An assessment of two evanescent field biosensors in the development of an immunoassay for tuberculosis

Abstract

Accurate diagnosis of active tuberculosis is required to improve treatment, reduce transmission of the disease and control the emergence of drug resistance. A rapid and reliable test would make a considerable contribution to the management of the TB epidemic, especially in HIV-burdened and resource-poor countries where access to diagnostic laboratories are limited. Surrogate marker antibody detection to mycobacterial lipid cell wall antigens gave promising results, in particular with cord factor. The specific advantage of using mycolic acids as lipid antigens in comparison to protein antigens is that mycolic acid is a CD1 restricted antigen with the ability to induce proliferation of CD4/CD8 double negative T-cells, which may explain the sustained antibody production in AIDS patients. Traditional end-point assays to detect anti-MA antibodies showed an unacceptable number of false positive and negative test results. Here a much improved biosensor method (the MARTI-assay, i.e. Mycolic acid Antibody Real-Time Inhibition assay) was developed to detect antibodies to mycolic acid in patient sera as surrogate markers of active tuberculosis. The test was assessed on an IAsys optical biosensor and gave an accuracy of 82%. The technology was transferred to an SPR (ESPRIT) biosensor to economise and simplify the assay. Mycolic acid containing liposomes were immobilized on the SPR gold surface pre-coated with octadecanethiol. The following parameters were optimized on the ESPRIT biosensor to enable reliable TB diagnosis: effect of degassed buffer, saponin blocking, first exposure to serum at low concentration and second exposure to antigen inhibited serum at high concentration. The IAsys biosensor system has a weakness in the double channel cuvette system, in which the channels often do not give matching results, while being ten times more expensive than the gold discs provided for the ESPRIT biosensor. The ESPRIT biosensor is provided with an adjustable laser setting to compensate for differences in the channel readings as well as an automated fluidic system that reduces variance from one sample to the next. First indications are that the test can also be used for prognosis of TB during treatment. It is hoped that the ESPRIT biosensor will improve the accuracy of the test to more than 90%. If the MARTI-assay technology could be made amenable for high throughput screening, it may provide the solution to the serodiagnosis of tuberculosis and monitoring of progress during TB treatment both in adult and children, thereby reducing the spread of TB within the communities.