Impact of Radiotherapy on Circulating Regulatory T-Cell Levels in Melanoma Patients

Background: Emerging data indicate that radiotherapy can modulate antitumour immune responses. Regulatory T cells (Tregs) are a key immunosuppressive subpopulation of interest. We aimed to evaluate the impact of conventional fractionated radiotherapy on circulating Treg levels in melanoma patients and to determine the correlation between these changes and survival. Understanding radiotherapy–immunity interactions may facilitate personalized combination therapies. Methods: Peripheral blood samples from 32 melanoma patients were analysed to quantify Tregs before and 1-3 months after regional lymph node radiotherapy using flow cytometry. Tregs were characterized as CD4+/CD25+/CD127-. Total CD4+ T cells were also evaluated. Changes were compared to those in a control group of 30 tumor-free patients. Overall survival was correlated with T-cell subset dynamics induced by radiotherapy. Results: At baseline, the median percentage of Tregs was not significantly lower in melanoma patients than in controls (7.4% vs 8.32%, p=0.17). Radiotherapy markedly increased the median Treg count to 10.01% (p=0.0008), with a concomitant increase in total CD4+ cells (to 11.71%, p=0.001), indicating systemic expansion. Patients exhibiting coordinated increases in Treg and CD4+ T cells after treatment had a significantly improved prognosis compared with nonresponders (median OS 27.8 vs 13.1 months, p=0.001). Conclusions: Our study demonstrated the significant effects of standard fractionated radiotherapy on systemic Treg levels in melanoma patients. Radiation-induced changes in T-cell subset proportions hold prognostic value. Monitoring of circulating lymphocyte subsets could facilitate personalized combination therapies.