| Background and objectiveTuberculosis (TB) is a chronic infectious disease caused by Mycobacterium tuberculosis (M.tb). According to the report of WHO, one-third of the global population contracted with M.tb and China is a country with high burden of tuberculosis. Although M.tb infection is so serious, latent TB infection (LTB) and tuberculosis are not so conveniently diagnosed because of the lack of powerful clinical or laboratory diagnosis methods, thus the so-called "iceberg phenomenon" formed. Tuberculin skin test (TST), although has been applied for more than 100 years, nowadays is still a widely used method for detection of TB infection. In TST test, the key reagent PPD (purified protein derivative) is composed of more than 200 proteins from M.tb, therefore the result can not differenciate BCG (bacilli calmette Guerin) inoculation, NTM (nontuberculous mycobacterium) infection and mycobacterium tuberculosis infection. It is necessary to set up a new diagnosis method to take place of TST. In recent years, with the development of immunology and comparative genetics, a detection of T cell-based in vitro interferon-y release response was established. The mechanism of this detection is that T lymphacyte met and sensitized by Mycobacterium tuberculosis can release interferon-y if given correlative antigen to sensitize and high level of interferon-γsignifies TB infection. If the antigen applied is specific for Mycobacterium tuberculosis while not exist in BCG and most of the NTMs, such as the protein of ESAT-6 (early secreting antigenic target 6000, ESAT-6), it can successfully avoid the interruption of the above factors and can greatly enhance the specificity of TB diagnosis.MethodsSubjects are in four groups:(1) ordinary tuberculosis patients.(2)patients in special ages:children, adolescents, senescent people,(3)The dynamic change of ESAT-6 response with TB treatment duration in ordinary TB patients,(4)HIV (human immunodeficiency virus) positive combined with tuberculosis infection. Heparinized whole blood was incubated in 37℃for 16-20 hours after ESAT-6 was added. Interferon-y in supernatant was detected by ELISA (enzyme linked immunosorbent assay) method.Results(1) The sensitivity of ESAT-6 response in TB close contact people and TB patients verified by sputum culture is high. ESAT-6 as a protein exists in Mycobacterium tuberculosis while not in BCG or most nontuberculous mycobacteria, its in vitro interferon-γrelease response showed a much higher specificity than TST test.(2) ESAT-6-induced in vitro interferon-y release response, with high sensitivity and specificity, was shown a much better value in TB diagnosis in children and adolescents.(3) ESAT-6 is an early secretion antigen in TB infection and the early period of active TB disease, ESAT-6-induced in vitro interferon-y release responses in 65% TB patients were found reduced and some to normal level with the duration of treatment, this may relate to the relatively lower sensitivity in old people.(4) ESAT-6-induced in vitro interferon-y release response also has a high value in TB diagnosis in HIV positive people. CD4 count did not influence the result.(5) The detection rate of ESAT-6-induced IFN-yresponse is 3 times of that in TST test and twice of that in PPD-induced interferon-y release response. ConclusionESAT-6-induced in vitro interferon-y release response is helpful in the diagnosis of TB infection and tuberculosis disease. The operation of the assay is simple and the sensitivity and specificity is much higher in children and adolescents, thus is promising in clinical use. |
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