Characterization of Peripheral T-Cell Dynamics in the Patients with Bone Metastasis from Breast Cancer Following Stereotactic Body Radiotherapy.

PURPOSE/OBJECTIVE(S): Preclinical studies suggest that radiotherapy (RT) elicits various effects on antitumor T-cell responses. However, systemic T-cell responses upon RT in real-world cancer patients, including those with metastatic breast cancer, are poorly characterized. This study aims to investigate the detailed dynamics of peripheral T cells upon stereotactic body radiotherapy (SBRT) in metastatic breast cancer patients.

MATERIALS/METHODS: Peripheral blood samples of 32 prospectively recruited patients who received SBRT to bone metastasis of breast cancer, which were acquired at pre-SBRT (W0), 1 week after SBRT (W1), and 4 weeks after SBRT (W4), were analyzed using multi-color flow cytometry and cytometric bead array. Most patients (n = 30) received SBRT of 1 fraction (n = 16) or 3 fractions (n = 14). We also performed a subgroup analysis of 22 patients who did not start a new systemic therapy within 1 month before SBRT (new systemic Tx >1mo) to exclude the effects of systemic therapies.

RESULTS: Peripheral PD-1+ CD8+ T cells, which are enriched for tumor-specific clonotypes, were activated with increased expression of Ki-67 at W1 compared to W0. Expression of Ki-67 on PD-1+ CD8+ T cells remained higher at W4 than W0, although it was not statistically significant. Moreover, expressions of Ki-67 and CTLA-4 on circulating regulatory T (TREG ) cells were increased at W1 compared to W0. The suppressive (Foxp3hi CD45RA- ) TREG cells also exhibited enhanced expressions of Ki-67 and CTLA-4 at W1. We defined immunologic responders (ImmRs) as patients with Ki-67 expression on PD-1+ CD8+ T cells at W1 greater than 1.5-times of W0, and otherwise immunologic non-responders (ImmNRs). Notably, fold changes in expressions of Ki-67 and CTLA-4 on TREG cells and proportion of suppressive TREG cells among total TREG cells at W1 over W0 were higher in ImmRs than ImmNRs. Similar phenotypical changes of T cells were observed in the subgroup analysis of the patients with new systemic Tx >1mo. The peripheral T-cell changes were not significantly different between the dose-fractionation schedules (1 fraction vs. 3 fractions) nor molecular subtypes of cancer. In patients with new systemic Tx >1mo, plasma level changes of TGF-β1, sCTLA-4, and s4-1BB at W1 compared to W0 were significantly higher in ImmRs compared to ImmNRs, while only change of s4-1BB was associated with the immunologic response.

CONCLUSION: Our results suggest that circulating PD-1+ CD8+ T cells are activated upon SBRT. However, SBRT also results in the activation of circulating TREG cells, which was more prominent in patients with a significant activation of circulating PD-1+ CD8+ T cells. Alterations in plasma levels of TGF-β1, sCTLA-4, and s4-1BB are associated with the peripheral T-cell responses. Dose-fractionation schedule is not associated with the peripheral T-cell responses.