Abstract
The health of old monkeys usually begins to deteriorate by 20 years of age, coinciding with the onset of a slowly progressing immune senescence. Changes in lymphocyte subsets and responses to several antigens have been characterized in geriatric primates, but systematic research has not been conducted on vaccination against influenza virus, a topic of considerable clinical concern for elderly humans. Antibody responses were significantly reduced to primary immunizations in old monkeys, but by administering a second vaccine at 1 month, it was possible to boost antibody titers up to the level found in young adults during their primary phase. The immune competence of unusually long-lived animals (26-37 years) was also compared to more typical aged monkeys (19-25 years). Antibody responses were low overall, although some monkeys in both age groups did respond to immunization. Among the oldest animals, the leukocytes of the responders with higher antibody titers were found to release more interleukin-2 following in vitro stimulation with an anti-CD3/anti-CD28 cocktail relative to their cellular reactions to staphylococcal enterotoxin B. The general decline in immune vigor, and the marked individual variation in how old monkeys age, provides a useful animal model for investigating factors associated with immunosenescence.