Abstract PS2-12-05: Programmed death-ligand 1expression in triple negative breast cancer: insights from an African ancestry-enriched multinational cohort

Abstract Programmed death-ligand 1 (PD-L1) is an immune checkpoint protein expressed on tumor and immune cells that promotes immune evasion by inhibiting T-cell activation. In advanced triple-negative breast cancer (TNBC), a subtype with aggressive behavior and limited targeted treatment options, PD-L1 expression has emerged as a predictive biomarker for response to immune checkpoint inhibitors. However, most studies have focused on predominantly European-ancestry populations, leaving gaps in understanding PD-L1’s expression and clinical relevance among women of African ancestry, who are disproportionately affected by TNBC. Given TNBC’s molecular heterogeneity across ancestral groups, examining PD-L1 expression in diverse populations is critical for equitable immunotherapy and understanding ancestry-specific tumor biology.The International Center for the Study of Breast Cancer Subtypes (ICSBCS) biobank provides a unique ancestrally diverse tissue repository, enabling biomarker studies across historically underrepresented populations. Leveraging this resource, we are conducting a large-scale immunohistochemical (IHC) analysis of PD-L1 on formalin-fixed paraffin-embedded (FFPE) TNBC tissue sections. The study aims to evaluate PD-L1 in 1,000 TNBC samples, equally distributed among Ghanaian, Ethiopian, African American (AA), and European American (EA) patients. Here, we report initial findings.To date, 301 cases have been processed (AA n = 47, Ethiopian n = 12, Ghanaian n = 205, EA n = 37). PD-L1 expression, measured as percent positive tumor cells by IHC, showed means (± SD, 95% CI) of 7.25% (± 6.28, 3.34-11.16) in Ethiopian, 6.55% (± 6.28, 4.36-8.73) in EA, 5.32% (± 6.28, 3.39-7.25) in AA, and 1.13% (± 6.28, 0.20-2.06) in Ghanaian cases. Tukey-Kramer HSD comparisons revealed significant differences between Ethiopian and Ghanaian (p=0.0156), EA and Ghanaian (p