C-type lectin 4 regulates broad-spectrum melanization-based refractoriness to malaria parasites

Anopheles gambiae melanization-based refractoriness to the human malaria parasite Plasmodium falciparum has rarely been observed in either laboratory or natural conditions, in contrast to the rodent model malaria parasite Plasmodium berghei that can become completely melanized by a TEP1 complement-like system-dependent mechanism. Multiple studies have shown that the rodent parasite evades this defense by recruiting the C-type lectins CTL4 and CTLMA2, while permissiveness to the human malaria parasite was not affected by partial depletion of these factors by RNAi silencing. Using CRISPR/Cas9-based CTL4 knockout, we show that A. gambiae can mount melanization-based refractoriness to the human malaria parasite, which is independent of the TEP1 complement-like system and the major anti-Plasmodium immune pathway Imd. Our study indicates a hierarchical specificity in the control of Plasmodium melanization and proves CTL4 as an essential host factor for P. falciparum transmission and one of the most potent mosquito-encoded malaria transmission-blocking targets.

Genome, proteome, and immunome data explain why 6 month controlled human malaria infection with sporozoites of the Pf7G8 clone of Plasmodium falciparum is a rigorous predictor of the efficacy of the PfNF54-based PfSPZ Vaccine in Africa

Controlled human malaria infection (CHMI) has supported Plasmodium falciparum (Pf) malaria vaccine development by providing preliminary estimates of vaccine efficacy (VE). Because CHMIs generally use Pf strains similar to vaccine strains, VE against antigenically heterogeneous Pf in the field has been required to establish VE. We increased the stringency of CHMI by selecting a Brazilian isolate, Pf7G8, which is genetically distant from the West African parasite (PfNF54) in our PfSPZ vaccines. Using two regimens to identically immunize US and Malian adults, VE over 24 weeks in the field was as good as or better than against CHMI at 24 weeks in the US. To explain this finding, we quantified differences in the genome, proteome and predicted CD8 T cell epitopes of PfNF54 relative to 709 Pf isolates from Africa and Pf7G8. Pf7G8 is more distant from PfNF54 than any African isolates tested. We propose VE against Pf7G8 CHMI for providing pivotal data for malaria vaccine licensure for travelers to Africa, and potentially for endemic populations, because the genetic distance of Pf7G8 from the Pf vaccine strain makes it a stringent surrogate for Pf parasites in Africa.

The Basal Complex Protein PfMORN1 Is Not Required for Asexual Replication of Plasmodium falciparum

Plasmodium falciparum parasites cause the most severe form of human malaria. During the clinically relevant blood stage of its life cycle, the parasites divide via schizogony.

Diagnostic Performance and Comparison of Ultrasensitive and Conventional Rapid Diagnostic Test, Thick Blood Smear and Quantitative PCR for Detection of Low-Density Plasmodium Falciparum Infections During a Controlled Human Malaria Infection Study in Equatorial Guinea

BackgroundProgress towards malaria elimination has stagnated, partly because infections persisting at low parasite densities comprise a large reservoir contributing to ongoing malaria transmission and are difficult to detect. We compared the performance of an ultrasensitive rapid diagnostic test (uRDT) designed to detect low density infections to a conventional RDT (cRDT), expert microscopy using Giemsa-stained thick blood smears (TBS), and quantitative polymerase chain reaction (qPCR) during a controlled human malaria infection (CHMI) study conducted in malaria exposed adults (NCT03590340). MethodsBlood samples were collected from healthy Equatoguineans aged 18-35 years beginning on day 8 after CHMI with 3.2x103 cryopreserved, infectious Plasmodium falciparum (Pf) sporozoites (PfSPZ Challenge, strain NF54) administered by direct venous inoculation. qPCR (18s ribosomal DNA), uRDT (AlereTM Malaria Ag P.f.), cRDT (Carestart Malaria Pf/PAN (PfHRP2/pLDH)), and TBS were performed daily until the volunteer became TBS positive and treatment was administered. qPCR was the reference for the presence of Pf parasites. Results279 samples were collected from 24 participants; 123 were positive by qPCR. TBS detected 24/123 (19.5% sensitivity [95% CI 13.1% – 27.8%]), uRDT 21/123 (17.1% sensitivity [95% CI 11.1% – 25.1%], cRDT 10/123 (8.1% sensitivity [95% CI 4.2% – 14.8%]; all were 100% specific. qPCR was the most sensitive test (p<0.001); TBS and uRDT were more sensitive than cRDT (TBS vs. cRDT p=0.015; uRDT vs. cRDT p=0.053), detecting parasitemias as low as 3.7 parasites/mL (p/mL) (TBS and uRDT) compared to 5.6 p/mL (cRDT) based on TBS density measurements. TBS, uRDT and cRDT did not detect any of the 70/123 samples positive by qPCR below 5.86 p/mL, the qPCR density corresponding to 3.7 p/mL by TBS. The median prepatent periods in days (ranges) were 14.5 (10-20), 18.0 (15-28), 18.0 (15-20) and 18.0 (16-24) for qPCR, TBS, uRDT and cRDT, respectively; qPCR detected parasitemia significantly earlier (3.5 days) than the other tests.ConclusionsTBS and uRDT had similar sensitivities, both were more sensitive than cRDT, and neither matched qPCR for detecting low density parasitemia. uRDT could be considered an alternative to TBS in selected applications such as CHMI or field diagnosis where qualitative, dichotomous results for malaria infection might be sufficient.

Stable Artesunate Resistance in A Humanized Mouse Model of Plasmodium falciparum

Plasmodium falciparum, the most devastating human malaria parasite, confers higher morbidity and mortality. Although efforts have been made to develop an effective malaria vaccine, stage- and species-specific short-lived immunity crippled these efforts. Hence, antimalarial drug treatment becomes a mainstay for the treatment of malaria infection in the wake of the unavailability of an effective vaccine. Further, there has been a wide array of antimalarial drugs effective against various developmental stages of P. falciparum due to their different structures, modes of action, and pharmacodynamics as well as pharmacokinetics. The development of resistance against almost all frontline drugs by P. falciparum indicates the need for combination therapy (artemisinin-based combination therapy; ACT) to treat patients with P. falciparum. A higher pool of parasitemia under discontinuous in vivo artemisinin drug pressure in a developed humanized mouse allows the selection of artesunate resistant (ART-R) P. falciparum. Intravenously administered artesunate, using either single flash doses or a 2-day regimen, to the P. falciparum-infected human blood chimeric NOD/SCID.IL-2Rγ−/− immunocompromised (NSG) mice, with progressive dose increments upon parasite recovery, was the strategy deployed to select resistant parasites. Parasite susceptibility to artemisinins and other antimalarial compounds was characterized in vitro and in vivo. P. falciparum has shown to evolve extreme artemisinin resistance as well as co-resistance to antimalarial drugs. Overall, the present information shall be very useful in devising newer therapeutic strategies to treat human malaria infection.