Modification of the effect of C. parvum on macrophage activity and tumour growth by X-irradiation.

The radiosensitivity of three forms of response to injection of C. parvum in mice has been investigated. The increase in phagocytin index evoked by IP or IV injection of 0.7 mg C. parvum (but not that evoked by 1.4 mg) was reduced but not abolished in mice given 350-500 rad whole-body irradiation 4 days before C. parvum injection. Irradiation (500-1,000 rad) 4 days after C. parvum injection had no such effect. The antitumour cytotoxicity in vitro of PE and spleen cells from C. parvum-treated mice was abolished by irradiation of the cell donor (400-800 rad) 4 days before C. parvum injection, but was not reduced by irradiation (800 rad) of the cell donor 4 days after C. parvum injection or of the effector cells in vitro. The antitumour response to systemic (IP) injection of C. parvum was reduced by 350-500 rad whole-body irradiation, irrespective of whether this was given 4 days before or 4 days after C. parvum injection. The response to intratumour injection of C. parvum was even more radiosensitive. It has been suggested in previous papers from this laboratory that the antitumour effect of C. parvum is related to its capacity to stimulate macrophage activity, although in addition T lymphocytes are necessary for local injection to be effective and non-T lymphocytes may be concerned in the response to both local and systemic injection. The present results in no way conflict with this view. They suggest in addition that the effect of C. parvum on the macrophage system is, to a considerable extent, due to stimulation of macrophage precursors to differentiate into actively phagocytic and cytotoxic mature cells.