Failure of killed Listeria monocytogenes vaccine to produce protective immunity

Abstract

We report here experiments which, in contrast to recently published data^1,2, suggest that protection against lethal doses of viable Listeria monocytogenes cannot be achieved by previous injection with killed Listeria vaccine neither by adding macrophage-blocking agents such as dextran sulphate nor by using C3H/HeJ mice having inborn macrophage defects. This discrepancy can be explained by the ineffective killing of bacteria at 56 °C, resulting in injection of small numbers of viable bacteria; such doses are normally subinfective and do not provide immunity. Blocking of the macrophage system by dextran sulphate³, however, or use of C3H/HeJ mice with inborn macrophage defects⁴ renders these doses fully infective, hence providing subsequent protective immunity⁵.