Haptoglobin-related protein mediates trypanosome lytic factor binding to trypanosomes

Trypanosome lytic factor (TLF-1) is an unusual high density lipoprotein (HD L) found inhuman serum that is toxic to Trypanosoma brucei bruce! and may b e critical in preventing human infections by this parasite. TLF-1 is compos ed of four major apolipoproteins: apolipoprotein AI, apolipoprotein AII, pa raoxonase, and the primate-specific haptoglobin-related protein (Hpr). Hpr is greater than 90% homologous to haptoglobin (Hp), an abundant acute phase serum protein. Killing of trypanosomes by TLF-1 requires cell sin-face bin ding, endocytosis, and subsequent lysosomal targeting. Low temperature bind ing studies reveal two receptors for TLF-1: one that is high affinity/low c apacity (K-d similar to 12 nM, 350 receptors per cell) and another that bin ds with low affinity/ high capacity (K-d similar to 1 muM, 60,000 receptors per cell). The low affinity binding is competed by nonlytic human HDL and is likely to be apolipoprotein Al-mediated. Purified human Hpr and human Hp bind to trypanosomes, are internalized, and are targeted to the lysosome. Furthermore,. Hpr shows competition for TLF-1 binding, and a monoclonal ant ibody against Hpr prevents both TLF-1 uptake and trypanosome killing. Based on these results, we propose that Hpr mediates the high affinity binding o f TLF-1 to T. b. brucei through a haptoglobin-like receptor.