R E V I E W Key roles of adjuvants in modern vaccines Steven G Reed, Mark T Orr & Christopher B Fox Vaccines containing novel adjuvant formulations are increasingly reaching advanced development and licensing stages, providing new tools to fill previously unmet clinical needs. However, many adjuvants fail during product development owing to factors such as manufacturability, stability, lack of effectiveness, unacceptable levels of tolerability or safety concerns. This Review outlines the potential benefits of adjuvants in current and future vaccines and describes the importance of formulation and mechanisms of action of adjuvants. Moreover, we emphasize safety considerations and other crucial aspects in the clinical development of effective adjuvants that will help facilitate effective next-generation vaccines against devastating infectious diseases. Adjuvants have proven to be key components in vaccines that are Modern adjuvant development, which in spite of many hurdles is today taken for granted. Indeed, many vaccines, comprised of whole or progressing, is based on enhancing and shaping vaccine-induced killed bacteria or viruses, have inherent immune-potentiating activity. responses without compromising safety by selectively adding well- However, attempts to develop a new generation of adjuvants, which defined molecules, formulations or both. Because vaccines are often will be essential for new vaccines, have been hindered somewhat by employed prophylactically in populations of very young people, it perceived, but most often undocumented, health risks and public is important that medical risks to the subject (that is, safety) and misinformation, rather than by verified safety issues. Nonetheless, it other adverse effects (that is, tolerability) are addressed. Vaccine is essential that vaccine and adjuvant developers fully utilize infor- adjuvants designed for therapeutic uses, such as in cancer, may mation on adjuvants’ modes of action, avoid using undefined com- have a different risk-benefit profile. Adjuvants currently employed ponents in adjuvant formulations and develop comprehensive data in human vaccines licensed for use in the US and/or Europe include packages on the safety, tolerability and efficacy of adjuvanted vac- aluminum salts, oil-in-water emulsions (MF59, AS03 and AF03), cines. Crucially, the inclusion of an adjuvant in a vaccine product or virosomes and AS04 (monophosphoryl lipid A preparation (MPL) therapeutic regimen must be justified—that is, it should fill an unmet with aluminum salt). © 2013 Nature America, Inc. All rights reserved. America, Inc. © 2013 Nature need. The degree of enthusiasm with which vaccine developers and Adjuvant and formulation selection may be based on several param- regulators approach new vaccine adjuvants will depend largely on eters, including the physical and chemical natures of the vaccine anti- the contribution of the adjuvant and the importance of the vaccine. gen, type of immune response desired, age of the target population This Review addresses the contribution of adjuvants in current and and route of vaccine administration. The desired qualities of each npg future vaccines, their formulation aspects and safety considerations, particular vaccine may necessitate adjuvants with specific properties. and progress in understanding their mechanisms of action. We do Indeed, the selection of the wrong adjuvant may render a particular not discuss other roles of adjuvant formulations as therapeutics, for vaccine antigen inadequate. Thus, vaccine antigen selection must take example, in treating cancer or allergy. into account adjuvant selection to avoid discarding potentially effec- Adjuvants, in the context of vaccines, are defined as components tive vaccine antigen candidates. capable of enhancing and/or shaping antigen-specific immune responses. Biotechnology advances have enabled modern vaccines Essential roles of adjuvants to be based on rationally designed recombinant antigens containing Immunization with purified protein antigens typically results in highly purified components with excellent safety profiles. Conversely, the induction of a modest antibody response with little or no T cell the immunogenicity of such well-defined vaccine antigens may be response. Additionally, multiple immunizations may be required to low compared to vaccines comprised of live attenuated or inacti- elicit sufficient antibody responses. Developers may seek to include vated pathogen preparations. Live attenuated or inactivated vaccines adjuvants in vaccine candidates to enhance the efficacy of weak may inherently contain natural adjuvants as they have heterogene- antigens, to induce appropriate immune responses not sufficiently ous compositions, which may include particulate forms