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ItemMolecular mechanisms of liver infection by the human malaria parasite Plasmodium falciparum( 2021)Malaria is the disease caused by Plasmodium parasites. The parasite infects the red blood cells giving rise to symptoms but it musts first infect the liver to reach the blood. Blocking liver infection would prevent both malaria disease and onward transmission as well as stimulate immunity. However, little is known about parasite-host interactions during liver infection of Plasmodium falciparum, the species responsible for the most lethal form of malaria in humans, as its pre-erythrocytic stages are challenging to study. Plasmodium sporozoites are injected in the dermis by the bite of an infected mosquito. They make their way from the skin to the bloodstream and finally the liver, where they invade and replicate within a hepatocyte. The sporozoite’s journey from the skin to the host liver is enabled by a remarkable process called cell traversal that allows parasites to migrate and penetrate deeper into host tissues by entering and then rupturing host cells. Little is known about the key molecular interactions involved in this mechanism especially with respect to the host cell. There is a lack of knowledge about the importance of host factors and proteins involved in sporozoite infectivity. A deeper understanding of cell traversal and hepatocyte invasion could lead to novel interventions. This work aimed to identify key proteins involved in cell traversal and hepatocyte invasion by P. falciparum. A robust sporozoite production protocol was initially established to ensure the feasibility of the project. Host factors involved in cell traversal were systematically investigated using a whole genome CRISPR/Cas9 knock-out screen. The unbiased screen was enabled by the design of a new positive selection cell traversal assay that kills traversed hepatocytes, permitting the enrichment of traversal-resistant cells. Validation of more than one hundred curated hits identified several human genes involved in infection by other pathogens that are putative proteins involved in P. falciparum cell traversal. Finally, antibodies targeting different regions of the most abundant P. falciparum sporozoite surface protein — the circumsporozoite protein (CSP) — were characterised for their inhibition potential. To do so, an improved method allowing both cell traversal and hepatocyte invasion by P. falciparum sporozoite to be quantified by flow cytometry was established before inhibition assays were performed. Different inhibition profiles were identified, highlighting a role for the N-terminus of CSP in hepatocyte invasion. Identifying essential factors and parasite-host interactions during this first step of the malaria parasite lifecycle will provide more insight into support of a prophylactic treatment for malaria.