High-dimensional Characterization of the Systemic Immune Landscape Informs on Synergism between Radiotherapy and Immune Checkpoint Blockade.

Abstract Background Improved anti-tumor responses have been observed in patients following combination radiotherapy (RT) and immune checkpoint blockade (ICB). Whether these clinical responses are linked to the host systemic immune system has not been elucidated. Methods In this study, peripheral blood was longitudinally collected from 10 patients with metastatic disease, who had responded to anti-PD-1/ anti-PD-L1 ICB and received radiotherapy (8-50 Gy in 1-5 fractions) upon disease progression, at the following timepoints: Baseline (pre-RT), 1-2 weeks post-RT and post-ICB#1 on re-introduction post-RT, as part of a single institution prospective observational study. To thoroughly characterize the interaction between combined RT-ICB with the host immune system, we performed high-dimensional, mass cytometry-based immunophenotyping of circulating lymphocytes using a 40 marker-panel addressing lineage, differentiation, activation, trafficking, cytotoxicity, co-stimulatory and inhibitory functions. Phenotypic expressions of circulating lymphocytes were compared across patients and timepoints and correlated with post-RT tumour responses. Results Foremost, we demonstrated excellent post-treatment clinical responses including 4 local responses with >50% reduction in radiated tumor size, 1 out-of-field response and 4 patients who resumed ICB for >1 year. Baseline and post-RT immune states were highly heterogeneous between patients. Despite this interindividual heterogeneity in baseline immune states, we observed a systemic immune reaction to RT-ICB common across patients, histology and radiation sites; a subset of pre-existing Ki-67+ CD8+ T cells were increased post-RT and further expanded upon re-introduction of ICB post-RT (2.3-fold increase, P = 0.02). Importantly, RT did not alter the phenotypic profile of these Ki-67+ CD8+ T cells, which was characterized by a distinct activated and differentiated effector phenotype. Conclusions Collectively, these findings point towards a sustained reinvigoration of host anti-tumor immunity following RT-ICB, and suggest an expansion in activated Ki-67+ CD8+ T cells as a possible demonstration of this synergy, thereby providing new insights which may support the development of optimal sequencing strategies.