Response of Leishmania chagasi promastigotes to oxidant stress

At the onset of infection, Leishmania promastigotes are phagocytized by mammalian macrophages. They must survive despite exposure to toxic oxidants such as hydrogen peroxide (H₂O₂) and superoxide (·O₂^-) generated during phagocytosis. We investigated the effects of these oxidants on Leishmania chagasi promastigotes and promastigote mechanisms for oxidant resistance. According to spin trapping and electron paramagnetic resonance spectrometry, ·O₂^- could be generated by exposure of promastigotes to the redox-cycling compound menadione. Incubation in either menadione or H₂O₂ caused a concentration-dependent loss of promastigote viability. However, incubation in sublethal concentrations of H₂O₂ or menadione caused a stress response in promastigotes. This oxidant-induced response was associated with an increase in the amount of heat shock protein hsp70. Induction of a stress response by exposure of promastigotes either to heat shock or to sublethal oxidants (H₂O₂ or menadione) caused promastigotes to become more resistant to H₂O₂ toxicity. Sublethal menadione also caused promastigotes to become more virulent in a BALB/c mouse model of leishmaniasis. We previously correlated H₂O₂ cytotoxicity for promastigotes with the formation of hydroxyl radical (·OH) from H₂O₂. However, according to electron paramagnetic resonance spectrometry, the increase in H₂O₂ resistance after exposure to sublethal oxidants was not associated with diminished generation (i.e., scavenging) of ·OH. These data suggest that there is a cross- protective stress response that occurs after exposure of L. chagasi promastigotes to heat shock or to sublethal H₂O₂ or ·O₂^-, exposures that also occur during natural infection. This response results in increased resistance to H₂O₂ toxicity and increased virulence for a mammalian host.