Murine models for triple-negative breast cancer with differential responsiveness to immunotherapy

ABSTRACT Background Breast cancer is the most common cancer diagnosis in women. Clinical studies with triple-negative breast cancer (TNBC) are encouraging for immunotherapy combined with chemotherapy (αPD-1 with paclitaxel and/or carboplatin). However, additional clinical advances may be accelerated by leveraging preclinical TNBC models including syngeneic mammary tumor cell lines. Here, we report two syngeneic mammary tumor cell lines that exhibit differential responsiveness to immunotherapy in vivo . Methods Spontaneous mammary tumors from C57BL/6J MMTV-Cre Trp53 fl/+ animals were passaged serially in cell culture and in vivo in the mammary fat pad of fully wildtype animals. The resulting lines, MM001i and MM008i, lost Trp53 and formed 1000 mm 3 tumors in the mammary fat pad within 21-28 days. Tumors from each group were analyzed for their response to αPd-1 and αCtla-4 immunotherapy, infiltrating immune populations, and transcriptional profiles. Results Despite originating from the same genetic background, these lines exhibit differential responses to immunotherapy. For αPd-1 therapy, MM001i is poorly responsive and MM008i is strongly responsive with near-complete tumor regression. In comparison, both MM001i and MM008i respond rapidly to αCtla-4 therapy. Both models express unique tumor antigens as evidenced by immunity to subsequent engraftments. Primary MM008i tumors exhibit greater T cell infiltration, and CD8-positive T lymphocytes are required for αPd-1 responses. Cancer epithelial cells within MM008i tumors demonstrate a greater abundance of Pd-l1 expressing cells, correlating with observations in human TNBC where PD-L1 expression is a biomarker for αPD-1 response. Conclusions These TNBC models are promising for further mechanistic studies and testing future single and combinatorial therapies. WHAT IS ALREADY KNOWN ON THIS TOPIC Immunotherapies such as αPD-1 have demonstrated modest clinical benefit for triple negative breast cancer (TNBC) patients with high PD-L1 expression. To improve therapy efficacy, there is a critical need to better understand mechanisms of sensitivity and resistance. Syngeneic models are a valuable tool for preclinical mechanistic studies of new therapeutics and immunotherapy in vivo . However, these models are rare, and those that exist are poorly responsive to immunotherapy and offer limited potential for mechanistic studies. WHAT THIS STUDY ADDS We generate two new syngeneic TNBC cell models derived from Trp53 fl/+ background in C57BL/6 mice. Despite a shared initial genotype and responsiveness to αCtla-4 therapy, these tumor lines are differentially responsive to αPd-1 therapy and demonstrate major differences in infiltrating immune populations. HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE, OR POLICY These new syngeneic model systems can be leveraged to better understand immunotherapy responses and immune microenvironments in a fully immune competent setting, enabling future mechanistic and therapeutic studies.