Thymostimulin effects on lymphoid organs in Ames dwarf mice

This work was undertaken to study the effects of thymostimulin (TP-1) on the immune function in Ames dwarf mice, and to relate these effects to PRL and/or GH deficiency in these animals. Male Ames dwarf mice implanted with pituitaries from normal mice under the kidney capsule, sham-operated dwarf mice and normal immature or adult mice were injected daily for five days with TP-1. In comparison to normal animals, sham-operated dwarf mice had markedly lower body, thymus and spleen weights, as well as a lower number of lymphocytes in the spleen and in the thymus and the natural killer (NK) activity of spleen lymphocytes. Ectopic pituitary transplants produced the expected enhancement of body weight gain and increased spleen and thymus weights, which reached the values found in normal (non-dwarf) animals. The numbers of lymphocytes in the spleen and thymus were significantly increased in pituitary-grafted dwarf mice, but the grafts did not modify the cytotoxic activity of NK spleen cells, or the number of peripheral white blood cells (PWBC). In sham-operated dwarf mice, TP-1 treatment did not modify the number of cells in the spleen and thymus, or the NK activity. In pituitary-grafted dwarf mice, treatment with TP-1 induced an increase in the number of spleen lymphocytes and in the NK activity of spleen cells without affecting the weight of lymphoid organs or the number of thymic cells. Plasma prolactin (PRL) and growth hormone (GH) levels of pituitary-grafted dwarf mice were not changed after TP-1 administration. Surprisingly, the NK activity of spleen lymphocytes in normal adult mice was greatly increased after TP-1 administration. These findings suggest that the thymic extract TP-1 can exert a major stimulatory influence on NK activity of spleen lymphocytes in adult mice, and potentiate some of the stimulatory effects of hormones secreted by ectopic pituitary transplants on the immune function of Ames dwarf mice. These effects are not mediated by modifications of the release of PRL or GH.