Dose-related effects of oestradiol on rat thymic and splenic T-lymphocyte responsiveness to mitogens

The dose-related effects of oestradiol on responses of thymic and splenic lymphocytes to mitogenic stimulation were studied in immature female Wistar rats. An attempt was made to relate responses not merely to dose but also to the circulating levels of the steroid. Lymphocytes were prepared from thymus and spleen and stimulated with phytohaemagglutinin (PHA) and concanavalin A (Con A) prior to measurement of 3H-thymidine incorporation. Oestradiol suppressed lymphoid tissue weight and cell number in a dose-related manner, but it was found, unexpectedly, that the dose of oestradiol used actually stimulated responsiveness of lymphocytes to mitogenic stimulation. Log dose-response curves indicate that the effects of oestradiol on responsiveness are complex, and the results suggest that the atrophic effects of oestradiol on lymphoid tissue and its enhancement of response to mitogens might be mediated by different mechanisms. Since the stimulant effects of the steroid were observed with serum levels of oestradiol attained both physiologically and pharmacologically, the results suggest a possible mechanism of action of oestradiol in abnormal cell proliferation, as occurs in neoplastic tissues, and might also help to explain certain sex-linked immune-related disorders.