Toxoplasma depends on lysosomal consumption of autophagosomes for persistent infection.

There are currently no options for curing chronic Toxoplasma infection. The lack of effective therapeutics is due partly to a poor understanding of essential pathways that maintain this long-term infection. Although it is known that Toxoplasma replicates slowly within intracellular cysts demarcated with a cyst wall, precisely how it sustains itself and remodels organelles in this niche is unknown. Here we identify a key role for proteolysis within the parasite lysosomal organelle (the vacuolar compartment/plant-like vacuole; hereafter termed the VAC) in turnover of autophagosomes and persistence during neural infection. We found that genetically or chemically disrupting a VAC-localized cathepsin L protease (CPL) compromised VAC digestive function and markedly reduced chronic infection in vitro and in vivo. Death of parasites lacking CPL was preceded by accumulation of large cytoplasmic inclusions containing markers for the VAC, the autophagic protein Atg8, and remnants of cytoplasmic organelles including the endoplasmic reticulum. Conditional expression or targeted deletion of Atg8 or Atg9, respectively, precluded formation of the undigested autophagosomes, confirming a requirement for the autophagy pathway. These findings suggest an unanticipated function for parasite lysosomal degradation in chronic infection and identify an intrinsic role for autophagy in T. gondii and its close relatives. This work also identifies a key element of Toxoplasma persistence and suggests that VAC proteolysis is a prospective target for pharmacologic development.