The contribution of stromal cells on drug response in primary tumors remains unclear. To determine how individual cells within the stroma of triple negative breast cancer (TNBC) allografts respond to chemotherapy, we used single cell sequencing to profile cells present in murine tumors with or without exposure to doxorubicin. Doxorubicin is a chemotherapeutic agent commonly used to treat TNBC by inhibiting cancer cell proliferation through intercalation of DNA and preventing topoisomerase II activity. In this study, murine TNBC 4T1 cell line was utilized to generate allograft tumors in immunocompetent BALB/c mice. Syngeneic 4T1 tumors exhibit a range of responsiveness to doxorubicin treatment. Tumor growth rates were monitored throughout the chemotherapeutic regiment then stratified into sensitive or resistant response. Cellular composition and behavior were then analyzed via single cell RNA sequencing (scRNA-seq) and flow cytometry 8 days post-doxorubicin chemotherapeutic administration mimicking clinical treatment. ScRNA-seq revealed decreases in tumor infiltrating lymphocytes after doxorubicin exposure. Furthermore, an increase in T cell abundance was discovered in tumors sensitive to doxorubicin treatment. This finding was further supported by flow cytometric analysis showing a tumor specific increase in all, CD4+, and CD8+ T cell populations relative to resistant tumors. Additionally, T-cell differentiation, exhaustion, and activation states were further examined from scRNA-seq data providing insights into functional properties of the tumor residing T cell populations undergoing chemotherapeutic treatment. Future work will focus on analyzing prognostic value of specific T-cell populations in disease regression following doxorubicin treatment. This study received funding from LLNL LDRD grant 19-SI-003. This work was conducted under the auspices of the USDOE by LLNL (DE-AC52-07NA27344).

Citation Format: Nicholas R. Hum, Aimy Sebastian, Sean F. Gilmore, David M. Gravano, Naiomy D. Rios-Arce, Kelly A. Martin, Elizabeth K. Wheeler, Matthew A. Coleman, Gabriela G. Loots. Single cell transcriptomics of triple negative breast cancer allografts following chemotherapy treatment reveals increased T cell abundance in doxorubicin-sensitive tumors [abstract]. In: Abstracts: AACR Virtual Special Conference: Tumor Immunology and Immunotherapy; 2020 Oct 19-20. Philadelphia (PA): AACR; Cancer Immunol Res 2021;9(2 Suppl):Abstract nr PO065.