Potential application of variable surface glycoproteins in diagnosis of Trypanosoma brucei rhodesiense

Abstract

Sleeping sickness, also known as human African trypanosomiasis (HAT), is a neglected tropical disease caused by two subspecies of Trypanosoma brucei namely T. b. gambiense and T. b. rhodesiense. Trypanosoma b. gambiense causes chronic form of HAT (Gambian HAT) in West and Central Africa while T. b. rhodesiense is responsible for the acute form (Rhodesian HAT) in East and Southern Africa. In both cases, the disease evolves through clinically distinct stages namely early/stage 1 and late/stage 2. The early stage is haemolymphatic, in which parasites develop in the blood, lymph and peripheral organs. The parasites then spread to the central nervous system (CNS) (late/encephaliticstage) where they cause serious neurological disorders, and results into death if untreated. This necessitates timely diagnosis, stage determination and treatment, with drugs used dependent on the stage of the disease. However, diagnostic tools applied are limiting, suffering from low sensitivity and specificity, high costs and inapplicability in field environments in rural areas where the disease is endemic. This calls for development of improved and/or novel diagnostic tools, specifically for Rhodesian HAT, which kills in weeks or months if not treated. One potential diagnostic approach is serodiagnosis using trypanosome surface proteins, the variable surface glycoproteins (VSGs), which are highly immunogenic and expressed in an ordered fashion early during infection. In this study, two VSGs previously shown to be predominantly expressed early during T. b. rhodesiense infection in monkeys were expressed in bacterial and insect expression systems, and the purified recombinant diagnostic antigens used for immunological detection by applying Ouchterlony double immunodiffusion test. The recombinant proteins generated from both expression systems detected anti-VSG antibodies in a panel of sera from infected monkeys, with a detection rate of 89%, 97% and 83% by VSG3 expressed in bacterial cells, VSG3 and VSG4 expressed in insect cells respectively. In addition, sera recovered up to 15 days post infection, and 19 days when the parasite has crossed into the CNS were seropositive, suggesting potential application of VSGs in diagnosis of sleeping sickness. This is a proof of concept that VSG recombinant antigens predominantly expressed during early stage of trypanosome infection have diagnostic potential, and could form a basis for the development of a simple, cheap, robust and sensitive screening tool applicable in the field setting for early disease detection.