Low-dose total body irradiation augments the therapeutic effect of interleukin-2 in a mouse model for metastatic malignant melanoma.

BACKGROUND: Low-dose total body irradiation (LTBI) is known for its antitumor immune modulatory effects. Moreover, there is theoretical ground suggesting that combining LTBI with interleukin-2 (IL-2) will have a synergistic immune-mediated antitumor effect. However, the use of LTBI in combination with IL-2 or other forms of immunotherapy has not been tested before. AIM OF THE WORK: To test the efficacy of combining LTBI and IL-2 in controlling lung metastases in a murine model for malignant melanoma compared to IL-2 alone.
MATERIAL AND METHODS: Ten-week-old female C57BL/6 mice were inoculated intravenously (on day 0) with 1 million B16F1 malignant melanoma cells. The mice received either no treatment (control group), LTBI alone (single fraction of 0.75 Gy), IL-2 treatment alone (30,000 CU x 2 daily for 5 consecutive days), or a combination of LTBI and IL-2. LTBI was given on day +7 and IL-2 treatment started day +8. On day +14, the mice were sacrificed and the lungs were removed and analyzed for tumor burden. Lung sections were also tested for tumor-infiltrating cells using immunohistochemical staining. Peripheral blood and splenic cells were collected and tested for the percentage of the various lymphocytic subsets using immunostaining and flow cytometry.
RESULTS: Tumor burden expressed as the percentage of lung area occupied with metastases (+/- 1 SD), was the same in the control group (8.1 +/- 4.9%), and in the group receiving LTBI alone (8.3 +/- 4.5%). Tumor burden was reduced to 6.4% (+/- 3.4%) in the IL-2 alone group (P = 0.3) and further reduced to 3% (+/- 1%) in the combined treatment group (P = 0.004). The difference in tumor burden between the IL-2 alone group and the combined treatment group was statistically significant (P = 0.006). The combined treatment caused a significant increase in the number of natural killer (NK) cells and macrophages infiltrating the metastatic sites. This was associated with a significant increase in the percentage of CD122+ (IL-2R beta) cells and NK cells in both peripheral blood and spleens.
CONCLUSION: We conclude that combining LTBI and IL-2 treatment is synergistic and therapeutically more effective than IL-2 alone. The data points to NK cells and macrophages as likely major effectors of the synergistic outcome of the combined treatment. This observation may have important clinical implications in the treatment of patients with metastatic malignant melanoma.