Pathogens and their hosts engage in perpetual molecular arms races. In one such evolutionary stand-off, the protagonists are trypanosome parasites and a human immune complex based on a high-density lipoprotein. See Letter p.430
African trypanosomes, the cause of African sleeping sickness, have waged war against humans for millennia. The epicentre of this struggle is the rapidly evolving protein apolipoprotein L1, which is found in humans and some other primates1, and is the key mediator of these hosts' innate immune responses to the parasites. But some trypanosomes have evolved to overcome this defence and can successfully establish long-term infections in humans. On page 430 of this issue, Uzureau et al.2 describe the complex suite of resistance adaptations used by the most prevalent disease-causing African trypanosome, Trypanosoma brucei gambiense.
African trypanosomes are single-celled parasites that live freely in their host's bloodstream. To escape immune destruction, they constantly change their surface coat by selecting from a repertoire of genes that encode variant surface glycoproteins. But some primates, including humans, have developed innate-immune complexes known as trypanolytic factors (TLFs) that can circumvent this antigenic variation and kill most species of trypanosome. TLFs are high-density lipoprotein molecules that have two key protein components: haptoglobin-related protein (HPR) and apolipoprotein L1 (APOL1). HPR binds to the protein haemoglobin and this complex binds to a receptor on trypanosomes. The trypanosomes then take up the TLF and transfer it to a digestive cellular organelle called the lysosome in order to use its haemoglobin and lipids for their own biosynthetic processes. But during this transit, APOL1 is released and inserted into the lysosome membrane, where it forms small ion-permeable pores that cause the parasite to burst and die3 (Fig. 1).
However, some human-infective trypanosomes, including T. b. gambiense, which causes 97% of human cases of sleeping sickness, can resist the activity of TLFs. By combining previous observations with new findings, Uzureau et al. propose that the parasites have three requirements for complete TLF resistance: the expression of T. b. gambiense-specific glycoprotein (TgsGP); modification of the TLF receptor; and changes in lysosomal physiology.
The protein TgsGP is unique to the subspecies T. b. gambiense type 1 (ref. 4) and is used as a marker for these parasites. Deletion of the TgsGP gene renders these trypanosomes sensitive to human TLFs, and adding back the gene restores resistance2. To determine which domain of the protein is responsible for TLF resistance, Uzureau et al. introduced a battery of modified TgsGP genes into TgsGP-deficient parasites. Despite a limitation of this approach — the potential for misfolding of the modified TgsGPs that interferes with their activity — the authors identified a particular stretch of amino-acid residues that was required for resistance and that, when synthesized as a peptide, stiffened lipid-bilayer membranes into which it was incorporated in a manner akin to the action of cholesterol. They propose that stiffening of the trypanosome lysosomal membrane by TgsGP prevents APOL1 membrane insertion. This concept remains to be verified, but it opens up the possibility that cholesterol-rich membranes may be resistant to APOL1 and inspires the question of whether cholesterol protects human cells from APOL1-mediated injury.
However, Uzureau and colleagues also show that introducing the TgsGP gene into a non-resistant trypanosome species does not confer resistance. Further studies of the TLF-uptake pathway suggested that resistant parasites have, in addition to TgsGP, altered lysosomal pH and protein-cleaving enzymatic activity that enhances their resistance to TLF.
T. b. gambiense type 1 is also known to have evolved variations in the TLF receptor, one of which is a single amino-acid change that is found in all strains of this subspecies5. This modification reduces the affinity of the receptor for its ligand6, and thereby reduces the amount of TLF accumulated by the parasite7. Uzureau et al. speculate that this receptor variant arose during evolution as a result of selection pressure from humans with chronic low levels of red-blood-cell lysis owing to diseases such as malaria. Lysis of red blood cells releases haemoglobin, which binds to the protein haptoglobin, an abundant component of serum. This results in the clearance of haptoglobin by immune cells, causing a condition known as hypohaptoglobinaemia. The released haemoglobin also binds to HPR on TLF and this, coupled with the removal of competing haptoglobin–haemoglobin complexes, results in increased TLF uptake by trypanosomes, leading to enhanced parasite killing. Uzureau et al. show that the mutation in the T. b. gambiense receptor, which reduces TLF uptake, allows parasite survival in hypohaptoglobinaemic serum.
Previous work from the same research group3 had shown that the East African human-infective trypanosome T. b. rhodesiense has evolved a different way to escape killing by TLF. This species has not acquired mutations in the TLF receptor, even though malaria (and therefore hypohaptoglobinaemia) is endemic throughout sub-Saharan Africa. Instead, the parasite has evolved a protein called the serum-resistance-associated protein (SRA)8, which alone is sufficient to confer complete resistance to the TLFs present in most humans. SRA binds to APOL1 with high affinity and prevents either the insertion and/or oligomerization of APOL1 in membranes9. However, some people produce APOL1 variants (called G1 and G2) that arose in Africans after the migration of ancestral humans out of Africa; sera from individuals with these variant proteins kill T. b. rhodesiense in vitro10, but their ability to protect against infection has yet to be confirmed. There are multiple African-restricted APOL1 variants in addition to G1 and G2, and one of these may prove to confer resistance to T. b. gambiense11.
One surprising consequence of these evolutionary events is that African-Americans who express only the G1 or G2 APOL1 variants have 7–30-fold higher rates of several types of kidney disease10,12. This scenario resembles the human mutations in haemoglobin that provide protection against malaria but lead to the blood disorder sickle-cell anaemia. It is possible that kidney disease or other deleterious effects of these trypanosome-protective APOL1 variants may have prevented them from becoming fixed throughout the population.
Uzureau et al. have contributed a significant advance to our understanding of natural selection involving African trypanosomes and humans, but their work points towards ever more complex and interrelated events involving different trypanosome species, and perhaps malaria parasites, in this long-running battle with humans. So who is winning this epic arms race? Perhaps it is the non-human primates — baboons, for example, express APOL1 variants that seem capable of killing human-infective trypanosomes13,14 but that do not cause kidney disease. Figuring out the secrets of baboon APOL1 may answer crucial questions that will help to solve the riddles of both African sleeping sickness and kidney disease.
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Nature Microbiology (2017)
International Journal for Parasitology: Parasites and Wildlife (2015)