Anti-neoplastic and immunostimulatory effects of low-dose X-ray fractions in mice.

Purpose: The exploration of immune mechanisms of the tumour-inhibitory effect of exposures to low-level fractions of X-rays.

Materials And Methods: BALB/c mice were exposed to whole-body daily irradiations with 0.01, 0.02, or 0.1 Gy X-rays per day for 5 days/week for two weeks. Then, mice were intravenously injected with L1 tumour cells, killed 14 days later, and neoplastic colonies were counted in the lungs. Natural killer (NK) cell-enriched splenocytes and activated peritoneal macrophages (Mϕ) were collected and cytotoxic activities of these cells against susceptible tumour targets were assayed. Concanamycin A (CMA) and antibody against the ligand for the Fas receptor (FasL) were used to inhibit the NK cell-mediated cytotoxicity. Production of nitric oxide (NO) was quantified using the Griess reagent. Secretion of interferon-γ (IFN-γ), interleukin-1β (IL-1β), interleukin-12 (IL-12), and tumour necrosis factor-α (TNF-α) was measured using the enzyme-linked immunosorbent assays.

Results: All the exposures to X-rays significantly reduced the number of the induced tumour colonies and enhanced cytotoxic properties of the NK cell-enriched splenocytes and activated Mϕ.

Conclusion: Suppression of the growth of pulmonary tumour colonies by irradiations of mice with low-dose fractions of X-rays may result from stimulation of anti-tumour reactions mediated by NK cells and/or cytotoxic macrophages.