Background Clinical trials in triple-negative breast cancer (TNBC) patients have demonstrated improved pathologic complete response to immunotherapy (ICI) in early stages and progression free survival in the metastatic setting, leading to FDA approval for TNBC. Though promising, patients show highly variable responses to the addition of ICI to conventional chemotherapy. We modeled ICI response in an in vivo murine model of TNBC to evaluate therapeutic resistance and response heterogeneity with the intent of identifying targets that may be exploited therapeutically to potentiate ICI efficacy in patients.

Methods Using an immunocompetent mammary tumor EMT6 model, we investigated the efficacy of aPD-L1. The primary tumor cellular microenvironment and peripheral blood from mice with differential responses were longitudinally assessed by single-cell (or bulk) RNA sequencing and T-cell receptor sequencing, respectively, to identify systemic genetic alterations and T-cell expansion.

Results We found that: 1) systemic PD-L1 blockade significantly reduces mammary tumor growth and promotes immune cell infiltration into the tumor microenvironment, 2) aPD-L1 treatment induces heterogeneous responses, even within genetically matched mice with identical tumors, encompassing complete response, disease stability with eventual acquired resistance, and intrinsic resistance, 3) fine-needle biopsies of tumors revealed marked transcriptional upregulation of cytotoxic T cell response and activation signatures, specifically inflammatory interferon signaling (both at baseline and post aPD-L1 administration) in mice responsive to aPD-L1 compared to mice with intrinsic resistance, 4) resistance to aPD-L1 corresponds to reduced T cell receptor expansion and enriched myeloid cell mobilization transcriptomic signatures in the peripheral blood of mice, suggesting the presence of a peripherally-detectable marker of ICI resistance, and finally 5) blocking myeloid cell recruitment using navaraxin (CXCR1/2 inhibitor) or anti-CSF1R enhanced response to ICI, further suppressing tumor growth and enhancing survival.

Conclusions We describe a heterogeneously ICI-responsive mammary murine model, which reflects heterogeneous patient response to ICI. Longitudinal host-specific signatures, specifically myeloid cells, correlating with differential response to ICI may serve as rationale for tracking ICI response in peripheral blood from breast cancer patients. By illuminating the mechanisms underlying immunotherapy resistance, this work has important clinical ramifications for reassessing the advantages of standard-of-care treatment for breast cancer patients. Completion of this project will leverage non-invasive peripheral blood samples as a surrogate for predicting response to ICI in the primary tumor. This may serve as a basis for selecting patients likely to benefit from ICI. This bench-to-bedside work may be tested via clinical trials to enhance patient selection and guide novel therapeutic strategies.