Abstract 1562: Syngeneic MC38 mouse colorectal tumor model: Immune profile and response to immunomodulatory agents and radiation

Abstract With 53,200 deaths expected in 2020, colorectal cancer (CRC) is the second most common cause of cancer deaths in the US. Metastatic lesions are common, and CRC remains incurable for many patients, leading to over 900,000 deaths per year worldwide. There is a need to develop safe and effective treatments since not all patients or tumors respond to current therapies and the use of appropriate animal models remains vital. One such model is the MC38 murine CRC cell line, derived from a grade-III adenocarcinoma chemically induced in a female C57BL/6 mouse. The aim of this study was to analyze the immune infiltration in subcutaneous MC38 tumors and track responses to various therapeutics. All animal work was performed in an AAALAC accredited facility, in alignment with applicable animal welfare regulations and with predetermined humane euthanasia criteria on all studies. As shown by flow cytometry, naïve tumors in the log phase of growth exhibited low infiltration of CD45+ cells. Infiltration of lymphoid cells, represented by CD8+ T cells and CD4+ T helper cells, was also minimal, characteristic of a cold immune-suppressive landscape. The myeloid population represented ~50% of the CD45+ cells and was characterized predominantly by M1-TAMs, M-MDSCs, M2-TAMs and G-MDSCs, also implicated in immune-suppressive environments. To evaluate the effect of immunotherapies, mice bearing established tumors were administered various checkpoint inhibitors or costimulatory antibodies. The costimulatory antibodies anti-GITR, anti-CD137 and anti-OX40 showed no evident anti-tumor effect. In contrast, anti-PD-L1 and anti-PD-1 treatments produced clear anti-tumor activities with 40 and 50% putative responders and 35 and 37% increase in time to progression, respectively, as compared to controls. Tumor volumes in anti-PD-L1 and anti-PD-1 treated mice were also smaller than those in isotype controls starting day 22 post-tumor inoculation (509+103 and 504+94mm3 vs 899+124mm3, respectively, on day 22). Because radiotherapy is thought to increase the immunogenicity of tumors, we investigated the effects of focal radiation (RT) treatment (10Gy), delivered by the Small Animal Radiation Research Platform (Xstrahl), alone or in combination with checkpoint inhibition. Treatment with RT alone resulted in tumor growth delay of 12 days and an increase in time to progression of 119 %, as compared to controls, and 20% partial regression. Treatment with RT combined with anti-PD-1 potentiated these effects, with tumor growth delay of 14 days, an increase in time to progression of 134%, 30% complete regression, 10% partial regression, and 30% tumor free survivors. These results demonstrate that RT can convert an immune-suppressive milieu as seen in the MC38 model, into a warmer environment, and underscore the value of such relevant mouse models in developing clinically relevant therapeutic multimodality strategies. Citation Format: Sylvie Kossodo, Sheri Barnes, David Draper, Derrick Germain, Scott Wise, Maryland Rosenfeld Franklin. Syngeneic MC38 mouse colorectal tumor model: Immune profile and response to immunomodulatory agents and radiation [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 1562.