New horizons in adjuvants for vaccine development

Enhancement of humoral and cellular immunity with an anti-glucocorticoid-induced tumour necrosis factor receptor monoclonal antibody

Adjuvants, including antibodies to tumour necrosis factor receptor superfamily members, augment immune responses. One member of this family, glucocorticoid-induced tumour necrosis factor receptor (GITR), is expressed at low levels on naive/resting T cells, B cells and macrophages, but at higher levels on T regulatory cells. The aim of this study was to determine the ability of a rat anti-mouse GITR monoclonal antibody, 2F8, to stimulate murine humoral and cellular immunity in a prime boost model with particular attention to posology and antigen-specific effects. 2F8 enhanced the humoral immune response to ovalbumin and haemagglutinin (HA) compared with controls and this enhancement was equal to or greater than that obtained in mice dosed with standard adjuvants. 2F8 F(ab')(2) fragments were as effective as intact antibody in boosting humoral immunity, indicating that FcR-mediated cross-linking of 2F8 is not required for efficacy. Moreover, the enhanced response was durable and antigen specific. Administration of 2F8 shifted the immune response towards a T helper type 1 response with significant enhancement of immunoglobulin G2a- and G2b-specific anti-HA antibodies, as well as enhanced cellular immunity as measured by ELISPOT. 2F8-treated mice also generated significantly more neutralizing antibodies to HA than control mice. Our findings show that anti-GITR is a robust, versatile adjuvant that, unlike commonly used adjuvants that primarily enhance humoral immunity, enhances both humoral and cellular immunity. These results support the continued development of anti-GITR for such indications as haematological and solid tumours, chronic viral infections, and as a vaccine adjuvant.

Production of H5N1 (NIBRG-14) inactivated whole virus and split virion influenza vaccines and analysis of immunogenicity in mice using different adjuvant formulations

Consecutive lots of H5N1 (A/Vietnam/1194/2004 - NIBRG-14) split virion and whole virus vaccines were produced in a pilot-scale laboratory. The average yields of vaccine doses (15 microg HA) per egg were 0.57 doses for H5N1 split virion vaccine and 1.12 for H5N1 whole virus vaccine, compared to 2.09 doses for the seasonal H3N2 split virion vaccine. H5N1 split virion vaccine lots complied with WHO protein content criteria, while some lots of the H5N1 whole virus vaccine showed protein content per dose higher than the limit established. All lots of both vaccines showed ovalbumin (OVA) concentration below the recommended limit. Dose sparing strategies using adjuvant formulations using aluminum hydroxide (Al(OH)(3)) and monophosphoryl lipid A (MPLA) from Bordetella pertussis were tested in mice. Both 3.75 microg HA and 7.5 microg HA of H5N1 split virion vaccine with Al(OH)(3) or Al(OH)(3) plus MPLA in aqueous suspension showed higher hemagglutination-inhibition (HAI) titers when compared to the same vaccine dose without any adjuvant. Immunization with the H5N1 inactivated whole virus vaccine was also performed using 3.75 microg HA and HAI titers were higher than those induced by the split virion vaccine. Moreover, the use of Al(OH)(3) with MPLA as an emulsion induced a further increase in HAI titers.

[Application of bioactive mutant TNF alpha to a mucosal vaccine adjuvant]

A large number of emerging pathogens, such as severe acute respiratory syndrome (SARS), human immunodeficiency virus (HIV), and influenza virus are mucosally transmitted and must cross mucosal barriers to infect the host. Thus, to induce a maximal protective effect, it is desirable to apply vaccines by the mucosal route where virus infections start. Mucosal vaccines administered either orally or nasally have been shown to be effective in inducing antigen-specific immune responses at both systemic and mucosal compartments. However the mucosal antigen-specific immune response is weak because most protein antigens can evoke only a weak immune response when they are applied mucosally. Therefore, one strategy to overcome the weakness of the immune response is a co-administration of mucosal adjuvant with the vaccine antigen. Unfortunately, the development of safe and effective mucosal adjuvant has proved to be challenging. Cytokines are promising adjuvants because they are human-derived safe material and display potent immune-modulating functions. In this regards, we have created a mutant tumor necrosis factor-alpha (TNF-alpha), mTNF-K90R, that exhibits high bioactivity and resistance to proteases. In this report, we examined the potential of mTNF-K90R as a mucosal adjuvant and evaluated its effectiveness and safety.

