MAPPING OF T-CELL EPITOPES OF THE 30-KDA EX ANTIGEN OF MYCOBACTERIUM-BOVIS STRAIN BACILLUS-CALMETTE-GUERIN IN PURIFIED PROTEIN DERIVATIVE (PPD)-POSITIVE INDIVIDUALS

The fibronectin-binding 30-kDa alpha Ag is a major secretory protein o f growing mycobacteria that stimulates in vitro lymphocyte blastogenes is in most healthy purified protein derivative-positive individuals, b ut only a minority of patients with active tuberculosis. T cell epitop es of the alpha Ag were assessed using blastogenic responses of PBMC f rom 12 healthy purified protein derivative-positive subjects to a set of synthetic peptides based on the 325-amino acid sequence of the alph a Ag of Mycobacterium bovis BCG. Because epitope-specific precursor ce lls are infrequent and randomly distributed, we used Poisson analysis to determine positive responses to 10 mu g/ml of each peptide in 12 re plicate culture wells. Seven immunodominant regions of the alpha Ag we re identified. Each subject responded to at least one of the two most dominant epitopes, which correspond to amino acids 131-155 and 233-257 (from N terminus). Peptides of these two epitopes induced production of IFN-gamma by sorted CD4(+) T cells. The immunodominant peptides may have use as components of a vaccine and as tools to study the evoluti on of the immune response to M. tuberculosis. The two most dominant ep itopes both occur in regions of the alpha Ag that differ from those of the atypical pathogens M. avium and M. kansasii. In addition, the M. bovis epitope of amino acids 133-155 differs from that of M. tuberculo sis by a single amino acid. It may be possible to exploit the sequence differences for development of diagnostic tests with increased specif icity.