title: Sunlight inhibits growth and induces markers of programmed cell death in Plasmodium falciparum in vitro.
abstract: BACKGROUND
  Plasmodium falciparum is responsible for the majority of global malaria deaths. During the pathogenic blood stages of infection, a rapid increase in parasitaemia threatens the survival of the host before transmission of slow-maturing sexual parasites to the mosquito vector to continue the life cycle. Programmed cell death (PCD) may provide the parasite with the means to control its burden on the host and thereby ensure its own survival. Various environmental stress factors encountered during malaria may induce PCD in P. falciparum. This study is the first to characterize parasite cell death in response to natural sunlight.
  
  
  METHODS
  The 3D7 strain of P. falciparum was cultured in vitro in donor erythrocytes. Synchronized and mixed-stage parasitized cultures were exposed to sunlight for 1 h and compared to cultures maintained in the dark, 24 h later. Mixed-stage parasites were also subjected to a second one-hour exposure at 24 h and assessed at 48 h. Parasitaemia was measured daily by flow cytometry. Biochemical markers of cell death were assessed, including DNA fragmentation, mitochondrial membrane polarization and phosphatidylserine externalization.
  
  
  RESULTS
  Sunlight inhibited P. falciparum growth in vitro. Late-stage parasites were more severely affected than early stages. However, some late-stage parasites survived exposure to sunlight to form new rings 24 h later, as would be expected during PCD whereby only a portion of the population dies. DNA fragmentation was observed at 24 and 48 h and preceded mitochondrial hyperpolarization in mixed-stage parasites at 48 h. Mitochondrial hyperpolarization likely resulted from increased oxidative stress. Although data suggested increased phosphatidylserine externalization in mixed-stage parasites, results were not statistically significant.
  
  
  CONCLUSIONS
  The combination of biochemical markers and the survival of some parasites, despite exposure to a lethal stimulus, support the occurrence of PCD in P. falciparum.
authors.affiliation: Department of Molecular Medicine and Haematology, Faculty of Health Sciences, School of Pathology, Wits Medical School, Wits Research Institute for Malaria, University of the Witwatersrand, 7th floor, 7 York Road, Parktown, Johannesburg, 2193, South Africa. dewaldtengelbrecht@gmail.com.
authors.forename: Dewaldt
authors.initials: D
authors.lastname: Engelbrecht
authors.affiliation: Department of Molecular Medicine and Haematology, Faculty of Health Sciences, School of Pathology, Wits Medical School, Wits Research Institute for Malaria, University of the Witwatersrand, 7th floor, 7 York Road, Parktown, Johannesburg, 2193, South Africa. theresa.coetzer@nhls.ac.za.|National Health Laboratory Service, Johannesburg, South Africa. theresa.coetzer@nhls.ac.za.
authors.forename: Thérèsa Louise
authors.initials: TL
authors.lastname: Coetzer
mesh_terms: D017209:Apoptosis / Q000528:radiation effects*; D002478:Cells, Cultured; D053938:DNA Fragmentation / Q000528:radiation effects; D016054:DNA, Protozoan / Q000528:radiation effects; D004912:Erythrocytes / Q000469:parasitology; D006801:Humans; D016778:Malaria, Falciparum / Q000469:parasitology; D008928:Mitochondria / Q000528:radiation effects; D010963:Plasmodium falciparum / Q000166:cytology / Q000235:genetics / Q000528:radiation effects*; D013472:Sunlight
chemical_list: D016054:DNA, Protozoan