Abscopal-like antitumor effect induced by localized low-temperature plasma application of normal tissue in mice

We have previously reported that the localized application of gaseous plasma to normal tissues suppresses distant tumor growth in mice, resembling the abscopal effect of radiotherapy. Plasma, a partially ionized gas generated by a high-voltage electrical discharge, is fundamentally distinct from ionizing radiation and produces diverse reactive oxygen and nitrogen species that interact with biological tissues. This study examined the abscopal-like effects of normal tissue plasma treatment in BALB/c mice with subcutaneous Colon 26 tumors. The left dorsal skin, 2-3 cm from the tumor, was exposed to plasma for 10 min per day for 5 consecutive days, which delayed the growth of distant tumors. Similar tumor suppression was observed with abdominal exposure, indicating that the effect was not site-specific. In C.B-17 SCID mice (lacking T and B cells) and BALB/c nu/nu mice (lacking T cells), dorsal treatment did not suppress tumor growth, suggesting that T cells are likely involved in the response. Flow cytometric analysis of tumor-infiltrating immune cells in BALB/c mice revealed significant reductions in macrophages and increases in monocytes, with a possible but nonsignificant increase in dendritic cells. No consistent changes were detected in CD8+ T-cell proportion or ICOS (inducible T-cell costimulatory) expression. However, the lack of antitumor effects in immunodeficient mice suggests that CD8+ T cells are involved.