The potential of recombinant antigens ESAT-6, MPT63 and mig for specific discrimination of Mycobacterium tuberculosis and M. avium infection.

The tuberculin skin test is currently the only accepted immunological diagnostic tool available to detect infection with Mycobacterium tuberculosis. Immunological diagnosis of active tuberculosis (TB) in childhood is essential, since chest X-ray findings are less characteristic than in adults and as many as 60%–70% of cases remain culture-negative. However, tuberculin is not specific for M. tuberculosis due to the presence of epitopes shared with M. bovis BCG and non-tuberculous mycobacteria (NTM), such as M. avium. Since therapy of infections with M. tuberculosis and NTM differs, we assessed whether defined antigens for in vitro testing may improve diagnostic strategies. Data from human studies have shown that one of these antigens, ESAT-6, elicits strong in vitro T-cell immune responses in adult TB patients but not in healthy tuberculin (PPD, purified protein derivative) skin test negative individuals (reviewed in [1]). MPT63 elicits specific humoral immune responses in humans with TB [3]. For the M. avium antigen mig specific antibody production in humans with M. avium infection (MAC) has been described [5]. Currently, there is only limited knowledge on the diagnostic potential of these antigens to discriminate between M. tuberculosis and MAC infection in T-cell-based tests in humans [2], and no data on paediatric mycobacterial infections are available. In our experiments, PPD induced in vitro proliferation in 29/30 (Fig. 1A) and interferon-c (IFNc) secretion in 20/24 of TB patients, but these responses were not specific for M. tuberculosis since it was also recognised by 8/10 of MAC patients and 8/21 of healthy PPD skin test negative controls (IFNc secretion in 2/9 and 0/20, respectively). Neither results in healthy skin test negative individuals nor in TB patients were influenced by BCG vaccination status. In contrast, ESAT-6 induced strong and highly specific in vitro proliferation in 20/27 and IFNc-secretion in 11/24 of TB patients, but not in MAC patients or controls (Fig. 1B). The differences in responder rates of TB patients and controls or TB and MAC patients were highly significant. Both responders and non-responders to ESAT-6 showed comparable reactivity to PPD (Table 1). MPT63 induced proliferation (Fig. 1C) and IFNc release from peripheral blood mononuclear cells (PBMC) from a subgroup of TB patients (6/30 and 2/24, respectively) but not from those of MAC patients or controls (responder rates TB versus MAC, P=0.01; TB versus controls, P=0.001). One TB patient showed in vitro recognition of MPT63 but not of ESAT-6. The magnitude of proliferative responses induced by ESAT-6 in TB patients was significantly higher than that by MPT63 (P<0.001), but was lower than that by PPD (P<0.001, Fig. 1A–C). Comparable differences were Eur J Pediatr (2003) 162: 534–536 DOI 10.1007/s00431-003-1199-6