Purpose: Radiotherapy (RT) has been shown to induce antitumor immune responses whereby the host immune system is stimulated to recognize tumor and activate mechanisms to clear it. RT-induced antitumor immunity is rare but once achieved it offers robust and long-lasting antitumor immunity. Because RT-induced antitumor immunity is thought to be mediated through DNA damage we set out to establish if inhibitors of DNA damage repair proteins, particularly targeting ataxia telangiectasia and Rad3-related protein (ATR), can induce greater immune responses than RT alone.
Methods: We used a triple negative breast cancer murine model, 4T1, on the hind leg of BALB/c mice. Animals were treated with vehicle or an ATR inhibitor (ATRi) (75 mg/kg, AZD6738) 2 h prior to each radiation fraction (3x6 Gy using 6 MV x-rays) or sham exposure. Tumors were harvested 7 days after the last fraction to isolate sub-populations of immune cells (CD8⁺ T-cells, CD4⁺ T-cells, T-regs, CD8⁺ PD-1⁺ T-cells, type 1 and 2 macrophages and classical dendritic cells type 1 and 2) within the tumor. Tumor growth delay and survival data were also acquired.
Results: RT delayed tumor growth and modestly increased survival. RT+ATRi enhanced these effects significantly. RT alone was able to increase the percentage of macrophages present in the tumor (12.9%) compared to vehicle alone (4.6%). RT+ATRi also increased macrophage numbers (24.8%) but also significantly increased CD4⁺ (9.3% vs vehicle 3.2%) and CD8⁺ (5.6 % vs vehicle 2.6%) T-cells. Dendritic cells were also increased after RT+ATRi.
Conclusion: RT alone can alter the tumor microenvironment by increasing macrophage recruitment. This effect can be increased with RT+ATRi. Furthermore, RT+ATRi significantly increased the number T-cells (CD4⁺ and CD8⁺) and non-significantly, dendritic cells. This suggests differential signaling at 7 days between RT and RT+ATRi. Further analysis is required to identify pro- and anti-tumor subpopulations within these sub-types.
Funding Support, Disclosures, and Conflict of Interest: This research was supported in part by RP170040, 1R21CA252411-01, Emerson Collective, and MD Anderson Cancer Center. Dr. Sawakuchi has research agreements with Alpha Tau Medical and Artios Pharma. Dr. Shaitelman has a research agreement with Exact Sciences.
Not Applicable / None Entered.