Thymic Humoral Factors

Abstract

Noncellular factors derived from thymic tissue have often been shown to influence reactivity of the lymphoid system and have been considered in attempts to understand the role of the thymus and the mechanism of its action. In the early years of this century, it had already been suggested that epithelial elements of the thymic medulla attract wandering lymphoid cells and stimulate their proliferation within the thymus (Maximow, 1909, 1912). A hormonal mechanism for this lymphoepithelial symbiosis was suggested by Grégoire, who, during 25 years devoted to thymic research, studied the regeneration of irradiated thymus and thymic tissue separated from outside lymphocytes by artificial barriers, the interaction of the thymus with the thyroid, adrenals, and gonads, and the hyperplastic changes induced in regional lymph nodes of mice injected with extracts of epithelial tissue from irradiated thymus (Grégoire, 1935, 1942, 1943, 1945, 1958; Grégoire and Duchateau, 1956; Grégoire and Grégoire, 1934). In the early 1960s, when experiments disclosed that the thymus contributes to the normal development of the lymphoid system (Miller, 1961; Good et al., 1962), among the hypotheses suggested for thymic function were (1) that the thymus serves as a source of competent cells which seed the peripheral lymphoid organs, (2) that the thymus provides a suitable environment for the maturation of lymphoid cells originating elsewhere, and (3) that thymic activity is mediated by a humoral factor. With the perspective of 10 more years of accumulated experience, we can today admit that the different hypotheses put forward then were not mutually exclusive and that each may contribute to current notions of thymic function.