TMIC-53 - Transglutaminase 2 expression in glioblastoma: evidence for a role in efferocytosis

Glioblastoma is the most common type of brain cancer in adults. It is currently incurable and there is a clear need for more effective treatments. The heterogeneity of this cancer and its ability to promote both local and systemic immunosuppression are major obstacles to the development of effective therapies. Previous studies have explored the use of transglutaminase 2 (TGM2) inhibitors for glioblastoma therapy. These studies focussed on TGM2 that is expressed in a subset of glioblastoma cells. We show here that glioblastoma-associated macrophages are a major source of TGM2 in glioblastoma tumours. Analysis of bulk and single cell RNAseq data showed that TGM2 mRNA was expressed at high levels in glioblastoma-associated macrophages. Immunohistochemical and immunofluorescence analysis of TGM2 expression was performed in a mouse xenograft model in which human mesenchymal subtype glioblastoma cells were grown intracerebrally in nude mice. In these xenograft tumours, glioblastoma cells were negative for TGM2 expression, while infiltrating macrophages were strongly positive. The most intense staining for TGM2 was observed in macrophages in the vicinity of necrotic regions. Apoptotic cells were frequently present within the necrotic regions and highly TGM2-positive macrophages were observed to be phagocytosing these. The same patterns of TGM2 expression were also observed in samples from glioblastoma patients. Consistent with a role in phagocytosis, TGM2 co-localized with the endosomal protein CD68 in glioblastoma-associated macrophages. The phagocytosis of apoptotic cells by macrophages, known as efferocytosis, is tolerogenic, both because it clears potentially antigenic material and because it induces an immunosuppressive phenotype in macrophages. As TGM2 has a previously established role in efferocytosis and autoimmunity suppression, our findings suggest that TGM2 has a role in maintaining the glioblastoma immunosuppressive environment and that its inhibition might enhance immunotherapy for this cancer.