Abstract 535: Preclinical use of focal radiation and immune checkpoint blockade to improve therapeutic response in an immunologically cold tumor
Authors: Franklin Maryland Rosenfeld , Draper David , Urs Sumithra , Wise Scott 
Published: August 2019

Abstract
Radiotherapy is a highly utilized clinical treatment modality. More than 50% of all cancer patients receive some type of radiation therapy during the course of their illness. In mouse models, radiation treatment has been shown to increase the level of tumor antigen presentation and the variety of peptides available for cross-presentation. Current work in the field focuses on using radiation as a tool to bridge the gap from tumor equilibrium to tumor elimination, which could improve the response rate of immuno-oncology agents.
4T1 is a murine breast cancer model known to have a large percentage of myeloid derived suppressor cells (MDSC) making the model resistant to many immunotherapies and is considered an immunologically cold tumor. We hypothesized that treatment with focal radiation (RT, Xstrahl) would sensitize 4T1 tumors to anti-CTLA-4 treatment.
To determine an appropriate dose of RT, mice with 4T1-Lu2 tumors were treated with a single, focal dose of 5, 10 or 20Gy RT. No response was seen at 5Gy, 10Gy resulted in 10 days growth delay, and 20Gy resulted in 16 days growth. In subsequent work, mice were placed into groups and treated with either isotype control (10mg/kg, MPC-11), anti-CTLA-4 (10mg/kg, 9D9), 10Gy RT, or the combination. Mice were monitored over time for changes in primary tumor volume by caliper measurements and for metastatic disease by bioluminescence imaging (BLI) of the thoracic region. Satellite groups were included for immunohistochemistry and the evaluation of changes in tumor infiltrating lymphocytes.
Median tumor growth delay with anti-CTLA-4, RT, or the combination, was 2.4, 3.5 and 9.4 days, respectively. Time to progression, to 1,200mm3 tumors, was increased by 25% in the combination group but not significantly in the monotherapy groups. While all mice showed evidence of metastatic disease, mice in the combination group displayed a lower level of BLI signal on day 30 when compared to other groups.
To examine the effects on immune cell infiltration, 11 subsets were profiled by flow cytometry. B cells and regulatory T cells were significantly reduced (&gt;95% and 90%, respectively) in the combination group when compared to an untreated group. B cell reductions that exceeded 90% were also observed in some animals within the isotype control group. Examination of the CD8+ T cell phenotype demonstrated that combination therapy, but not monotherapies, increased expression of both CD69 and PD-1 on CD8+ T cells suggesting an enhancement of anti-tumor cytotoxicity in these cells. Interestingly, we found that the combination selectively reduced the levels of phosphorylated STAT3 in MDSC subsets. Collectively, this intimates that anti-CTLA-4 and RT selectively disrupt STAT3 signaling in MDSCs and combine to enhance CD8+ T cell activity, which may play a role in the 4T1 tumor growth delay observed.
Citation Format: Maryland Rosenfeld Franklin, David Draper, Sumithra Urs, Scott Wise. Preclinical use of focal radiation and immune checkpoint blockade to improve therapeutic response in an immunologically cold tumor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 535.