Programmed Cell Death Ligand 1 (PD-L1) and Major Histocompatibility Complex Class I (MHC Class I) Expression Patterns and Their Pathologic Associations in Triple-Negative Breast Cancer.

Abstract

Purpose: This study aims to investigate the clinicopathological characteristics of triple-negative breast cancer (TNBC) in relation to programmed cell death ligand 1 (PD-L1) and major histocompatibility complex class I (MHC class I) expression, with a focus on their prognostic significance.

Patients and methods: A retrospective analysis was conducted on formalin-fixed paraffin-embedded (FFPE) tissue samples from 148 TNBC patients diagnosed between 2008 and 2021. Immunohistochemical analysis evaluated PD-L1 and MHC class I expression. PD-L1 was assessed using Combine Positive Scores (CPS), with the threshold set at CPS ≥ 1 and CPS ≥ 10. MHC class I expression was categorized into low and high levels. Associations between these markers, clinicopathological features, overall survival (OS), and disease-free survival (DFS) were analyzed. PD-L1 expression was also compared between older FFPE blocks (2008-2018) versus newer blocks (2019-2021).

Results: PD-L1 expression was observed in 29.1% of cases with a Combined Positive Score (CPS) ≥1 and 8.8% of CPS ≥10 cases. MHC class I expression was evenly split between low and high levels. Older FFPE blocks (2008-2018) showed lower PD-L1 expression than newer blocks (2019-2021). There was no significant association between PD-L1 expression and overall survival (OS) or disease-free survival (DFS). However, high MHC class I expression was strongly associated with improved OS (HR = 0.469, 95% CI: 0.282-0.780, p=0.004). Patients with negative PD-L1 and high MHC class I expression had the most favorable prognosis, with significant OS for CPS ≥1 (HR = 0.447, 95% CI: 0.236-0.846, p=0.013) and CPS ≥10 (HR = 0.516, 95% CI: 0.307-0.869, p=0.013).

Conclusion: These findings support the potential of PD-L1 and MHC class I expression as prognostic markers for TNBC, offering insights to guide treatment decisions and improve patient outcomes.