Phase I testing of a malaria vaccine composed of hepatitis B virus core particles expressing Plasmodium falciparum circumsporozoite epitopes

Abstract

We report the first phase I trial to assess the safety and immunogenicity of a malaria vaccine candidate, ICC-1132 (Malarivax), composed of a modified hepatitis B virus core protein (HBc) containing minimal epitopes of the Plasmodium falciparum circumsporozoite (CS) protein. When expressed in Escherichia coli, the recombinant ICC-1132 protein forms virus-like particles that were found to be highly immunogenic in preclinical studies of mice and monkeys. Twenty healthy adult volunteers received a 20- or a 50-microg dose of alum-adsorbed ICC-1132 administered intramuscularly at 0, 2, and 6 months. The majority of volunteers in the group receiving the 50-microg dose developed antibodies to CS repeats as well as to HBc. Malaria-specific T cells that secreted gamma interferon were also detected after a single immunization with ICC-1132-alum. These studies support ICC-1132 as a promising malaria vaccine candidate for further clinical testing using more-potent adjuvant formulations and confirm the potential of modified HBc virus-like particles as a delivery platform for vaccines against other human pathogens.

FIG. 1.

(A) Illustration showing the location of immunodominant B-cell epitope and the T1 epitope within the central repeat region and the universal T* epitope in the C terminus of P. falciparum CS protein. (B) Diagram of ICC-1132 monomer showing the CS protein T1 and B repeat epitopes inserted in the loop region (light and hatched boxes) and the T* epitope inserted at the C terminus of the truncated HBc protein (dark box). HBc monomer α-helical regions are represented as cylinders, and HBc amino acid residues are shown in circles. Reprinted and modified from Nature (6) with permission of the author and publisher.

FIG. 2.

Kinetics of the IgG antibody response measured by ELISA using ICC-1132 (A), HBc protein without CS inserts (B), and P. falciparum CS repeat peptide (C). Results are shown as GMTs for all vaccinees in the 50-μg dose (circles) and 20-μg dose (triangles) cohorts at various time points postvaccination (vaccinations are indicated by arrows).

FIG. 3.

CS-specific antibody responses in sera obtained 2 weeks after the third immunization with 20-μg (A) or 50-μg (B) doses, as measured by IFA (dark bars) or CS repeat peptide ELISA (hatched bars). All sera were tested in a CSP assay using viable transgenic sporozoites expressing P. falciparum CS repeats. +, sera with strong CSP reactions (greater than one-half the length of the sporozoite); ±, weak positive reactions.

FIG. 4.

IgM and IgG subtypes measured using ICC-1132 (A), HBc (B), or CS repeats (C) as the ELISA antigen. Results shown are the ΔOD (the OD of antigen-stimulated wells minus the OD of wells coated with bovine serum albumin) obtained with a 1:100 dilution of sera from vaccinees in the 20-μg (volunteers 10 and 12) and 50-μg (volunteers 13, 15, 17, and 21) dose groups. Volunteer 23 was a saline control. A positive control serum, Polyox 10, was obtained from a volunteer in a previous phase I trial of a polyoxime peptide vaccine containing the same CS epitopes (24).

FIG. 5.

IFN-γ ELISPOT carried out using cultured PBMC obtained from volunteers in the 50-μg dose group 56 days after the first immunization (A) and 14 days after the second immunization (day 70) (B). IFN-γ SFC were measured following overnight incubation with recombinant proteins (ICC-1132, HBc, or rCS) or malaria CS peptides (T* or repeats). Due to the wide range of backgrounds in unstimulated wells obtained with cells from different volunteers (day 56 range, 24 to 384 SFC/10⁶ cells in unstimulated wells; day 70 range, 7 to 563 SFC/10⁶ cells in unstimulated wells), the results are shown as the increase (n-fold) in the number of SFC (mean SFC in triplicate wells with antigen divided by mean SFC in wells without antigen). Volunteers 16-C and 23-C were saline placebo controls.