Elsevier

Biomolecular Engineering

Volume 18, Issue 3, 15 October 2001, Pages 69-85
Biomolecular Engineering

Review
Recent developments in adjuvants for vaccines against infectious diseases

https://doi.org/10.1016/S1389-0344(01)00101-0Get rights and content

Abstract

New generation vaccines, particularly those based on recombinant proteins and DNA, are likely to be less reactogenic than traditional vaccines, but are also less immunogenic. Therefore, there is an urgent need for the development of new and improved vaccine adjuvants. Adjuvants can be broadly separated into two classes, based on their principal mechanisms of action; vaccine delivery systems and ‘immunostimulatory adjuvants’. Vaccine delivery systems are generally particulate e.g. emulsions, microparticles, iscoms and liposomes, and mainly function to target associated antigens into antigen presenting cells (APC). In contrast, immunostimulatory adjuvants are predominantly derived from pathogens and often represent pathogen associated molecular patterns (PAMP) e.g. LPS, MPL, CpG DNA, which activate cells of the innate immune system. Once activated, cells of innate immunity drive and focus the acquired immune response. In some studies, delivery systems and immunostimulatory agents have been combined to prepare adjuvant delivery systems, which are designed for more effective delivery of the immunostimulatory adjuvant into APC. Recent progress in innate immunity is beginning to yield insight into the initiation of immune responses and the ways in which immunostimulatory adjuvants may enhance this process. However, a rational approach to the development of new and more effective vaccine adjuvants will require much further work to better define the mechanisms of action of existing adjuvants. The discovery of more potent adjuvants may allow the development of vaccines against infectious agents such as HIV which do not naturally elicit protective immunity. New adjuvants may also allow vaccines to be delivered mucosally.

Introduction

Vaccines have traditionally consisted of live attenuated pathogens, whole inactivated organisms or inactivated toxins. In many cases, these approaches have been very successful at inducing immune protection, mainly based on antibody responses. However, to develop vaccines against more ‘difficult’ pathogens, which often establish chronic infections, e.g. HIV, HCV, TB and malaria, the induction of cell mediated immunity (CMI) is likely to be necessary. Unfortunately, non-living vaccines have generally proven ineffective at inducing CMI. In addition, although live vaccines may induce CMI, some live attenuated vaccines can cause disease in immunosuppressed individuals and some pathogens are difficult or impossible to grow in culture (e.g. HCV). Moreover, many traditional inactivated vaccines (e.g. Bordetella pertussis) also contain components that can cause undesirable effects and safety problems. As a result of these problems, several new approaches to vaccine development have emerged, which may have significant advantages over more traditional approaches. These approaches include: (1) recombinant protein subunits; (2) synthetic peptides; (3) protein polysaccharide conjugates; and (4) plasmid DNA. While these new approaches may offer important safety advantages, a general problem is that the vaccines alone are often poorly immunogenic. Traditional vaccines contain many components, some of which can elicit additional T cell help or function as adjuvants, e.g. bacterial DNA in whole cell vaccines. However, these components have been eliminated from many new generation vaccines. Therefore, there is an urgent need for the development of potent and safe adjuvants that can be used with newer generation vaccines, including DNA vaccines. In the recent years there has been great interest in DNA vaccines, since they appear to offer significant potential for the induction of potent CMI [1], which has been very difficult to achieve with non-living vaccines. Nevertheless, the potency of DNA vaccines in humans has so far been disappointing, particularly in relation to their ability to induce humoral responses [2], [3]. This has prompted investigators to work both on adjuvants and delivery systems for DNA vaccines [4] and also to use DNA in a prime/boost setting with alternative modalities, e.g. live viruses [5], [6], [7].

Immunological adjuvants were originally described by Ramon [8] as ‘substances used in combination with a specific antigen that produced a more robust immune response than the antigen alone’. This broad definition encompasses a very wide range of materials [9]. However, despite extensive evaluation of a large number of candidates over many years, the only adjuvants currently approved by the U.S. Food and Drug Administration are aluminum based mineral salts (generically called alum). Alum has a good safety record, but comparative studies show that it is a weak adjuvant for antibody induction to protein subunits and a poor adjuvant for CMI [10]. Moreover, alum adjuvants can induce IgE antibody responses and have been associated with allergic reactions in some subjects [10], [11]. Although Alum has been used as an adjuvant for many years, its mechanism of action remains poorly defined. It was originally thought to provide a ‘depot’ effect, resulting in persistence of antigen at the injection site. However, more recent studies involving radio-labelled antigens have shown that Alum does not establish an antigen depot at the injection site [12]. Recent work in vitro has indicated that Alum upregulates co-stimulatory signals on human monocytes and promotes the release of IL-4 [13]. Alum adsorption may also contribute to a reduction in toxicity for some vaccines, due to the adsorption of contaminating endotoxin [14].

