Manganese promotes anti-tumor effect of radiotherapy through reshaping the immune microenvironment

Abstract

Objective To explore the sensitizing effect of manganese for radiotherapy against tumors and its possible mechanisms. Methods A total of 300 male C57BL/6 mice (6~8 weeks old, weighing 20~23 g) with subcutaneous tumor were randomly divided into 4 groups: control group, radiotherapy group, manganese treatment group, and combined radiotherapy and manganese treatment group. Nasal drip with 10 μg manganese adjuvant was applied to the mice from the latter 2 groups on day 9 of tumor bearing, then single dose of 20 Gy radiation was locally administered on day 10. Tumor growth and mouse survival were monitored regularly. The sensitizing effect of manganese on radiotherapy was determined by monitoring and comparing the tumor growth among different unilateral mouse models. Bilateral tumor-bearing model was used to examine the effect of manganese on abscopal effects induced by radiotherapy. Flow cytometry was used to illustrate the changes in tumor-infiltrating immune cells in unilateral tumor bearing model. Immunohistochemical staining was employed to evaluate the spleen function in unilateral tumor bearing mice. Results Based on repeated validation of 3 different unilateral tumor-bearing models, radiotherapy combined with manganese therapy significantly inhibited tumor growth and prolonged survival of mice (P < 0.05). The results of bilateral tumor-bearing model showed that manganese therapy enhanced abscopal effects of radiotherapy, and significant regression was observed in both side of tumor under radiotherapy or not (P < 0.05). Flow cytometry revealed that manganese further increased radiation-induced CD8+T infiltration (P < 0.05) and decreased radiation-induced infiltration of Treg cells and myeloid-derived suppressor cells (MDSCs) (P < 0.05). Furthermore, manganese increased lymphocyte reserve pool of the spleen and improved its function. Conclusion Manganese adjuvant could act as a sensitizing agent for radiotherapy, by improving the function of spleen and reprogramming the tumor microenvironment synergistically.

Keywords

• radiotherapy
• manganese adjuvant
• abscopal effect
• radiation sensitizer