Abstract
P48/45 is a conserved gametocyte antigen involved in Plasmodium parasite fertilization. A recombinant Plasmodium vivax P48/45 (Pvs48/45) protein expressed in Escherichia coli (E. coli) was highly antigenic and immunogenic in experimental animals and elicited specific transmission-blocking (TB) antibodies in a previous pilot study. Here, a similar Pvs48/45 gene was expressed in Chinese Hamster Ovary (CHO) cells and we compared its immunoreactivity with the E. coli product. Specific antibody titers were determined using plasma from Colombian individuals (n=227) living in endemic areas where both P. vivax and P. falciparum are prevalent and from Guatemala (n=54) where P. vivax is highly prevalent. In Colombia, plasma seroprevalence to CHO-rPvs48/45 protein was 46.3%, while for E. coli-rPvs48/45 protein was 36.1% (p<0.001). In Guatemala, the sero prevalence was 24.1% and 14.8% (p<0.001), respectively. Reactivity index (RI) against both proteins showed an age-dependent increase. IgG2 was the predominant subclass and the antibody avidity index evaluated by ELISA ranged between 4-6 mol/L. Ex vivo P. vivax mosquito direct membrane feeding assays (DMFA) performed in presence of study plasmas, displayed significant parasite transmission-blocking (TB), however, there was no direct correlation between antibody titers and oocysts transmission reduction activity (%TRA). Nevertheless, DMFA with CHO rPvs48/45 affinity purified IgG showed a dose response; 90.2% TRA at 100 μg/mL and 71.8% inhibition at 10 μg/mL. In conclusion, the CHO-rPvs48/45 protein was more immunoreactive in most of the malaria endemic places studied, and CHO-rPvs48/45 specific IgG showed functional activity, supporting further testing of the protein vaccine potential.
Highlights
Malaria is transmitted in ~106 countries in tropical and subtropical regions where over 219 million clinical cases and 405,000 deaths were reported in 2018 [1]
The different MW of the two proteins is due to: 1) an additional thioredoxin protein fragment added to the E. coli protein (108 aa) inserted to avoid inclusion bodies formation [24, 31] whereas the Chinese Hamster Ovary (CHO)-rPvs48/45 has a shorter thioredoxin fragment (42aa), and a theoretical mass of 51,766.22
Both proteins have a ~89% similar sequence and were both recognized by sera from P. vivax malaria semi-immune subjects as well as from plasma of Aotus monkeys immunized with the E. coli-rPvs48/45 protein (Figure 1B: lane 1 and 2), and confirm the similarity of both proteins and previous results [24]
Summary
Malaria is transmitted in ~106 countries in tropical and subtropical regions where over 219 million clinical cases and 405,000 deaths were reported in 2018 [1]. Guatemala reported 3,521 cases in 2018; most of them (99%) were caused by P. vivax [3] and, together with other countries of the region, reached conditions to initiate malaria elimination programs [5]. It has been observed that individuals continuously exposed to malaria infection in endemic regions develop immune responses that reduce or completely block Plasmodium transmission from humans to mosquitoes, which would decrease parasite spreading in endemic populations. This TB immunity is essential to develop vaccines that could accelerate malaria elimination [8, 9]
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