Influence of Thymosin on Cell-Mediated and Humoral Immune Responses in Normal and in Immunologically Deficient Mice

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Abstract
Thymosin, a cell-free, lymphocytopoietic factor prepared from calf thymus, was examined for its influence on cell-mediated and humoral immune responses in normal and immunologically deficient mice. Thymosin was administered to the following groups of mice: a) normal adult; b) neonatally thymectomized; c) adult, lethally irradiated; and d) adult, thymectomized, lethally irradiated animals given syngeneic bone marrow cells. Immune responses of the mice in each group were assessed on the basis of their response to a skin allograft and their capacity to form humoral antibody. Thymosin restored the skin allograft response in neonatally thymectomized mice, thus supporting our conclusion that this thymic function may be endocrine in nature. The capacity to form 19S antibody in neonatally thymectomized or adult thymectomized, lethally irradiated mice was not markedly altered by thymosin treatment, suggesting the lack of a primary function for thymosin in these experimental conditions. Thymosin had no stimulatory effect on the hemolysin response to sheep erythrocytes in normal and x-irradiated mice when given after the initial antigenic challenge. When thymosin was administered prior to antigenic challenge, there was a reduction in hemolysin titers. A hypothesis is presented suggesting that control by the thymus of cell-mediated immunity resides primarily in an endocrine function of the gland, with maturation of peripheral stem cells occurring under the influence of thymosin. In contrast, maturation of cells involved in humoral immune responses may require an in situ thymic locus under the influence of one or more humoral factors and/or interaction with other stem cells, and may be initiated and essentially completed prior to birth.

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