CSIG-09. RADIOTHERAPY WITH CONCURRENT INHIBITION OF CHEK2 ACTIVATES STING IN GLIOMAS

Using an in vivo CRISPR screen approach, we demonstrated the contribution of glioma cell intrinsic checkpoint kinase 2 (Chek2) in the evasion of tumor cells from CD8⁺ T cell recognition. Genetic depletion or pharmacological inhibition of Chek2 with blood-brain-barrier permeable drugs that are being evaluated in clinical trials, in combination with standard of care radiotherapy and PD-1/PD-L1 blockade, led to survival benefit in multiple preclinical glioma models. Moreover, we observed that Chek2 inhibition or depletion is associated with activation of cytosolic DNA-sensing STING pathway in the glioma cells. A positive correlation was identified between STING pathway activity and T cell infiltration in the TCGA glioblastoma dataset. STING activation has been shown to promote robust immune response in glioblastoma. Building upon these results, we evaluated the impact of radiotherapy on Chek2 and STING signaling. After applying 2 Gy radiotherapy, a 20 fold increase of Chek2 activation was observed within the first hour in the in vitro grown glioblastoma patient derived cells, specifically GBM6 and GBM43 cells. Chek2 activation levels returned to the baseline within 6 hours in these cells. However, STING activation was moderate to absent at different time intervals within the first 48 hours post 2 Gy radiotherapy. Interestingly, when the in vitro-grown GBM6 cells were pretreated with Chek1/2 inhibitor prexasertib for 48 hours prior to the 2 Gy radiotherapy exposure, there was 10 fold reduction in the activation of Chek2 as compared to the radiotherapy alone group. Concomitantly, a 2-fold increase in the activation of STING was noted for up to 48 h in the pretreated group, as compared to the radiotherapy alone group. Our in vitro findings suggest an inverse relationship between the activation of Chek2 and STING. In conclusion, radiotherapy with concurrent inhibition of Chek2 offers a novel therapeutic combination to modulate STING activation in gliomas.