Chemical synthesis and proinflammatory responses of monophosphoryl lipid A adjuvant candidates

Lipopolysaccharides (LPS), which are structural components of the outer surface membrane of Gram-negative bacteria, trigger innate immune responses through activation of Toll-like receptor 4 (TLR4). Such responses may be exploited for the development of adjuvants and in particular monophosphoryl lipid A (MPLA) obtained by controlled hydrolysis of LPS of Salmonella minnesota, exhibits low toxicity yet possesses beneficial immuno-stimulatory properties. We have developed an efficient synthetic approach for the preparation of a major component of MPLA (1), which has as a key feature the use of allyloxycarbonates (Alloc) as permanent protecting groups for the C-3 and C-4 hydroxyls of the proximal glucosamine unit. The latter protecting groups greatly facilitated deprotection of the fully assembled compound. Furthermore, the amino functions were protected as N-2,2,2-trichloroethoxycarbamates (Troc), which performed efficient neighboring group participation to give selectively 1,2-trans-glycosides and could easily be removed under mild conditions without affecting the permanent Alloc carbonates and anomeric dimethylthexylsilyl (TDS) ether. The synthetic methodology was also employed for the preparation of a monophosphoryl lipid A (2) derivative that has the anomeric center of the proximal sugar modified as a methyl glycoside. Compound 1 was not able to induce cytokine production in mouse macrophages whereas methyl glycoside 2 displayed activity, however it has a lower potency and efficacy than lipid A obtained by controlled hydrolysis S. minnesota. This indicates compound 2 is an attractive candidate for adjuvant development and that 1 is not the active substance of MPLA obtained by controlled hydrolysis of LPS.

Synthetic and natural TLR4 agonists as safe and effective vaccine adjuvants

Natural derivatives and synthetic analogues of lipopolysaccharide are potent stimulators of the mammalian immune system. Retained adjuvant activity with reduced toxicity was obtained by the development of monophosphoryl lipid A (MPL((R))), which is approved for use in several vaccine products. Ongoing research and development of synthetic TLR4 agonists may offer increased purity and biological activity with reduced cost. Extensive research has elucidated the mechanism of action of TLR4 agonists and structure-function relationships. Moreover, the formulation of TLR4 agonists has been shown to significantly affect the type and magnitude of elicited immune response. TLR4 agonists comprise a promising class of adjuvants for safe and effective vaccines.

A review of adjuvants for Leishmania vaccine candidates

Over the last decade, there has been a flurry of research on adjuvants for vaccines, and several novel adjuvants are now licensed products or in late stage clinical development. The success of adjuvants in enhancing the immune response to antigens has led many researchers to re-focus their vaccine development programs. Although several vaccine candidates have been tested against leishmaniasis, there is yet no effective vaccine against this parasitic disease. Recent research has documented that efforts to develop effective Leishmania vaccine have been limited due to lack of an appropriate adjuvant. In view of this, this review paper outlines some of the adjuvants that have been used in Leishmania vaccine candidates and cites a few of the responses obtained from these studies. The aim of the present review is to consolidate these findings to facilitate the application of these adjuvants in general and experimental vaccinology.

