Hemoglobin Is a Co-Factor of Human Trypanosome Lytic Factor

Abstract

Trypanosome lytic factor (TLF) is a high-density lipoprotein (HDL) subclass providing innate protection to humans against infection by the protozoan parasite Trypanosoma brucei brucei. Two primate-specific plasma proteins, haptoglobin-related protein (Hpr) and apolipoprotein L-1 (ApoL-1), have been proposed to kill T. b. brucei both singularly or when co-assembled into the same HDL. To better understand the mechanism of T. b. brucei killing by TLF, the protein composition of TLF was investigated using a gentle immunoaffinity purification technique that avoids the loss of weakly associated proteins. HDL particles recovered by immunoaffinity absorption, with either anti-Hpr or anti-ApoL-1, were identical in protein composition and specific activity for T. b. brucei killing. Here, we show that TLF-bound Hpr strongly binds Hb and that addition of Hb stimulates TLF killing of T. b. brucei by increasing the affinity of TLF for its receptor, and by inducing Fenton chemistry within the trypanosome lysosome. These findings suggest that TLF in uninfected humans may be inactive against T. b. brucei prior to initiation of infection. We propose that infection of humans by T. b. brucei causes hemolysis that triggers the activation of TLF by the formation of Hpr–Hb complexes, leading to enhanced binding, trypanolytic activity, and clearance of parasites. African trypanosomes are parasites that can infect a wide range of mammals, including domestic animals and humans. Several hundred thousand humans are infected with African sleeping sickness, but this number would be much higher if not for a natural defense molecule found in human blood. The trypanosome lytic factor (TLF) is a minor subclass of high-density lipoprotein that contains two proteins found only in primates, apolipoprotein L-1 and haptoglobin-related protein (Hpr). In this paper, we show that Hpr contributes to TLF toxicity to trypanosomes because it binds hemoglobin (Hb). We found that when Hb is bound to TLF, it is rapidly taken up by the parasite and activated within the acidic environment of the parasite's digestive organelle, the lysosome. Within the lysosome, Hb releases iron, inducing a chemical reaction that produces free radicals that damage membranes and contributes to trypanosome killing. Usually, free Hb is rapidly cleared from the circulation of mammals because of the organ damage free Hb can cause. Trypanosome infection results in breakage of red blood cells and the release of large amounts of Hb. We postulate that trypanosome infection causes increased vascular levels of Hb, resulting in the formation of TLF–Hb complexes that may be important in “arming” the human innate immune system to clear the circulation of certain African trypanosomes.