Age-dependent differences in outcome of infections, with special reference to experiments in mice.

The host defects that increase either the frequency or the severity of certain infections in older persons are reviewed briefly. The major defect is in the functioning of the lymphocyte-macrophage system; this is expressed as deficits in cutaneous reactivity and T cell response after antigenic stimulation. A review of induced infections in experimental animals shows results generally consistent with the data in humans. Infections due to Listeria monocytogenes, Salmonella typhimurium, and Toxoplasma gondii all appear to be worse in senescent mice; each of these organisms is dealt with primarily by the lymphocyte-macrophage system. We have recently completed studies on aged mice infected with Staphylococcus aureus, an organism dealt with primarily by polymorphonuclear leukocytes rather than the lymphocyte-macrophage system. Surprisingly, staphylococcal infections were much worse in older animals. These studies suggest that in older mice there may also be a defect either in polymorphonuclear leukocyte mobilization and/or function or in the ability of older mice to resist the lethal effects of staphylococcal toxins.