Adjuvants for malaria vaccines

Summary

There is a renewed enthusiasm about subunit vaccines for malaria coincident with the formation of new alliances and partnerships raising international public awareness, attracting increased resources and the re-focusing of research programs on adjuvant development for infectious disease vaccines. It is generally accepted that subunit vaccines for malaria will require adjuvants to induce protective immune responses, and availability of suitable adjuvants has in the past been a barrier to the development of malaria vaccines. Several novel adjuvants are now in licensed products or in late stage clinical development, while several others are in the earlier development pipeline. Successful vaccine development requires knowing which adjuvants to use and knowing how to formulate adjuvants and antigens to achieve stable, safe, and immunogenic vaccines. For the majority of vaccine researchers this information is not readily available, nor is access to well-characterized adjuvants. In this minireview, we outline the current state of adjuvant research and development as it pertains to effective malaria vaccines.

Introduction

The causative agent of malaria, the Plasmodium parasite, is transmitted by mosquitoes and has a complex life cycle. When an infected Anopheline mosquito takes a blood meal sporozoites from the proboscis enter the bloodstream of the human host. These cells can reach the liver within half an hour and infect hepatocytes. The sporozoites reproduce asexually in a symptomless liver stage and after 1 week they rupture their host cells releasing thousands of merozoites. These infect red blood cells where they continue to multiply. During this erythrocytic stage the hallmark signs of malaria are exhibited including fever, anaemia, impaired renal function and coma. Some merozoites develop into male and female gametocytes, which can be taken up by mosquitoes, sexually combining and completing the cycle by developing into new infective sporozoites. The traditional approach to develop malaria vaccines has been the targeting of the different stages of parasite development [reviewed in Girard et al. (1)]. The majority of malaria vaccine candidates focus on inducing immune responses to parasite antigens, which can vary greatly between strains and through expression of variants within gene families. It is believed that comprehensive immune responses are required for protection against malaria. Antibodies can mediate sporozoite killing via opsonization, blocking sporozoite invasion and eliminating infected cells either directly, with complement or via antibody-dependent cellular cytotoxicity. CD4⁺ and CD8⁺ T cells also play a part in immunity through the production of cytokines and other immune mediators released during activation or by natural killer (NK) and other cells that directly kill infected hepatocytes or erythrocytes. CD8⁺ or CD4⁺ cytotoxic T cell (CTL) against the antigens expressed by infected hepatocytes can prevent merozoite release and directly destroy infected cells via perforins and granzymes, or through apoptosis (2). A T-helper 1 (T_H1) immune profile skewed towards IFN-γ and IL-12 production has been established by in vitro and in vivo studies as critical for effective immunity against malaria.

Understanding the foundation of immunity that comes into play during the complex life cycle is critical for the rational design and development of effective vaccines against a pathogen as complex as Plasmodium. The different stages of the parasite life cycle present a variety of antigen targets for vaccines. Today, most licensed vaccines generate antibodies against extracellular pathogens. These antibodies can be measured, and often correlate with protection. Such vaccines comprise whole inactivated microorganisms or subunits of microorganisms with appropriate adjuvants. For malaria vaccine development, adjuvants or antigen delivery systems have been prioritized based in part on the target antigen. The most advanced of the malaria vaccine candidate, GlaxoSmithKline’s (GSK; Belgium) RTS,S which focuses on the pre-erythrocytic stage has been shown to require induction of both antibody and cell-mediated immunity to achieve a modest level of protection, and complex adjuvants containing both a saponin and a Toll-like receptor 4 (TLR4) agonist formulated in particulate systems are required to enable this (3). Another type of vaccine in development focuses on the induction of high titre antibodies in humans that will target sexual stages, the so-called transmission blocking vaccines. Such vaccines would not prevent infection of the immunized individual, but would prevent transmission of the parasite to a new host.

In addition to the usual challenges of achieving adequate safety and efficacy, malaria vaccines must also induce lasting immunity and be affordable for populations with limited resources. In malaria-endemic environments natural immunity is slow to develop and is not complete even after years of exposure to the parasite in endemic areas. There is mounting evidence that effective memory and protective immunity are short-lived, and lost when an individual is not continually exposed to the parasite (4). Novel concepts in malaria vaccine development must thus include intervention strategies that take into account the elicitation of effective antigen-specific B and T-helper cell responses, and the generation and maintenance of specific memory B and T cells. These must persist without natural boosting or periodic re-vaccination. Here, we will focus on novel concepts in malaria vaccine development, highlighting recent advances in adjuvants and formulations in development for use with protein antigens.

