Priming of memory but not effector CD8 T cells by a killed bacterial vaccine.

Killed or inactivated vaccines targeting intracellular bacterial and protozoal pathogens are notoriously ineffective at generating protective immunity. For example, vaccination with heat-killed Listeria monocytogenes (HKLM) is not protective, although infection with live L. monocytogenes induces long-lived, CD8 T cell-mediated immunity. We demonstrate that HKLM immunization primes memory CD8 T lymphocyte populations that, although substantial in size, are ineffective at providing protection from subsequent L. monocytogenes infection. In contrast to live infection, which elicits large numbers of effector CD8 T cells, HKLM immunization primes T lymphocytes that do not acquire effector functions. Our studies show that it is possible to dissociate T cell-dependent protective immunity from memory T cell expansion, and that generation of effector T cells may be necessary for long-term protective immunity.