Preimmune resistance to Toxoplasma gondii in aged and young adult mice

Aged individuals are more susceptible to certain infections than are young adults. To investigate the relative resistance capabilities of aged and young adult mice, responses that are induced within the first week of a Toxoplasma gondii infection, which are known to be involved in preimmune resistance, were compared in young adult and aged mice. Aged mice did not differ reproducibly from young adults in numbers of induced Thy-1+ CD4- CD8- cells or interferon-gamma levels. Numbers of induced CD4+ and CD8+ T cells, associated with acquired immunity, were as high in aged mice as in young adults. Natural killer cell activity, although induced to a high level, was lower in aged mice. Aged mice thus are capable of inducing a mechanism of preimmune resistance to T. gondii and presumably other infectious agents. Nonetheless, aged mice died within 8-12 days after intraperitoneal or peroral inoculation of 500 T. gondii cysts, whereas young adult mice survived. Causes other than an age-related impairment in preimmune resistance mechanisms are apparently responsible for the increased susceptibility of aged mice to T. gondii infection.