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Immuno-diagnosis of Tuberculosis
Diagnosis of TB is traditionally based on identification of M. tuberculosis by microscopy or culture of relevant patient material (eg. sputum). However, microscopy is labour intensive, not particular sensitive and prone to error under field conditions, while culturing of bacteria can take several weeks before the result is ready. Furthermore, a substantial number of TB patients are negative by microscopy or culture and the diagnosis of these patients is complicated and often delayed due to the need for invasive diagnostic procedure. For almost a century, the tuberculin skin test, based on measuring delayed-type hypersensitivity responses to purified protein derivative (PPD) has been used to support TB diagnosis and to identify infections with M. tuberculosis. The main drawback with the use of the PPD skin test is the lack of specificity which is due to the fact that PPD contains many components (proteins) that are widely shared between different mycobacterial species. A PPD-based skin test can therefore not distinguish between sensitisation with avirulent environment mycobacteria, prior BCG vaccination or infection with M. tuberculosis. Improved and specific tests for use in diagnosis of TB are therefore urgently needed.
The aim for a novel TB immuno-diagnostic tool is the detection of active or subclinical TB through TB specific antibodies, T cell signals or antigen detection assays. We have focused on antigen discovery of proteins that are specific for a M. tuberculosis infection and not influenced by the BCG vaccination status of the individual or by prior exposure to environmental mycobacteria (see figure). More than 250 proteins have been produced and evaluated and we are currently pursuing 3 different lines of research:
- Diagnosis of latent or subclinical M. tuberculosis infections is based on detection of T cell responses in the suspected individual, and a number of highly specific and frequently recognized antigens have been identified. Two of these are ESAT-6 and CFP10 which can be used for the detection of both latent M. tuberculosis infection and active TB in patients. In contrast to the current methods for TB diagnosis, these antigens are very efficient at identifying an M. tuberculosis infection based on a blood sample from the suspected individual. These specific antigens are the main components in two different blood-based kits which have recently been released and developed by Cellestis and Oxford Immunotec. ESAT-6 and CFP10 also show great potential as components in an improved skin test reagent as the reaction is specific for an infection with M. tuberculosis.
- Diagnosis of active TB can also be achieved by detecting the presence of TB-specific antibodies in patients and antibody based assays have the advantage that they do not require live cells, making transport of samples much easier. We have screened and evaluated more than 100 serological proteins and have identified and produced a number of seroantigens suitable for use in such kits.
- Diagnosis of active, contagious TB: To stop the spread of TB with the minimum of resources it is necessary to be able to identify the contagious TB patients. We are therefore also evaluating the potential of antigens and antibodies to be used in antigens detection assays for identification of patients with mycobacterial components in sputum or urine.
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Mycobacterial antigens |
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| Different mycobacteria have different antigens, which are specific to their species but also closely related or identical antigens that can cause cross-reactivity. |
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