Separation of Plasmodium falciparum late stage-infected erythrocytes by magnetic means

The direct binding of Plasmodium vivax AMA1 to erythrocytes defines a RON2-independent invasion pathway

We used a transgenic parasite in which Plasmodium falciparum parasites were genetically modified to express Plasmodium vivax apical membrane antigen 1 (PvAMA1) protein in place of PfAMA1 to study PvAMA1-mediated invasion. In P. falciparum, AMA1 interaction with rhoptry neck protein 2 (RON2) is known to be crucial for invasion, and PfRON2 peptides (PfRON2p) blocked the invasion of PfAMA1 wild-type parasites. However, PfRON2p has no effect on the invasion of transgenic parasites expressing PvAMA1 indicating that PfRON2 had no role in the invasion of PvAMA1 transgenic parasites. Interestingly, PvRON2p blocked the invasion of PvAMA1 transgenic parasites in a dose-dependent manner. We found that recombinant PvAMA1 domains 1 and 2 (rPvAMA1) bound to reticulocytes and normocytes indicating that PvAMA1 directly interacts with erythrocytes during the invasion, and invasion blocking of PvRON2p may result from it interfering with PvAMA1 binding to erythrocytes. It was previously shown that the peptide containing Loop1a of PvAMA1 (PvAMA1 Loop1a) is also bound to reticulocytes. We found that the Loop1a peptide blocked the binding of PvAMA1 to erythrocytes. PvAMA1 Loop1a has no polymorphisms in contrast to other PvAMA1 loops and may be an attractive vaccine target. We thus present the evidence that PvAMA1 binds to erythrocytes in addition to interacting with PvRON2 suggesting that the P. vivax merozoites may exploit complex pathways during the invasion process.

Activation of TCR Vδ1+ and Vδ1-Vδ2- γδ T Cells upon Controlled Infection with Plasmodium falciparum in Tanzanian Volunteers

Our understanding of the human immune response to malaria remains incomplete. Clinical trials using whole-sporozoite-based vaccination approaches such as the Sanaria PfSPZ Vaccine, followed by controlled human malaria infection (CHMI) to assess vaccine efficacy offer a unique opportunity to study the immune response during Plasmodium falciparum infection. Diverse populations of T cells that are not restricted to classical HLA (unconventional T cells) participate in the host response during Plasmodium infection. Although several populations of unconventional T cells exist, the majority of studies focused on TCR Vγ9Vδ2 cells, the most abundant TCR γδ cell population in peripheral blood. In this study, we dissected the response of three TCR γδ cell subsets and mucosal-associated invariant T cells in healthy volunteers immunized with PfSPZ Vaccine and challenged by CHMI using Sanaria PfSPZ Challenge. Using a flow cytometry-based unbiased analysis followed by T cell cloning, several findings were made. Whereas major ex vivo alterations were not detectable after immunization with PfSPZ Vaccine, TCR Vδ2, and mucosal-associated invariant T cells expanded after asexual blood-stage parasitemia induced by CHMI. CHMI, but not vaccination, also induced the activation of TCR Vδ1 and Vδ1^-Vδ2^- γδ T cells. The activated TCR Vδ1 cells were oligoclonal, suggesting clonal expansion, and upon repeated CHMI, showed diminished response, indicating long-term alterations induced by blood-stage parasitemia. Some TCR Vδ1 clones recognized target cells in the absence of parasite-derived Ags, thus suggesting recognition of self-molecules. These findings reveal the articulate participation of different populations of unconventional T cells to P. falciparum infection.

Evidence against a Role of Elevated Intracellular Ca2+ during Plasmodium falciparum Preinvasion

Severe malaria is primarily caused by Plasmodium falciparum parasites during their asexual reproduction cycle within red blood cells. One of the least understood stages in this cycle is the brief preinvasion period during which merozoite-red cell contacts lead to apical alignment of the merozoite in readiness for penetration, a stage of major relevance in the control of invasion efficiency. Red blood cell deformations associated with this process were suggested to be active plasma membrane responses mediated by transients of elevated intracellular calcium. Few studies have addressed this hypothesis because of technical challenges, and the results remained inconclusive. Here, Fluo-4 was used as a fluorescent calcium indicator with optimized protocols to investigate the distribution of the dye in red blood cell populations used as P. falciparum invasion targets in egress-invasion assays. Preinvasion dynamics was observed simultaneously under bright-field and fluorescence microscopy by recording egress-invasion events. All the egress-invasion sequences showed red blood cell deformations of varied intensities during the preinvasion period and the echinocytic changes that follow during invasion. Intraerythrocytic calcium signals were absent throughout this interval in over half the records and totally absent during the preinvasion period, regardless of deformation strength. When present, calcium signals were of a punctate modality, initiated within merozoites already poised for invasion. These results argue against a role of elevated intracellular calcium during the preinvasion stage. We suggest an alternative mechanism of merozoite-induced preinvasion deformations based on passive red cell responses to transient agonist-receptor interactions associated with the formation of adhesive coat filaments.