Guinea pig cellular immune responses to proteins secreted by Mycobacterium tuberculosis

To study the immunological activity of proteins secreted by Mycobacterium tuberculosis, we carried out comparative studies in guinea pigs infected intravenously with 2.5 x 10(3) CFU of this organism or with 2.5 x 10(4) CFU of Mycobacterium bovis BCG. Groups of infected guinea pigs were skin tested with fractions of secreted proteins covering well-defined narrow-molecular-mass regions, or such fractions were used for lymphocyte stimulation experiments. The lymphocyte stimulation experiments showed that the fraction containing proteins with molecular masses below 10 kDa had a superior stimulating capacity in tuberculous guinea pigs whereas the 24- to 30-kDa fraction gave significantly higher skin reactions in this group compared with BCG-vaccinated guinea pigs. A precise mapping within the region from 23 to 35 kDa by using a combination of narrow overlapping fractions and purified proteins enabled the identification of the 24-kDa antigen MPT64 as a molecule specific for tuberculous infection. Thus, MPT64 is a promising candidate for a specific diagnostic skin test reagent for human tuberculosis.