Abstract 920: Characterization of the orthotopic MMTV PyMT murine mammary carcinoma model following radiation and immune checkpoint blockade
Authors: Trachet Erin E. , Germain Derrik Germain , Urs Sumithra , Draper David , Franklin Maryland Rosenfeld 
Published: September 2020

Abstract
Radiotherapy (RT) is a highly utilized clinical treatment approach for certain types of breast cancer. The use of RT, in combination with checkpoint blockade, to prime and activate the immune system is an area of intensive research. Developing preclinical models for breast cancer research with well characterized tumor immune profiles is highly sought after. MMTV PyMT was originally created as a transgenic mouse model that we have optimized as an orthotopic transplantable model. The baseline immune profile shows an equal distribution of CD8+ and CD4+ T cells, with considerable presence of NK cells. MMTV PyMT tumors have a large myeloid population represented predominantly by granulocytic myeloid-derived suppressor cells and M1 tumor-associated macrophages. In this transplantable model, we tested anti-tumor responses to RT and immunomodulatory agents to determine if the model is more in line with an immunosuppressive “cold” tumor or an immuno-responsive “warm” tumor. Tumor-bearing mice were treated with checkpoint blockade antibodies against PD 1, PD L1, or CTLA 4 alone or in combination with focal RT (SARRP; Xstrahl). Single agent checkpoint inhibitors showed minimal responses while single agent focal RT proved to be highly effective, producing dose dependent tumor regressions at all dosage levels tested (5, 10 & 20Gy). The combination of low dose focal RT (5Gy) with anti CTLA 4 (10mg/kg) resulted in one partial regression and stagnate tumor growth. To examine combination effects on the immune response, we examined infiltration of immune cell types in the tumor using flow cytometry at 2 time points. No remarkable changes occurred in any treatment group, at either time point. Moreover, positive expression of PD 1 remained low, both with and without treatment (&lt;36% of tumor-derived CD8+ T cells). In most cases, ex vivo TIL stimulation with PMA and Ionomycin failed to induce cytokine expression (IFNγ, TNFα, and IL 2) in above 10% of T and NK cell subsets. IL-2 production was increased in several subsets from mice treated with radiation and/or anti mCTLA 4. However, no additive effect was observed in the combined group. Finally, the frequency of polyfunctional helper CD4+ T cells producing both IL 2 and TNFα correlated with in vivo efficacy response, regardless of treatment suggesting that activated helper T cells may be important for the anti-tumor response. In the MMTV PyMT transplant model, early assessment of immune cell populations were predictive for the lack of in vivo efficacy with checkpoint blockade. Interestingly, the modulation in cytokines (IL 2 and TNFα) that was produced by the combination of focal RT with or without anti mCTLA 4 treatment was indicative of an efficacious therapy.
Citation Format: Erin E. Trachet, Derrik Germain Germain, Sumithra Urs, David Draper, Maryland Rosenfeld Franklin. Characterization of the orthotopic MMTV PyMT murine mammary carcinoma model following radiation and immune checkpoint blockade [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 920.