Toll-like receptors and B-cell receptors synergize to induce immunoglobulin class-switch DNA recombination: relevance to microbial antibody responses

Differentiation of naïve B cells, including immunoglobulin class-switch DNA recombination, is critical for the immune response and depends on the extensive integration of signals from the B-cell receptor (BCR), tumor necrosis factor (TNF) family members, Toll-like receptors (TLRs), and cytokine receptors. TLRs and BCR synergize to induce class-switch DNA recombination in T cell-dependent and T cell-independent antibody responses to microbial pathogens. BCR triggering together with simultaneous endosomal TLR engagement leads to enhanced B-cell differentiation and antibody responses. Te requirement of both BCR and TLR engagement would ensure appropriate antigen-specific activation in an infection. Co-stimulation of TLRs and BCR likely plays a significant role in anti-microbial antibody responses to contain pathogen loads until the T cell-dependent antibody responses peak. Furthermore, the temporal sequence of different signals is also critical for optimal B cell responses, as exemplified by the activation of B cells by initial TLR engagement, leading to the up-regulation of co-stimulatory CD80 and MCH-II receptors, which result in more efficient interactions with T cells, thereby enhancing the germinal center reaction and antibody affinity maturation. Overall, BCR and TLR stimulation and the integration with signals from the pathogen or immune cells and their products determine the ensuing B-cell antibody response.

Immunological mechanisms underlying protection mediated by RTS,S: a review of the available data

The RTS,S/AS candidate malaria vaccine has demonstrated efficacy against a variety of endpoints in Phase IIa and Phase IIb trials over more than a decade. A multi-country phase III trial of RTS,S/AS01 is now underway with submission as early as 2012, if vaccine safety and efficacy are confirmed. The immunologic basis for how the vaccine protects against both infection and disease remains uncertain. It is, therefore, timely to review the information currently available about the vaccine with regard to how it impacts the human-Plasmodium falciparum host-pathogen relationship. In this article, what is known about mechanisms involved in partial protection against malaria induced by RTS,S is reviewed.

Toward the discovery of vaccine adjuvants: coupling in silico screening and in vitro analysis of antagonist binding to human and mouse CCR4 receptors

Background

Adjuvants enhance or modify an immune response that is made to an antigen. An antagonist of the chemokine CCR4 receptor can display adjuvant-like properties by diminishing the ability of CD4+CD25+ regulatory T cells (Tregs) to down-regulate immune responses.

Methodology

Here, we have used protein modelling to create a plausible chemokine receptor model with the aim of using virtual screening to identify potential small molecule chemokine antagonists. A combination of homology modelling and molecular docking was used to create a model of the CCR4 receptor in order to investigate potential lead compounds that display antagonistic properties. Three-dimensional structure-based virtual screening of the CCR4 receptor identified 116 small molecules that were calculated to have a high affinity for the receptor; these were tested experimentally for CCR4 antagonism. Fifteen of these small molecules were shown to inhibit specifically CCR4-mediated cell migration, including that of CCR4⁺ Tregs.

Significance

Our CCR4 antagonists act as adjuvants augmenting human T cell proliferation in an in vitro immune response model and compound SP50 increases T cell and antibody responses in vivo when combined with vaccine antigens of Mycobacterium tuberculosis and Plasmodium yoelii in mice.

Adjuvants in tuberculosis vaccine development

Tuberculosis remains a major public health problem around the world. Because the Mycobacterium bovis Bacilli-Calmette-Guerin (BCG) vaccine fails to protect adults from pulmonary tuberculosis, there is an urgent need for improved vaccine formulations. Unlike BCG, recombinant vaccines purified from bacterial expression vectors, as well as naked DNA, require an additional adjuvant. Recent improvements in our understanding of disease immunopathology, together with advances in biochemical and molecular techniques, have permitted the successful development of promising tuberculosis vaccine delivery and adjuvant combinations for human use. Here, we summarize the current state of adjuvant development and its impact on tuberculosis vaccine progress.

