Induction of protective CD8+ T lymphocytes by an attenuated Listeria monocytogenes actA mutant

We tested the ability of an attenuated actA mutant of Listeria monocytogenes to induce protective immunity in mice. This mutant can enter and multiply in the cytosol of the infected host cell, but is deficient in actin-dependent cell-to-cell spread. It was found to be of attenuated virulence for inbred C3H mice: the LD50 after i.v. injection was 1000-fold higher than that of the wild-type strain. Mutant bacteria multiplied up to the fourth day in the liver, but only for 1 day in the spleen. A single infection with the maximum sublethal dose of the actA mutant induced long-lasting immunity; the LD50 of virulent wild-type L. monocytogenes increased 100-fold and growth of wild-type L. monocytogenes was controlled in liver and spleen of these mice. The presence of Listeria-reactive T cells in spleen of C3H mice infected 7 days previously with the actA mutant was monitored, through their ability to protect naive syngeneic recipients against wild-type L. monocytogenes. Protection was mainly conferred by Thy-1+ CD8+ T lymphocytes; depletion of CD4+ T cells had no significant effect on the level of transferred protection. Such attenuated mutants may be used to develop live vector vaccines for delivery of heterologous proteins into the cytosol, thereby favoring the induction of a CD8+ T cell response.