Prospective Immunophenotyping of CD8+ T Cells and Associated Clinical Outcomes of Patients With Oligometastatic Prostate Cancer Treated With Metastasis-Directed SBRT

Purpose This study examined the effects of metastasis-directed stereotactic body radiation therapy (mdSBRT) on CD8 + T-cell subpopulations and correlated post-mdSBRT immunophenotypic responses with clinical outcomes in patients with oligometastatic prostate cancer (OPCa). Methods and Materials Peripheral blood mononuclear cells were prospectively isolated from 37 patients with OPCa (≤3 metastases) who were treated with mdSBRT. Immunophenotyping identified circulating CD8 + T-cell subpopulations, including tumor-reactive (T TR ), effector memory, central memory (T CM ), effector, and naive T cells from samples collected before and after mdSBRT. Univariate Cox proportional hazards regression was used to assess whether changes in these T-cell subpopulations were potential risk factors for death and/or progression. The Kaplan-Meier method was used for survival. Cumulative incidence for progression and new distant metastasis weas estimated, considering death as a competing risk. Results Median follow-up was 39 months (interquartile range, 34-43). Overall survival at 3 years was 78.2%. Cumulative incidence for local progression and new distant metastasis at 3 years was 16.5% and 67.6%, respectively. Between baseline and day 14 after mdSBRT, an increase in the T CM cell subpopulation was associated with the risk of death (hazard ratio, 1.22 [95% confidence interval, 1.02-1.47]; P  = .033), and an increase in the T TR cell subpopulation was protective against the risk of local progression (hazard ratio, 0.80 [95% confidence interval, 0.65-0.98]; P  = .032). Conclusions An increase in the T TR cell subpopulation was protective against the risk of disease progression, and an increase in the T CM cell subpopulation was associated with the risk of death in patients with OPCa treated with mdSBRT. Disease control may be further improved by better understanding the CD8 + T-cell subpopulations and by enhancing their antitumor effect.