Aging and the immune response. I. Antibody formation and chronic infection in Toxoplasma gondii-infected mice.

Studies were performed to determine the effect of aging on the antibody response and cyst formations after infection with a relatively avirulent strain of the intracellular parasite Toxoplasma gondii. When compared with young mice (4 months), aged mice showed a significant decrease in the magnitude of humoral immune response to infection. This decrease was observed at the peak of the acute infection and also during chronic infection. Evaluation of the presence of Toxoplasma cysts, a measure of the latent infection, revealed that the numbers of tissue cysts present 11 weeks after infection increased with the age of the mice at time of infection. The larger numbers of cysts in older mice which had received the same inoculum size of T. gondii as young mice, together with our previous observations of increased susceptibility of these older mice to T. gondii, suggest that an age-related decrease in the early immune response to this infection allows an increased multiplication of the organism in vivo, leading to increased cyst numbers or death.