Dependency of Radiation Therapy and Combinatorial Radio-Immunotherapy Responses on the Systemic T Cell Immune Response

Combinatorial immune checkpoint blockade (ICB) with radiotherapy (RT) potentiates anti-tumor response via modulation of the immune microenvironment. However, detailed host-specific mechanisms underpinning dramatic clinical responses of RT-ICB are poorly understood. Here, we performed in depth characterization of the systemic immune response in circulating T cells following treatment with RT and RT-ICB. We recruited a cohort of 29 patients with biopsy-proven metastatic cancers (10 prostate, 10 EBV+ nasopharynx, 9 others) who underwent RT (N = 13) or RT-ICB (N = 16; anti-PD1/-PDL1/-CTLA4), under a prospective observational study protocol. All patients received ablative RT (8-50 Gy in 1-5 fractions). Patient blood samples were longitudinally collected at the following timepoints: baseline, 2 d, 7 d, and 14 d post-RT/RT-ICB. Circulating T cells were deeply profiled by using a customized CyTOF panel consisting of 41 phenotypic and functional marker molecules and high-performance dimensional reduction and clustering analysis (t-SNE and PhenoGraph). Median follow-up was 3.6 mo (0.5–8 mo). At time of reporting, 26 of 29 patients had evaluable lesions; response of any kind was observed in 9 of 12 cases in the RT cohort and 12 of 14 cases in the RT-ICB cohort. However, we observed increased complete response rates at 1 mo in the RT-ICB than RT group (50% vs 8%). Additionally, we observed abscopal responses in 2 of 14 RT-ICB cases. We detected significant shifts in the CD8+ and CD4+ T cells that peaked 7 d post-RT (P <0.001). Interestingly, the majority of responses (67%) involved the expansion of a distinct immunophenotype of elevated TbetDimCD28high effector memory CD8+ and CD4+ T cells post-RT. These responses were reproduced in the RT-ICB cohort; in particular, CD28highCD27high CD4+ T cells were increased in the exceptional responders (P = 0.027). Last, for a patient with abscopal response, this was associated with a 25% reduction of TH2 CD4+ T cells. Here, we characterized the systemic immune repertoire of circulating CD8+ and CD4+ T cells in response to RT, either alone or in combination with immunotherapy. Our data suggests that expansion of a distinct CD28highCD27high CD4+ T cell population may account for the dramatic responses to RT-immunotherapy.