Adjuvants and autoimmunity

Some adjuvants may exert adverse effects upon injection or, on the other hand, may not trigger a full immunological reaction. The mechanisms underlying adjuvant adverse effects are under renewed scrutiny because of the enormous implications for vaccine development. In the search for new and safer adjuvants, several new adjuvants were developed by pharmaceutical companies utilizing new immunological and chemical innovations. The ability of the immune system to recognize molecules that are broadly shared by pathogens is, in part, due to the presence of special immune receptors called toll-like receptors (TLRs) that are expressed on leukocyte membranes. The very fact that TLR activation leads to adaptive immune responses to foreign entities explains why so many adjuvants used today in vaccinations are developed to mimic TLR ligands. Alongside their supportive role, adjuvants were found to inflict by themselves an illness of autoimmune nature, defined as ‘the adjuvant diseases’. The debatable question of silicone as an adjuvant and connective tissue diseases, as well as the Gulf War syndrome and macrophagic myofaciitis which followed multiple injections of aluminium-based vaccines, are presented here. Owing to the adverse effects exerted by adjuvants, there is no doubt that safer adjuvants need to be developed and incorporated into future vaccines. Other needs in light of new vaccine technologies are adjuvants suitable for use with mucosally delivered vaccines, DNA vaccines, cancer and autoimmunity vaccines. In particular, there is demand for safe and non-toxic adjuvants able to stimulate cellular (Th1) immunity. More adjuvants were approved to date besides alum for human vaccines, including MF59 in some viral vaccines, MPL, AS04, AS01B and AS02A against viral and parasitic infections, virosomes for HBV, HPV and HAV, and cholera toxin for cholera. Perhaps future adjuvants occupying other putative receptors will be employed to bypass the TLR signaling pathway completely in order to circumvent common side effects of adjuvant-activated TLRs such as local inflammation and the general malaise felt because of the costly whole-body immune response to antigen. Lupus (2009) 18, 1217—1225.

Enhancement of in vivo and in vitro immune functions by a conformationally biased, response-selective agonist of human C5a: implications for a novel adjuvant in vaccine design

A conformationally biased, agonist of human C5a(65-74) (EP67) was assessed for its adjuvant activities in vitro and in vivo. EP67 induced the release of the inflammatory (Th1) type cytokines from C5a receptor (CD88)-bearing antigen presenting cells (APC). EP67 did not induce the release of these cytokines from splenic APCs obtained from C5a receptor knockouts (CD88(-/-)). Serum from mice immunized with EP67-ovalbumin (OVA) contained high OVA-specific antibody (Ab) titers [IgG1, IgG2a (IGg2c), IgG2b]. Mice receiving OVA alone produced only IgG1 Abs, indicating the ability of EP67 to induce a Th1-like Ab class switch. Spleen cell cultures from wild type mice but not CD88(-/-) mice showed an enhanced OVA-specific proliferative response in vitro. These results indicate the ability of EP67 to drive a Th1-mediated immune response and its potential use as a unique adjuvant.

Adjuvants for malaria vaccines

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.

Vaccines: the fourth century

Vaccine development, which began with Edward Jenner's observations in the late 18th century, has entered its 4th century. From its beginnings, with the use of whole organisms that had been weakened or inactivated, to the modern-day use of genetic engineering, it has taken advantage of the tools discovered in other branches of microbiology. Numerous successful vaccines are in use, but the list of diseases for which vaccines do not exist is long. However, the multiplicity of strategies now available, discussed in this article, portends even more successful development of vaccines.

Novel vaccines to human rabies

Rabies, the most fatal of all infectious diseases, remains a major public health problem in developing countries, claiming the lives of an estimated 55,000 people each year. Most fatal rabies cases, with more than half of them in children, result from dog bites and occur among low-income families in Southeast Asia and Africa. Safe and efficacious vaccines are available to prevent rabies. However, they have to be given repeatedly, three times for pre-exposure vaccination and four to five times for post-exposure prophylaxis (PEP). In cases of severe exposure, a regimen of vaccine combined with a rabies immunoglobulin (RIG) preparation is required. The high incidence of fatal rabies is linked to a lack of knowledge on the appropriate treatment of bite wounds, lack of access to costly PEP, and failure to follow up with repeat immunizations. New, more immunogenic but less costly rabies virus vaccines are needed to reduce the toll of rabies on human lives. A preventative vaccine used for the immunization of children, especially those in high incidence countries, would be expected to lower fatality rates. Such a vaccine would have to be inexpensive, safe, and provide sustained protection, preferably after a single dose. Novel regimens are also needed for PEP to reduce the need for the already scarce and costly RIG and to reduce the number of vaccine doses to one or two. In this review, the pipeline of new rabies vaccines that are in pre-clinical testing is provided and an opinion on those that might be best suited as potential replacements for the currently used vaccines is offered.

