Modulation of Thymic Endocrine Function by Thyroid and Steroid Hormones

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Abstract
The thymic epithelium is responsible for the secretion of thymic hormones that intervene in some steps of intra- and extra-thymic T cell differentiation. In the present paper, we studied the in vivo and in vitro influences of thyroid and steroid hormones on the secretion of thymulin, one of the chemically-defined thymic hormones. Triodothyronine injected in young mice or applied into supernatants of cultured thymic epithelial cells resulted in an increase of thymulin synthesis and secretion, respectively evaluated by the analyses of thymulin containing cells and thymulin levels. The influence of steroids was investigated in vivo by steroid depletion (adrenalectomy and/or castration) and in vitro by the addition of various steroids and/or their specific antagonists to cultured thymic epithelial cells. Surgical ablation of adrenals and/or gonads induced a transient depletion of circulating thymulin and, by feedback mechanism, an increase in the numbers of thymulin containing cells. From the in vitro data we could conclude that the various steroid hormones (that exhibited a stimulatory effect) can act directly on the thymic epithelium, probably via specific receptors. The bulk of data above described represents a strong evidence for the physiological involvement of the neuroendocrine network, on the hormonal function of the thymic epithelium.