Mycolic acid as antigen or analyte in tuberculosis

Abstract

Tuberculosis has become one of the world’s most devastating diseases, with more than two million deaths and eight million new cases occurring annually due to the development of drug-resistant strains of Mycobacterium tuberculosis, the breakdown of the immune system of its host by HIV, lapses in public health programmes and the fact that diagnosis of TB is not 100% reliable. Early, affordable, unsophisticated and accurate diagnosis of TB to facilitate timely and proper treatment has become of highest priority to public health. Mycolic acid (MA) is the major lipid cell wall component of Mycobacterium tuberculosis and is unique to mycobacteria and closely aligned genera. Mycolic acids have been shown to be unique antigens for TB diagnosis and have been utilized in standard serodiagnostic techniques, but sensitivity and specificity was found to be unsatisfactory. Two vastly different techniques were investigated in this study – one making use of antibodies and MA, the other, just MA and its unique physical properties of interaction with other MA using fluorescently labelled MA. In the first approach, Sepharose protein-A was employed to trap patient IgG antibodies. The anti-MA antibodies were then quantified by probing with liposomes containing fluorescently labelled MA. Although it generally worked well, a few false –positive and –negative results were obtained. This assay appeared to be more accurate than the standard ELISA immunoassay but it is more labour intensive and not even remotely as amenable to large-scale screening and automation as ELISA. The second approach is based on the release of fluorescent MA from immobilized liposomes on glass by means of the specific attraction that MA in test liposomes or TB patient serum was perceived to have on the immobilized MA. The end-point measured was the remaining fluorescent MA on the surface. Differences were observed between the control and patients’ sera at a very high dilution but not between the HIV negative, TB positive and HIV positive, TB positive patients. This was merely an exploratory investigation and more work still needs to be done before the test is ready for validation with large numbers of serum samples. If subsequent studies confirm these findings, then this concept may be converted into a simple, rapid and affordable TB diagnostic test or be used in combination with the IAsys affinity biosensor to provide a more thorough diagnosis