Thymosin, a product of the endocrine system, was used to further define the effects of
immunomodulation of
metastasis. Adult thymectomized C57BL/6 mice, 4 wk post-irradiation (400 R) had a decrease in the number of pulmonary
metastasis (compared to controls) following tail vein injection of 5 X 10(4)
B16 melanoma cells.
Thymosin fraction 5 (fr. 5) administration (200 micrograms/mouse, 3 times weekly beginning 2 days post-
thymectomy) returned the number of
metastasis to the nonthymectomized values.
Thymosin treatment of
sham-operated,
sham-operated irradiated, or thymectomized nonirradiated mice did not significantly elevate the number of
metastases compared to the respective controls. Variant
tumors which have an increase in
metastasis following
thymectomy and irradiation were also used.
Thymosin administration reversed the effects of
thymectomy in such variants, resulting in a decrease in
metastasis.
Metastases in
thymosin-treated control mice were not significantly altered. A role for the thymus in
metastasis via an endocrine product (
thymosin) is suggested by these studies. Since
thymosin did not increase
metastasis in intact mice with
tumors, further clinical trials with
thymosin in
cancer patients are not counterindicated by our results. These experiments confirm that
thymosin fr. 5 is an important probe of the immunoendocrine events involved in
tumor growth and
metastasis.