A review of combination adjuvants for malaria vaccines: a promising approach for vaccine development

It is obvious that there is a critical need for an efficient malaria vaccine to accelerate malaria eradication. Currently, recombinant subunit vaccination against malaria using proteins and peptides is gaining attention. However, one of the major drawbacks of this approach is the lack of an efficient and durable immune response. Therefore, subunit vaccines require adjuvants to make the vaccine sufficiently immunogenic. Considering the history of the RTS,S vaccine, it seems likely that no single adjuvant is capable of eliciting all the protective immune responses required in many malarial subunit vaccines and the use of combination adjuvants will be increasingly important as the science of malaria vaccines advances. In light of this, it appears that identifying the most effective mixture of adjuvants with minimal adverse effects offers tremendous opportunities in improving the efficacy of vaccines against malaria. Owing to the importance of a multi-adjuvanted approach in subunit malaria vaccine development, this review paper outlines some of the best known combination adjuvants used in malaria subunit vaccines, focusing on their proposed mechanisms of action, their immunological properties, and their notable results. The aim of the present review is to consolidate these findings to aid the application of these combination adjuvants in experimental malaria vaccines.     

The Bordetella type III secretion system: its application to vaccine development

B. pertussis is a causative agent of whooping cough (pertussis) in humans. Despite wide-scale vaccination in many countries, there is serious concern about pertussis as a re-emerging disease. Re-emergence of pertussis may be explained by several factors: the short duration of protection by the currently available acellular pertussis vaccine, an increase in asymptomatic adult carriers and expansion of strains with certain antigenic variations which are not covered by currently available vaccines. To develop safer and more efficacious vaccines which confer more prolonged protection, researchers are focusing on identification and characterization of new virulence factors. One candidate for protective antigens is the type III secretion system and its secreted proteins.     

Reverse immunogenetics: from HLA-disease associations to vaccine candidates

Analysis of the human leukocyte antigens (HLA) in patients with either infectious or autoimmune diseases has led to the identification of several HLA alleles associated with either resistance or susceptibility to disease. Understanding the role of HLA molecules in the presentation of peptide antigens to T cells has led to the use of ‘reverse immunogenetics’: a novel approach to analysing the key antigenic peptides that are presented by the relevant HLA molecules. Recent advances in the analysis of naturally occurring peptides bound to HLA molecules has allowed the direct identification of antigenic peptides from living cells and has supported the development of vaccine candidates, such as the liver-stage antigen 1 in malaria.     

Genes to genetic immunization: identification of bacterial vaccine candidates

Pathogenic bacteria still represent a major threat to human health worldwide and the need for new vaccines is great. Virulence factors, particularly surface-located molecules, often make good vaccine targets because they are essential for access to their niche within the host. The advent of whole-genome sequencing of bacterial pathogens has revolutionized the methods by which these organisms are studied and provides us with the possibility of identifying potential targets for vaccines by sequence scanning alone. Other developments in molecular biology, such as whole genome expression and mass mutagenesis, are also contributing to the identification of potential vaccine targets. In this chapter, we review approaches that exploit whole genome sequence data to identify potential virulence determinants and vaccine antigens.     

Yellow fever vector live-virus vaccines: West Nile virus vaccine development

By combining molecular-biological techniques with our increased understanding of the effect of gene sequence modification on viral function, yellow fever 17D, a positive-strand RNA virus vaccine, has been manipulated to induce a protective immune response against viruses of the same family (e.g. Japanese encephalitis and dengue viruses). Triggered by the emergence of West Nile virus infections in the New World afflicting humans, horses and birds, the success of this recombinant technology has prompted the rapid development of a live-virus attenuated candidate vaccine against West Nile virus.     

Multi high-throughput approach for highly selective identification of vaccine candidates: the Group A Streptococcus case

We propose an experimental strategy for highly accurate selection of candidates for bacterial vaccines without using in vitro and/or in vivo protection assays. Starting from the observation that efficacious vaccines are constituted by conserved, surface-associated and/or secreted components, the strategy contemplates the parallel application of three high throughput technologies, i.e. mass spectrometry-based proteomics, protein array, and flow-cytometry analysis, to identify this category of proteins, and is based on the assumption that the antigens identified by all three technologies are the protective ones. When we tested this strategy for Group A Streptococcus, we selected a total of 40 proteins, of which only six identified by all three approaches. When the 40 proteins were tested in a mouse model, only six were found to be protective and five of these belonged to the group of antigens in common to the three technologies. Finally, a combination of three protective antigens conferred broad protection against a panel of four different Group A Streptococcus strains. This approach may find general application as an accelerated and highly accurate path to bacterial vaccine discovery.     