Adjuvants for Malaria Vaccines

A range of adjuvants, formulations, and viral/bacterial vectors is available for use with antigen candidates. Acceptable adjuvants enhance the potency, longevity and quality of specific immune response to antigens, but cause minimal toxicity or long-lasting immune effects on their own (5,6). The addition of adjuvants to antigens should both enhance and direct a specific immune response. Adjuvants have limited efficacy unless properly formulated; therefore both adjuvant selection and appropriate formulation is critical for enhancing vaccine potency. The formulation should aim to provide an optimal physical state for the adjuvant (e.g. colloid, emulsion, etc.) which affects the bioavailability and biological activity, as well as careful control of biophysical parameters such as particle size and lipid/water partitioning, interaction with the antigen, and overall stability.

Adjuvants have traditionally been broadly classified into two major classes according to their component sources, physiochemical properties or mechanisms of action, namely: (i) immunostimulants such as TLR ligands, cytokines, saponins and bacterial exotoxins that directly act on the immune system to increase responses to antigens and (ii) vehicles such as mineral salts, emulsions, liposomes, virosomes and biodegradable polymer microspheres that present vaccine antigens and co-administered immununostimulants to the immune system in an optimal manner. In recent years it has become apparent that many of these vehicles also have a direct effect on the immune system and can be considered immunostimulants.

Traditional live vaccines based on attenuated pathogens typically do not require the addition of adjuvants. The exception is whole cell vaccines such as those comprised of irradiated sporozoites, for which adjuvants may or may not increase protective responses or allow for dose sparing. Likewise, vaccines based on inactivated viruses or bacteria are often sufficiently immunogenic without added adjuvants, although some of these [e.g. split flu virus, hepatitis A virus (HAV) or whole cell Pertussis] may be formulated with adjuvants to further enhance the immune responses. However, most malaria vaccine candidates in development are recombinant proteins, and these will require adjuvants to induce effective immunity.

Rational design of adjuvants for human use has resulted in several leads in early and late stage development. Over the past two decades molecules have been discovered that are recognized by pattern-recognition receptors (PRRs), which include the TLRs (7), C-type lectin-like receptors (8,9), retinoic acid inducible gene-based-I-like receptors (10,11) and cytosolic nucleotide oligomerization domain-like receptors (12,13). In nature, these receptors have broad specificity to recognize pathogen-associated molecular patterns (PAMPs). These microbial ligands, which include cell wall components, lipoproteins, proteins, LPSs, DNA and RNA of bacteria, viruses, protozoa, and fungi, trigger different types of immune responses (14,15), and are the basis of many adjuvants (16). Discovery and development of such molecules into effective adjuvants must take into account several important aspects: The variation in responses between model animal species and humans such as seen in the unmethylated deoxycytidyl-deoxyguanosines (CpGs), which behave differently in different species, due at least in part to the variability of the TLR9 receptor between these species, and the differential expression of the TLR receptor on different cell types across species. Another issue to be addressed early on is one of safety: some adjuvants such as the water-in-oil emulsions (Montanide ISA-720, ISA-51) have induced sterile abscesses at the site of injection, a phenomenon which appears to be antigen dependent, and frequently not detected in preclinical studies; some TLR ligands have been implicated in the induction or enhancement of auto-immune responses in preclinical studies (17,18). Such effects must be considered prior to developing an adjuvant for clinical use. Another major issue that has been often ignored by vaccine developers is development of appropriate formulations where the stability and bio-availability of the immunostimulant and antigen are optimized. For example, numerous studies found the saponin adjuvant, QS21, to be chemically unstable and locally reactogenic. Formulation of QS21 with oil-in-water emulsions or liposomes is the basis of GSK’s AS02 and AS01 adjuvant formulations where the stability and local reactogenicity are improved. Other examples include the evaluation of CpG-containing oligonucleotides, and resiquimod (a TLR 7/8 ligand) which have entered clinical development without adequate consideration of appropriate formulation, which could enable the molecules to more effectively target intracellular TLR, thus potentially increasing efficacy while lowering dose and improving the risk-benefit profile.

