Phase Ia clinical evaluation of the Plasmodium falciparum blood-stage antigen MSP1 in ChAd63 and MVA vaccine vectors

Abstract

Efficacy trials of antibody-inducing protein-in-adjuvant vaccines targeting the blood-stage Plasmodium falciparum malaria parasite have so far shown disappointing results. The induction of cell-mediated responses in conjunction with antibody responses is thought to be one alternative strategy that could achieve protective efficacy in humans. Here, we prepared chimpanzee adenovirus 63 (ChAd63) and modified vaccinia virus Ankara (MVA) replication-deficient vectors encoding the well-studied P. falciparum blood-stage malaria antigen merozoite surface protein 1 (MSP1). A phase Ia clinical trial was conducted in healthy adults of a ChAd63-MVA MSP1 heterologous prime-boost immunization regime. The vaccine was safe and generally well tolerated. Fewer systemic adverse events (AEs) were observed following ChAd63 MSP1 than MVA MSP1 administration. Exceptionally strong T-cell responses were induced, and these displayed a mixed of CD4(+) and CD8(+) phenotype. Substantial MSP1-specific serum immunoglobulin G (IgG) antibody responses were also induced, which were capable of recognizing native parasite antigen, but these did not reach titers sufficient to neutralize P. falciparum parasites in vitro. This viral vectored vaccine regime is thus a leading approach for the induction of strong cellular and humoral immunogenicity against difficult disease targets in humans. Further studies are required to assess whether this strategy can achieve protective efficacy against blood-stage malaria infection.

Figure 1

Flow chart of the study. All vaccinations were administered intramuscularly. Chimpanzee adenovirus 63 (ChAd63) merozoite surface protein 1 (MSP1) dose-escalation was assessed (groups 1 versus 2), as well as the effect of modified vaccinia virus Ankara (MVA) MSP1 boosting (groups 1A versus 1B, and groups 2A versus 2B+C). The dose of MVA MSP1 was 5 × 10⁸ plaque forming units (pfu). The three volunteers in group 2C (otherwise identical to group 2B) were subsequently recruited into a phase IIa sporozoite challenge safety and efficacy study (S.H. Sheehy et al., manuscript in preparation) at the day 84 time-point. Safety and immunogenicity data are included for these three volunteers up until that time-point.

Figure 2

Systemic and local adverse events (AEs) deemed definitely, probably, or possibly related to immunization. Only the highest intensity of each AE per subject is listed. Local and systemic reactogenicity was evaluated at clinic visits and graded for severity (mild, moderate, severe), outcome and association to vaccination as per the criteria outlined in Supplementary Tables S1–S4. Data are combined for all AEs for all volunteers receiving the same vaccine at the stated dose. There were no immunization related serious AEs. Immunizations took place between November 2009 and January 2010 (during a time of high local incidence of upper respiratory tract infections). (a) Local and (b) systemic AEs post-chimpanzee adenovirus 63 (ChAd63) merozoite surface protein 1 (MSP1). (c) Local and systemic AEs post-modified vaccinia virus Ankara (MVA) MSP1. “Other” AEs are detailed in Supplementary Materials and Methods.

Figure 3

Cellular immunogenicity of chimpanzee adenovirus 63 (ChAd63) merozoite surface protein 1 (MSP1) and ChAd63-modified vaccinia virus Ankara (MVA) MSP1 immunization regimes. Median ex-vivo interferon-γ (IFN-γ) ELISPOT responses in peripheral blood mononuclear cells (PBMC) to the MSP1 insert (summed response across all the individual peptide pools) are shown over time for (a) groups 1A and 2A, and (b) group 1B and groups 2B+C (d0–d84 time-points include data combined for groups 2B+2C, d140 time-point includes only data from group 2B). (c) The percentage of the amino acid sequence within the MSP1 vaccine insert (Supplementary Figure S1) that is attributable to each block is shown (top). Data show the median total response to each block of sequence within the MSP1 insert according to group (1 or 2) and immunization regime (Ad = ChAd63, AdM = ChAd63-MVA) at the peak time-point (d14 after ChAd63 and d63/d84 after ChAd63-MVA).

Figure 4

Multifunctionality of the CD3⁺ T-cell responses was assessed by polychromatic flow cytometry and intracellular cytokine staining (ICS) following chimpanzee adenovirus 63 (ChAd63)-modified vaccinia virus Ankara (MVA) immunization at d84. PBMC from (a) group 1B and (b) group 2B+C were restimulated with a pool of merozoite surface protein 1 (MSP1) peptides or cryopreserved iRBCs. Individual data points and the median are shown for the % CD4⁺ and CD8⁺ T cells positive for CD107a, interferon-γ (IFN-γ), interleukin-2 (IL-2), or tumor necrosis factor-α (TNF-α). Responses <0.01% are not shown.

Figure 5

Antibody immunogenicity of chimpanzee adenovirus 63 (ChAd63) merozoite surface protein 1 (MSP1) and ChAd63-modified vaccinia virus Ankara (MVA) MSP1 immunization regimes. (a) Total immunoglobulin G (IgG) enzyme-linked immunosorbent assay (ELISA) responses against 3D7 PfMSP1₁₉ (ETSR/Mad20 allele) as measured in the serum over time following immunization. The geometric mean response is shown for each group and the dotted line represents the limit of detection of the assay. (a) groups 1A and 2A, and (b) group 1B and groups 2B+C (d0–d84 time-points include data combined for groups 2B+2C, d140 time-point includes only data from group 2B). (c) Spearman's correlation of serum IgG ELISA titers against PfMSP1₁₉ for the 3D7 ETSR versus Wellcome QKNG alleles after ChAd63 MSP1 priming at d28, n = 16 (left panel), and at the peak time-point (d84) after ChAd63-MVA MSP1 immunization, n = 12 (right panel). (d) Immunofluorescence (IFA) showing the recognition of 3D7 strain P. falciparum schizonts by immunoglobulin G (IgG) (green) in the sera of ChAd63-MVA MSP1 vaccinated volunteers. Day 84 sera from all 12 volunteers in groups 1B and 2B+C tested positive, but two representative results from vaccines (V) are shown (top row). DNA was counterstained with DAPI (blue). A representative control (C) testing pooled preimmunization sera was negative (bottom right), and a human monoclonal antibody (mAb) specific for PfMSP1₁₉ was included as a positive control (+) (bottom left). (e) ELISA titers against 3D7 and FVO PfMSP1₄₂ in µg/ml for group 1B (n = 4) and groups 2B+C (n = 8) at the peak time-point (d84). Individual data points and the median are shown. Groups 1B and 2B (open symbols), group 2C (closed symbols). (f) Relationship between 3D7 strain % GIA using purified immunoglobulin G (IgG) at 10 mg/ml and serum 3D7 PfMSP1₁₉-specific IgG ELISA titer. Samples testing less than the minimal detection level by ELISA were set at 5.0 ELISA units. All individual data points for all time-points assayed are shown (n = 30).