Modification by soluble antigen of the immune response to mycobacterial infection.

After inoculation with viable BCG organisms, spleen cells from mice developed cellular immune reactivity to purified protein derivative antigen by day 10, which declined by day 25; after rechallenge on day 25, a vigorous secondary response rapidly developed (by day 29). The inclusion of autochthonous serum (from the BCG-infected mice) in the in vitro cellular immune reactivity tests altered this pattern considerably: the primary response was shortened, and the secondary response totally abated, suggesting the presence of blocking factors active against BCG-specific cellular immune reactivity in the sera of infected animals. Subsequent experiments indicated that BCG-specific serum blocking factor activity could be generated in the serum of control mice by the intravenous inoculation of purified protein derivative.