Mechanism of action of C. parvum on a solid, subcutaneous mouse tumour.

The effects of i.v. C. parvum on the growth of a.s.c. inoculum of the Lewis lung carcinoma were studied in normal C57 B1 mice, and in those in which separate components of the immune response were impaired. C. parvum given either at the same time as tumour inoculation or when the carcinoma was 1 cm in diameter, and fully vascularised, reduced tumour growth. Macrophages were impaired by silica (Si), cortisone acetate (CA) or trypan blue (TB), and T cells by thymectomy and sublethal irradiation (TXR) or antilymphocyte serum (ALS). TB did not affect tumour size, whereas Si, CA, TXR, or ALS (initiated before tumour inoculation) reduced it. When C. parvum was given in combination with TB, or ALS (initiated after tumour inoculation), its antitumour effects were unchanged. C. parvum given after ALS (before tumour), TXR, Si, or CA had no further effect on tumour growth. We have shown that C. parvum inhibits the growth of inoculated Lewis tumour. However, it has not been possible to clearly define in vivo the immunological mechanisms involved.