PARP Inhibitor Plus Radiotherapy Reshapes IDH1 Mutation Tumor Immune Suppression Microenvironment Potentiating the Efficiency of Immune Checkpoint Inhibitor

Abstract

Isocitrate dehydrogenase 1 (IDH1) mutations confer gain-of-function activity by converting α-ketoglutarate (α-KG) to the oncometabolite D-2-hydroxyglutarate (D-2HG). IDH mutant tumors have fewer tumor-infiltrating CD8+ T cells and reduced PD-L1 expression compared with their wild type (WT) counterparts. In addition, 2-HG can directly inhibit the killing and proliferative functions of CD8+ T lymphocytes, and suggesting that 2-HG promotes an immunosuppressive TME. Several studies have shown that 2-HG can inhibit homologous recombination (HR) and weaken the DNA damage response (DDR), making them more sensitive to poly (ADP-ribose) polymerase (PARP) inhibitors and radiotherapy (RT). At the same time, RT and PARP inhibition (PARPi) have been considered to be a new direction to stimulate antitumor immunity. Therefore, our study intends to use RT + PARPi to reverse the immunosuppressive microenvironment caused by IDH1 mutations, thereby promoting the therapeutic effect of immune checkpoint inhibitors. We compared the immune responses of clinical tissue samples and TCGA data from either IDH1mut or IDH1WT low-grade gliomas. We then established IDH1mut-overexpressing MC38 and GL261 cell lines to determine the antitumor effect of RT + PARPi. Apoptosis and immunogenic death markers were detected by flow cytometry, western blot (WB) and ELISA in these cell lines. Tumor growth and mouse survival curves were observed in both an MC38 subcutaneous and GL261 orthotopic tumor model. Changes in the composition of the immune microenvironment were assessed using flow cytometry. The mechanisms underpinning these compositional shifts were then further interrogated using various techniques, including WB, immunofluorescence, qRT-PCR, CRISPR/Cas9, and CD8+ T cell migration experiments. We observed that CD8+ T cell infiltration and expression of the chemokines CXCL10 and CCL5 of CD8+ T cells in IDH1mut tumors were significantly downregulated by immunohistochemistry and TCGA analysis. Gene enrichment analysis using the TCGA database found that IDH1 mutations downregulated interferon (IFN)-related signaling pathways. RT + PARPi induces more DNA damage and actives the CGAS-STING pathway compared with monotherapy, leading to more expression of IFN-β, CXCL10 and CCL5 at mRNA and protein level. In the MC38 subcutaneous tumor model, we found that RT + PARPi increased the infiltration of CD8+ T cells while enhancing the killing function of CD8+ T cells. We observed these same effects in the GL261 orthoma model, as well as increased proliferation function of CD8+ T cells. In addition, RT + PARPi increased the expression of PD-L1 and enhanced the therapeutic effect of immune checkpoint inhibitors. RT + PARPi reshapes the IDH1mut tumor immune suppression microenvironment, thereby potentiating the antitumor effect and efficiency of immune checkpoint inhibitor.