The Importance of Adjuvant Formulation

Criteria involved in selecting the formulation for a given vaccine include: stability and bio-availability of antigen and immunostimulant; safety and cost considerations; the nature of the antigen components; the type of immune response desired; the preferred route of delivery; and immunogenicity of the final vaccine. Furthermore, the ideally formulated adjuvant will be well defined chemically and physically to facilitate quality control that will ensure reproducible manufacturing and potency. Effective adjuvant formulations may utilize multiple mechanisms to achieve the desired immunological effects, which can include the generation of antigen depots, concomitant delivery of antigen and immunostimulant, as well as increased immunological presentation of vaccine antigens by DCs and macrophages. These effects occur through the engagement of PRR or damage-associated molecular pattern (DAMP) receptors, and induction of CD8⁺ CTL responses and/or CD4⁺ T_H lymphocyte responses (T_H1 or T_H2) (19–21).

Unfortunately, decisions by vaccine developers regarding the appropriateness of a particular adjuvant and/or its formulation are often poorly informed and based on an empirical or historical approach rather than a scientific decision matrix. Certainly, incomplete understanding of immune correlates of protection or of the types of immune responses a particular adjuvant formulation induces in humans often precludes a rational approach, and a more empirical approach must be taken. A good example is the malaria RTS,S antigen which, when mixed with Alum plus MPL^® (Montana, USA) (AS04) or an oil-in-water emulsion (AS03) – although inducing high antibody titres – failed to protect immunized subjects against a Plasmodium falciparum challenge, while the same antigen in an oil-in-water emulsion containing MPL^® and QS21 (AS02) succeeded in inducing protection (3). Clearly, well informed and rationale selection of adjuvants and formulations will contribute to development of effective new vaccines. The use of adjuvants that are already in approved vaccines would facilitate the approval of a malaria vaccine, but as described below, such adjuvants currently appear inadequate and hence the development of new experimental adjuvants is likely to be required.

Adjuvants Approved for Human Vaccines

Adjuvants and delivery systems that have been approved for human clinical trial testing or are components of licensed vaccines, and have been used in malaria vaccines include aluminum salts (alum), MF59™ (an oil-in-water emulsion) (Switzerland), MPL^® (monophosphoryl lipid A; a glycolipid), virus-like particle, immunopotentiating reconstituted influenza virosomes and cholera toxin (22,23).

Alum

Aluminum salt-based adjuvants, referred to generically as ‘alum’, are used as noncrystalline gels. Alum adjuvants are components of several licensed human vaccines, including diphtheria-pertussis-tetanus, diphtheria-tetanus (DT), DT combined with hepatitis B (HBV), haemophilus influenza B or inactivated polio virus, HAV, Streptococcus pneumonia, meningococcal and human papilloma virus (HPV) (24). The advantages of aluminum adjuvants include their safety record, augmentation of antibody responses (faster, higher antibody titres and longer-lasting antibody responses), antigen stabilization and relatively simple formulation for large-scale production. The major limitations of aluminum adjuvants include their inability to elicit cell-mediated T_H1 and CTL responses that are required to control most intracellular pathogens such as those that cause malaria (25).

MF59™

MF59™ is an oil (squalene)-in-water emulsion composed of droplets of <250 nm and is approved for use in many European countries as an adjuvant for Fluad^® (Switzerland), a seasonal influenza vaccine for the elderly (26–28), as well as for use in Focetrea^® (Switzerland), a pandemic influenza vaccine. MF59™ has also been tested in candidate vaccines against malaria (29), HBV (30) and HIV (31). Overall, MF59™ has an acceptable safety profile, and with several antigens generates higher antibody titres with more balanced IgG subclass composition than those obtained with alum (32). In the clinic, strong helper T-cell responses were also observed as a result of vaccination (32,33). MF59™ is believed to act through direct stimulation of cytokine (33) and chemokine production by monocytes, macrophages, and granulocytes (34). Nevertheless, like alum, MF59™ has limited ability to induce increased CD4⁺ T_H1 immune responses (35).

