<i>Plasmodium berghei</i> serine repeat antigen 3 (PbSERA3) is required for hepatic merozoite egress

The intracellular stages of <i>Plasmodium</i> that replicate asexually reside within a vacuole delimited by a parasitophorous vacuolar membrane (PVM). A family of serine-rich antigens (SERAs), with a cysteine residue in its catalytic site, is implicated in liberating these parasites from PVM. In <i>P. berghei</i>, a rodent malaria parasite, PbSERA3, an ortholog of PfSERA6, is indispensable for the parasite. However, its maturation by another parasite protease called subtilisin 1 (SUB1) is critical for its effector functions. During EEF development, the processed PbSERA3 is translocated across the PVM and possibly implicated in hepatic takeover. A direct role of PbSERA3 in liver stages is lacking to date. Our study generated conditional mutants of PbSERA3 and demonstrated normal development of the mutant in hepatocytes, but an inability to cause blood-stage infection. These observations point to the role of PbSERA3 in hepatic egress. We further demonstrated the extracellular nature of PbSERA3 in the ookinete, midgut, and salivary gland sporozoite stages, with a bona fide processing pattern similar to that of blood stages. Our studies demonstrated the essentiality of PbSERA3 in liver stages, making it an attractive target for antimalarial therapy. As PbSERA3 mutants manifest a late developmental arrest in the liver, they have implications in eliciting cross-stage immunity, owing to a shared repertoire of antigens with blood stages.