T cell proliferation and
interferon (IFN)-gamma secretion were analyzed in 45
leprosy contacts stimulated with
antigen 85 (
Ag85), the major culture filtrate
antigen from Mycobacterium bovis bacille Calmette-Guérin. All 14 Mitsuda reaction-positive contacts reacted to Mycobacterium leprae and
Ag85. Three Mitsuda reaction-negative contacts reacted weakly to M. leprae and
Ag85. The other 28 Mitsuda reaction-negative contacts did not react to M. leprae, but 9 reacted to
Ag85. Thirty-four contacts were retested 16 months later. Eleven contacts initially positive by the Mitsuda test remained
lepromin positive and reactive to M. leprae and
Ag85. Fourteen contacts initially negative by the Mitsuda test converted, and all reacted in vitro to M. leprae and
Ag85. Finally, 9 contacts remained Mitsuda test-negative, and 7 were unreactive to
Ag85. In vitro reactivity to
Ag85 at baseline in Mitsuda test-negative contacts was associated with subsequent conversion to
lepromin reactivity in 7 of 9 subjects. These data suggest that reactive T cells against
Ag85 develop very early during M. leprae
infection and that
Ag85 is a potentially protective T cell immunogen.