Functional characterization and comparison of Plasmodium falciparum proteins as targets of transmission-blocking antibodies

Plasmodium falciparum malaria continues to evade control efforts, utilizing highly specialized sexual-stages to transmit infection between the human host and mosquito vector. In a vaccination model, antibodies directed to sexual-stage antigens, when ingested in the mosquito blood meal, can inhibit parasite growth in the midgut and consequently arrest transmission. Despite multiple datasets for the Plasmodium sexual-stage transcriptome and proteome, there have been no rational screens to identify candidate antigens for transmission-blocking vaccine (TBV) development. This study characterizes 12 proteins from across the P. falciparum sexual-stages as possible TBV targets. Recombinant proteins are heterologously expressed as full-length ectodomains in a mammalian HEK293 cell system. The proteins recapitulate native parasite epitopes as assessed by indirect fluorescence assay and a proportion exhibits immunoreactivity when tested against sera from individuals living in malaria-endemic Burkina Faso and Mali. Purified IgG generated to the mosquito-stage parasite antigen enolase demonstrates moderate inhibition of parasite development in the mosquito midgut by the ex vivo standard membrane feeding assay. The findings support the use of rational screens and comparative functional assessments in identifying proteins of the P. falciparum transmission pathway and establishing a robust pre-clinical TBV pipeline

Plasmodium falciparum liver stage infection to stable blood stage infection in liver-humanized and blood-humanized FRGN KO mice enables testing of blood stage inhibitory antibodies (reticulocyte-binding protein homolog 5) in vivo

The invention of liver-humanized mouse models has made it possible to directly study the preerythrocytic stages ofPlasmodium falciparum. In contrast, the current models to directly study blood stage infectionin vivoare extremely limited. Humanization of the mouse blood stream is achievable by frequent injections of human red blood cells (hRBCs) and is currently the only system with which to study human malaria blood stage infections in a small animal model. Infections have been primarily achieved by direct injection ofP. falciparum-infected RBCs but as such, this modality of infection does not model the natural route of infection by mosquito bite and lacks the transition of parasites from liver stage infection to blood stage infection. Including these life cycle transition points in a small animal model is of relevance for testing therapeutic interventions. To this end, we used FRGN KO mice that were engrafted with human hepatocytes and performed a blood exchange under immune modulation to engraft the animals with more than 50% hRBCs. These mice were infected by mosquito bite with sporozoite stages of a luciferase-expressingP. falciparumparasite, resulting in noninvasively measurable liver stage burden byin vivobioluminescent imaging (IVIS) at days 5-7 postinfection. Transition to blood stage infection was observed by IVIS from day 8 onward and then blood stage parasitemia increased with a kinetic similar to that observed in controlled human malaria infection. To assess the utility of this model, we tested whether a monoclonal antibody targeting the erythrocyte invasion ligand reticulocyte-binding protein homolog 5 (with known growth inhibitory activityin vitro) was capable of blocking blood stage infectionin vivowhen parasites emerge from the liver and found it highly effective. Together, these results show that a combined liver-humanized and blood-humanized FRGN mouse model infected with luciferase-expressingP. falciparumwill be a useful tool to studyP. falciparumpreerythrocytic and erythrocytic stages and enables the testing of interventions that target either one or both stages of parasite infection

Plasmodium falciparum liver stage infection to stable blood stage infection in liver-humanized and blood-humanized FRGN KO mice enables testing of blood stage inhibitory antibodies (reticulocyte-binding protein homolog 5) in vivo

The invention of liver-humanized mouse models has made it possible to directly study the preerythrocytic stages ofPlasmodium falciparum. In contrast, the current models to directly study blood stage infectionin vivoare extremely limited. Humanization of the mouse blood stream is achievable by frequent injections of human red blood cells (hRBCs) and is currently the only system with which to study human malaria blood stage infections in a small animal model. Infections have been primarily achieved by direct injection ofP. falciparum-infected RBCs but as such, this modality of infection does not model the natural route of infection by mosquito bite and lacks the transition of parasites from liver stage infection to blood stage infection. Including these life cycle transition points in a small animal model is of relevance for testing therapeutic interventions. To this end, we used FRGN KO mice that were engrafted with human hepatocytes and performed a blood exchange under immune modulation to engraft the animals with more than 50% hRBCs. These mice were infected by mosquito bite with sporozoite stages of a luciferase-expressingP. falciparumparasite, resulting in noninvasively measurable liver stage burden byin vivobioluminescent imaging (IVIS) at days 5-7 postinfection. Transition to blood stage infection was observed by IVIS from day 8 onward and then blood stage parasitemia increased with a kinetic similar to that observed in controlled human malaria infection. To assess the utility of this model, we tested whether a monoclonal antibody targeting the erythrocyte invasion ligand reticulocyte-binding protein homolog 5 (with known growth inhibitory activityin vitro) was capable of blocking blood stage infectionin vivowhen parasites emerge from the liver and found it highly effective. Together, these results show that a combined liver-humanized and blood-humanized FRGN mouse model infected with luciferase-expressingP. falciparumwill be a useful tool to studyP. falciparumpreerythrocytic and erythrocytic stages and enables the testing of interventions that target either one or both stages of parasite infection

Structural basis for RIFIN-mediated activation of LILRB1 in malaria

The&nbsp;Plasmodium&nbsp;species that cause malaria are obligate intracellular parasites, and disease symptoms occur as they replicate within human blood. Despite risking immune detection, the parasite delivers proteins that bind host receptors to infected erythrocyte surfaces. In the causative agent of the most deadly human malaria,&nbsp;Plasmodium falciparum, RIFINs form the largest erythrocyte surface protein family1. Some RIFINs can bind inhibitory immune receptors, acting as targets for unusual antibodies containing a LAIR1 ectodomain2&ndash;4, or as ligands for LILRB15. RIFINs stimulate LILRB1 activation and signalling5, thereby potentially dampening human immune responses. To understand this process, we determined a structure of a RIFIN bound to LILRB1. We show that the RIFIN mimics the natural activating ligand of LILRB1, MHC class I, in its LILRB1-binding mode. A single RIFIN mutation disrupts the complex, blocks LILRB1 binding by all tested RIFINs and abolishes signalling in a reporter assay. In a supported lipid bilayer system, which mimics NK cell activation by antibody-dependent cell-mediated cytotoxicity, both RIFIN and MHC are recruited to the NK cell immunological synapse and reduce cell activation, as measured by perforin mobilisation. Therefore, LILRB1-binding RIFINs mimic the binding mode of the natural ligand of LILRB1 and suppress NK cell function.</p