Abstract 4152: Longitudinal local and peripheral immunologic changes associated with PD-L1 response in a murine breast cancer model
Authors: Hanna Ann , Sun Xiaopeng , Tran Emily , Sheng Quanhu , Taylor Brandie C. , Opalenik Susan R. , Balko Justin M. 
Published: April 2023

Abstract
Immune checkpoint inhibitors (ICI) have significantly enhanced patient survival in many cancers but yield limited success in breast cancer. ICIs activate anti-tumor immunity by overriding the inhibition of tumor infiltrating lymphocytes (TILs). Clinical trials in triple negative breast cancer (TNBC) patients, who harbor TILs within tumor stroma, have demonstrated increased survival (IMpassion130) and pathologic complete response (KEYNOTE-522) to ICI leading to FDA-approval of ICI and chemotherapy combinations in metastatic TNBC. However, ICI benefit is heterogeneous among patients. We sought to model ICI response in vivo to evaluate therapeutic resistance and response heterogeneity and to ascertain predictive biomarkers for favorable ICI outcomes. An immunocompetent EMT6 orthotopic mammary tumor model was used to investigate the efficacy of anti-PD-L1. Matched longitudinal samples of the tumor microenvironment (TME) (collected by fine-needle aspiration) and peripheral blood (PBMC) from mice were profiled by bulk RNA and T-cell receptor sequencing.
Anti-PD-L1 robustly suppressed primary tumor growth and extended survival beyond the control group. The addition of chemotherapy demonstrated moderate therapeutic efficacy but failed to enhance ICI benefit. Phenotypic profiling of the TME revealed increased T cells, DCs, and NK cells in anti-PD-L1 only and chemotherapy combination groups. Despite using a genetically identical tumor model and host, PD-L1 blockade induced heterogeneous responses, like clinical outcomes in TNBC patients, ranging from complete response (CR) to intrinsic resistance (IR). The primary TME showed upregulated signatures of cytotoxic T cell response and activation, specifically inflammatory interferon signaling (both prior to and post ICI administration) that corresponded to favorable outcomes to anti-PD-L1 in individual mice. Longitudinal analysis of the peripheral blood identified modest changes among mice at baseline that progressively deviated by response type (IR-vs-CR). Mice harbored enriched myeloid signatures and clonal T cell expansion during therapy corresponding to ICI resistance and response, respectively. Further investigations of matched peripheral blood and the primary TME signatures may identify systemic biomarkers and tumor antigen-specific T cell clones to accurately predict ICI response in patients and uncover mechanisms for sensitizing tumors refractory to ICI.
Thus, we identify an in vivo model that emulates TNBC patient heterogeneous outcomes to ICI combinatorial approaches. We describe host-specific signatures, specifically from myeloid cells, that correlate with differential responses to ICI, which may serve as a basis for peripheral blood tracking of breast cancer patient responses.
Citation Format: Ann Hanna, Xiaopeng Sun, Emily Tran, Quanhu Sheng, Brandie C. Taylor, Susan R. Opalenik, Justin M. Balko. Longitudinal local and peripheral immunologic changes associated with PD-L1 response in a murine breast cancer model. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4152.