Immunity,Age and Loss of Immunohomeostasis1

An interpretative discussion on the cellular basis of the association between immunosenescence and loss of immunologic homeostasis was developed from results of studies performed to resolve the issues (a) of whether cells in certain stages of differentiation are more vulnerable to aging than those in other stages and (b) whether the regulatory cells participating in modulating immune responses are prime targets of aging. Young and old mice were exposed to (1) 6-thioguanine (6-TG) to study the activity of their mitotically active and inactive cells and (2) 500R of irradiation to study regeneration of their immune cells. Many alterations were detected in old mice including the existence of a relatively large pool of mitotically inactive pluripotent stem cells in the bone marrow, heightened stem cell regenerative activity, and altered patterns of immunologic regeneration. These kinetic abnormalities reflect age-related changes in stem cells and their progenies, and in the lymphohematopoietic stromal cells.