Inducing Humoral and Cellular Responses to Multiple Sporozoite and Liver-Stage Malaria Antigens Using Exogenous Plasmid DNA

Abstract

A vaccine candidate that elicits humoral and cellular responses to multiple sporozoite and liver-stage antigens may be able to confer protection against Plasmodium falciparum malaria; however, a technology for formulating and delivering such a vaccine has remained elusive. Here, we report the preclinical assessment of an optimized DNA vaccine approach that targets four P. falciparum antigens: circumsporozoite protein (CSP), liver stage antigen 1 (LSA1), thrombospondin-related anonymous protein (TRAP), and cell-traversal protein for ookinetes and sporozoites (CelTOS). Synthetic DNA sequences were designed for each antigen with modifications to improve expression and were delivered using in vivo electroporation (EP). Immunogenicity was evaluated in mice and nonhuman primates (NHPs) and assessed by enzyme-linked immunosorbent assay (ELISA), gamma interferon (IFN-γ) enzyme-linked immunosorbent spot (ELISpot) assay, and flow cytometry. In mice, DNA with EP delivery induced antigen-specific IFN-γ production, as measured by ELISpot assay and IgG seroconversion against all antigens. Sustained production of IFN-γ, interleukin-2, and tumor necrosis factor alpha was elicited in both the CD4(+) and CD8(+) T cell compartments. Furthermore, hepatic CD8(+) lymphocytes produced LSA1-specific IFN-γ. The immune responses conferred to mice by this approach translated to the NHP model, which showed cellular responses by ELISpot assay and intracellular cytokine staining. Notably, antigen-specific CD8(+) granzyme B(+) T cells were observed in NHPs. Collectively, the data demonstrate that delivery of gene sequences by DNA/EP encoding malaria parasite antigens is immunogenic in animal models and can harness both the humoral and cellular arms of the immune system.

Malaria vaccine candidate antigen targeting the pre‐erythrocytic stage ofPlasmodium falciparumproduced at high level in plants

“…Even though Plassmeyer et al [41] have demonstrated that >60% sporozoite inhibition can be achieved using PfCSP_TSR-specific monoclonal antibodies, our results indicate that CCT needs to be improved either by using a larger portion of either PfTRAP or PfCSP to introduce additional B-cell and/or T-cell epitopes, or by optimizing the immunization procedure by different dose regimens, adjuvants and formulations, as well as the choice of mouse strain [42]. Reduction of the apparent immuno dominance of the PfCSP_TSR, an observation that has also been made by Ferraro et al [43] after DNAvaccination using PfCSP, PfTRAP, PfCelTOS, and PfLSA1, could be a strategy to induce a more balanced immune response against the three antigens to improve the efficacy of this vaccine candidate.…”

Malaria parasite CelTOS targets the inner leaflet of cell membranes for pore-dependent disruption

“…Individuals living in malaria endemic regions mount IFN-γ responses to CelTOS suggesting that CelTOS is a target of protective immunity (Anum et al, 2015). CelTOS is also unique as it is a promising multi-stage malaria vaccine target (Bergmann-Leitner et al, 2010; Ferraro et al, 2013; Bergmann-Leitner et al, 2011, 2013) for both infection-blocking and transmission-blocking vaccines currently in clinical trials.…”

“…Additionally, CelTOS has been identified as a promising malaria vaccine candidate referred to as Antigen 2 (Doolan et al, 2003). Immunization of mice with recombinant CelTOS results in humoral and cellular immune responses that reduced infection, demonstrating that targeting CelTOS is a viable approach for developing a malaria vaccine (Bergmann-Leitner et al, 2010; Ferraro et al, 2013; Bergmann-Leitner et al, 2011, 2013). Even though CelTOS is critical during cell traversal by malaria parasites and is a leading transmission- and infection-blocking malaria vaccine candidate, its function remains unknown as it has no sequence similarity to proteins of known function.…”

Evaluation of Plasmodium vivax Cell-Traversal Protein for Ookinetes and Sporozoites as a Preerythrocytic P. vivax Vaccine

“…berghei sporozoite challenge (18). In addition, a DNA vaccine coding for Pf CelTOS has been shown to induce humoral and cellular responses against the protein in mice and nonhuman primates (19). …”