A path to tackle liver-stage parasites: Malaria parasites are evolutionarily prepared to resist drug attack. Resistance is emerging to even the latest frontline combination therapies, which target the blood stages of the Plasmodium parasite. As an alternative strategy, Antonova-Koch et al. investigated the possibilities of drugs against liver-stage parasites (see the Perspective by Phillips and Goldberg). To do so, they devised a luciferase-reporter drug screen for the rodent parasite Plasmodium berghei. Three rounds of increasingly stringent screening were used. From this regime, several chemotypes that inhibit Plasmodium mitochondrial electron transport were identified. Excitingly, several new scaffolds, with as-yet-unknown modes of action but solely targeting the parasites' liver stages, emerged as promising drug leads for further development. Science , this issue p. eaat9446 ; see also p. 1112

The exoerythrocytic stage of Plasmodium infection is a critical window for prophylactic intervention. Using genome-wide dual RNA sequencing of flow-sorted infected and uninfected hepatoma cells we show that the human mucosal immunity gene, mucin-13 (MUC13), is strongly upregulated during Plasmodium exoerythrocytic hepatic-stage infection. We confirm MUC13 transcript increases in hepatoma cell lines and primary hepatocytes. In immunofluorescence assays, host MUC13 protein expression distinguishes infected cells from adjacent uninfected cells and shows similar colocalization with parasite biomarkers such as UIS4 and HSP70. We further show that localization patterns are species independent, marking both P. berghei and P. vivax infected cells, and that MUC13 can be used to identify compounds that inhibit parasite replication in hepatocytes. This data provides insights into host-parasite interactions in Plasmodium infection, and demonstrates that a component of host mucosal immunity is reprogrammed during the progression of infection.

Potent and durable vaccine responses will be required for control of malaria caused by Plasmodium falciparum (Pf). RTS,S/AS01 is the first, and to date, the only vaccine that has demonstrated significant reduction of clinical and severe malaria in endemic cohorts in Phase 3 trials. Although the vaccine is protective, efficacy declines over time with kinetics paralleling the decline in antibody responses to the Pf circumsporozoite protein (PfCSP). Although most attention has focused on antibodies to repeat motifs on PfCSP, antibodies to other regions may play a role in protection. Here, we expressed and characterized seven monoclonal antibodies to the C-terminal domain of CSP (ctCSP) from volunteers immunized with RTS,S/AS01. Competition and crystal structure studies indicated that the antibodies target two different sites on opposite faces of ctCSP. One site contains a polymorphic region (denoted α-ctCSP) and has been previously characterized, whereas the second is a previously undescribed site on the conserved β-sheet face of the ctCSP (denoted β-ctCSP). Antibodies to the β-ctCSP site exhibited broad reactivity with a diverse panel of ctCSP peptides whose sequences were derived from field isolates of P. falciparum whereas antibodies to the α-ctCSP site showed very limited cross reactivity. Importantly, an antibody to the β-site demonstrated inhibition activity against malaria infection in a murine model. This study identifies a previously unidentified conserved epitope on CSP that could be targeted by prophylactic antibodies and exploited in structure-based vaccine design.

Most phenotypic screens aiming to discover new antimalarial chemotypes begin with low cost, high-throughput tests against the asexual blood stage (ABS) of the malaria parasite lifecycle. Compounds active against the ABS are then sequentially tested in more difficult assays that predict whether a compound has other beneficial attributes. Although applying this strategy to new chemical libraries may yield new leads, repeated iterations may lead to diminishing returns and the rediscovery of chemotypes hitting well-known targets. Here we adopted a different strategy to find starting points, testing ~70,000 open source small molecules from the Global Health Chemical Diversity Library for activity against the liver stage, mature sexual stage and asexual blood stage malaria parasites in parallel. In addition, instead of using an asexual assay that measures accumulated parasite DNA in the presence of compound (SYBR green), a real time luciferase-dependent parasite viability assay was used that distinguishes slow acting (delayed death) from fast-acting compounds. Among 382 scaffolds with activity confirmed in dose response (<10µM), we discovered 68 novel delayed-death, 84 liver stage, and 68 stage V gametocytes inhibitors as well. Although 89% of evaluated compounds had activity in only a single lifecycle stage, we discovered six potent (half-maximal inhibitory concentration <1µM) multistage scaffolds, including a novel cytochrome bc1 chemotype. Our data further show the luciferase-based assays have higher sensitivity. Chemoinformatic analysis of positive and negative compounds identified scaffold families with a strong enrichment for activity against specific or multiple stages.