IMMU-46. MONITORING T-CELL DYNAMICS AND CHECKPOINT EXPRESSION IN THE IMMUNE MICROENVIRONMENT OF PEDIATRIC HIGH GRADE BRAIN TUMORS: THE MIMIC PROGRAM

Abstract

Abstract BACKGROUND Pediatric high grade brain tumors (pHGBT) pose a significant challenge due to limited treatment options and poor prognosis. Understanding the immune landscape, in particular T cell profiles and functionality, is of high interest as numerous immunotherapeutic approaches focus on or utilize T-cells. This study aimed to characterize T-cell phenotypes and their potential for immunotherapeutic interventions in tumor fragments (obtained from cavitron ultrasonic surgical aspiration (CUSA)) and tumor tissue (biopsy), in comparison with peripheral blood, collected from pHGBT patients. METHODS Spectral flow cytometry was employed to characterize the presence of T-cell subsets and their functional phenotype. RESULTS T cells were detected in all materials with similar CD8/CD4 ratios. At diagnosis, T-cells in the tumor microenvironment exhibited significantly increased expression of PD1, LAG3 and TIM3 checkpoint receptors compared to peripheral blood. This difference was more pronounced in biopsy samples compared to CUSA fragments. T-cell composition and phenotype in peripheral blood were similar to healthy adult controls at diagnosis. However, during therapy, the T-cell compartment showed a shift towards a higher CD8/CD4 ratio, accompanied by increased activation markers (CD25, CD69) and checkpoint receptor expression . Notably, CD8+ T-cells in both peripheral blood and tumor microenvironment showed high production of Granzyme B. CONCLUSION Our findings demonstrate that despite the high expression of checkpoint receptors on CD8+ T-cells, these cells retain the capability to produce significant amounts of Granzyme B, indicating their potential for cytotoxic activity. Furthermore, variations in checkpoint receptor expression on T-cells were observed when comparing CUSA fragments to biopsy material. Functional assays will further validate these findings and elucidate the functional status of these cells. Understanding the function of T-cells within the tumor microenvironment is essential for designing novel combination therapies to enhance T-cell cytotoxicity against pHGBT.