Enhancement of tumour-specific immune responses in vivo by 'MHC loading-enhancer' (MLE)

Background

Class II MHC molecules (MHC II) are cell surface receptors displaying short protein fragments for the surveillance by CD4+ T cells. Antigens therefore have to be loaded onto this receptor in order to induce productive immune responses. On the cell surface, most MHC II molecules are either occupied by ligands or their binding cleft has been blocked by the acquisition of a non-receptive state. Direct loading with antigens, as required during peptide vaccinations, is therefore hindered.

Principal Findings

Here we show, that the in vivo response of CD4+ T cells can be improved, when the antigens are administered together with ‘MHC-loading enhancer’ (MLE). MLE are small catalytic compounds able to open up the MHC binding site by triggering ligand-release and stabilizing the receptive state. Their enhancing effect on the immune response was demonstrated here with an antigen from the influenza virus and tumour associated antigens (TAA) derived from the NY-ESO-1 protein. The application of these antigens in combination with adamantane ethanol (AdEtOH), an MLE compound active on human HLA-DR molecules, significantly increased the frequency of antigen-specific CD4+ T cells in mice transgenic for the human MHC II molecule. Notably, the effect was evident only with the MLE-susceptible HLA-DR molecule and not with murine MHC II molecules non-susceptible for the catalytic effect of the MLE.

Conclusion

MLE can specifically increase the potency of a vaccine by facilitating the efficient transfer of the antigen onto the MHC molecule. They may therefore open a new way to improve vaccination efficacy and tumour-immunotherapy.

Squalene emulsions for parenteral vaccine and drug delivery

Squalene is a linear triterpene that is extensively utilized as a principal component of parenteral emulsions for drug and vaccine delivery. In this review, the chemical structure and sources of squalene are presented. Moreover, the physicochemical and biological properties of squalene-containing emulsions are evaluated in the context of parenteral formulations. Historical and current parenteral emulsion products containing squalene or squalane are discussed. The safety of squalene-based products is also addressed. Finally, analytical techniques for characterization of squalene emulsions are examined.

Chitosan-based systems for the delivery of vaccine antigens

This review discusses the current status of chitosan and its derivatives as adjuvants and delivery systems in vaccine delivery, and their future possibilities and challenges. After a brief introduction to adjuvants and delivery systems, chitosan will be described in detail in regard to vaccine formulation. Applications of chitosan and its derivatives will be reviewed and their proposed mechanisms in the enhancement of immune responses will be discussed.

Protective immunity to influenza: lessons from the virus for successful vaccine design

The development of an effective influenza vaccine would require the ability to protect against infection with multiple influenza viral strains. In particular, mucosal and T-cell-mediated immunity may offer a more cross-reactive vaccine approach for the prevention of epidemic or potentially pandemic influenza. Thus, it is imperative to more fully understand the molecular events that occur in the host upon infection with a live virus and, in particular, to better evaluate the role of the distinct signaling pathways involved in developing protective immune responses. The paper under evaluation here introduces the notion that activation of caspase-1 inflammasomes in the hematopoietic cells in vivo are required for the establishment of Th1, cytotoxic T-lymphocyte and IgA responses to influenza virus infection.

Detailed analysis for inducing specific CD8 T cells via a CpG-DNA adjuvant

An important question for vaccine development is how the adaptive immune system incorporates antigenic, costimulatory and inflammatory signals. Here, Cui et al. addressed the concept that antigenic and inflammatory signals were individually or cooperatively involved in effector CD8 T-cell expansion and differentiation by using a peptide-pulsed dendritic cell immunization system. The paper under evaluation introduced IL-12-enhanced effector CD8 T-cell expansion and differentiation to short-lived effector cells (SLECs). The authors also suggested that antigenic stimulation and inflammation needed to be coupled to induce SLEC formation, and that IL-12 produced by bystander cells played an important role in SLEC formation. IFN-gamma could be required for optimal IL-12 production. Their data also showed that CpG-B adjuvant increased SLEC formation, but did not increase the potential of CD8 T cells to differentiate into memory cells. Data in the paper should be considered in research into the development of new adjuvants.

