[Evolution of IGRA researches].Kekkaku. 2008 Sep; 83(9):641-52.K
The progress of genomic analysis in mycobacterium including M. tuberculosis (Mtb) allowed us to find Mtb-specific antigens, ESAT-6 and CFP-10, which induce strong interferon-gamma (IFN-gamma) from sensitized T cells. Shortly after discovery of these antigens, diagnostic tests for tuberculosis (TB) infection were developed using these antigens. Since ESAT-6 and CFP-10 are absent from all BCG substrains and most of non-tuberculous mycobacterium, these diagnostic tests are not confounded with BCG vaccination and infection of most of non-tuberculosis mycobacterium. These diagnostic tests are called as Interferon-Gamma Release Assays (IGRAs), and currently there are two commercially available tests. One of them, QuantiFERON-TB Gold (it is called QuantiFERON TB-2G in Japan, QFT-2G) based on ELISA method has been approved in Japan, and the other is T-SPOT. TB which is based on ELISPOT method and has not been approved in Japan yet. As in general T-SPOT. TB has been shown to be more sensitive than QFT-2G, approval of T-SPOT. TB in Japan would be expected. However, there are many questions to be solved in IGRAs, since we have just started to use these tests. A paper which integrated these questions was published last year, and it would be helpful. In this mini-symposium, Dr. Peter Andersen reported the progress of development of diagnostic tests for tuberculosis infection, the possibility to distinguish between active TB and latent TB infection (LTBI) which the current IGRAs do not, and the prognostic use of IGRAs (The Japanese content was reported by Chairpersons). Dr. Ariga reported the application of QFT-2G for specimens other than blood. He also reported the interesting data on which T cells responded in QFT-2G. Dr. Higuchi comprehensively reported data on several questions in the QFT-2G test. Currently the use of IGRAs is expanding rapidly. Under this circumstance, it would be very important to properly understand the characteristics of IGRAs. We hope that this mini-symposium may help for understanding these issues. 1. Interferon-Gamma Release Assays (IGRA) and antigens for detection of latent infection and prediction of disease: Peter ANDERSEN (Department of Infectious Disease Immunology and the SSI Centre for Vaccine Research, Statens Serum Institut, Denmark) One of the most important challenges in global tuberculosis control is the diagnosis and treatment of latent tuberculosis infection. The currently used method for detection of latent tuberculosis infection, the tuberculin skin test, has low specificity. The identification of antigens specific for Mycobacterium tuberculosis to replace purified protein derivative has therefore been a major international research priority. We have performed a rigorous assessment of the diagnostic potential of antigens that are lacking from the M. bovis bacille Calmette-Guérin vaccine strains, as well as from most nontuberculous mycobacteria. We have identified three antigens with a major diagnostic potential: ESAT-6, CFP-10 and TB 7.7. These antigens are currently used in IGRA tests such as the QuantiFERON that measure the production of interferon-gamma from sensitized T lymphocytes, thereby signalling ongoing infection. In the EU, US and Japan, where these tests have entered the market, the value of this approach in contact tracing has rapidly become apparent. I will suggest that such tests can be modified to identify the individuals among the latently-infected, at most risk of developing active contagious TB. Targeted treatment of this part of the population offers the possibility of preventing TB before it becomes infectious, which would greatly contribute to the eventual control of this global epidemic. 2. Immune responses specific for M. tuberculosis antigen--peripheral blood and sites of inflammation: Haruyuki ARIGA (National Hospital Organization Tokyo National Hospital) To develop a more accurate method for diagnosing active tuberculous pleuritis, as well as peritonitis, meningitis and pericarditis of tuberculous origin, we established an antigen-specific interferon-gamma (IFN-gamma) release assay using cavity fluid specimens. Study subjects were 30 patients with bacteriologically confirmed active tuberculous serositis and 49 patients with definitive nontuberculous etiology. Culture was performed for 18 h with fluid mononuclear cells in the supernatant of the effusion together with saline or Mycobacterium tuberculosis-specific antigenic peptides, early secretory antigenic target 6 and culture filtrate protein 10. IFN-gamma concentrations in the culture supernatants were measured by ELISA. In patients with active tuberculous serositis, antigen-specific IFN-gamma responses of cavity fluid samples were significantly higher than those of nontuberculous effusion samples. Area under the receiver operating characteristic curve was significantly greater for cavity fluid IFN-gamma response than for cavity fluid adenosine deaminase and whole-blood IFN-gamma release assay. The cavity fluid IFN-gamma release assay could be a noninvasive method for accurately and promptly diagnosing tuberculous serositis in patients in whom active tuberculosis in the cavity space is clinically suspected but for which no bacteriological evidence can be obtained. 3. Several questions in IGRAs: Kazue HIGUCHI (Research Institute of Tuberculosis, Japan Anti-Tuberculosis Association). Although it has been recommended to use QFT-2G for contact investigations in the revised guideline for contact investigation last year, there are several questions in QFT-2G. In this mini-symposium, data on several questions in the QFT-2G test were presented. These included the application of QFT-2G to vulnerable individuals in immune system such as infants and HIV-positives, the effects of chemotheraphy on the QFT-2G test, the prognosis of development of active TB by QFT-2G, the next generation of QFT-2G, quality assurance of the QFT-2G test, and some problems of the current QFT-2G test. It should be important to research these questions and improve IGRAs based on the basic immunology.