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Genetic crosses are most powerful for linkage analysis when progeny numbers are high, parental alleles segregate evenly and numbers of inbred progeny are minimized. We previously developed a novel genetic crossing platform for the human malaria parasite Plasmodium falciparum, an obligately sexual, hermaphroditic protozoan, using mice carrying human hepatocytes (the human liver-chimeric FRG NOD huHep mouse) as the vertebrate host. We report on two genetic crosses—(1) an allopatric cross between a laboratory-adapted parasite (NF54) of African origin and a recently patient-derived Asian parasite, and (2) a sympatric cross between two recently patient-derived Asian parasites. We generated 144 unique recombinant clones from the two crosses, doubling the number of unique recombinant progeny generated in the previous 30 years. The allopatric African/Asian cross has minimal levels of inbreeding and extreme segregation distortion, while in the sympatric Asian cross, inbred progeny predominate and parental alleles segregate evenly. Using simulations, we demonstrate that these progeny provide the power to map small-effect mutations and epistatic interactions. The segregation distortion in the allopatric cross slightly erodes power to detect linkage in several genome regions. We greatly increase the power and the precision to map biomedically important traits with these new large progeny panels., Button-Simons et al. report on two different genetic crosses of Plasmodium falciparum derived from Asian and African origin parasites, and generate large numbers of recombinant progeny. These crosses are characterized and have the potential to map biologically important traits across parasite genomes.

Nutrient acquisition/metabolism pathways provide potent targets for drug design. We conducted crosses between African (NF54) and Asian (NHP4026) malaria parasites, and compared genome-wide allele frequency changes in independent progeny populations grown in human serum or AlbuMAX, a commercial bovine serum formulation. We detected three QTLs linked with differential growth that contained strong candidate genes: aspartate transaminase AST (chromosome 2), cysteine protease ATG4 (chr. 13) and EBA-140 (chr. 14). Alleles inherited from NF54 (chr. 2 and 14) and from NHP4026 (chr. 13) were positively selected in AlbuMAX, while the same alleles were selected against in serum. Selection driving differential growth was strong (s = 0.10 – 0.23 per 48-hour lifecycle) and observed in all biological replicates. These results demonstrate the effectiveness of bulk segregant approaches for revealing nutritional polymorphisms in Plasmodium falciparum. This approach will allow systematic dissection of nutrient acquisition/metabolism pathways that are potential targets for intervention against P. falciparum.

Background The assessment of antibody responses after immunization with radiation-attenuated, aseptic, purified, cryopreserved Plasmodium falciparum sporozoites (Sanaria PfSPZ Vaccine) has focused on IgG isotype antibodies. Here, we aimed to investigate if P. falciparum sporozoite binding and invasion-inhibitory IgM antibodies are induced following immunization of malaria-preexposed volunteers with PfSPZ Vaccine. Methods Using serum from volunteers immunized with PfSPZ, we measured vaccine-induced IgG and IgM antibodies to P. falciparum circumsporozoite protein (PfCSP) via ELISA. Function of this serum as well as IgM antibody fractions was measured via in vitro in an inhibition of sporozoite invasion assay. These IgM antibody fractions were also measured for binding to sporozoites by immunofluorescence assay and complement fixation on whole sporozoites. Results We found that in addition to anti-PfCSP IgG, malaria-preexposed volunteers developed anti-PfCSP IgM antibodies after immunization with PfSPZ Vaccine and that these IgM antibodies inhibited P. falciparum sporozoite invasion of hepatocytes in vitro. These IgM plasma fractions also fixed complement to whole P. falciparum sporozoites. Conclusions This is the first finding that PfCSP and P. falciparum sporozoite-binding IgM antibodies are induced following immunization of PfSPZ Vaccine in malaria-preexposed individuals and that IgM antibodies can inhibit P. falciparum sporozoite invasion into hepatocytes in vitro and fix complement on sporozoites. These findings indicate that the immunological assessment of PfSPZ Vaccine-induced antibody responses could be more sensitive if they include parasite-specific IgM in addition to IgG antibodies. Clinical trials registration NCT02132299.