Editorial

2002 © Ashley Publications Ltd ISSN 1471-2598 119

Ashley Publicationswww.ashley-pub.com

Vaccines & Antibodies

Toward the development of better vaccine adjuvantsJeffrey B UlmerVaccines Research, Chiron Corporation, Emeryville, CA 94608, USA

Keywords: adjuvant, CpG, innate immunity, PAMP, pathogen-associated molecular patterns, TLR, toll-like receptors, vaccine

Expert Opin. Biol. Ther. (2002) 2(2):119-121

Various types of vaccines have been developed over the past two centuries and thesehave been very effective in preventing diseases caused by many pathogens. Table 1lists the main types of vaccines in use today and selected disease targets againstwhich they are used. These types include whole organisms (live attenuated or inacti-vated) or subunits derived from the organisms (polysaccharides, modified toxins,partially purified or recombinant proteins). While quite efficacious for their specificapplications, these types of vaccines alone may not be adequate or appropriate forinduction of protective immune responses against many pathogens. For example,safety issues related to live attenuated organisms set a high barrier for use against cer-tain pathogens that establish chronic infections with devastating consequences (e.g.,HIV). Also, many of these vaccine types are potent for generating systemichumoural immune responses but are ill suited for the induction of mucosal and/orcellular immunity. The prevention of infection by mucosal pathogens may requirethe presence of local immunity at the site of initial infection and the clearance ofviral and bacterial infections is thought to be mediated by cellular immuneresponses, such as cytokine-producing CD4+ T-cells and cytolytic CD8+ T-cells(CTL). Finally, vaccines capable of inducing stronger immune responses may benecessary for application to cancer immunotherapy, where one must break toleranceagainst self-antigens expressed by the tumour. Therefore, new vaccine technologieswill be key to success against infectious diseases caused by pathogens for which thereexist no current vaccines (e.g., HIV, malaria) against cancer and for improving exist-ing vaccines (e.g., tuberculosis).

One area of vaccine technology gaining renewed attention is adjuvants. Mineralsalts have long been used as vaccine adjuvants but only a few others have beenlicensed for human use, with most adjuvants still at an experimental stage (seeTable 2). This is due, in part, to unacceptable toxicity associated with the mostpotent adjuvants, as well as a poor understanding of their mechanisms of action.However, recent discoveries in signalling of the innate immune system has providedinsights into how adjuvants function and how improvements can be made on iden-tifying novel compounds with adjuvant activity.

Conditions of stress (e.g., tissue injury, heat shock) and exposure to pathogensresult in the local and rapid production of pro-inflammatory cytokines. In the caseof infection, components of the incoming pathogen (pathogen-associated molecularpatterns or PAMPs) are recognised by membrane-bound receptors termed toll-likereceptors (TLRs). Such engagement of TLRs present on immune cells, such as anti-gen-presenting cells (APCs), results in a signal transduction cascade leading to theactivation of transcription factors, such as NFκB, which upregulate cytokine geneexpression. This rapid recognition of pathogens (within minutes) constitutes theinnate immune system and provides a first line of defence prior to the onset of anantigen-specific (adaptive) immune response, which usually takes days to weeks.The nature of TLR ligands is being elucidated and it is now known that variousPAMPs are recognised by specific TLRs (see Table 3). In addition, several adjuvants

Toward the development of better vaccine adjuvants

120 Expert Opin. Biol. Ther. (2002) 2(2)

Table 1. Main types of human vaccines.

Vaccine type Disease target (e.g.)

Live organism Smallpox

Inactivated organism Influenza

Polysaccharide conjugate Haemophilus

Toxoid Tetanus

Recombinant protein Hepatitis B

Table 2. Types of vaccine adjuvants.

Adjuvant Composition Use

Mineral salts Aluminium or calcium salts Approved for humans

MF59 Oil-in-water emulsion Approved for humans

MPL Modified bacterial lipid A Approved for humans

ISCOMS Immune-stimulating complexes Experimental

Saponins Related compounds isolated from tree bark Experimental

CpG Immune-stimulating oligonucleotides Experimental

Cytokines Recombinant protein or plasmid DNA Experimental

CpG: Cytosine phoshoryl guanine; ISCOMS: Immunostimulating complexes; MPL: Monophosphoryl lipid A.

Table 3. Selected toll-like receptor ligands.

