A Plasmodium falciparum VAR2CSA DBL4 specific monoclonal antibody maps to an interface between DBL3X and DBL4 blocks infected-RBC binding to Chondroitin Sulfate A Women become resistant to PM with the acquisition of antibodies against CSA-binding Pf-IE over successive pregnancies. The variant surface antigen VAR2CSA mediates adhesion to CSA, and antibodies against this antigen are acquired over successive pregnancies and correlate with resistance. We examined whether antibodies against VAR2CSA DBL domains 4 and 5 could inhibit Pf-IE binding to CSA. We find that polyclonal antibodies generated against E. coli expressed and refolded recombinant VAR2CSA DBL4domains but not those against recombinant VAR2CSA DBL5 inhibited homologous Pf-IE binding to CSA by a static binding inhibition assay (BIA). As previously reported, recombinant DBL4 bound poorly to CSA while DBL5 bound with high affinity to CSA by Surface Plasmon Resonance. A panel of mouse monoclonal antibodies (mAb) were raised against DBL4 one of which (8F8), demonstrated homologous blocking activity in the BIA. The functional epitope of 8F8 was mapped to a short polymorphic region of DBL4 positioned at an interface with DBL3X. These results suggest that antibodies which target this polymorphic region may effectively inhibit Pf-IE binding to placental CSA and could lead to the development of a combination vaccine for placental malaria. Dynamics of VAR2CSA antibody responses and plasma functional activity over successive pregnancies in an Aotus model of placental malaria During pregnancy, susceptibility to P. falciparum infection increases despite pre-existing immunity acquired from years of exposure. Susceptibility is attributed to the emergence of a distinct parasite phenotype that expresses VAR2CSA on the surface of infected erythrocytes and that binds Chondroitin sulfate A (CSA) in intervillous spaces to cause placental malaria (PM). VAR2CSA antibody levels increase over successive pregnancies as women become resistant to PM, supporting VAR2CSA as a PM vaccine candidate, and N-terminal VAR2CSA constructs have entered clinical trials. VAR2CSA includes several Duffy Binding-Like (DBL) domains and inter-domain (ID) regions, and several lines of evidence suggest that epitopes targeted by functional antibodies are not limited to a single DBL domain. The specificities of naturally acquired anti-VAR2CSA antibodies related to protection remain unclear. During FY16, we used our newly established Aotus model of PM to map the antibody response to VAR2CSA epitopes, assaying plasma samples collected from a single monkey infected with the same CS2 parasite clone over four successive pregnancies. Plasma samples were assessed for functional activity by binding inhibition assay, then samples from 5 time-points were selected to screen a microarray of 1727 15-mer peptides generated from full-length VAR2CSA sequences of FCR3 (isogenic to CS2 parasite) and NF54 variants. While the nave sample did not meaningfully react to any peptides, moderate to strong antibody responses were observed against 581 peptides using plasma collected after the pregnancy (P)-2, with the strongest response found in a sample collected after P-4. Antibody titers and functional activity decreased by 8 months after P-4 and were boosted by a challenge infection when animal was non-pregnant. Asparagine-rich peptides with consensus motifs found in DBL2 and DBL6 domains displayed the greatest reactivity. Reactivity to several peptides throughout VAR2CSA was related to the functional activity of the corresponding plasma sample by correlation analysis. Peptides related to functional activity can be tested in future as a PM vaccine cocktail. Exploring immunopathogenesis in the etiology of severe malarial anemia using two primate malaria models Severe malarial anemia (SMA; defined as a hemoglobin of <5g/dL in malaria-infected individuals) is the most common complication of P. falciparum infection in holoendemic areas, and a major cause of morbidity and mortality in young children. The pathogenesis of SMA is multi-factorial, with contributions from hemolysis, inadequate erythropoiesis, and potentially inflammatory responses. In preliminary comparative studies, we find that rhesus macaques infected with P. coatneyi suffer SMA with rapid onset similar to African children with P. falciparum; the acute drop in hematocrit is accompanied by consumption of haptoglobin (indicative of hemolysis) and a poor reticulocyte response. As we previously reported, activated CD4 and CD8 T cells (Ki67+ and CD69+) appear in the blood and bone marrow of infected rhesus macaques, and thus could be involved in the etiology of SMA. P. coatneyi causes milder infection and only mild to moderate anemia in the cynomolgus macaque, a closely related species and the parasites natural host. We have initiated studies in FY16 to compare rhesus and cynomolgus macaques during infection to interrogate factors involved in the pathogenesis of severe anemia. We are examining whether differences in immune activation, cytokine or chemokine profiles during the course of infection, including early time points when parasitemia is comparable in both species, can contribute to the difference in susceptibility to SMA. Seasonality and the epidemiology of malaria in young children An improved understanding of the role of seasonality in the epidemiology of malaria will be valuable for the rational design of malaria control programs and for the identification of novel interventional opportunities. During FY16, we integrated existing birth cohort data from a perennial transmission region of Tanzania (2002 to 2006) with new data from a seasonal transmission region of Mali (2010 to 2016), to describe how the intra-annual patterning of Plasmodium falciparum risk could influence the epidemiology of parasitemia, clinical malaria, and severe malarial anemia in children under five. During a total of 78,293 study visits with 2544 children (n=880, Tanzania; n=1664, Mali), we observed 7265 malaria infections (n=3929, Tanzania; n=3336, Mali), 5522 clinical malaria episodes (n=2816, Tanzania; n=2706, Mali), and 74 severe malarial anemia events (n=41, Tanzania; n=33, Mali). Although the two cohorts differed in the distributions of parasitemia and clinical malaria across the calendar year, both cohorts similarly experienced high transmission seasons with a concentration of severe malarial anemia cases and relatively elevated parasite densities as compared to the respective low transmission seasons. These data also provide ecological evidence that children in the seasonal Malian environment experienced both a later peak age at risk and a wider window of vulnerability to malarial disease relative to their peers in the perennial Tanzanian setting. These data underscore the importance of accounting for seasonality in public health planning and suggest interruptions in parasitic exposure may contribute to a delayed acquisition of clinical immunity to malaria in young children in highly seasonal settings. Developing and internally validating a risk prediction model for pediatric severe malarial anemia in a Tanzanian region of perennial malaria transmission In regions where population-wide approaches to malaria control may be limited due to ecological patterns, resource availability, and/or parasitic resistance to antimalarial medications, it could be valuable to have a tool to identify individuals at elevated risk of severe malaria for targeted intervention. In this pilot study conducted in FY16, we used data collected in the MOMS Project birth cohort (n=880), which was based in a region of perennial malaria transmission in Muheza, Tanzania, (2002 to 2006) to develop and internally validate a prognostic model for severe malarial anemia in children under three.
Showing the most recent 10 out of 48 publications
