African Sleeping Sickness: Ploys Between Host and Parasite

The evolutionary cat and mouse between parasitic organism and host is defined further by scientists who examined the protozoan that causes sleeping sickness. Specifically a receptor on the organism Trypanosoma* that takes up the host's heme-containing proteins is also exploited by the host to induce parasitic lysis. The parasite's receptor was identified by Belgian and Danish investigators and reported in the latest issue of Science.

By using mice experiments and human sera, the investigators observed that the Trypanosoma receptor binds haptoglobin-hemoglobin complexes from the host with high affinity, so that heme may be incorporated into the parasite's intracellular proteins. However, the parasite receptor was also discovered to bind a similar host complex, composed of haptoglobin-related protein (HpR), with similarly high affinity. 

HpR is known to be a component of human HDL particles, which also contain apoliprotein L1 (apo L1)—a known trypanolytic factor. The investigators conclude that parasitic recognition of HpR on human HDL particles triggers internalization of the hemoglobin-exposed portion of these particles, which also contain apo L1. Apo L1 then causes parasitic lysis by inducing uncontrolled vacuole swelling in the organism. So bye, bye trypanosome (at least for now).

HDL = high density lipoprotein.

*Specifically the subspecies T. brucei brucei (which causes animal African trypanosomiasis) and T. brucei gambiense or brucei rhodesiense (which cause forms of human African trypanosomiasis).

Giemsa-stained light photomicrograph (1000X) of Trypanosoma brucei in thin film blood smear from CDC/Dr. Mae Melvin (1977).