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

## TL;DR

Treating normal tissue with low-temperature plasma in mice can slow distant tumor growth, possibly through immune mechanisms involving T cells.

## Contribution

This study demonstrates that plasma treatment of normal tissue can induce abscopal-like antitumor effects, mediated by T cells.

## Key findings

- Plasma treatment of normal tissue delayed tumor growth in mice.
- The effect was not site-specific and was absent in immunodeficient mice.
- Tumor immune cell composition changed, with reduced macrophages and increased monocytes.

## Abstract

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.