PAMP Source Toll-like receptor

Lipoprotein Bacteria TLR2

Zymosan Yeast TLR2

GPI Parasites TLR2

dsRNA Viruses TLR3

LPS Bacteria TLR4

Taxol Plants TLR4

Flagellin Bacteria TLR5

CpG DNA Bacteria, viruses TLR9

CpG: Cytosine phoshoryl guanine; dsRNA: Double-stranded RNA; GPI: Glycosylphosphatidylinositol; LPS: Lipopolysaccharide; PAMP: Pathogen-associated molecular pattern; TLR: Toll-like receptors.

have been shown to activate APCs to produce cytokines.Hence, it is not a coincidence that many adjuvant-active com-pounds are PAMPs derived from bacteria, viruses and plants.

The innate and adaptive immune systems are not mutuallyexclusive. In addition to cytokine production, activation ofAPCs by PAMPs stimulates more effective presentation ofantigens to T- and B-cells and concomitant priming of anti-gen-specific immune responses. Furthermore, the type ofPAMP may influence the phenotype of the adaptive immuneresponse, owing to differences in TLR distribution, require-ment for co-receptors, pattern recognition and cytokineinduction. Recent analysis of APCs infected with differentpathogens revealed distinct gene expression profiles in thecells, illustrating how the immune system responds to patho-

gens in a manner tailored to the specific circumstance. On amolecular level, it has been shown that signalling via TLR2(e.g., with lipoproteins from Gram-positive bacteria) versusTLR4 (e.g., with lipopolysaccharides from Gram-negativebacteria) stimulates the production of different sets ofcytokines. Thus, the coincident presence of PAMPs and anti-gens (spatially and temporally) from the pathogen during aninfection can drive and influence the nature and potency ofthe antigen-specific immune response. This is, in essence, howadjuvants are thought to work. A good recent example of aTLR-signalling PAMP being effective as a vaccine adjuvant isthe immunostimulatory CpG oligonucleotides. CpG signalsvia TLR9 to stimulate production of cytokines, including IL-12. As an adjuvant, CpG is very potent at enhancing Th1-

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Expert Opin. Biol. Ther. (2002) 2(2) 121

type immune responses in mice (IgG2a antibodies, IFN-γsecreting CD4+ T-cells and CTL), likely as a consequence ofits ability to induce IL-12. Importantly, preliminary data fromclinical trials has suggested that CpG may be effective as anadjuvant for humans.

These and other data illustrate the intimate relationshipbetween the innate and adaptive immune systems and how itmay be possible to capitalise on this link for identification anddevelopment of novel adjuvants. The growing base of infor-mation pertaining to the physicochemical nature of the inter-action between PAMPs and TLRs and the downstream effectsof this interaction will allow a more rational approach todesigning compounds with adjuvant activity. Using a differentstrategy, it should also be possible to take an unbiased, high-

throughput approach to search for adjuvant ompounds with-out regard to their structure. The existence of cell-based assaysto measure cytokine production by APCs in vitro makes thisapproach feasible. Finally, since TLR signalling appears tooccur primarily at the cell surface, the often confoundingproblem of intracellular delivery of a drug may not be a signif-icant barrier for vaccine adjuvants. Thus, the near futureshould yield new and improved adjuvants that will be impor-tant for making better vaccines.

Acknowledgements

The author would like to thank Drs Derek O’Hagan andNicholas Valiante for their helpful suggestions.

Suggested ReadingAKIRA S, TAKEDA K, KAISHO T: Toll-like receptors: critical proteins linking innate and acquired immunity. Nature Immunol. (2001) 2:675-680.

HUANG Q, LIU D, MAJEWSKI P et al.: The plasticity of dendritic cell responses to pathogens and their components. Science (2001) 294:870-875.

MOINGEON P, HAENSLER J, LINDBERG A: Towards the rational design of Th1 adjuvants. Vaccine (2001) 19:4363-4372.

O’HAGAN DT, MACKICHAN ML, SINGH M: Recent developments in adjuvants for vaccines against infectious diseases. Biomol. Eng. (2001) 18:69-85.

AffiliationJeffrey B Ulmer PhDVaccines Research, Chiron Corporation, 4560 Horton St., Mail stop 4-3, Emeryville, CA 94608, USATel.: +1 510 923 5140; Fax: +1 510 923 2586;E-mail: jeffrey_ulmer@chiron.com