Difference in the induction of macrophage interleukin-1 production between viable and killed cells of Listeria monocytogenes.

T-cell-mediated immunity to Listeria monocytogenes in mice, as determined by delayed-type hypersensitivity and acquired resistance, was induced by immunization with viable bacteria but not with killed bacteria, even when killed cells were injected in a high dose or repeatedly. T cells obtained from mice immunized with viable L. monocytogenes were readily stimulated with killed-bacterial antigens, resulting in T-cell proliferation in vitro and expression of a delayed footpad reaction in vivo. After immunization with killed-bacterial vaccine, T-cell responsiveness to interleukin 2 (IL-2) never developed but a lower level of responsiveness to IL-1 appeared later than with T cells from mice immunized with viable bacteria. When IL-1 production by macrophages was examined in vitro, viable L. monocytogenes stimulated a high level of IL-1 release while killed bacteria did not. Avirulent strains which were ineffective in the induction of T-cell mediated immunity were incapable of inducing IL-1 production as well. The impaired ability of killed bacteria to stimulate IL-1 production was confirmed by the level of IL-1 mRNA expression. These results suggested that the ineffectiveness of killed L. monocytogenes vaccine is not due to loss or lack of antigenic epitopes but may be ascribed to insufficient induction of IL-1 production in the initial stage of the immune response in vivo.