Evaluation of the safety and immunogenicity in rhesus monkeys of a recombinant malaria vaccine for Plasmodium vivax with a synthetic toll-like receptor 4 agonist formulated in an emulsion

Plasmodium vivax is the major cause of malaria outside sub-Saharan Africa and inflicts debilitating morbidity and consequent economic impacts in developing countries. In order to produce a P. vivax vaccine for global use, we have previously reported the development of a novel chimeric recombinant protein, VMP001, based on the circumsporozoite protein (CSP) of P. vivax. Very few adjuvant formulations are currently available for human use. Our interest is to evaluate second-generation vaccine formulations to identify novel combinations of adjuvants capable of inducing strong, long-lasting immune responses. In this study rhesus monkeys were immunized intramuscularly three times with VMP001 in combination with a stable emulsion (SE) or a synthetic Toll-like receptor 4 (TLR4) agonist (glucopyranosyl lipid A [GLA]) in SE (GLA-SE). Sera and peripheral blood mononuclear cells (PBMCs) were tested for the presence of antigen-specific humoral and cellular responses, respectively. All groups of monkeys generated high titers of anti-P. vivax IgG antibodies, as detected by enzyme-linked immunosorbent assays (ELISAs) and immunofluorescence assays. In addition, all groups generated a cellular immune response characterized by antigen-specific CD4⁺ T cells secreting predominantly interleukin-2 (IL-2) and lesser amounts of tumor necrosis factor (TNF). We conclude that the combination of VMP001 and GLA-SE is safe and immunogenic in monkeys and may serve as a potential second-generation vaccine candidate against P. vivax malaria.