RECALL OF LONG-LIVED IMMUNITY TO MYCOBACTERIUM-TUBERCULOSIS INFECTIONIN MICE

Our study investigates the recall of immunity in the mouse model of me mory immunity to tuberculosis infection. The results provide evidence that recall of immunity is expressed as an accelerated accumulation of potent effector cells in the infected target organs. These effector c ells were recruited from the resting pool of memory cells and were imm ediately triggered to exert their effector functions, leading to a mas sive release of Th1 cytokines detectable both in splenic extracts and in the serum within the first 24 h of infection. During a primary infe ction, in contrast, a 14-day delay was observed before significant cyt okine levels were reached. After the initial effector phase, the cells blasted and entered into clonal expansion, resulting in a rapid incre ase in the total number of CD4 CD45RB(low) cells in the spleen. The re call of memory immunity was highly efficient and controlled an infecti ous challenge within the first week. The molecules recognized by the m emory effector subset were the proteins secreted from Mycobacterium tu berculosis during growth. By separating the CD4 population into CD45RB (high) and CD45RB(low) subsets, the memory effector cells were demonst rated to reside predominantly in the activated population of CD45RB(lo w) CD44(high) LFA-1(high) L-selectin(low) cells. The key antigenic tar gets recognized by these cells were identified as Ag85B and a secreted 6-kDa protein (ESAT-6) that elicited the release of exceedingly high revels of IFN-gamma. ESAT-6 was biochemically purified, characterized, and the gene encoding the protein was cloned.