Molecular vaccines for malaria.

The basic premise of vaccination is the triggering of host immune responses leading to the induction of adaptive immunity having sufficient magnitude and duration to provide long term protection. This has been achieved by many licensed vaccines, the majority based on attenuated or inactivated organisms, although often the protective antigens and underlying molecular mechanisms have not been identified. However, this traditional approach has not led to the development of a licensed vaccine for malaria or for several other devastating infectious diseases. Recently, substantial efforts have been focused on applying rational molecular design principles toward the development of novel vaccines for these refractory pathogens. In this review, we discuss the molecular aspects of antigen design, adjuvant advancement and the development of vaccine delivery systems as they are being applied to malaria vaccines.

Novel malaria antigen Plasmodium yoelii E140 induces antibody-mediated sterile protection in mice against malaria challenge.

Only a small fraction of the antigens expressed by malaria parasites have been evaluated as vaccine candidates. A successful malaria subunit vaccine will likely require multiple antigenic targets to achieve broad protection with high protective efficacy. Here we describe protective efficacy of a novel antigen, Plasmodium yoelii (Py) E140 (PyE140), evaluated against P. yoelii challenge of mice. Vaccines targeting PyE140 reproducibly induced up to 100% sterile protection in both inbred and outbred murine challenge models. Although PyE140 immunization induced high frequency and multifunctional CD8+ T cell responses, as well as CD4+ T cell responses, protection was mediated by PyE140 antibodies acting against blood stage parasites. Protection in mice was long-lasting with up to 100% sterile protection at twelve weeks post-immunization and durable high titer anti-PyE140 antibodies. The E140 antigen is expressed in all Plasmodium species, is highly conserved in both P. falciparum lab-adapted strains and endemic circulating parasites, and is thus a promising lead vaccine candidate for future evaluation against human malaria parasite species.

Liposomes containing monophosphoryl lipid A and QS-21 serve as an effective adjuvant for soluble circumsporozoite protein malaria vaccine FMP013.

Malaria caused by Plasmodium falciparum continues to threaten millions of people living in the tropical parts of the world. A vaccine that confers sterile and life-long protection remains elusive despite more than 30years of effort and resources invested in solving this problem. Antibodies to a malaria vaccine candidate circumsporozoite protein (CSP) can block invasion and can protect humans against malaria. We have manufactured the Falciparum Malaria Protein-013 (FMP013) vaccine based on the nearly full-length P. falciparum CSP 3D7 strain sequence. We report here immunogenicity and challenge data on FMP013 antigen in C57BL/6 mice formulated with two novel adjuvants of the Army Liposome Formulation (ALF) series and a commercially available adjuvant Montanide ISA 720 (Montanide) as a control. ALF is a liposomal adjuvant containing a synthetic monophosphoryl lipid A (3D-PHAD®). In our study, FMP013 was adjuvanted with ALF alone, ALF containing aluminum hydroxide (ALFA) or ALF containing QS-21 (ALFQ). Adjuvants ALF and ALFA induced similar antibody titers and protection against transgenic parasite challenge that were comparable to Montanide. ALFQ was superior to the other three adjuvants as it induced higher antibody titers with improved boosting after the third immunization, higher serum IgG2c titers, and enhanced protection. FMP013+ALFQ also augmented the numbers of splenic germinal center-derived activated B-cells and antibody secreting cells compared to Montanide. Further, FMP013+ALFQ induced antigen-specific IFN-γ ELISPOT activity, CD4+ T-cells and a TH1-biased cytokine profile. These results demonstrate that soluble CSP can induce a potent and sterile protective immune response when formulated with the QS-21 containing adjuvant ALFQ. Comparative mouse immunogenicity data presented here were used as the progression criteria for an ongoing non-human primate study and a regulatory toxicology study in preparation for a controlled human malaria infection (CHMI) trial.

Vaccine Strain-Specificity of Protective HLA-Restricted Class 1 P. falciparum Epitopes.

A DNA prime/adenovirus boost malaria vaccine encoding Plasmodium falciparum strain 3D7 CSP and AMA1 elicited sterile clinical protection associated with CD8+ T cell interferon-gamma (IFN-γ) cells responses directed to HLA class 1-restricted AMA1 epitopes of the vaccine strain 3D7. Since a highly effective malaria vaccine must be broadly protective against multiple P. falciparum strains, we compared these AMA1 epitopes of two P. falciparum strains (7G8 and 3D7), which differ by single amino acid substitutions, in their ability to recall CD8+ T cell activities using ELISpot and flow cytometry/intracellular staining assays. The 7G8 variant peptides did not recall 3D7 vaccine-induced CD8+ T IFN-γ cell responses in these assays, suggesting that protection may be limited to the vaccine strain. The predicted MHC binding affinities of the 7G8 variant epitopes were similar to the 3D7 epitopes, suggesting that the amino acid substitutions of the 7G8 variants may have interfered with TCR recognition of the MHC:peptide complex or that the 7G8 variant may have acted as an altered peptide ligand. These results stress the importance of functional assays in defining protective epitopes. Clinical Trials Registrations: NCT00870987, NCT00392015.

Correction: Vaccine Strain-Specificity of Protective HLA-Restricted Class 1 P. falciparum Epitopes.

[This corrects the article DOI: 10.1371/journal.pone.0163026.].