Phase I Clinical Trial of a Recombinant Blood Stage Vaccine Candidate for Plasmodium falciparum Malaria Based on MSP1 and EBA175

Abstract

Background: A phase I randomised, controlled, single blind, dose escalation trial was conducted to evaluate safety and immunogenicity of JAIVAC-1, a recombinant blood stage vaccine candidate against Plasmodium falciparum malaria, composed of a physical mixture of two recombinant proteins, PfMSP-1(19), the 19 kD conserved, C-terminal region of PfMSP-1 and PfF2 the receptor-binding F2 domain of EBA175.

Method: Healthy malaria naïve Indian male subjects aged 18-45 years were recruited from the volunteer database of study site. Fifteen subjects in each cohort, randomised in a ratio of 2:1 and meeting the protocol specific eligibility criteria, were vaccinated either with three doses (10 μg, 25 μg and 50 μg of each antigen) of JAIVAC-1 formulated with adjuvant Montanide ISA 720 or with standard dosage of Hepatitis B vaccine. Each subject received the assigned vaccine in the deltoid muscle of the upper arms on Day 0, Day 28 and Day 180.

Results: JAIVAC-1 was well tolerated and no serious adverse event was observed. All JAIVAC-1 subjects sero-converted for PfF2 but elicited poor immune response to PfMSP-1(19). Dose-response relationship was observed between vaccine dose of PfF2 and antibody response. The antibodies against PfF2 were predominantly of IgG1 and IgG3 isotype. Sera from JAIVAC-1 subjects reacted with late schizonts in a punctate pattern in immunofluorescence assays. Purified IgG from JAIVAC-1 sera displayed significant growth inhibitory activity against Plasmodium falciparum CAMP strain.

Conclusion: Antigen PfF2 should be retained as a component of a recombinant malaria vaccine but PfMSP-1(19) construct needs to be optimised to improve its immunogenicity.

Trial registration: Clinical Trial Registry, India CTRI/2010/091/000301.

Fig 1. Volunteer Flow Chart (CONSORT diagram).

Fig 2. Antibodies to PfF2 and PfMSP-1₁₉ after vaccination with JAIVAC-1.

Anti-PfF2 (A) and PfMSP-1₁₉ (B) antibody levels measured by ELISA. Geometric mean antibody levels (AU) to recombinant PfF2 and PfMSP-1₁₉ measured by ELISA in sera collected from Day 0 to Day 365 in 10μg, 25μg, and 50μg JAIVAC-1/Montanide ISA720 and Hepatitis B vaccine recipients. Study participants were immunized on Days 0, 28 and 180 and sera were collected on Days 0, 28, 56, 180, 208 and 365. Antibody levels measured by ELISA were expressed as Geometric Mean Titres (GMT) for the 10 subjects. Anti-PfF2 specific IgG subclass profiles in the sera of individuals vaccinated with 25μg (C) and 50μg (D) JAIVAC-1 as measured by ELISA are shown.

Fig 3. Recognition of native antigens in blood stage P. falciparum schizonts.

Serum from one representative subject immunised with JAIVAC-1 was used to detect native PfMSP1 and EBA 175 in P. falciparum schizonts by Immunofluoroscence Assay (IFA). The punctate staining at the apical end of merozoites in late-stage schizonts reflects the known distribution of EBA 175.

Fig 4. IFA titer.

Dose-dependence (A) and kinetics (B) of antibody response as measured by IFA using sera from subjects who received 10μg, 25μg and 50μg of JAIVAC-1 and Hepatitis B vaccine. The arrows indicate days of immunisation.

Fig 5. Vaccine induced in vitro Growth Inhibitory Activity (GIA).

The percent growth inhibition in cultures was measured by pLDH using two different strains of P. falciparum (CAMP and 3D7). (A) and (B) represent the percent growth inhibition of P. falciparum CAMP and 3D7 parasite strains after addition of 10 mg/ml purified IgG from sera of Day 0 and Day 208 of 50μg of JAIVAC-1 vaccine and Hepatitis vaccine group. (C) and (D) represent the average percent growth inhibition of—P. falciparum parasite strains CAMP and 3D7 with Day 0 and Day 208 sera from individuals immunized with 50μg of JAIVAC-1 and Hepatitis vaccine.