MPL^®

MPL^® is a nontoxic derivative of the LPS from Salmonella minnesota (36) and is a potent stimulator of T_H1 responses. LPS consists of two basic structures: a hydrophilic polysaccharide portion and a hydrophobic lipid core moiety (lipid A) (37). As a TLR4 agonist, structural alterations of lipid A would presumably influence the binding affinity to myeloid differentiation factor 2 (MD2) as a ligand for TLR4. MPL^® was the first immunostimulant capable of activating T-cell effector responses to be used in licensed vaccines (fendrix, HBV; cervix, HPV) (36, 38–40) where it is combined with aluminium salts in the proprietary AS04 formulation. MPL^® has been evaluated in clinical trials of malaria vaccines in the form of liposomal-MPL^® combined with alum (41) however without success. More recently, MPL^®-saponin combinations including AS01B and AS02A, have been included in promising malaria vaccines clinical trials (42,43). MPL^® has been given to hundreds of thousands of individuals, and is a safe, well-tolerated and potent adjuvant. However, MPL^®, a natural product, is a heterogeneous mixture, which may affect its effectiveness. A newer generation of TLR-4 agonists include the synthetic aminoalkyl glucosamimide phosphates (44), and glucopyranosyl lipid adjuvants (in press).

Adjuvants in Development for Malaria

The development of additional adjuvants has been driven principally by the shortcomings of aluminum adjuvants (failure to stimulate T-cell responses, including CTL, loss of potency if frozen and causing granulomas at injection sites). Some promising malaria vaccines in development are discussed in the following paragraphs. In many instances, several adjuvants have been combined in one formulation to obtain synergistic or additive effects.

Montanides (ISA51, ISA720)

Montanides (including ISA51, ISA720) are water-in-oil emulsions containing squalene and mannide-monooleate as an emulsifier. Montanides, similar in physical character to incomplete Freund’s adjuvant (IFA) but biodegradable, have been developed in response to safety concerns with IFA in animal studies (45,46). Montanides have been extensively used in malaria, HIV, and cancer vaccine clinical trials (47). In several malaria studies, ISA 720 has been effective in promoting strong immune responses. However, occasional unacceptable local reactogenicity has been an issue and has frequently prevented further development of the candidate vaccines (48). In addition, instability of antigens in ISA 720 formulations has been reported (49). A further complication to the use of this type of adjuvant is that the formulation process can not be readily done while dosing requiring preformulation of the antigen in the emulsion, which adds to the cost and complexity of performing investigatory trials.

Saponins (Quil-A, ISCOM, QS-21, AS02 and AS01)

Saponins [Quil-A, immunostimulating complexes (ISCOMs), QS-21, AS02 and AS01] are triterpene glycosides isolated from plants. The most widely used in adjuvant research is Quil-A and its derivatives, extracted from the bark of the Quillaja saponaria tree (50). Quil-A is composed of a heterogeneous mixture of triterpene glycosides that vary in their adjuvant activity and toxicity. Partially purified fractions of Quil-A have also been used in ISCOMs composed of antigen, phospholipids, cholesterol and Quil-A fractions. ISCOMs are approximately 40 nm cage-like particles trapping the protein antigen through hydrophobic interactions, while ISCOMATRIX^® (Australia) (51) (preformed antigen-free particles) provides for more general applications by later accommodating nonhydrophobic antigens. QS-21 is a purified component of Quil-A that demonstrates low toxicity and maximum adjuvant activity. In a variety of animal models, QS-21 has augmented the immunogenicity of protein, glycoprotein and polysaccharide antigens (52). QS-21 has been shown to stimulate both humoral and cell-mediated T_H1 and CTL responses to subunit antigens (53). Clinical trials are in progress with QS-21, alone or in combination with carriers and other immunostimulants (e.g. in AS01 and AS02) for vaccines against infections including malaria, influenza and cancers. The use of QS21 is complicated by the chemical lability of this molecule, as well as its lytic activity which can cause local reactogenicity. These barriers may be overcome by appropriate formulation with carriers.

MPL^® formulations and combinations (MPL^®-SE, AS01, AS02 and AS04)

MPL^®-SE is a formulation comprised of a mixture of 3-deacylatedmonophosphoryl lipid A, squalene oil, other excipients (inactive substances used as carriers for the active ingredient), and water formulated as a stable oil-in-water emulsion. MPL^®-SE is an excellent promoter of T_H1 responses and is currently being evaluated in several clinical trials to treat and prevent leishmaniasis (54). The AS series of adjuvants are proprietary (GSK Biologics) formulations, several of which contain MPL^®. AS02 is an oil-in-water emulsion containing MPL and QS-21 that induces both strong humoral and T_H1 responses. AS02 is being evaluated in vaccine clinical trials for malaria (39,42,55), and other infectious diseases (56–58). In infants, the RTS,S/AS02D vaccine had a promising safety profile, did not interfere with the immunologic responses to co-administered Expanded Program on Immunization antigens, and reduced the incidence of malaria infection (55). AS01, a liposomal formulation containing MPL^® and QS21 which induces potent humoral and cell-mediated responses including CTL responses is currently being evaluated in clinical trials for malaria (59,60). Recently, the efficacy of RTS,S/AS01E was evaluated in children 5–17 months of age, a target population for vaccine licensure. Overall, there were 38 episodes of clinical malaria among recipients of RTS,S/AS01E, as compared with 86 episodes among those receiving the control vaccine, with an adjusted rate of efficacy against all malarial episodes of 56% (95% CI: 31–72; P ≤ 0·001) (59).

