Effect of aging on antimicrobial immunity: old mice display a normal capacity for generating protective T cells and immunologic memory in response to infection with Listeria monocytogenes.

Old (19 to 30 mo) and young adult (11 to 16 wk) AB6F1 mice of both sexes were compared in terms of their capacity to resist infection with Listeria monocytogenes. The LD50 was found to be two to four times higher for old than for young mice, and the time to death was longer for old mice. Enumeration of bacteria in the livers and spleens showed that old mice restricted growth of Listeria more effectively than young mice during the preimmune phase of infection, the difference being detectable as early as 12 to 24 hr after bacterial inoculation. Therefore, to ensure a similar level of infection in old and young mice, old mice had to be given a larger inoculum. Indeed, it was found that, provided the size of the bacterial inoculum was adjusted to make the level of immunizing infection the same, old mice generated similar levels of anti-Listeria immunity as young mice, as measured by their ability to generate splenic T cells capable of adoptively immunizing young recipients against lethal challenge infection. Furthermore, the level of memory immunity to reinfection 28 to 117 days after immunizing infection was similar in old and young mice. The results indicate, therefore, that old mice have no defect in their capacity to generate T cell-mediated anti-Listeria immunity.