Nonviable bacterial antigens administered with IL-12 generate antigen-specific T cell responses and protective immunity against Listeria monocytogenes.

The development of effective vaccine strategies for intracellular pathogens, including bacteria, viruses, and parasites, is one of the major frontiers of scientific research. For the studies described here, the murine model of Listeria infection was used to evaluate the adjuvant effects of IL-12 when used as an immunization component. These studies revealed that typically nonimmunogenic doses of heat-killed Listeria monocytogenes, or soluble listerial Ag preparations, elicit intense Th1-type Listeria-specific T cell responses when administered i.p. along with recombinant murine IL-12. In addition to the Ag-specific production of IL-2 by CD4+ peritoneal cells that was elicited, several other correlates of protective responses were noted, including dramatic induction of CD3+ and alpha beta TCR+ cell populations in the peritoneal cavity and increased expression of class II MHC and production of IL-12 (upon in vitro restimulation) by peritoneal macrophages. Protection studies demonstrated that the T cell responses elicited by a IL-12-potentiated, heat-killed L. monocytogenes vaccine were sufficient to effectively protect mice against challenge with a large dose of virulent Listeria.