Alteration of non-specific resistance to infection with Listeria monocytogenes.

The experimental infection of murine hosts with Listeria monocytogenes is often used as a model for cell-mediated immunity. However, the natural immunity or non-specific resistance to listeriosis can be influenced by the parasite itself and also by a wide array of endogenous and exogenous host factors. The most important host factor in inbred mouse strains is their genetically determined susceptibility or resistance to Listeria monocytogenes. Secondly, the age of the mice is crucial for the outcome of infection. Resistance is only slowly developed by newborn mice, while aged mice possess an increased non-specific resistance as compared to young adult animals. Resistance is further influenced by the nutritional status, by pregnancy or by a simultaneous second antigenic stimulation. Regarding exogenous factors, macrophage blocking agents can totally abolish the resistance to listeriosis, while a lot of immunomodulating agents, such as BCG, killed Bordetella pertussis or Propionibacterium acnes organisms, lipopolysaccharides, suramin etc., can either increase or decrease the resistance. The mononuclear phagocyte system seems to be the main target of all these immunomodifiers. The timing between listeria infection and application of the immunomodulator determines the effect on non-specific resistance. A simultaneous injection of parasite and immunomodulator results in a decrease of resistance, while the application of immunoadjuvants several days before infection can dramatically increase the resistance to listeriosis. The delicate equilibrium of the mononuclear phagocyte system must therefore be taken into account, when infection with Listeria monocytogenes is used to test for immune-modifying agents, which are intended for use in humans or animals.