Abstract 3442: TiME evolution in the context of diffuse astrocytoma post-therapy relapse and progression

Abstract Tumor recurrence and progression are common in IDH-mutant diffuse astrocytomas, which are typically diagnosed as low-grade central nervous system tumors, but frequently progress to grade 4 after conventional therapy, leading to worsened patient prognosis. The biological mechanisms underlying this pattern of recurrence, particularly the contribution of the tumor immune microenvironment (TiME) to treatment resistance and post-therapy progression, remain poorly understood.To investigate the evolution of the TiME in these tumors, we analyzed a cohort of more than 80 patients with IDH-mutant astrocytoma. For most patients, longitudinal tumor samples were available from both pre-treatment and post-therapy relapse or progression. Using advanced spatial profiling approaches, including highly multiplex immunohistochemistry (mIHC), we characterized immune and cancer cell subtype populations and assessed how their abundance and spatial organization change as tumors recur and progress to higher grade. All findings are being evaluated in the context of patient treatment history and overall survival.Our preliminary analyses, based on a subset of the full cohort, reveal differences in overall tumor composition both across patients and within matched primary and relapse tumor pairs, highlighting inter- and intra-patient heterogeneity in the TiME. When examining progression-related changes in recurrent tumors, we observe a marked accumulation of CD163+ macrophages and microglia, accompanied by a decrease in CD163- myeloid phenotypes after chemotherapy. Relapse tumors also show increased infiltration of both CD4+ and CD8+ T cells, suggesting enhanced lymphocytic response during recurrence.Treatment-specific differences were similarly apparent. Tumors from patients who received combined radiation and chemotherapy showed a higher overall accumulation of immune cells compared to those treated with radiation therapy alone, indicating a potential additive impact on immune cell recruitment. Furthermore, tumor grade appeared to influence overall myeloid composition, with low-grade tumors (WHO grade 2 to 3) exhibiting a higher proportion of microglia compared to high-grade (grade 4) tumors.Together, this longitudinal dataset provides insights into both recurrent and patient-specific alterations in the TiME that accompany tumor relapse and progression. As these observations are derived from preliminary analyses of a partial cohort, they will be validated in the complete dataset. Ultimately, this work aims to improve our understanding of treatment failure mechanisms and arising opportunities in diffuse astrocytomas. Citation Format: Aliisa Tiihonen, Iida Salonen, Ella Raulamo, Eelis Mikkonen, Emilia Mäki, Maria Annala, Tomi Hoikka, Ismaïl Hermelo, Anna Dénes, Thomas Olsson Bontell, Helena Carén, Asgeir Jakola, Hannu Haapasalo, Kirsi Rautajoki. TiME evolution in the context of diffuse astrocytoma post-therapy relapse and progression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3442.