On the basis that multiple interactions exist between thyroid hormones and immune system, and ageing is accompanied by changes in thyroid hormone secretion, it seems possible that thyroid hormones may be involved in the age-related immune dysfunction. The present study was conducted to evaluate in vivo and in vitro effects of thyroxine (T(4)) treatment on both cell-mediated and humoral immune responses of aged mice. In a trial to improve age-associated immune dysfunction, T(4) (0.2, 1.0 and 5.0 microg) was subcutaneously supplemented to BALB/c mice (over 18 months old) for 30 consecutive days. The present results showed that exogenous treatment of aged mice with T(4) was associated with a marked increase in serum T(4) level, and the total number of peripheral blood leukocytes as well as the total cellularity of thymus, spleen, peripheral lymph nodes (PLNs), mesenteric lymph nodes (MLNs) and bone marrow (BM). T(4) treatment also caused a significant increase in the total and differential numbers of peritoneal exudate cells (PECs), while it caused a slight increase in macrophages' phagocytic activity of PEC. Moreover, T(4) treatment elicited a statistically significant increase in both plaque-forming cell and rosette-forming cell responses. In vitro results showed that the addition of T(4) at concentrations of 0.001, 0.005 and 0.025 microg/well substantially potentiated the ability of splenocytes from aged mice to proliferate in the presence of concanavalin-A mitogen. Histological examination of thymuses from T(4)-treated aged mice revealed that the cortex was preferentially enlarged and repopulated with immature thymocytes. The present study postulates that thyroid hormones may be involved in the observed decrease in the immune responsiveness during ageing, and that T(4) treatment to aged mice is able to restore the age-related decline of the immune efficiency.

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