Malaria, caused by infection with Plasmodium spp., is one of the oldest and most lethal diseases in human
history. It continues to be a leading cause of morbidity and mortality, especially amongst children under five.
Despite over a decade of progress in reducing the burden of malaria, the last four years have seen a
stagnation due to recalcitrance in highly endemic areas and interruptions in eradication efforts. As for any
infectious disease, sustained progress will require an effective vaccine. For malaria, antibodies have been
shown particularly potent in stopping the “sporozoite” form of Plasmodium during its ~30 minute journey from
the mosquito, into the skin and then into a liver hepatocyte. Here, the parasite completes the asymptomatic
“pre-erythrocytic” (PE) stages over ~1 week before emerging back into the blood for the erythrocytic stage,
where it infects red blood cells and causes all morbidity, mortality and transmission. Stopping the parasite at
the PE stage (i.e. sterile protection) is the ultimate goal of a malaria vaccine. Unfortunately, antibody-based
vaccine candidates targeting the major surface protein (circumsporozoite protein, CSP) exposed at the
sporozoite surface have failed to provide the long-lasting sterile protection needed for sustained control and
elimination as this requires extremely high titers of antibodies which can only be maintained over short periods
or with frequent boosting. One strategy to enhance CSP-based vaccines is to target additional antigens.
However, demonstration that non-CSP targeting antibodies which can function on their own or in concert with
CSP has been lacking. Current efforts have focused on surface/secreted proteins exposed during the
extracellular sporozoite stage in line with dogma that once the parasite is within the hepatocyte, it is no longer
accessible to antibodies. Surprisingly, we have found that antibodies targeting a liver stage-specific
protein, UIS3, can efficiently kill the parasite during the intracellular hepatocyte stage. Furthermore, a
low dose of these antibodies can synergize with a low dose of anti-CSP antibodies to raise sterile
protection from 0% with each antibody alone to 64% in combination. These data not only add to the
small number of bona fide PE antibody targets, but also open an entirely new parasite stage for
antibody discovery and are the first demonstration of synergy between multiple antigens at the PE
stage. While such intracellularly targeted antibodies (iAbs) have been described for other pathogens, their
mechanisms of action are not well understood. Here, we aim to use monoclonal antibodies to define the basic
functional characteristics of anti-UIS3 antibodies and to identify the core mechanisms of anti-UIS3 iAb action.
These data will support the rational design of superior CSP-based vaccines and monoclonal antibody regimens
with an iAb component, with the goal that these Ab-based interventions can achieve the high levels of long-
lasting sterile protection to drive malaria to elimination.