A key issue in adjuvant development is toxicity, since safety concerns have restricted the development of adjuvants since Alum was first introduced more than 50 years ago [15]. Many experimental adjuvants have advanced to clinical trials and some have demonstrated high potency, but most have proven too toxic for routine clinical use. For standard prophylactic immunization in healthy individuals, only adjuvants that induce minimal adverse effects will prove acceptable. In contrast, for adjuvants which are designed to be used in life-threatening situations e.g. cancer vaccines, the acceptable level of adverse events would likely be increased. This review will focus predominantly on adjuvants to be used in vaccines against infectious diseases, although the potential use of similar adjuvants in cancer vaccines will be covered when appropriate. Developments in cancer vaccines have recently been reviewed [16]. There has been much concern recently that potent adjuvants might activate immunity to such an extent that auto-immune conditions might be triggered. This might be a particular concern for adjuvants which mimic components of pathogenic microorganisms and provide potent pro-inflammatory signals. Clearly, the timing and localization of certain stimuli may prove to be important in this context and limiting distribution of adjuvant actives to key cells is likely to be beneficial. To date, autoimmunity has only been linked to immunization in exceptional cases. Nevertheless, as more potent adjuvant actives become available, particularly those which activate innate immune responses, this needs to be monitored closely. Additional practical issues which are important for adjuvant development include biodegradability, stability, ease of manufacture, cost and applicability to a wide range of vaccines. Ideally, for ease of administration and enhanced patient compliance, the adjuvant should allow the vaccine to be administered by a mucosal route, but this has proven difficult. Examples of some of the adjuvants that have been evaluated in clinical trials are shown in Table 1. Although the mechanisms of action of adjuvants often remain poorly understood [9], [15], there is currently great interest in the effects of adjuvants on non-specific, or innate immunity.

Section snippets

The role of adjuvants in vaccine development

Adjuvants can be used to improve the immune response to vaccine antigens in several different ways: (1) adjuvants can increase the immunogenicity of weak antigens; (2) enhance the speed and duration of the immune response; (3) modulate antibody avidity, specificity, isotype or subclass distribution; (4) stimulate cell mediated immunity (CMI), (5) promote the induction of mucosal immunity; (6) enhance immune responses in immunologically immature, or senescent individuals; (7) decrease the dose

An immunologic perspective on adjuvants

Some of the key components involved in induction and maintenance of an immune response are summarized briefly in Table 2. Immune protection following vaccination depends predominantly on the generation of immunologic memory, mediated by B and T lymphocytes of the acquired immune system, which have highly restricted antigen specificity. Vaccines are effective either due to prevention of infection, or to prevention of disease. Antibodies are thought to be most important for prevention of

Cell types and effector mechanisms of innate immunity

Initiation of an immune response by vaccination relies on innate immune cells, and is mediated in large part by neutrophils and macrophages. These cells phagocytose and kill pathogens, but additionally co-ordinate the adaptive response by secreting a range of inflammatory mediators and cytokines. Adjuvants can elicit cytokine and chemokine production by APC, they can recruit cells to the local tissue and node, and can direct the development of humoral and CMI.

When APC are activated they become

Induction of acquired cellular immunity

CD4+T cells recognize antigens after they have been processed through the exogenous pathway by APC, expressing major histocompatability complex class II (MHC II) molecules. After activation, CD4+T cells differentiate into functional subsets, termed T helper 1 (Th1) and Th2. The division into these subsets is based largely on their secretion of different cytokines [55], [56], [57]. Th1 responses are typically characterized by the secretion of IFN-γ and the generation of delayed

Immunostimulatory adjuvants

As discussed earlier, MPL adjuvant is a PAMP which is derived from bacterial cell walls and interacts with TRL4. In a number of pre-clinical studies, MPL has been shown to induce the synthesis and release of cytokines, particularly IL-2 and IFN-γ, which promotes the generation of Th1 responses [60], [61]. In addition, MPL appears to increase the migration and maturation of DC [62]. MPL as a single adjuvant does not appear to be very potent for antibody induction, although it appears effective

