Failure of killed Listeria monocytogenes vaccine to produce protective immunity.

Abstract

We report here experiments which, in contrast to recently published data1,2, 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.