Immunostimulatory oligonucleotides

Synthetic oligodeoxynucleotides, containing unmethylated CpG motifs, act through TLR-9 and induce activation of DCs and secretion of pro-inflammatory cytokines such as TNF-α, IL-1 and IL-6. TLR-9 activation also leads to secretion of the pro-inflammatory cytokines IFN-α, IFN-γ and IL-12. CpGs are extremely efficient inducers of T_H1 immunity and CTL responses (61) and induce protection against infectious diseases, allergy and cancer in mice and primate models (62,63). Ongoing clinical studies indicate that CpGs are relatively safe and well-tolerated in humans (64,65) but due to variation in response between species, which makes evaluation of safety complicated, individual variable efficacy and safety concerns their use may well be limited.

Imidazoquinolines

These include synthetic compounds that induce the maturation and activation of professional APC and the secretion of inflammatory cytokines and chemokines (66–68). The small molecule imidazoquinolines, imiquimod and resiquimod, are TLR-7 and TLR-7/8 ligands respectively (69,70). Imiquimod applied as a topical cream has demonstrated efficacy in human clinical trials for leishmaniasis (71), and is licensed for treatment of HPV and basal cell carcinoma (BCC). The exact mechanism of action of imiquimod is unknown but it is thought that its activity as a TLR-7 agonist mimics a microbial antigen inducing the expression of different cytokines like IL-1, IL-6, IL-12, IFN-alpha and TNF-alpha, which stimulate or enhance both the innate immune system and the cell-mediated immune response, enhance migration of Langerhans’ cells to regional lymph nodes, as well as stimulate apoptosis in BCC (72) and diminish pathology associated with Leishmania infection (73).

Adjuvants to Enable Future Vaccines

Advances in genomics and proteomics have accelerated the identification of recombinant and synthetic vaccine molecules, but have also heightened the need for improved adjuvants and formulations beyond those currently available. In conjunction with these advances, recent insights into how immune responses are activated have facilitated the discovery of new and improved adjuvants (74). Activation of DC is paramount to effective adjuvants as this results in enhanced antigen up-take, migration to the draining lymph nodes, acquisition of co-stimulatory molecules, and presentation of antigenic peptides on MHC class I and II to the T-cell receptor (TCR). Stimulation of T cells through the TCR-complex in absence of co-stimulation of CD28 by CD80 or CD86 (signal 2) usually results in T-cell tolerance rather than activation. It is of interest to note that adjuvants possibly engaging DAMPs-like alum and MF59™ tend to induce T_H2 and B cell responses, while those containing TLR ligands (PAMPs) tend to favour more T_H1 and CTL responses. In addition, the particulate nature of some adjuvants such as virosomes, liposomes and ISCOMs seems to help in antigen cross-presentation and priming of CD8⁺ T cells.

Our understanding of the molecular structure of TLRs and increasing capabilities to synthesize complex molecules in a cGMP compliant manner has assisted in the development of a new generation of ligands that could become potent activators of appropriate immune responses that will propel subunit-based protein vaccines to a new level. While MPL^® has a long-standing track record as a safe immunostimulant (75). It is a heterogeneous mixture of glycolipids containing variable numbers of acyl chains. Because of this, assay standardization and guaranteeing even potency between lots of this natural product becomes difficult. To address some of these concerns, researchers have developed synthetic TLR-4 agonists (44,76–78) that have interesting binding and signalling profiles and may allow for better manufacture and fine tuning of the immune response. The published crystal structures of the TLR3 ectodomain (79,80) and the recently reported structures of the TLR1 and TLR2 heteroduplex (81) and the TLR-4/MD2 complex (82) have allowed insights into the mechanism of binding and requirements on the ligands needed for signalling (83). Molecular modelling studies and neural network guided selection strategies are currently being used to identify new, potent molecules that will stimulate these receptors in a rationally designed fashion.

