EVALUATION OF THE RECOMBINANT 38-KILODALTON ANTIGEN OF MYCOBACTERIUM-TUBERCULOSIS AS A POTENTIAL IMMUNODIAGNOSTIC REAGENT

The diagnosis of infection caused by Mycobacterium tuberculosis is of increased public health concern following increases in the number of c ases in developed countries and major increases in developing countrie s associated with the spread of human immunodeficiency virus (HIV) inf ection. The specificity of purified protein derivative skin testing fo r the detection of infection is compromised by exposure to environment al mycobacteria. Examination of sputum detects the most infectious pat ients, but not those with extrapulmonary disease. The 38-kDa antigen o f M. tuberculosis contains two M. tuberculosis-specific B-cell epitope s. We overexpressed the gene for this antigen in Escherichia coli and evaluated the recombinant product in in vitro assays of T-cell functio n and as a target for the antibody response in humans. The sensitivity and specificity of the antigen as a skin test reagent were also asses sed in outbred guinea pigs. We found that 69% of healthy sensitized hu mans recognize the antigen in vitro, as manifested by both cell prolif eration and the production of gamma interferon. Untreated patients ini tially have a lower frequency of response (38%); this recovers to 72% during therapy. A total of 292 patients (20 with HIV coinfection) and 58 controls were examined for production of antibody to the 38-kDa ant igen by using a commercially available kit. The sensitivity of the tes t in comparison with that of culture was 72.6%, and the specificity wa s 94.9%. The antigen was also tested for its ability to induce skin re actions in outbred guinea pigs sensitized by various mycobacterial spe cies. The antigen provoked significant skin reactions in M. tuberculos is-, M. bovis BCG-, and M. intracellulare-sensitized animals. The sign ificance of these findings and the usefulness of this antigen in immun odiagnosis are discussed.