Mechanism of Trypanosoma brucei gambiense resistance to human serum

Abstract

The African parasite Trypanosoma brucei gambiense accounts for 97% of human sleeping sickness cases. T. b. gambiense resists the specific human innate immunity acting against several other tsetse-fly-transmitted trypanosome species such as T. b. brucei, the causative agent of nagana disease in cattle. Human immunity to some African trypanosomes is due to two serum complexes designated trypanolytic factors (TLF-1 and -2), which both contain haptoglobin-related protein (HPR) and apolipoprotein LI (APOL1). Whereas HPR association with haemoglobin (Hb) allows TLF-1 binding and uptake via the trypanosome receptor TbHpHbR (ref. 5), TLF-2 enters trypanosomes independently of TbHpHbR (refs 4, 5). APOL1 kills trypanosomes after insertion into endosomal/lysosomal membranes. Here we report that T. b. gambiense resists TLFs via a hydrophobic β-sheet of the T. b. gambiense-specific glycoprotein (TgsGP), which prevents APOL1 toxicity and induces stiffening of membranes upon interaction with lipids. Two additional features contribute to resistance to TLFs: reduction of sensitivity to APOL1 requiring cysteine protease activity, and TbHpHbR inactivation due to a L210S substitution. According to such a multifactorial defence mechanism, transgenic expression of T. b. brucei TbHpHbR in T. b. gambiense did not cause parasite lysis in normal human serum. However, these transgenic parasites were killed in hypohaptoglobinaemic serum, after high TLF-1 uptake in the absence of haptoglobin (Hp) that competes for Hb and receptor binding. TbHpHbR inactivation preventing high APOL1 loading in hypohaptoglobinaemic serum may have evolved because of the overlapping endemic area of T. b. gambiense infection and malaria, the main cause of haemolysis-induced hypohaptoglobinaemia in western and central Africa.

Evolution of the primate trypanolytic factor APOL1

“…None of the other animal-infective trypanosomes have acquired SRA, and thus, they cannot infect humans. In contrast, human-infective T. b gambiense has evolved different mechanisms of resistance to TLF (13)(14)(15) and is resistant to G1 and G2 sera in vitro (1). The dramatic increase in frequency of G1 in West Africa, despite the presence in the gene pool of the more effective T. b. rhodesiense killer G2, hints that the two alleles were selected in response to more than one pathogen.…”

“…Human and gorilla sera protect against Trypanosoma brucei brucei, whereas baboon sera protect against T. b. brucei, Trypanosoma brucei rhodesiense, and potentially, Trypanosoma brucei gambiense (8,9). Human-infective T. b. rhodesiense, which causes acute African sleeping sickness, arose from animal-infective T. b. brucei through the evolution of a virulence factor called serum resistance-associated protein (SRA) (10) that binds to (11) and prevents human APOL1-mediated lysis (12), whereas T. b. gambiense evolved different mechanisms to neutralize human APOL1 (13)(14)(15).…”

Structural basis for trypanosomal haem acquisition and susceptibility to the host innate immune system

“…brucei HpHbR (Leu210) leads to reduced affinity for human Hp-Hb and Hpr-Hb 7,27,28 . In combination with lowlevel expression of HpHbR 26 and the protective effect of TgsGP 22 , this evolutionary amino-acid substitution contributes to T.b. gambiense resistance against the human TLF particles.…”

“…rhodesiense) and Trypanosoma brucei gambiense (T.b. gambiense) express resistance proteins counteracting apolipoprotein L1 activity enabling these parasite strains to evade the lethal action of TLF particles [22][23][24][25] . T.b.…”

Human trypanolytic factor APOL1 forms pH-gated cation-selective channels in planar lipid bilayers: Relevance to trypanosome lysis

“…Whereas multifactorial mechanisms were proposed to account for resistance of T. b. gambiense to human serum (13)(14)(15)(16), resistance in the case of T. b. rhodesiense is determined by the parasite's serum resistance-associated (SRA) protein, which localizes to parasite endosomes and binds to APOL1 under acidic conditions (10,17). Primate APOL1 orthologs, and two African human APOL1 variants, contain mutations in the C-terminal SRA-binding site that reduce binding affinity for SRA, allowing these variants to lyse SRA-producing trypanosomes (18)(19)(20).…”