An understanding of the mode of action of adjuvants and resulting immune responses will enable the development of malaria vaccines for difficult patient groups like infants and the elderly who have weaker immune responses. T cell-independent B-cell (antibody) responses are markedly compromised in the first year of life. T cell-dependent antibody responses mature much earlier, but neonates and infants may require multiple immunizations to achieve or sustain titres comparable to those in older individuals. Neonates can mount effective antigen-specific T-cell responses, but CD4⁺ T-cell responses are often slower to develop, less readily sustained, and in general more easily biased towards a T_H2 type response, most likely due to the decreased efficiency of neonatal DCs to establish T_H1 CD4⁺ T cell responses (84). This limitation can be overcome given appropriate stimuli, including adjuvants, delivered in the context of early priming and subsequent boosting (84).

Another major obstacle to the development of any active immunotherapeutic vaccine is the immunosuppressive environment including the induction of tolerogenic DCs and CD4⁺CD25⁺ regulatory T cells, which suppress the development of protective effector T-cell responses. This can be compounded by the use of TLR ligand-containing adjuvants as immunotherapeutics as TLR agonists can generate suppressive as well as inflammatory responses in innate immune cells and can promote the induction of regulatory as well as effector T cells (85).

Several barriers must be overcome to meet the demands for new adjuvants. Unacceptable side-effects and toxicity remain hurdles for many candidates, particularly for the development of paediatric vaccines. In addition, regulatory standards for adjuvant approval have increased substantially as the approval of alum. Currently, adjuvants do not receive FDA approval as stand alone products, but as part of a registered vaccine adjuvant-antigen combination. Therefore, potential adjuvant-antigen combinations have not been developed because of the huge costs and efforts involved in gaining FDA approval for each adjuvant-antigen combination. In addition, most vaccine companies keep their adjuvant formulations proprietary until the adjuvant is registered with a potential vaccine product. This limits the development of the adjuvant for other vaccine applications.

Strategy to Develop and Test New Adjuvants and Formulations for Malaria

Today, most researchers working on vaccines are focusing on the antigens, and testing them with the few available adjuvants that utilize only a single immunostimulant. Lack of either the knowledge or capacity to formulate complex adjuvant systems comprising immunostimulants and delivery vehicles, no readily available published methods for such systems, and often difficult access to new immunostimulants because of intellectual property and complicated material transfer agreements are major hurdles for most researchers working on vaccine development. A strategy to solve these important issues needs to address adjuvant access and new adjuvant development. In light of this, the Bill & Melinda Gates Foundation has awarded grants that will be used to develop and provide adjuvants for priority vaccine antigen candidates for malaria, HIV and neglected diseases.

New adjuvant development for malaria is needed to identify novel combinations of adjuvants and formulations capable of inducing strong, long-lasting humoral and cellular immune responses in humans. Ideally, these new adjuvants and formulations would generate a protective immune response with a reduced number of administrations. This will result in rational knowledge-based selection of adjuvant systems for the development of new vaccines targeting predominant humoral and/or cellular responses. Finally, the development of adjuvants with freedom to operate that are not largely controlled by large pharmaceutical companies will globally benefit the development of novel promising vaccines by providing researchers with the best available adjuvants and formulations to test with their antigens.

Numerous challenges remain related to adjuvant development. In effect, it is unlikely that any single immunostimulant or delivery system will be sufficient to induce the broad and long-lasting immunity that is required for new malaria vaccines. Effective adjuvant systems are likely to require synergy between one or more immunostimulants and a carrier/delivery system. Furthermore, each antigen has a different intrinsic immunogenicity, interacts differently with immunostimulants and carriers, and no reliable algorithms currently exist to permit selection of optimal adjuvants based on physico-chemical or immunological properties of an antigen.

Final Comments

Vaccination has been the most cost-effective health intervention for a range of infectious diseases, and this will 1 day include malaria. To ensure that new and existing adjuvants will be accessible for use in vaccines and therapeutics for malaria, the development path of the adjuvant candidates should include checking for patent or license fences, cost of goods and compliance with current and foreseeable regulatory standards. Despite the social, economic and environmental challenges we are optimistic that it is possible to bring malaria under control with the help of effective vaccines.