Alga-Produced Malaria Transmission-Blocking Vaccine Candidate Pfs25 Formulated with a Human Use-Compatible Potent Adjuvant Induces High-Affinity Antibodies That Block Plasmodium falciparum Infection of Mosquitoes

Abstract

A vaccine to prevent the transmission of malaria parasites from infected humans to mosquitoes is an important component for the elimination of malaria in the 21st century, yet it remains neglected as a priority of malaria vaccine development. The lead candidate for Plasmodium falciparum transmission-blocking vaccine development, Pfs25, is a sexual stage surface protein that has been produced for vaccine testing in a variety of heterologous expression systems. Any realistic malaria vaccine will need to optimize proper folding balanced against cost of production, yield, and potentially reactogenic contaminants. Here Chlamydomonas reinhardtii microalga-produced recombinant Pfs25 protein was formulated with four different human-compatible adjuvants (alum, Toll-like receptor 4 [TLR-4] agonist glucopyranosal lipid A [GLA] plus alum, squalene-oil-in-water emulsion, and GLA plus squalene-oil-in-water emulsion) and compared for their ability to induce malaria transmission-blocking antibodies. Alga-produced recombinant Pfs25 plus GLA plus squalene-oil-in-water adjuvant induced the highest titer and avidity in IgG antibodies, measured using alga-produced recombinant Pfs25 as the enzyme-linked immunosorbent assay (ELISA) antigen. These antibodies specifically reacted with the surface of P. falciparum macrogametes and zygotes and effectively prevented parasites from developing within the mosquito vector in standard membrane feeding assays. Alga-produced Pfs25 in combination with a human-compatible adjuvant composed of a TLR-4 agonist in a squalene-oil-in-water emulsion is an attractive new vaccine candidate that merits head-to-head comparison with other modalities of vaccine production and administration.A vaccine to prevent gametocytemic humans from infecting mosquitoes by inducing antibodies against sexual stage Plasmodium antigens, termed a transmission-blocking vaccine (TBV), was first demonstrated experimentally as proof of principle using a chicken model of Plasmodium gallinaceum more than 30 years ago (1, 2). TBV development has been stated to be a key priority toward achieving the global goals of malaria control and elimination (3, 4), yet it lags behind other malaria vaccine development despite candidate proteins such as Pfs25 being deeply studied as a TBV. Pfs25, the first molecularly cloned Plasmodium sexual stage protein, was identified more than 25 years ago and remains a lead TBV candidate (5, 6). Parasite-produced Pfs25 contains 4 epidermal growth factor (EGF)-like domains and is not glycosylated, which makes it biotechnologically challenging to produce the properly folded, conformationally correct recombinant protein(s) required for the induction of effective transmission-blocking antibodies (7-9). Heterologous expression systems used to produce Pfs25 as a recombinant subunit immunogen include Escherichia coli (10), Saccharomyces cerevisiae (11), Pichia pastoris (7, 12), baculovirus (13), Nicotiana benthamiana (14), Triticum vulgare (wheat germ) extract (6,15,16), and most recently, the chloroplast of the mic...

Strategies & recent development of transmission-blocking vaccines against Plasmodium falciparum

“…Various TBV candidates have been successfully produced in plant system707172 with high recovery and desired immunogenicity (Table IV). Algae are also a promising system for the production of TBV that are orally delivered to avoid expensive purification and injectible delivery737475. These results sustain the feasibility of expressing TBV antigens in the plant based system.…”

Non-conventional expression systems for the production of vaccine proteins and immunotherapeutic molecules

“…In fact Pfs25 formulated with different adjuvants induced high-affinity antibodies and block P. falciparum infection in mice. 110 Staphylococcus and HPV proteins have also been expressed in this system, 107 as well as monoclonal antibodies.…”

Tailoring Microalgae for Efficient Biofuel Production

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