Neonatal inoculation with the protein-bound polysaccharide PSK increases resistance of adult animals to challenge with syngeneic tumor cells and reduces azoxymethane-induced precancerous lesions in the colon.

We have investigated the results of neonatal inoculation with a protein-bound polysaccharide, PSK, as it affects the defense mechanism of animals against cancer. Male BALB/c mice received a single i.p. injection of 10 mg/kg PSK within 48 h of birth. When the mice were 8 weeks of age, colon adenocarcinoma 26 (C26 tumor) cells were transplanted s.c. Injection of PSK increased the number of tumor-rejecting mice from 10 to 50% compared with the control mice transplanted with 5 x 10(3) tumor cells and prolonged the median survival period to 174% of control mice with tumors. When the number of transplanted tumor cells was increased to 1 x 10(6), PSK injection significantly prolonged the survival period, although tumors grew in all mice. The survival period was also significantly prolonged in male C57BL/6 mice that received an injection neonatally with PSK and were given a s.c. transplant of Lewis lung carcinoma or B16 melanoma at 8 weeks of age. The effect on survival was dependent on the PSK dose and the number of transplanted tumor cells. PSK was as effective for male mice 30 weeks of age as for mice 8 weeks of age treated with PSK during the neonatal period. However, prolongation of the survival period of tumor-bearing mice was not observed in the offspring (F1). Neonatal injection of PSK also significantly reduced the number of metastatic foci in the liver of mice inoculated with 1 x 10(5) C26 tumor cells in the splenic vein after 8 weeks of age. In addition, neonatal injection of PSK significantly reduced the number of aberrant crypts and aberrant crypt foci, the precancerous lesions in the colon of F344 rats that received injections s.c. with azoxymethane after 7 weeks of age, to 47% of that of rats that received an injection with saline at the same age. The effect on precancerous lesions was dependent on the timing of PSK injection and the dose. Regarding the mechanism, when animals thymectomized during the neonatal period or when congenitally athymic animals were used instead of healthy animals, the effect on survival or precancerous lesions did not appear. Neonatal injection of PSK significantly reduced the number of CD4+ CD8+ T cells and significantly increased the number of CD4+ CD8- and CD4- CD8+ T cells in the thymus of healthy mice 10 weeks of age and C26 tumor-bearing mice. Furthermore, neonatal injection of PSK significantly elevated the T-cell differentiation induced by a mouse thymus extract 10 weeks of age. These findings suggest that neonatal injection of PSK induces resistance in adult mice to challenge by syngeneic tumor cells and reduces the azoxymethane-induced precancerous lesions in the colon of adult rats via the thymus functions.