Mechanism of the antineoplastic action of lysozyme: evidence for host mediated effects.

The effects of samples of whole plasma obtained from mice orally treated with hen egg-white lysozyme on lung metastasis development were studied in syngeneic CBA animals bearing MCa mammary carcinoma. 50 microliters of whole plasma obtained from mice treated with lysozyme 100 mg/kg/day for 7 consecutive days causes a pronounced and statistically significant reduction of spontaneous lung metastases formed from i.m. implants of MCa mammary carcinoma cells; the use of whole plasma samples prepared from tumor bearing mice is equally effective. The use of 25 microliters of whole plasma or the fractionation of 50 microliters into two injections of 25 microliters is much less effective. No appreciable cytotoxicity for tumor cells was measured by in vivo bioassay of tumor cells kept in vitro with the plasma samples employed. These data indicate that the antitumor activity exhibited by orally administered lysozyme in the treated mice can be totally transferred to other syngeneic hosts through a sample of whole plasma. Furthermore, these results show the intervention of host responses in the antitumor activity of orally administered lysozyme. This conclusion supports the hypothesis put forward that the antitumor activity of oral lysozyme is mediated by the elicitation of host "reactivities", different from lysozyme itself, towards tumor cells, and gives credit to data showing the long lasting antimetastatic effects in mice treated with oral lysozyme 2 weeks before tumor implantation.