Cluster 2 - Platforms and Adjuvants

WP2.1. Preclinical development of new virosomally formulated vaccine components

Virosomes are spherical, unilammelar vesicles, prepared by detergent removal from a mixture of natural and synthetic phospholipids and influenza surface glycoproteins. They have been shown to be a highly effective means of enhancing the immune response to a variety of antigens. The first two registered Immunopotentiating Reconstituted Influenza Virosomes (IRIVs)-based vaccines for human use, a virosomal hepatitis A vaccine and a trivalent influenza vaccine have shown very good immunogenicity and safety profiles. Our revious research identified synthetic peptide antigens of CSP and AMA1 , which have been shown to elicit parasite binding and inhibitory antibodies when delivered with the IRIV-based antigen delivery system to experimental animals. Compared to alum-adjuvanted formulations, antigens coupled to the surface of IRIVs by phospholipid anchors exhibit a more native conformation thus eliciting superior parasite-binding and inhibitory antibodies. Furthermore, immune responses elicited by IRIV-based formulations are higher than those of liposomal formulations, demonstrating the enhancing effect of the influenza haemagglutinin. Preclinical data of the malaria antigens (CSP and AMA1) were confirmed in a recently completed Phase I clinical trial, which has shown very good safety and immunogenicity properties of the two vaccine components, both alone and in combination. Additional components will no doubt have to be added to produce an effective multi-stage, multi-component malaria vaccine formulation. Using the same iterative structural optimisation process as for AMA1 and CSP, development of synthetic vaccine candidates of further malaria blood stage proteins is currently in process. Promising synthetic lead structures have been identified for the C-terminal EGF homologous region of MSP-1 antigen. This project will be compared to the consortium's agreed MSP1 "reference" antigens, by developing formulations of these antigens for final pre-clinical profiling and clinical testing.

Work Package Leader: PB

 WP2.2. Generation of live attenuated recombinant measles virus based vaccines

A fundamental problem facing malaria vaccine development is that existing subunit vaccines can induce high titre antibodies but lack the ability to induce the long-term immunological memory needed to prevent severe malaria syndromes in endemic populations. Attenuated measles virus (MeV) vaccines have been given to literally billions of people and are among the safest and most effective vaccines invented, providing life-long immunity against measles after a single application to infants at about nine months of age. Live attenuated measles and mumps viruses (MeV and MuV) are thus exceptionally safe childhood vaccines, inducing remarkably long-lived immunity after a single immunisation dose. MeV vectors allow insertion and stable expression over multiple replication rounds of various genes (> 5 kb of additional genetic material) from different genome positions, allowing multiple levels of expression. Recombinant MeVs have been shown to induce comparable immunity against MeV proteins and vectored antigens (Hepatitis B, HIV and other pathogens). Therefore, the induction of a significant anti-malaria immune response is expected if antigens are properly selected and expressed by the recombinant viruses. Experiments will be conducted with this attractively immunogenic viral platform to express the critical determinants from merozoite antigen malaria vaccine candidates. To test this exciting concept, initially the C-terminal MSP-119 fragment will be cloned into MeV and expressed by the virus. The ability of MeV to induce cellular and humoral neutralising immunity will be systematically tested in transgenic mice susceptible to MeV infection in our pre-clinical assays. Previous experience has shown that p19, when expressed alone without the preceding portion of MSP-1 p42 in mammalian cells, it is properly folded and transported to the cell surface.

Work Package Leader: ETNA

 WP2.3. Vaccination with live attenuated adenovirus priming and modified vaccinia virus boosting of an MSP–1 antigen insert.

This product is designed to elicit protective antibodies against the 19 kDa fragment of MSP1 and protective T cells against other conserved regions that contain more T cell epitopes. Vectored vaccines provide a particularly powerful approach to inducing effector T cell responses and have protected humans from sporozoite challenge when used to encode pre-erythrocytic proteins. To date, Modified Vaccinia Ankara (MVA) has been used in numerous clinical trials of malaria vaccines in Africa and has induced liver-stage protection in heterologous prime-boost regimes. Adenovirus-MVA is chosen for clinical development in order to induce both strong anti-merozoite antibodies, and also effector T cells that should protect against blood-stage as well as late liver-stage parasites. In recent pre-clinical studies the adenovirus prime-MVA boost, with a MSP 42kDa insert, has induced both high titre antibodies and very strong T cell immunogenicity, as well as complete protection against blood-stage challenge in the P. yoelii model with a two dose immunisation regime. The only published data showing comparable efficacy required a five dose regime using complete Freund’s adjuvant.

Vectored vaccines are used without adjuvant and are amongst the most advanced malaria vaccine candidates in clinical trials. Generic manufacturing processes for both MVA and adenoviral vectors are well established. Simian adenovirus vectors have generated considerable excitement as novel vectors because they retain the high CD8 T cell immunogenicity of the human adenovirus five serotype but are not impaired by immunity to human adenoviruses.

Work Package Leader: UOXF

 WP2.4. Pre-clinical immunological studies of a potent adjuvant system for enhancing immune responses to blood-stage vaccine candidates

Current lead malaria blood stage vaccine candidates all confer at least partial protection by vaccination in animal models. However, our understanding of the mechanisms underlying protective immunity is incomplete. Vaccine efficacy trials have to date only shown, at best, partial protection. This may not be so much due to shortcomings of the vaccine candidates as to the adjuvant formulations. Clinical immunity against blood stage malaria has traditionally been considered predominantly antibody dependent, and it is evident that antibodies are important in mediating protection. In the development of malaria subunit vaccines it is clearly very important to optimise humoral immune responses by potent adjuvant systems. However, recent studies of both human malaria and mouse models indicate that cellular mediated immune responses are more important than previously assumed. Repeated ultra low dose infection and rapid cure of volunteers confer protection against low dose challenge without detectable antibodies but with a cellular mediated immune response against parasite antigen. In addition, IFN-gamma responses to both liver- and blood stage antigens have been correlated to protection. This WP will investigate a potent novel adjuvant system for malaria based on enhanced cationic liposomes, which was originally developed for vaccination against tuberculosis. This adjuvant system promotes strong humoral immune responses, and at the same time strong cellular mediated Th1 type immune responses. The hypothesis is that this will enhance the protective efficacy of current blood stage vaccines candidates.

Work Package Leader: SSI