Abstract 1663: Activation of STING in response to partial-tumor radiation exposure
Authors: Mathieu Mickael , Nepali Prerna R. , Budhu Sadna , Powell Simon N. , Humm John L. , Deasy Joseph O. , Haimovitz-Friedman Adriana 
Published: July 2021

Abstract
Purpose: To determine whether STING activation is involved in partial volume radiation therapy.
Background: The potential of radiotherapy (RT) to induce immune recognition of cancer cells is a growing topic of research. It has been suggested that partial volume irradiation used in GRID therapy/Lattice RT, a type of RT in which radiation does not cover the entire tumor intentionally but rather is spatially fractionated, sometimes induces an immune response. We previously showed that a single dose of radiation delivered to half of the tumor (50% RT) activated an anti-tumor immune response comparable to the response in a fully-irradiated tumor in the immunogenic 67NR murine tumor model (breast carcinoma) and in the less immunogenic and more radioresistant Lewis lung carcinoma (LLC) tumor model. We have also demonstrated that this immune response was due to the infiltration of CD8+ T cells along with an increased expression of ICAM adhesion molecules. Treatment with either anti-CD8 or anti-ICAM antibodies abrogated the hemi-RT response. Furthermore, a significant abscopal effect was observed after partial irradiation with a single dose of 10Gy in a bilateral 67NR tumors model. It has been shown that ionizing radiation can mediate antitumor immunity via the activation of the cytosolic DNA sensor cGAS/STING pathway. Therefore, in this study, we tested whether the hemi-irradiation-mediated immune response involves the cGAS/STING canonical pathway, or a non-canonical activation of STING, in the 67NR or LLC tumor models. It has been reported that STING can be activated, independently of cGAS, via non-canonical activation of STING, involving ATM and TRAF6, among other factors.
Brief methods: We investigated 67NR murine orthotopic breast tumors in Balb/c mice and LLC cells injected in the flank of C57Bl/6, cGAS or STING KO mice. RT was delivered to 50% or 100% of the tumor volume using a 2X2 cm collimator on a microirradiator allowing precise irradiation. Tumors were collected at different time points post-RT and assessed for different measurements.
Results: There is significant activation of the cGAS/STING pathway in the hemi-irradiated tumors as compared to control and to 100% exposed 67NR tumors. Interestingly, the increased expression of the cGAS/STING pathway was found in the hemi-irradiated tumor but, also in the non-irradiated part of the tumor. In the LLC model, a non-canonical activation of STING is involved. Using both cGAS and STING KO mice, we demonstrated that the partial exposure RT-mediated immune response is dependent on STING activation in the host while cGAS is dispensable.
Conclusions: Identifying the upstream pathways responsible for STING activation in the partial RT-mediated immune response in different tumor types would improve this therapy and its potential combination with immune checkpoint blockade.
Citation Format: Mickael Mathieu, Prerna R. Nepali, Sadna Budhu, Simon N. Powell, John L. Humm, Joseph O. Deasy, Adriana Haimovitz-Friedman. Activation of STING in response to partial-tumor radiation exposure [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1663.