Childhood brain tumors establish immunotolerance by secreting cytokines into the cerebrospinal fluid that educate skull bone marrow hematopoietic stem/progenitor cells to function as MHC-II+ antigen-presenting cells, which present tumor neoantigens as self-antigens to CD4+ T cells, driving both myeloid-biased differentiation and regulatory T cell polarization—a circuit that can be therapeutically disrupted by GM-CSF pathway blockade to induce tumor regression across multiple pediatric brain tumor types.

Recent research has revealed a remarkable role for immunosurveillance in healthy and diseased brains, dispelling the notion that this organ is a passive immune-privileged site. Better understanding of how this immunosurveillance operates could improve the treatment of neurological diseases. Here, using a novel genetically engineered mouse model of ZFTA-RELA ependymoma–a childhood brain tumour–we characterised an immune circuit between the tumour and antigen presenting, haematopoietic stem/progenitor cells (HSPCs) in the skull bone marrow. The presentation of antigens in the cerebrospinal fluid (CSF) by HSPCs to CD4+ T cells, biased HSPC lineages toward myelopoiesis and polarised CD4+ T-cells to regulatory T cells (T- regs), culminating in tumour immunotolerance. Remarkably, a single infusion of antibodies directed against cytokines enriched in the CSF of mice bearing ZFTA-RELA ependymomas, choroid plexus carcinomas or Group-3 medulloblastoma–all aggressive childhood brain tumours–disrupted this process and caused profound tumour regression. These data unmask a mechanism by which skull bone marrow-derived HSPCs and CD4+ T cells cooperate to promote the immunotolerance of childhood brain tumours. Antibodies that disrupt this immunosurveillance could prove an effective therapy for these cancers that are less toxic than current treatments.