Abstract BACKGROUND Glioblastomas are referred to as immunologically “cold” tumors since their cellular environment promotes an anti-inflammatory environment leading to an unresolved barrier to immunotherapy. Although the role of reactive astrocytes in other inflammatory diseases has been investigated in several studies, the immunoregulatory functions of astrocytes in the tumor environment remains poorly understood. MATERIAL AND METHODS We purified reactive astrocytes from de-novo glioblastoma and non-infiltrated cortex specimens by immunoprecipitation and analyzed the transcriptional phenotype by RNA sequencing. In order to investigate the origin of astrocytic transformation, we used a microglia loss-of-function model in human organotypic slices. Microglia was depleted by clodronat stimulation. We injected tumor cells and analyzed gene expression of the astrocytes after 7d incubation by RNA sequencing. Environmental cytokines were analyzed by multi-ELISA. Immunostainings of slices were processed by confocal microscopy and 3D reconstruction. RESULTS Here we address the immunological impact of tumor-associated astrocytes, we were able to identify a novel reactive subtype marked by JAK/STAT pathway activation and CD274 expression. Our results show a distinct astrocytic transcriptional phenotype that mutually arises from both microglia and astrocytes of the tumor environment. This interaction leads to a large release of anti-inflammatory cytokines such as TGFß and IL10. The reactive subtype switch of astrocytes was recovered by inhibition of the JAK pathway, which caused an increase of pro-inflammatory environment. CONCLUSION Our results can form the basis for a novel therapeutic approach by directly targeting tumor-associated astrocytes in order to transformimmunologically “cold” into “warm” tumors.