Abstract B46: Varying the inoculation site of mouse syngeneic tumors can result in aberrant inclusion of lymph node tissue at tumor harvest

In recent years, growing interest in immunotherapies for the treatment of cancer has led to increased reliance on murine syngeneic tumor models for preclinical study in immunocompetent animals. These studies typically examine changes in the quantities and phenotypes of tumor-infiltrating immune cells; thus, it is crucial to distinguish between these cells and extratumoral immune cells during tumor harvest. Here, we show that 4T1 breast tumors inoculated in mammary fat pads (MFP) numbers 4 or 9 (4/9) contained greater frequencies of B cells and naive T cells than tumors inoculated in MFP numbers 5 or 10 (5/10). Immunohistochemical analysis for B-cell presence demonstrated that MFP 4/9 tumors encroached on, and often encapsulated, inguinal lymph nodes, such that lymph node tissue was unknowingly collected during tumor harvest. This prevented accurate enumeration and phenotyping of infiltrating immune cells by flow cytometry, as the majority of lymphocytes in the affected samples derived from lymph nodes, and had not infiltrated the tumor per se. We observed the same phenomenon in peritibial tumors using immunohistochemical analysis, which began to encapsulate popliteal lymph nodes at large tumor volumes (>1500 mm 3 ). Our data highlight the importance of choosing an appropriate inoculation site for obtaining accurate and consistent results when evaluating immune cell infiltration in syngeneic tumor models. Citation Format: Renee Clift, Barbara Blouw, Susan Zimmerman, Sanna Rosengren, Curt Thompson, Benjamin Thompson. Varying the inoculation site of mouse syngeneic tumors can result in aberrant inclusion of lymph node tissue at tumor harvest [abstract]. In: Proceedings of the AACR Special Conference: Advances in Modeling Cancer in Mice: Technology, Biology, and Beyond; 2017 Sep 24-27; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(10 Suppl):Abstract nr B46.