Radiation Immunodynamics as the Foundation of Immune-Guided Radiotherapy: Changes in Peripheral CD14+PD-L1+ Myeloid Cells over the Course of Chemoradiation and Association with Survival for New Diagnosis of Glioblastoma

Abstract

<h3>Purpose/Objective(s)</h3> Radiation immunodynamics, or the understanding of immune markers over a fractionated course of radiotherapy (RT), is a critical intermediate step in developing personalized RT approaches, including Immune-guided Radiotherapy (IM-GRT). Peripheral-origin myeloid cells are a major component of the glioblastoma (GBM) tumor microenvironment. The effect of adjuvant chemoradiation (CRT) on programmed death-ligand 1 (PD-L1)-expressing suppressive myeloid cells is unknown. We hypothesize that PD-L1⁺ monocytes increase from baseline in the peripheral blood of patients with GBM undergoing CRT. <h3>Materials/Methods</h3> Tumor and peripheral blood were collected from a prospective, single-institution, biospecimen protocol of newly diagnosed GBM participants undergoing CRT. A neuropathologist selected regions of interest within baseline formalin-fixed, paraffin-embedded tissue for digital spatial profiling. Expression for PD-L1 on CD163⁺ regions of interest is quantified relative to isotype and presented as fold change. Flow cytometry was performed on peripheral blood immune cells at seven time points in relation to CRT: one prior to start, five weekly during, and one month after completion. Flow cytometry data are expressed as percent frequency (%fx) of CD14⁺ monocytes. Correlative studies focusing on plasma analytes, metabolic proteins, and bulk RNA sequencing were also performed. Relationships between PD-L1 and overall survival (OS) or progression-free survival (PFS) were assessed by Cox model. "Survivors" were alive 2 years after diagnosis. <h3>Results</h3> A total of 15 cases with baseline PD-L1 assessment are included. Digital spatial profiling (n=10) of tumor identified PD-L1 expression within regions of interest with CD163⁺ myeloid cells is increased in non-survivors compared to survivors (log2FC=0.28; padj<0.05). The median %fx of peripheral blood CD14⁺ PD-L1⁺ cells at the pre-CRT baseline is 39.2% among survivors and 16.0% among non-survivors, and not associated with OS (HR=0.98, p=0.36) or PFS (HR=0.99, p=0.7). Comparing to baseline, the %fx of CD14⁺PD-L1⁺ cells at week 4 during CRT decreases (median fold change 0.49, range 0.08-0.92) in survivors but increases (median fold change 1.86, range 0.31-4.32) in non-survivors. Such a mid-CRT change (in terms of fold change) is significantly associated with OS (HR=2.21, p=0.008) but not PFS (HR=1.33, p=0.17). One month after CRT, the median %fx is 4.9% in survivors and 19.7% in non-survivors, and not associated with OS (HR=1.04, p=0.11) or PFS (HR=1.02, p=0.33). <h3>Conclusion</h3> This is the first study to demonstrate changes in peripheral CD14⁺PD-L1⁺ myeloid cells over the course of CRT and to associate these changes with survival outcome. Dynamic biomarker assessments, like radiation immunodynamics, may identify patients who would benefit from immune adjuvants while undergoing CRT in neuro-oncology and other disease sites.