Identification of sVSG117 as an Immunodiagnostic Antigen and Evaluation of a Dual-Antigen Lateral Flow Test for the Diagnosis of Human African Trypanosomiasis

Background: The diagnosis of human African trypanosomiasis (HAT) caused by Trypanosoma brucei gambiense relies mainly on the Card Agglutination Test for Trypanosomiasis (CATT). There is no immunodiagnostic for HAT caused by T. b. rhodesiense. Our principle aim was to develop a prototype lateral flow test that might be an improvement on CATT. Methodology/Principle Findings: Pools of infection and control sera were screened against four different soluble form variant surface glycoproteins (sVSGs) by ELISA and one, sVSG117, showed particularly strong immunoreactivity to pooled infection sera. Using individual sera, sVSG117 was shown to be able to discriminate between T. b. gambiense infection and control sera by both ELISA and lateral flow test. The sVSG117 antigen was subsequently used with a previously described recombinant diagnostic antigen, rISG65, to create a dual-antigen lateral flow test prototype. The latter was used blind in a virtual field trial of 431 randomized infection and control sera from the WHO HAT Specimen Biobank. Conclusion/Significance: In the virtual field trial, using two positive antigen bands as the criterion for infection, the sVSG117 and rISG65 dual-antigen lateral flow test prototype showed a sensitivity of 97.3% (95% CI: 93.3 to 99.2) and a specificity of 83.3% (95% CI: 76.4 to 88.9) for the detection of T. b. gambiense infections. The device was not as good for detecting T. b. rhodesiense infections using two positive antigen bands as the criterion for infection, with a sensitivity of 58.9% (95% CI: 44.9 to 71.9) and specificity of 97.3% (95% CI: 90.7 to 99.7). However, using one or both positive antigen band(s) as the criterion for T. b. rhodesiense infection improved the sensitivity to 83.9% (95% CI: 71.7 to 92.4) with a specificity of 85.3% (95% CI: 75.3 to 92.4). These results encourage further development of the dual-antigen device for clinical use. Author Summary: Human African Trypanosomiasis (HAT) is caused by infection with Trypanosoma brucei gambiense or T. b. rhodesiense. The diagnosis of T. b. gambiense infections currently relies primarily on a Card Agglutination Test for Trypanosomiasis (CATT), which has acknowledged limitations, and there is no simple test for T. b. rhodesiense infection. Our overall aim is to produce a simple lateral flow test device with a similar or better sensitivity and specificity than CATT but with better stability and ease of use at point of care. In this study, we identified a particular variant surface glycoprotein, sVSG117, with good diagnostic potential and combined it with a previously identified recombinant diagnostic antigen, rISG65, to produce a prototype dual-antigen lateral flow test. We performed a virtual field trial by testing the device blind with 431 randomized serum samples provided by the WHO HAT Specimen Biobank. The results show that, although the prototype lateral flow test is un-optimized, it was able to diagnose T. b. gambiense HAT with a sensitivity and specificity of 97.3% and 83.3% and T. b. rhodesiense HAT with a sensitivity and specificity of 83.9% and 85.3%.