Trypanosome Lytic Factor-1 Initiates Oxidation-Stimulated Osmotic Lysis of Trypanosoma brucei brucei [Membrane Biology]

December 8th, 2015 by Styer, A. L., Hajduk, S. L.

Human innate immunity against the veterinary pathogen Trypanosoma brucei brucei is conferred by trypanosome lytic factors (TLFs), against which human-infective T. b. gambiense and T. b. rhodesiense have evolved resistance. TLF-1 is a subclass of high density lipoprotein particles defined by two primate specific apolipoproteins: the ion-channel forming toxin apolipoproteinL1 (ApoL1), and the hemoglobin (Hb) scavenger haptoglobin related protein (Hpr). The role of oxidative stress in the TLF-1 lytic mechanism has been controversial. Here we show that oxidative processes are involved in TLF-1 killing of T. b. brucei. The lipophilic antioxidant DPPD protected TLF-1 treated T. b. brucei from lysis. Conversely, lysis of TLF-1 treated T. b. brucei was increased by addition of peroxides or thiol-conjugating agents. Previously, the Hpr-Hb complex was postulated to be a source of free radicals during TLF-1 lysis. However, we found that the Fe-containing heme of the Hpr-Hb complex was not involved in TLF-1 lysis. Furthermore, neither high concentrations of transferrin nor knockout of cytosolic lipid peroxidases prevented TLF-1 lysis. Instead, purified ApoL1 was sufficient to induce lysis, and ApoL1 lysis was inhibited by the antioxidant DPPD. Swelling of TLF-1 treated T. b. brucei was reminiscent of swelling under hypotonic stress. Moreover, TLF-1 treated T. b. brucei became rapidly susceptible to hypotonic lysis. T. b. brucei cells exposed to peroxides or thiol-binding agents were also sensitized to hypotonic lysis in the absence of TLF-1. We postulate that ApoL1 initiates osmotic stress at the plasma membrane which sensitizes T. b. brucei to oxidation-stimulated osmotic lysis.