Clinical and Pathologic Features Associated with PD-L1 (SP142) Expression in Stromal Tumor-Infiltrating Immune Cells of Triple-Negative Breast Carcinoma

The Ventana PD-L1 SP142 immunohistochemistry (IHC) assay is the FDA-approved companion diagnostic assay for atezolizumab therapy selection for patients with PD-L1-positive locally advanced or metastatic triple-negative breast carcinoma (TNBC). We aimed to elucidate clinical, pathologic, and molecular features associated with PD-L1 expression in TNBCs. Clinical, pathologic, and next-generation sequencing (NGS)-based molecular data for TNBCs tested with PD-L1 (SP142) IHC at our institution between 11/2018 and 12/2019 were retrieved and reviewed. PD-L1 positivity was defined as ≥1% IC staining. Patients with metastatic TNBC treated at first line with atezolizumab regimens were evaluated for treatment response and for time to treatment failure (TTF). Among 156 TNBCs, PD-L1 was positive in 47.4% of cases. Primary TNBCs were significantly more frequently PD-L1 positive, compared with recurrent/metastatic samples (p = 0.002). PD-L1-positive TNBCs had increased stromal IC, compared with PD-L1-negative samples (p