A Process Similar to Autophagy Is Associated with Cytocidal Chloroquine Resistance in Plasmodium falciparum

Resistance to the cytostatic activity of the antimalarial drug chloroquine (CQ) is becoming well understood, however, resistance to cytocidal effects of CQ is largely unexplored. We find that PfCRT mutations that almost fully recapitulate P. falciparum cytostatic CQ resistance (CQR CS ) as quantified by CQ IC 50 shift, account for only 10–20% of cytocidal CQR (CQR CC ) as quantified by CQ LD 50 shift. Quantitative trait loci (QTL) analysis of the progeny of a chloroquine sensitive (CQS; strain HB3)×chloroquine resistant (CQR; strain Dd2) genetic cross identifies distinct genetic architectures for CQR CS vs CQR CC phenotypes, including identification of novel interacting chromosomal loci that influence CQ LD 50 . Candidate genes in these loci are consistent with a role for autophagy in CQR CC , leading us to directly examine the autophagy pathway in intraerythrocytic CQR parasites. Indirect immunofluorescence of RBC infected with synchronized CQS vs CQR trophozoite stage parasites reveals differences in the distribution of the autophagy marker protein PfATG8 coinciding with CQR CC . Taken together, the data show that an unusual autophagy – like process is either activated or inhibited for intraerythrocytic trophozoite parasites at LD 50 doses (but not IC 50 doses) of CQ, that the pathway is altered in CQR P. falciparum , and that it may contribute along with mutations in PfCRT to confer the CQR CC phenotype.