ISCOMs and ISCOMATRIX

Immunostimulatory complexes (ISCOMs) are particulate antigen delivery systems composed of antigen, cholesterol, phospholipid and saponin, while ISCOMATRIX is a particulate adjuvant comprising cholesterol, phospholipid and saponin but without antigen. The combination of an antigen with ISCOMATRIX is called an ISCOMATRIX vaccine. ISCOMs and ISCOMATRIX combine the advantages of a particulate carrier system with the presence of an in-built adjuvant (Quil A) and consequently have been found to be more immunogenic, while removing its haemolytic activity of the saponin, producing less toxicity. ISCOMs and ISCOMATRIX vaccines have now been shown to induce strong antigen-specific cellular or humoral immune responses to a broad range of antigens of viral, bacterial, parasite origin or tumor in a number of animal species including non-human primates and humans. These vaccines produced by well controlled and reproducible processes have also been evaluated in human clinical trials. In this review, we summarize the recent progress of ISCOMs and ISCOMATRIX, including preparation technology as well as their application in humans and veterinary vaccine designs with particular emphasis on the current understanding of the properties and features of ISCOMs and ISCOMATRIX vaccines to induce immune responses. The mechanisms of adjuvanticity are also discussed in the light of recent findings.

Plants as bioreactors for the production of vaccine antigens

Plants have been identified as promising expression systems for commercial production of vaccine antigens. In phase I clinical trials several plant-derived vaccine antigens have been found to be safe and induce sufficiently high immune response. Thus, transgenic plants, including edible plant parts are suggested as excellent alternatives for the production of vaccines and economic scale-up through cultivation. Improved understanding of plant molecular biology and consequent refinement in the genetic engineering techniques have led to designing approaches for high level expression of vaccine antigens in plants. During the last decade, several efficient plant-based expression systems have been examined and more than 100 recombinant proteins including plant-derived vaccine antigens have been expressed in different plant tissues. Estimates suggest that it may become possible to obtain antigen sufficient for vaccinating millions of individuals from one acre crop by expressing the antigen in seeds of an edible legume, like peanut or soybean. In the near future, a plethora of protein products, developed through ‘naturalized bioreactors’ may reach market. Efforts for further improvements in these technologies need to be directed mainly towards validation and applicability of plant-based standardized mucosal and edible vaccines, regulatory pharmacology, formulations and the development of commercially viable GLP protocols. This article reviews the current status of developments in the area of use of plants for the development of vaccine antigens.

Prospects for an influenza vaccine that induces cross-protective cytotoxic T lymphocytes

Our approach to vaccination against influenza is unique. For no other pathogen do we construct and produce a new vaccine every year in the face of uncertainty about the strains that will be circulating when it is used. The huge global cooperative effort that underpins this process reflects our awareness of the need to control this major pathogen. Moreover, the threat of devastation by a pandemic due to a newly emerging viral subtype has triggered an intense effort to improve and accelerate the production of vaccines for use if a pandemic arises. However, type A influenza viruses responsible for seasonal epidemics and those with the potential to cause a pandemic share amino acid sequences that form the targets of cytotoxic T lymphocytes (CTL). CTL activated by currently circulating viruses, therefore, offer a possible means to limit the impact of infection with future variant seasonal strains and even new subtypes. This review examines how cross-protective CTL can be exploited to improve influenza vaccination and issues that need to be considered when attempting to induce this type of immunity. We discuss the role of CTL responses in viral control and review the current knowledge relating to specificity and longevity of memory CD8(+) T cells, how vaccine antigen can be loaded into antigen-presenting cells to prime these responses and factors influencing the class of response induced. Application of these principles to the next generation of influenza vaccines should lead to much greater control of infection.