An intravenous immunizing infection with the facultative, intracellular parasite, Listeria monocytogenes results in the production in the spleen of a population of immunologically-committed lymphocytes which can adoptively immunize normal recipients against a lethal challenge infection. These cellular mediators of immunity are first produced in the spleen between days 2 and 4 of infection and reach peak production on day 6. Their production then progressively decreases until about day 20 when their presence can no longer be detected.

Increased production of cellular mediators is coincident with major increases in cell division, cellularity, and spleen weight. Decreased production of cellular mediators, on the other hand, is associated with decreases in cell division, cellularity, and spleen weight. Again, the level of delayed sensitivity to Listeria antigens expressed by the host at any one time is proportional to the number of cellular mediators in the spleen.

Increased production of cellular mediators is also associated with major increases in the total numbers of replicating T cells and B cells in the spleen. That the cellular mediators of immunity are part of the replicating T cell population, rather than the B cell population, is evidenced by their susceptibility to anti-θ serum and by their resistance to anti-Ig serum. Furthermore, they can be completely eliminated from the spleen by a brief pulse of the antimitotic drug, vinblastine. This study allows the conclusion that the cellular mediators of anti-Listeria immunity belong to an expanded population of rapidly dividing, short-lived T cells. It is suggested that they have the same properties as the T cell effectors of allograft immunity.

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