Phase Ia clinical evaluation of the safety and immunogenicity of the Plasmodium falciparum blood-stage antigen AMA1 in ChAd63 and MVA vaccine vectors

Abstract

Background: Traditionally, vaccine development against the blood-stage of Plasmodium falciparum infection has focused on recombinant protein-adjuvant formulations in order to induce high-titer growth-inhibitory antibody responses. However, to date no such vaccine encoding a blood-stage antigen(s) alone has induced significant protective efficacy against erythrocytic-stage infection in a pre-specified primary endpoint of a Phase IIa/b clinical trial designed to assess vaccine efficacy. Cell-mediated responses, acting in conjunction with functional antibodies, may be necessary for immunity against blood-stage P. falciparum. The development of a vaccine that could induce both cell-mediated and humoral immune responses would enable important proof-of-concept efficacy studies to be undertaken to address this question.

Methodology: We conducted a Phase Ia, non-randomized clinical trial in 16 healthy, malaria-naïve adults of the chimpanzee adenovirus 63 (ChAd63) and modified vaccinia virus Ankara (MVA) replication-deficient viral vectored vaccines encoding two alleles (3D7 and FVO) of the P. falciparum blood-stage malaria antigen; apical membrane antigen 1 (AMA1). ChAd63-MVA AMA1 administered in a heterologous prime-boost regime was shown to be safe and immunogenic, inducing high-level T cell responses to both alleles 3D7 (median 2036 SFU/million PBMC) and FVO (median 1539 SFU/million PBMC), with a mixed CD4(+)/CD8(+) phenotype, as well as substantial AMA1-specific serum IgG responses (medians of 49 µg/mL and 41 µg/mL for 3D7 and FVO AMA1 respectively) that demonstrated growth inhibitory activity in vitro.

Conclusions: ChAd63-MVA is a safe and highly immunogenic delivery platform for both alleles of the AMA1 antigen in humans which warrants further efficacy testing. ChAd63-MVA is a promising heterologous prime-boost vaccine strategy that could be applied to numerous other diseases where strong cellular and humoral immune responses are required for protection.

Trial registration: ClinicalTrials.gov NCT01095055.

Conflict of interest statement

Competing Interests: AJS, MDJD, SCG, AVSH and SJD are named inventors on US 12/595 574 and UK PCT/GB2008/01262, US 12/595 351 and UK PCT/GB2008/01271 novel adenovirus patent applications covering malaria vectored vaccines and immunization regimes. This does not alter our adherence to all the PLoS ONE policies on sharing data and materials. Authors from Okairòs are employees of and/or shareholders in Okairòs, which is developing vectored vaccines for malaria and other diseases. This does not alter our adherence to all the PLoS ONE policies on sharing data and materials.

Figures

Figure 1. Flow chart of the study.

All vaccinations were administered intramuscularly. ChAd63 AMA1 was administered on day 0 and MVA AMA1 on day 56. Six volunteers were excluded following screening for the following reasons: psychiatric morbidity; recurrent severe urticaria; elevated alanine aminotransferase (87 IU/L); unexplained microscopic haematuria and proteinuria; and withdrawal of consent (two individuals).

Figure 2. Local and systemic AEs deemed definitely, probably or possibly related to ChAd63 AMA1.

Only the highest intensity of each AE per subject is listed. Data are combined for all AEs for all volunteers receiving the same vaccine at the stated dose. There were no immunization related serious AEs. (A) Local AEs post ChAd63 AMA1. (B) Systemic AEs post ChAd63 AMA1. ‘Other’ AEs post 5×109 vp ChAd63 AMA1 included cough, coryzal symptoms, abdominal pain and dysmenorrhoea. ‘Other’ AE post 5×1010 vp ChAd63 AMA1 was coryzal symptoms. All ‘other’ AEs were considered possibly related to vaccination due to a temporal association.

Figure 3. Local and systemic AEs deemed definitely, probably or possibly related to MVA AMA1.

Only the highest intensity of each AE per subject is listed. Data are combined for all AEs for all volunteers receiving the same vaccine at the stated dose. There were no immunization related serious AEs. (A) Local and systemic AEs post 5×108 pfu MVA AMA1. Local ‘other’ AE was mild bruising at vaccination site. Systemic ‘other’ AEs included two cases of rigor, one case of delirium, loss of appetite and chills. (B) Local and systemic AEs post 2.5×108 pfu MVA AMA1. ‘Other’ AE post 2.5×108 pfu MVA AMA1 was dizziness. (C) Local and systemic AEs post 1.25×108 pfu MVA AMA1.

Figure 4. Cellular immunogenicity of ChAd63 AMA1 and ChAd63-MVA AMA1 immunization regimes.

(A) Groups 1A and 2A and (B) groups 1B and 2B median ex-vivo IFN-γ ELISPOT responses in PBMC to the AMA1 insert (summed response across all the individual peptide pools). Individual responses are shown in Figure S1. Note due to allele-specific peptide overlap some responses can be potentially counted twice. Individual breakdowns of the ELISpot responses are shown in Figure S2, and data in Figure S3C show that on average one quarter to one third of the total summed response can be attributed to FVO allele-specific peptides. (C) Median and individual IFN-γ ELISPOT responses at day 63 and (D) at day 140 that are functional to the individual alleles 3D7 (circles) and FVO (squares). (E,F) PBMC from day 84 for group 1B (closed symbols) and group 2B (open symbols) were re-stimulated with a pool of AMA1 peptides or cryopreserved iRBCs. Individual data points and the median are shown for (E) the % CD4+ and (F) CD8+ T cells positive for CD107a, IFN-γ, IL-2 or TNFα. The dotted line indicates the 0.05% level and any response <0.03% is not shown.

Figure 5. Antibody immunogenicity of ChAd63 AMA1 and ChAd63-MVA AMA1 immunization regimes.

(A) Groups 1A and 2A and (B) groups 1B and 2B total IgG ELISA responses against 3D7 AMA1 as measured in the serum over time. The geometric mean response is shown for each group and individual responses are shown in Figure S7. The horizontal dotted line indicates the limit of detection of the assay. (C) AMA1-specific ELISA titers against 3D7 (circles) and FVO (squares) in µg/mL for group 1B (n = 4) and group 2B (n = 4) at the peak time-point (day 84). Individual data points and the median are shown. (D) % GIA against 3D7 and FVO parasites at day 0 and day 84 in group 1B (open symbols) and 2B (closed symbols). (E) Relationship between 3D7 strain % GIA and serum 3D7 AMA1-specific IgG ELISA titer at day 0 and day 84 in groups 1B (open symbols) and 2B (closed symbols).