The mixed lymphocyte response of senescent mice: sensitivity to alloantigen and cell replication time

Immunostimulatory activity of prothymosin-alpha in senescence

In this study we evaluated the immunorestorative activity of prothymosin-alpha (ProT-alpha) in senescence. Aged rats were repeatedly injected with ProT-alpha after antigen challenge. Both humoral and cell-mediated immune reactions were tested. The results show that ProT-alpha exerted a marked immunopotentiating effect in aged rats. Moreover, ProT-alpha induced enhancement of PFC response, and skin hypersensitivity reactions were more pronounced in senescent rats than in young adult controls. ProT-alpha treatment increased thymus and spleen weights in aged rats, and induced qualitative and quantitative improvements of the cellular make-up of the thymus and thymus-dependent areas of the spleen. The results imply that "restoring" thymus endocrine function by exogenous administration of ProT-alpha may improve immune system homeostasis in senescence.

Age-related decline in the lethal hit but not the binding stage of cytotoxic T-cell activity in mice

The activity of allogeneic cytotoxic T lymphocytes (CTL) declines with aging, as measured at the population level in chromium release assays, and is associated with a decreased proliferative response to the inducing alloantigens. At the single cell level, no difference was noted between target binding cells obtained from alloantigen-stimulated spleen cells of young and old mice. However, a consistent, large, and significant difference was observed between CTL generated from the spleens of young and old mice in the percentage of bound effector cells with lysed targets, i.e., the percentage of killer cells. These results show age-related changes in the lethal hit stage and not the binding stage of CTL activity. This change could be used as a probe to identify the important mechanism(s) which mediates CTL activity.

Age-related increases in mitogenic responses and natural immunity to a syngeneic fibrosarcoma in rats

An increase in natural (innate) immunity with age (through 65 weeks) of NBR rats to a syngeneic methylcholanthrene-induced fibrosarcoma (MCA) is described. This increase is characterized by the appearance of spontaneous tumor regressor animals and an increase in the incidence of tumor non-takes by rats 45 weeks of age and older when compared to younger animals. Age-related changes were observed in the in vitro proliferative responses of normal unfractionated and nylon wool fractionated spleen cells to Con A, PHA and irradiated MCA tumor cells. Certain of these different proliferative responses either increased and/or decreases independently at different ages. Relative to normal spleen cell proliferative responses, tumor progressors have decreased proliferative responses and tumor regressors display increased proliferative responses. The significance of this in light of current aging research is discussed.

The thymus: clock for immunologic aging?

A brief overview of the effects of age on T cell function is presented. Normal immune functions can begin to decline shortly after an individual reaches sexual maturity. Foremost among the cellular changes are those in the stem cells as reflected in their growth properties and the availability of precursor T cells, and in the T cells, in which a shift in subpopulations may be occurring. Present evidence indicates that thymic involution precedes, and therefore may be responsible for, the age-dependent decline in the ability of the immune system to generate functional T cells. It now appears that the primary effect of thymic involution is on a T cell differentiation pathway; the more mature T cells are affected first, the less mature T cells only later. Thus, the thymus may be the aging clock for the immune system. Current studies are centered on processes regulating growth and atrophy of the thymus, and methods for restoring the impaired immune function of elderly individuals.

T-cell division and aging

The age-dependent drop in mixed lymphocyte reactivity and responsiveness to concanavalin A of lymph node and spleen cells of C57Bl/6J female mice were studied. The relative decrease in mixed lymphocyte reactivity was shown to be the same whether mounted against H-2 or Mls incompatibile stimulator cells. The time of peak response in vitro as well as the sensitivity to stimulator cell concentration are not altered with age. Cell cycle studies demonstrate that those cells which respond in vitro to alloantigens or to concanavalin A do so with a cell cycle which does not change with the age of the lymphocyte donor. In addition, regardless of the age of the donor, those cells which divide in vitro demonstrate identical capacities to redivide. These experiments suggest that the decline in observed T-cell proliferation in mixed lymphocyte and mitogen reactivity of senescent mice is not to a decreased cell generation time or to a reduced capacity to divide and redivide but rather to a smaller population of reactive cells.

An overview of immune aging

A brief overview of the effect of age on the function of cells of the immune system is presented. Normal immune functions can begin to decline shortly after an individual reaches sexual maturity. Foremost among the cellular changes are those in the stem cells as reflected in their growth properties and the availability of precursor T cells, and in the T cell where a shift in subpopulations may be occurring. Present evidence indicates that thymic involution precedes and therefore may be responsible for the age dependent decline in the ability of the immune system to generate functional T cells. It now appears that the primary effect of thymic involution is on a T cell differentiation pathway affecting the more mature T cells first and only later the less mature T cells. Thus, the thymus may be the aging clock for the immune system. Future studies should be centered around processes regulating growth and atrophy of the thymus.