Evaluation of alternative prognostic thresholds for SP142 and 22C3 immunohistochemical PD-L1 expression in triple-negative breast cancer: results from a population-based cohort

Background: Immune checkpoint inhibitors are now a part of the treatment arsenal for triple-negative breast cancer (TNBC) but refinement of PD-L1 as a prognostic and predictive biomarker is a clinical priority. We aimed to evaluate the relevance of novel PD-L1 immunohistochemical (IHC) thresholds in TNBC with regard to PD-L1 gene expression, prognostic value, tumor infiltrating lymphocytes (TILs), and TNBC molecular subtypes. Material & methods: PD-L1 was scored in a tissue microarray with the SP142 (immune cell (IC) score) and the 22C3 (combined positive score; CPS) IHC assays and TIL abundance evaluated in whole slides in a population-based cohort of 237 early-stage TNBC patients. Survival analysis was performed and RNA sequencing data employed for molecular profiling. Results: As expected, PD-L1 positivity (IC ≥ 1% and/or CPS ≥ 1) was significantly associated with better prognosis compared to zero PD-L1 expression. Importantly however, also patients with intermediate expression (IC > 0%, < 1%; CPS > 0, < 1) showed a trend toward improved outcome. Tumors with intermediate PD-L1 IHC expression also had intermediate PD-L1 (CD274) gene expression (mRNA). Patients who were both low in TILs (< 30%) and PD-L1 (IC < 1%; CPS < 1) tended to have the poorest prognosis. PD-L1 positive tumors clustered significantly more often as Immunomodulatory-high and Basal-Like 1-high TNBC molecular subtypes and were enriched in immune response and cell cycle/proliferation signaling pathways. PD-L1-zero tumors on the other hand were enriched in cell growth, differentiation, and metastatic potential pathways and clustered more prevalently as Luminal-Androgen-Receptor-high and Mesenchymal-high. PD-L1-intermediate tumors categorized with neither PD-L1-positive nor PD-L1-zero tumors on the hierarchical clustering level, consigning them as a unique subgroup. Conclusion: With both SP142 and 22C3, we identified an intermediate IHC PD-L1 group within TNBCs that was supported on the molecular level. Any PD-L1 IHC expression, even though it is < 1, tended to have positive prognostic impact. We suggest that the generally accepted threshold of PD-L1 IHC positivity in TNBC should be investigated further. The Swedish Cancerome Analysis Network – Breast (SCAN-B) study was retrospectively registered 2nd Dec 2014 at ClinicalTrials.gov; ID NCT02306096.