NK cell-derived GZMB (granzyme B) suppresses glioblastoma radioresistance by blocking SDC1-mediated autophagosome maturation

Radiotherapy is a fundamental step in the combined treatment of glioblastoma (GBM), while radioresistance of GBM causes limitation of therapeutic efficacy. Natural killer (NK) cells, a potential target of immunotherapy, have attracted considerable attention due to the robust cancer cell-targeted cytotoxicity in combined treatment with radiotherapy, suggesting NK cell regulation might be a radiosensitization strategy. Here we show that a cytotoxic subset of NK cells could be stimulated by ionizing radiation (IR) and accumulate in the GBM tumor microenvironment (TME). Co-culturing with NK cells significantly enhances the GBM cell response to IR, and pharmaceutically depleting NK cells in mice elevates IR-induced tumor growth delay. Specifically, GZMB should be the radiosensitization effector secreted by NK cells. Suppressing GZMB activity remarkably impairs NK-mediated GBM radiosensitization. Meanwhile, administrating exogenous GZMB improves irradiation dose-survival response in vitro or in a xenograft model. Mechanically, GZMB blocks autophagosome-lysosome fusion in GBM cells by directly recognizing and cleaving SDC1, a key regulator of autophagosome maturation, at the valine 225 and aspartate 228 sites. Uncleavable mutation of SDC1 reverses GZMB-mediated radiosensitization in GBM. Further studies demonstrate that cleavage of SDC1 obstructs the localization of TGM2, a key MAP1LC3/LC3 recognizer, on the lysosome surface. Clinical data reveal GBM patients with an SDC1 valine 225 or aspartate 228 mutation display lower response to radiotherapy. In this study, we disclose the critical role of NK cells in tumor radiotherapy through secreting GZMB and impeding autophagosome maturation, as well as propose a potential strategy combining radiotherapy and NK-based immunotherapy against radioresistant GBM.Abbreviations: DEGs: differentially expressed genes; GBM: glioblastoma; GZMB: granzyme B; IL: interleukin; IR: ionizing radiation; IRS: immunoreactive score; LAMP: lysosomal associated membrane protein; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; mSDC1: mutant SDC1; NK: natural killer; PRF1: perforin 1; SDC1: syndecan 1; SNAP29: synaptosome associated protein 29; SQSTM1: sequestosome 1; STX17: syntaxin 17; TGM2: transglutaminase 2; TME: tumor microenvironment; TGD: tumor growth delay; VAMP8: vesicle associated membrane protein 8; WT: wild type.