Enzymatic shaving of the tegument surface of live schistosomes for proteomic analysis: a rational approach to select vaccine candidates

Background

The membrane-associated and membrane-spanning constituents of the Schistosoma mansoni tegument surface, the parasite''s principal interface with the host bloodstream, have recently been characterized using proteomic techniques. Biotinylation of live worms using membrane-impermeant probes revealed that only a small subset of the proteins was accessible to the reagents. Their position within the multilayered architecture of the surface has not been ascertained.

Methodology/Principal Findings

An enzymatic shaving approach on live worms has now been used to release the most accessible components, for analysis by MS/MS. Treatment with trypsin, or phosphatidylinositol-specific phospholipase C (PiPLC), only minimally impaired membrane integrity. PiPLC-enriched proteins were distinguished from those released in parasite vomitus or by handling damage, using isobaric tagging. Trypsin released five membrane proteins, Sm200, Sm25 and three annexins, plus host CD44 and the complement factors C3 and C4. Nutrient transporters and ion channels were absent from the trypsin fraction, suggesting a deeper location in the surface complex; surprisingly, two BAR-domain containing proteins were released. Seven parasite and two host proteins were enriched by PiPLC treatment, the vaccine candidate Sm29 being the most prominent along with two orthologues of human CD59, potentially inhibitors of complement fixation. The enzymes carbonic anhydrase and APD-ribosyl cyclase were also enriched, plus Sm200 and alkaline phosphatase. Host GPI-anchored proteins CD48 and CD90, suggest ‘surface painting’ during worm peregrination in the portal system.

Conclusions/Significance

Our findings suggest that the membranocalyx secreted over the tegument surface is not the inert barrier previously proposed, some tegument proteins being externally accessible to enzymes and thus potentially located within it. Furthermore, the detection of C3 and C4 indicates that the complement cascade is initiated, while two CD59 orthologues suggest a potential mechanism for its inhibition. The detection of several host proteins is a testimonial to the acquisitive properties of the tegument surface. The exposed parasite proteins could represent novel vaccine candidates for combating this neglected disease.     

Successes and failures: Worldwide vaccine development and application

The impact that vaccines have had on world health has been great. The misery prevented and the lives saved have been impressive. But all has not been good. As one looks at the success, one can also see the missed opportunities. This discussion takes a broad, worldwide view of vaccines - from early research, through development and application. It examines our successes and our failures and looks with great optimism towards a future having great potential to prevent much of today’s suffering from infectious diseases.     

The development of vaccines: how the past led to the future

The history of vaccine development has seen many accomplishments, but there are still many diseases that are difficult to target, and new technologies are being brought to bear on them. Past successes have been largely due to elicitation of protective antibodies based on predictions made from the study of animal models, natural infections and seroepidemiology. Those predictions have often been correct, as indicated by the decline of many infections for which vaccines have been made over the past 200 years.     

Lyophilisation and sterilisation of liposomal vaccines to produce stable and sterile products

The advantages of liposomes as delivery systems for peptide, protein and DNA vaccines is well-recognised, unfortunately their application has been stinted by their instability during storage and their limited shelf-life. Further, sterilisation of these systems has been problematic, with degradation of the liposomes being reported after sterilisation using the various techniques available. Work form our laboratory has investigated techniques that can be applied to particulate liposomal vaccines such that they can be prepared in a freeze-dried and sterile format. In this article, we describe techniques for the lyophilisation, cryoprotection and sterilisation of liposomal vaccines. Applying these methods allows for the retention of both the chemical integrity of the lipids and the key physico-chemical characteristics of the liposomes (e.g., particle size, zeta potential, and dynamic viscosity), thus supporting the enhanced transition of liposomal vaccines from the bench to the clinic.     

Development of vaccines to control bovine tuberculosis in cattle and relationship to vaccine development for other intracellular pathogens

Vaccination of cattle against bovine tuberculosis could be an important strategy for the control of disease either where there is a wildlife reservoir of Mycobacterium bovis infection or in developing countries where it is not economically feasible to implement a 'test and slaughter' control program. Advances in the understanding of protective immune responses to M. bovis infection in cattle and the advent of new molecular biological techniques, coupled with the sequencing of the M. bovis genome have provided opportunities for the rational development of improved tuberculosis vaccines. A number of new tuberculosis vaccines including attenuated M. bovis strains, killed mycobacteria, protein and DNA vaccines are under development and many are being assessed in cattle. Recent results have revealed several promising vaccine candidates and vaccination strategies. Ways of distinguishing between vaccinated and infected cattle are becoming available and the possibility of new approaches to the eradication of tuberculosis from domestic livestock is discussed. Similarities between the mechanisms of protective immunity against M. bovis and against other intracellular parasites continue to be found and discoveries from vaccine studies on bovine tuberculosis may provide helpful insights into requirements for vaccines against other intracellular pathogens.     

