Using Plasmodium knowlesi as a model for screening Plasmodium vivax blood-stage malaria vaccine targets reveals new candidates

Abstract

Plasmodium vivax is responsible for the majority of malaria cases outside Africa. Unlike P. falciparum, the P. vivax life-cycle includes a dormant liver stage, the hypnozoite, which can cause infection in the absence of mosquito transmission. An effective vaccine against P. vivax blood stages would limit symptoms and pathology from such recurrent infections, and therefore could play a critical role in the control of this species. Vaccine development in P. vivax, however, lags considerably behind P. falciparum, which has many identified targets with several having transitioned to Phase II testing. By contrast only one P. vivax blood-stage vaccine candidate based on the Duffy Binding Protein (PvDBP), has reached Phase Ia, in large part because the lack of a continuous in vitro culture system for P. vivax limits systematic screening of new candidates. We used the close phylogenetic relationship between P. vivax and P. knowlesi, for which an in vitro culture system in human erythrocytes exists, to test the scalability of systematic reverse vaccinology to identify and prioritise P. vivax blood-stage targets. A panel of P. vivax proteins predicted to function in erythrocyte invasion were expressed as full-length recombinant ectodomains in a mammalian expression system. Eight of these antigens were used to generate polyclonal antibodies, which were screened for their ability to recognize orthologous proteins in P. knowlesi. These antibodies were then tested for inhibition of growth and invasion of both wild type P. knowlesi and chimeric P. knowlesi lines modified using CRISPR/Cas9 to exchange P. knowlesi genes with their P. vivax orthologues. Candidates that induced antibodies that inhibited invasion to a similar level as PvDBP were identified, confirming the utility of P. knowlesi as a model for P. vivax vaccine development and prioritizing antigens for further follow up.

Plasmodium knowlesi: the game changer for malaria eradication

“…vivax 6-cysteine protein P12 ( Pv12 ) and P . vivax Asparagine-rich Protein ( PvARP ) have been identified [ 197 ]. However, it is important to note that P. knowlesi does not share many of the morphological characteristics of P. vivax and does not form hypnozoites.…”

Knowledge, Attitudes and Perceptions Regarding Endemic Vivax Malaria in Inhabitants and Patients in Two Cities of Northern Gyeonggi-do, Korea, 2020

“…An effective vaccine against P. vivax blood stages would limit symptoms and pathology from such recurrent infections, but vaccine development for P. vivax lingers behind that for P. falciparum . Although a vaccine against vivax malaria could play a significant role in malaria control, there is currently no effective vaccine against the blood stage of any malaria parasite, and progress on P. vivax vaccine development has been particularly hampered [ 18 ]. The correct answer rate of the patient group for the question about vaccines was statistically significantly higher than that of the general population ( Fig.…”

Plasmodium vivax malaria serological exposure markers: assessing the degree and implications of cross-reactivity with P. knowlesi

“…There remains interest in this candidate 42, 43 , and the high levels of cross-reactivity in our study with P. knowlesi suggest that potential cross-species immunity could be possible 15 . Recent work has used P. knowlesi as a model for screening inhibitory activity of P. vivax antibodies 44 . Polyclonal antibodies against the P. vivax proteins DBP, two 6-cysteine proteins (Pv12, Pv14) and the GPI-anchored micronemal antigen (GAMA) were able to inhibit invasion of wild-type P. knowlesi , demonstrating that cross-species functional immunity is possible (at least in vitro ).…”

“…In addition to MSP1-19, we observed high levels of potential cross-reactive antibody responses against the P. vivax proteins MSP8, Pv-fam-a (PVX_096995) and RAMA. These antigens could thus be considered for assessment of growth inhibitory activity in the P. knowlesi model 44 and considered as potential targets for cross-species protection. This is an important consideration given the currently limited landscape of vaccine development specifically for P. knowlesi 45 .…”