Particulate antigen delivery systems

The use of particulate adjuvants, or antigen delivery systems, as alternatives to immunostimulatory adjuvants has been evaluated by several groups. Particulate adjuvants (e.g. emulsions, microparticles, iscoms, liposomes, virosomes and virus-like particles) have comparable dimensions to the pathogens which the immune system evolved to combat (Table 3). Immunostimulatory adjuvants may also be included in particulate delivery systems to enhance the level of response, or to focus the response

Lipid particles as adjuvants

A potent oil-in-water (o/w) adjuvant, the syntex adjuvant formulation (SAF) [92] was developed using a biodegradable oil (squalane) in the 1980s, as a replacement for Freund's adjuvants. Freund's adjuvants are potent but toxic water in mineral oil adjuvants, which may also contain killed mycobacteria [93]. However, SAF contained a bacterial cell wall based synthetic adjuvant, threonyl muramyl dipeptide (MDP), and a non-ionic surfactant, poloxamer L121, and proved too toxic for widespread use in

Microparticles as adjuvants

Antigen uptake by APC is enhanced by association of antigen with polymeric microparticles, or by the use of polymers or proteins which self-assemble into particles. The biodegradable and biocompatible polyesters, the polylactide-co-glycolides (PLG) are the primary candidates for the development of microparticles as adjuvants, since they have been used in humans for many years as suture material and as controlled release drug delivery systems [134], [135]. However, the adjuvant effect achieved

Alternative routes of immunization

Although most vaccines have traditionally been administered by intramuscular or subcutaneous injection, mucosal administration of vaccines offers a number of important advantages; including easier administration, reduced adverse effects and the potential for frequent boosting. In addition, local immunization induces mucosal immunity at the sites where many pathogens initially establish infection of hosts. Oral immunization would be particularly advantageous in isolated communities, where access

Mucosal immunization with microparticles

In mice, oral immunization with PLG microparticles has been shown to induce potent mucosal and systemic immunity to entrapped antigens [152], [159], [160], [161], [162]. In addition, mucosal immunization with microparticles induced protection against challenge with B. pertussis [163], [164], [165], [166] Chlamydia trachomatis [167] and Salmonella typhimurium [168]. In primates, mucosal immunization with inactivated SIV in microparticles induced protective immunity against intravaginal challenge

Adjuvants for mucosal immunization

The most potent mucosal adjuvants currently available are the bacterial toxins from Vibrio cholerae cholera toxin (CT) and Escherichia coli heat-labile enterotoxin (LT). However, since CT and LT are too toxic for use in humans, they have been genetically manipulated to reduce toxicity [178], [179], [180]. Single amino acid substitutions in the enzymatic A subunit of LT allowed the development of mutant toxins that retained potent adjuvant activity, but showed negligible or dramatically reduced

Adjuvants for therapeutic vaccines

It seems increasingly likely that novel adjuvants may prove sufficiently potent to allow the development of therapeutic vaccines. Rather than prevent infection, therapeutic vaccines would be designed to eliminate or ameliorate existing diseases, including: [1] chronic infectious diseases, e.g. those caused by HSV, HIV, HCV, HBV, HPV or H. pylori [2]; tumors, e.g. melanoma, breast or colon cancer; and [3] allergic or autoimmune disorders, e.g. multiple sclerosis, Type I diabetes and rheumatoid

Future developments in vaccine adjuvants

Several recent problems have served to highlight the urgent need for the development of new and improved vaccines. These problems have included: (1) the inability of traditional approaches to allow the successful development of vaccines against ‘difficult’ organisms, including those that establish chronic infections, e.g. HIV and HCV; (2) the emergence of new diseases, e.g. Ebola, West Nile and nvCJD; (3) the re-emergence of ‘old’ infections e.g. TB; and (4) the continuing spread of antibiotic

Acknowledgements

We would like to acknowledge the contributions of our colleagues in Chiron Corporation to the ideas contained in this review: particularly Rino Rappuoli, Sergio Abrignani, Michael Houghton, John Donnelly and Gary Ott. We would also like to thank Nelle Cronen for help in the manuscript preparation. We are grateful to Terry Ulrich and Charlotte Read-Kensil for the provision of clinical data on MPL and QS21 adjuvants, respectively.

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