Alterations in PD-L1+ Myeloid Cells and Immune Infiltration Are Associated with Atezolizumab and Paclitaxel Therapy Success in a Triple-Negative Breast Cancer Model

<i>Background and Objectives:</i> A combination of chemotherapy and immunotherapy may improve cancer treatment outcomes; however, determining which patient groups will benefit from immunotherapy is critical. Triple-negative breast cancer (TNBC) achieves limited benefit from immune checkpoint inhibitors (ICIs) and anti-PD-L1 blockade therapy. <i>Materials and Methods:</i> In this study, PD-L1 expression levels in myeloid-derived cells in the tumor microenvironment were determined in an experimental TNBC model. <i>Results:</i> Compared with tumor cells, granulocytes, monocytes, and macrophages had significantly higher PD-L1 expression. CD206⁺ tumor-associated macrophages (TAMs) expressed the highest level of PD-L1. PD-L1 positivity in TAMs was also significantly high in the lung, liver, lymph node, and spleen. Despite treatment initiation in late-stage tumorigenesis, the combination of paclitaxel and the anti-PD-L1 monoclonal antibody atezolizumab significantly reduced tumor growth. In addition, lung metastasis burden was reduced with combined treatment compared with chemotherapy or anti-PD-L1 monotherapy alone. <i>Conclusions:</i> As a result, alterations in PD-L1⁺ myeloid cells and immune infiltration can be associated with atezolizumab and paclitaxel therapy success in triple-negative breast cancer.