with sufficient coverage. New methods to fully capture the benefits of vaccination and appropriate communication strategies to increase vaccine acceptance by the public are two key elements that all stakeholders involved in the whole vaccine development cycle, including scientists, must consider very carefully.

The adjuvant MF59 induces ATP release from muscle that potentiates response to vaccination.

Vaccines are the most effective agents to control infections. In addition to the pathogen antigens, vaccines contain adjuvants that are used to enhance protective immune responses. However, the molecular mechanism of action of most adjuvants is ill-known, and a better understanding of adjuvanticity is needed to develop improved adjuvants based on molecular targets that further enhance vaccine efficacy. This is particularly important for tuberculosis, malaria, AIDS, and other diseases for which protective vaccines do not exist. Release of endogenous danger signals has been linked to adjuvanticity; however, the role of extracellular ATP during vaccination has never been explored. Here, we tested whether ATP release is involved in the immune boosting effect of four common adjuvants: aluminum hydroxide, calcium phosphate, incomplete Freund's adjuvant, and the oil-in-water emulsion MF59. We found that intramuscular injection is always associated with a weak transient release of ATP, which was greatly enhanced by the presence of MF59 but not by all other adjuvants tested. Local injection of apyrase, an ATP-hydrolyzing enzyme, inhibited cell recruitment in the muscle induced by MF59 but not by alum or incomplete Freund's adjuvant. In addition, apyrase strongly inhibited influenza-specific T-cell responses and hemagglutination inhibition titers in response to an MF59-adjuvanted trivalent influenza vaccine. These data demonstrate that a transient ATP release is required for innate and adaptive immune responses induced by MF59 and link extracellular ATP with an enhanced response to vaccination.

Immunology of TLR-independent vaccine adjuvants.

Vaccine adjuvants target the innate immune system to enhance the immunogenicity of coadministered antigens. Dendritic cells (DCs) are responsible for antigen uptake and presentation to naïve T cells and represent a key target of adjuvant activity. Adjuvants derived from microbial components, such as Toll-like receptor (TLR) agonists, elicit innate immune receptors expressed by DCs. By contrast, particulate adjuvants, like mineral salts, oil-in-water emulsions, and microparticles, do not activate DCs directly, and their mechanism of action is poorly characterized. In the last two years it has been reported that particulate adjuvants induce chemokine production in accessory cells like macrophages, monocytes, and granulocytes, leading to cell recruitment at injection site followed by the differentiation of monocytes into activated DCs. The NLRP3 inflammasome complex is one of the molecular targets of particulate adjuvants and it is required for alum adjuvanticity. Other TLR-independent adjuvants were found to target DCs directly or by other accessory cells like iNKT or mast cells. These findings highlight that the activation of DCs plays a central role in the mechanism of action of all classes of vaccine adjuvants but can occur by a multitude of different pathways and cellular interactions.