SMAC mimetics drive microglia phenotype and glioblastoma immune microenvironment

Background Tumor-associated macrophages/microglia (TAMs) are highly plastic and heterogeneous immune cells that can be immune-supportive or tumor-supportive depending on the microenvironment. TAMs are the most abundant immune cells in glioblastoma (GB) and play a key role in immunosuppression. Therefore, TAMs reprogramming toward immune-supportive cells is a promising strategy to overcome treatment resistance. In this study, we investigated the effect of inhibiting the Inhibitor of Apoptosis proteins (IAPs) by SMAC mimetics (SM) on TAMs reprogramming. Methods We used scRNAseq human GB databases to identify IAPs expression by TAMs. We treated explants and cultured immune cells isolated from human GB samples with SM to determine IAPs involvement in TAMs phenotypes and activities as evaluated by immunofluorescences, flow cytometry and cytokine array. We set up co-cultures of microglia and spheroids/tumoroids to determine the role of microglia in SM-response. Then we designed a relevant immunogenic mouse GB model to decipher the spatio-temporal densities, distribution, phenotypes and function of TAMs with or without SM treatment. We used light-sheet microscopy, two-photon imaging, a transgenic mouse with fluorescent TAM subsets and mass cytometry. Results We showed that IAPs are expressed in TAMs. SM treatment promoted microglia pro-apoptotic and anti-tumoral function via caspase-3 pro-inflammatory cleavage and the inhibition of tumoroids growth. In vivo , SM promoted microglia activation, antigen-presenting function and their tumor infiltration. In addition, we observed a remodeling of the blood vessels, a decrease in anti-inflammatory macrophages and an increased level of monocytes and their monocyte-derived dendritic cell progeny. This reprogramming of the TAM landscape was associated with an increase in CD8 T cell density and activation. Conclusion Altogether, these results demonstrated that SM drive microglia toward an active phenotype with pro-apoptotic and anti-tumoral function and modify the GB immune landscape. This identifies SM as a molecule of choice for TAMs reprogramming.