BDNF-TrkB.T1 receptor signaling modulate an immunosuppressive tumor microenvironment and epithelial-mesenchymal transition in gliomas

Introduction: Gliomas are the most common and aggressive brain tumors in children and adults. The tumor microenvironment (TME), including neurons, glial cells, blood-brain barrier, and immune components, plays a key role in glioma progression and therapy resistance. Brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin receptor kinase B (TrkB), encoded by NTRK2, are emerging as central players in glioma biology. Among TrkB isoforms, truncated TrkB.T1 is the most abundantly expressed variant in human gliomas and various adult and pediatric tumors. Recent studies show TrkB.T1 is the predominant isoform during embryogenesis and organogenesis, with high expression along mesenchymal trajectories, suggesting a role in glioma epithelial-mesenchymal transition (EMT). Previously, we showed TrkB.T1 overexpression in human neural stem cells downregulated genes in the MHC Class II and interferon-gamma signaling pathways, both essential for antigen presentation and cytotoxic T cell recruitment. This study aims to elucidate BDNF-TrkB.T1 signaling in modulating glioma TME and EMT phenotype. Approaches: We performed in silico analyses using publicly available single-cell RNA sequencing data (GSE182109), stratified into NTRK2 high, medium, and low clusters, and correlated with PTPRC (CD45) expression. Using the PDGFB RCAS-tva model, we generated in vivo gliomas via intracranial RCAS-PDGFB + RCAS-TrkB.T1 injection in Nestin/tv-a;Ink4a/Arf-/- mice at postnatal day 1-2. Tumors were collected at 3.5-5 weeks post-injection for immunofluorescence and flow cytometry. We also created a TrkB.T1 knockout (KO) mouse model and performed blood immune profiling. For in vitro studies, we used 448T (TrkB.T1 high) and 559T (TrkB.T1 low) glioma stem cells (GSCs) to assess BDNF-TrkB.T1 signaling effects on cytokine profiles and neutrophil chemotaxis. EMT changes were examined via western blot and transwell migration assays. Results: NTRK2 expression in gliomas was inversely correlated with PTPRC, and higher NTRK2 levels were linked to reduced immune cell infiltration. In vivo, TrkB.T1 expression reduced classical dendritic cells and T cell-activating cytokines, while increasing microglial density and neutrophil infiltration, indicating an immunosuppressive TME. Conversely, TrkB.T1 KO mice showed increased effector CD8+ T cells. In vitro, BDNF-TrkB.T1 signaling promoted chemokine release linked to neutrophil recruitment and T-cell inactivation, and enhanced neutrophil migration in GSC-BDNF chemotaxis assays. BDNF stimulation also increased N-cadherin and GSC migration, indicating an EMT phenotype. Conclusion: These findings reveal the role of BDNF-NTRK2 splice variant TrkB.T1 in promoting immunosuppressive TME and EMT in gliomas, suggesting a therapeutic opportunity to target TrkB.T1 in future translational and clinical studies.