Inhibition of HVEM suppresses invasion and growth of mesenchymal glioblastoma

Mesenchymal glioblastoma is a subtype of glioblastoma multiforme (GBM) characterized by pronounced inflammatory features and resistance to conventional therapies. Proneural GBM acquires a mesenchymal phenotype through proneural-mesenchymal transition (PMT), in which NF-{kappa}B signaling plays a central role. Through RNA-sequencing analysis of glioma-initiating cells (GICs), we found that expression of herpes virus entry mediator (HVEM, also known as tumor necrosis factor (TNF) receptor superfamily member 14 or TNFRSF14) is highly expressed in mesenchymal GBM cells. Functional analyses revealed that HVEM promotes GIC proliferation, neurosphere formation, and invasive capacity in vitro, and enhances tumor formation following intracranial transplantation of GICs in mice. Among the TNF superfamily ligands, a proliferation-inducing ligand (APRIL, also known as TNF superfamily 13 or TNFSF13) binds to HVEM and activates NF-{kappa}B signaling, thereby inducing a mesenchymal phenotype in GBM cells. To therapeutically target this pathway, we have generated nanobodies from camelid-derived heavy-chain-only antibodies against human HVEM. An anti-human HVEM nanobody significantly inhibited the invasion of mesenchymal GICs in organotypic cultures and suppressed tumor growth in a mouse xenograft model. Furthermore, HVEM expression contributed to resistance to anticancer drugs, which was relieved by knockout of HVEM expression in the mesenchymal GICs. Collectively, these findings suggest that beyond its known role in immune evasion through interaction with B and T lymphocyte attenuator (BTLA), activation of HVEM on GBM cells promotes invasion and proliferation of mesenchymal GICs. Thus, HVEM represents a promising therapeutic target for the treatment of mesenchymal GBM.