of proteins, induced in the absence of adjuvant or both. For example, although lipids and oligonucleotides, albeit in an undefined context1 (Fig. 1). there has been considerable investment in the development of recom- binant influenza vaccines to better prepare for a pandemic, the candi- dates developed thus far require relatively high doses owing to their Infectious Disease Research Institute, Seattle, Washington, USA. weak immunogenicity, which has a negative impact on the potential Correspondence should be addressed to S.G.R. ([email protected]). for a global supply. Adjuvants enable the use of lower vaccine doses, Received 19 December 2012; accepted 27 September 2013; published online greatly expanding supply. This use and other practical applications 5 December 2013; doi:10.1038/nm.3409 of adjuvants are described below (Fig. 2). NATURE MEDICINE VOLUME 19 | NUMBER 12 | DECEMBER 2013 1597 REVIEW Year Vaccine Adjuvant and mechanism Scientific findings This may be accomplished by the addition of adjuvants to the target 1885 Rabies ssRNA antigens, as exemplified by the addition of the AS04 adjuvant to TLRs 7 and 8 hepatitis B antigen in GlaxoSmithKline’s (GSK’s) Fendrix, which ena- 1886 Briegen describes endotoxin 4,5 Coley shows tumor necrosis bled a reduction of a three-dose regimen to two doses . 1889 with bacterial extracts 1911 Typhoid LPS, DNA Antibody response broadening. Many pathogens, such as influenza TLRs 1, 2, 4, 5, 6 and 9 More durable immune viruses, HIV, human papilloma virus (HPV) and the malaria parasite, 1916 Lipovaccine response to typhoid vaccine display substantial antigenic drift, strain variations or both. Thus, the 1921 BCG for TB DNA, lipoprotein ability of adjuvants to broaden an immune response profile could be TLRs 1, 2, 6 and 9 Enhanced antibody crucial to the success of vaccines against such targets. Experimentally, 1926 Aluminum salts responses to diphtheria vaccine massively parallel sequencing has shown that the broadening effect of Incomplete Freund’s Enhanced cellular and adjuvants may be mediated via expansion of B cell diversity, not merely 1937 adjuvant (IFA) antibody responses to TB 6 (water-in-oil emulsion) through increased titers . Clinically, antibody response broadening by Diphtheria, pertussis 7–9 1942 LPS, DNA adjuvants has been demonstrated in influenza and HPV vaccines . and tetanus TLRs 1, 2, 4, 5, 6 and 9 Whole-cell 1949 ssRNA influenza TLRs 7 and 8 Antibody response magnitude and functionality. It is well accepted Inactivated 1955 ssRNA that widely used adjuvants such as aluminum salts or oil-in-water polio vaccine TLRs 7 and 8 1966 LPS structure determined emulsions induce a greater magnitude of antibody responses to vac- Ribi makes detoxified cine antigens. There is now an increased appreciation of the capacity 1979 endotoxin MPL of adjuvants to increase not just overall antibody titer but greater 1991 Hepatitis A MPL tested in clinic numbers of functional antibodies, antibodies with higher affinity for 1996 TLRs discovered vaccine antigens or both10,11. 1997 Fluad MF59 (oil-in-water emulsion) Developing vaccines for effective T cell responses. Several vac- Epaxal (for hepatitis A) 1997 Virosome Inflexal (for influenza) cines in development are aimed at targeting T cell responses, which LPS shown to be 1998 are not optimally induced by the most commonly used adjuvants in TLR ligand Invivac vaccines approved for human use, including alum and oil-in-water 2004 Virosome (for influenza; Europe) ­emulsion–based adjuvants. A more refined objective may be to elicit Fendrix 2005 MPL (for hepatitis B; Europe) more effective engagement of T helper cells for optimizing the quality Defined TLR4 + 2007– Pandemic influenza MF59, AS03 and durability of antibody responses or to induce effector CD4 or 2009 vaccines (Europe) (oil-in-water emulsion) CD8+ T cells to kill intracellular pathogens. Therefore, the new gen- Cervarix (for HPV16 2009 MPL and HPV18; USA) Defined TLR4 eration of vaccines often incorporates agonists for Toll-like receptors (TLRs) and other innate immune receptors that facilitate the genera- Debbie Maizels tion of T helper cell responses. This has been particularly important Figure 1 A timeline of adjuvant development. The history of vaccines in the development of vaccines against pathogens that are controlled containing adjuvants is shown, indicating the development from natural © 2013 Nature America, Inc. All rights reserved. America, Inc. © 2013 Nature by cellular immune responses, including those causing malaria, tuber- adjuvants to defined adjuvants. BCG, bacillus Calmette-Guérin; LPS, lipopolysaccharide; ssRNA, single-stranded RNA; TB, tuberculosis. culosis and leishmaniasis. 2 Classes of adjuvants npg Dose sparing. A recently issued report specifically