P02.07.A IMMUNOLOGICAL AND EPIGENETIC FEATURES OF RADIATION NECROSIS IN GLIOBLASTOMA PATIENTS

Abstract

Abstract BACKGROUND Radiation necrosis is a rare but highly relevant complication after radiotherapy in glioblastoma patients. Immunological and epigenetic features of these delayed tissue changes are not yet understood and no reliable biomarker to identify patients at risk of developing radiation necrosis is available. AIM To assess methylation patterns, T cell receptor repertoires and immune cell infiltration in patients with radiation necrosis and identify possible predictive epigenetic and immunological features. METHODS 405 glioblastoma patients who underwent tumor resection between 2008 and 2018 were retrospectively screened for histology-proven or MR-spectroscopy/FET-PET-diagnosed radiation necrosis (RN). After identifying patients with RN, a control cohort matched for age, sex and MGMT-status was established. For both cohorts, epigenetic profiles of primary tumor tissues were assessed via DNA methylation arrays. Immunohistochemistry was performed for CD3, CD8, CD68 and CD163. For patients with histology-proven RN, additional TCRß-bulk sequencing of DNA extracted from FFPE tissue of primary tumor and RN was performed. RESULTS 14/405 (3.5%) patients were identified with RN. The MGMT-promotor was hypermethylated in 50% of patients. The most prominent methylation subclasses of primary tumor tissue of patients in the RN cohort were GBM RTK I and GBM RTK II (5/8). None of these tumors was classified as GBM MES, which in contrast was the most prominent subclass in the control cohort (4/8). T cell infiltration and microglia/macrophage density did not differ between RN cohort and control cohort (CD3: 15.04 vs 20.28 counts/ROI, p= 0.55; CD8: 7.32 vs 8.3, p= 0.82; CD68: 7.42 vs 11.86, p=0.13; CD163: 14.29 vs 16.2, p= 0.73). In total, 1762 unique productive TCRß sequences were identified, overall productive clonality was low (0.2, range 0.06-0.4). A clonal expansion of specific TCRs from primary tumor to RN was identified in 2 patients. A more clonal repertoire in RN was observed in 50% of patients. 2 TCRß sequences were shared between 2 different patients, no sequences were shared between more than 2 patients. CONCLUSION Our data indicate that the methylation subclasses RTK I and RTK II may constitute a risk factor for RN whereas a MES subclass may be protective. Immune cell profiles in primary tumors of GBM patients that develop radiation necrosis during their treatment do not differ from primary tumors of control patients. TCR repertoire shows an overall low abundance of TCRs with polyclonal repertoires, but clonal expansion could be detected in a subset of patients, suggesting antigen-specific T cell responses to RN. Further analysis and validation of these findings in a larger cohort is ongoing.