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Enhanced resistance to Listeria monocytogenes due to non-specifically activated macrophages in aged mice.

Age-related modification of protection to L. monocytogenes by macrophages was examined. Cumulative mortality rates of old (15-month-old) or very old (24 to 26-month-old) mice were lower than that of young (3-month-old) mice after an intravenous inoculation of L. monocytogenes. The numbers of bacteria in the livers and spleens of old or very old mice were smaller than that of young mice at an early stage of infection (1 day or 3 days). Adversely, the numbers of bacteria in old or very old mice were larger than in young mice at a late stage (7 days). Enhanced resistance to L. monocytogenes was shown not only in the case of systemic infection after an intravenous bacterial challenge, but also in the case of local infection in the thigh muscle or subcutaneous air cavity. The number of macrophages accumulating to the infected sites in old mice was not larger over a five day period than that in young mice in the case of local infection in a subcutaneous air cavity. Peritoneal macrophages derived from old mice suppressed more effectively the bacterial growth within macrophages in vitro than did those derived from young mice. These results suggest that the macrophage function of aged mice is not impaired and non-specifically activated macrophages contribute to the protection to L. monocytogenes in aged mice.