Induction of immunity with avirulent Listeria monocytogenes 19113 depends on bacterial replication.

Events necessary for triggering the cell-mediated response to intracellular parasites are poorly understood. Here we show that extremely high doses of avirulent Listeria monocytogenes 19113 (greater than 10(9] induce a modest and short-lived state of resistance in BALB/c mice. Induction of this protective state could not be achieved with nonviable bacteria and was blocked by inhibiting replication of viable L. monocytogenes 19113 through antibiotic treatment. The immune response was antigen specific and could be adoptively transferred with lymphoid cells. However, unlike the prototypic acquired cellular resistance induced by virulent Listeria strains, the protective response induced by L. monocytogenes 19113 was extremely short-lived, lasting less than 2 weeks, with a precipitous decline in the activity of the immune cells involved. An intervening in vitro culture period with concanavalin A greatly enhanced the activity of the adoptively transferred immune cells.