The effect of cholesterol in a liposomal Muc1 vaccine

A liposomal Muc1 mucin vaccine for treatment of adenocarcinomas was formulated by incorporating a synthetic Muc1 mucin-based lipopeptide and Lipid A into a DPPC/cholesterol bilayer. Vaccination of mice with the liposomal formulation produced a peptide-specific immune response dependent on the cholesterol content. The response occurred at a threshold of 20-23 mol% cholesterol, and was optimal at cholesterol levels of > or =30 mol%. To understand this cholesterol dependency, we studied the effect of cholesterol on the liposomal bilayer and surface properties. Freeze-fracture electron microscopy showed a unique surface texture that was codependent upon cholesterol (> or =20 mol%) and lipopeptide content. Fluorescence anisotropy measurements exhibited a significant decrease in the rotational motion of 1,6-diphenyl-1,3,5-hexatriene in formulations containing >20 mol% cholesterol and only in the presence of the lipopeptide. At 20 mol% cholesterol and with lipopeptide, DSC showed a significant increase in the main phase transition of the DPPC bilayers, while Raman spectroscopy indicated a more ordered arrangement of DPPC molecules compared to control liposomes containing DPPC/cholesterol alone. Taken together, the data suggest the presence of lipopeptide-rich microdomains at and above a threshold of 20 mol% cholesterol that may play a role in the induction of a peptide-specific immunological response.     

Psoroptes ovis: identification of vaccine candidates by immunoscreening

Serum from successful vaccine trials against the sheep scab mite, Psoroptes ovis, was used to immunoscreen a cDNA library constructed from mixed-stage and gender P. ovis to identify potential recombinant vaccine candidates. Immunodominant recombinant proteins recognised by IgG in these sera were selected for further analysis. Two candidates were identified in this way; a catchin-like protein (CLP) and a novel mu class glutathione S-transferase (GST). Both candidates were expressed in bacteria as recombinant proteins, the GST as an active enzyme, and combined with four other recombinant allergens in a multi-component recombinant vaccine. Strong serum IgG responses were induced in sheep against each of the components of the recombinant vaccine, however, the protective efficacy of the vaccine could not be determined because of variability in the establishment of a challenge infection.     

Surface proteome mining for identification of potential vaccine candidates against Campylobacter jejuni: an in silico approach

Functional & Integrative Genomics - Campylobacter jejuni remains a major cause of human gastroenteritis with estimated annual incidence rate of 450 million infections worldwide. C. jejuni is a...     

Schistosomiasis vaccine:research to development

In this article, Robert Bergquist and Dan Colley deal with the consolidated, international efforts to generate a schistosomiasis vaccine; in particular, they summarize the deliberations of a series of meetings, held in Cairo, Egypt, 21-25 May 1997, with the aim of reviewing the current status of affairs in this respect in order to make recommendations for the future course of schistosomiasis vaccine development.     

Prostate cancer vaccines: the long road to clinical application

Cancer vaccines as a modality of immune-based cancer treatment offer the promise of a non-toxic and efficacious therapeutic alternative for patients. Emerging data suggest that response to vaccination largely depends on the magnitude of the type I immune response generated, epitope spreading and immunogenic modulation of the tumor. Moreover, accumulating evidence suggests that cancer vaccines will likely induce better results in patients with low tumor burden and less aggressive disease. To induce long-lasting clinical responses, vaccines will need to be combined with immunoregulatory agents to overcome tumor-related immune suppression. Immunotherapy, as a treatment modality for prostate cancer, has received significant attention in the past few years. The most intriguing characteristics that make prostate cancer a preferred target for immune-based treatments are (1) its relative indolence which allows sufficient time for the immune system to develop meaningful antitumor responses; (2) prostate tumor-associated antigens are mainly tissue-lineage antigens, and thus, antitumor responses will preferentially target prostate cancer cells. But, also in the event of eradication of normal prostate epithelium as a result of immune attack, this will have no clinical consequences because the prostate gland is not a vital organ; (3) the use of prostate-specific antigen for early detection of recurrent disease allows for the initiation of vaccine immunotherapy while tumor burden is still minimal. Finally, for improving clinical outcome further to increasing vaccine potency, it is imperative to recognize prognostic and predictive biomarkers of clinical benefit that may guide to select the therapeutic strategies for patients most likely to gain benefit.