Background Immunotherapies such as PD-L1 inhibitors have shown promising efficiency in breast cancer (BC) patients. However, treatment responses for immunotherapies are diverse since the immunogenicity of breast cancer is heterogeneous. Specific subtypes such as hormone receptor (HR)-positive, human EGF receptor 2 (HER2)-positive, and triple-negative breast cancer (TNBC) have shown heterogeneity in immunogenicity. Therefore, precisely identification of patients potentially benefit from immunotherapy is important. Previous studies have proved PD-L1 expression and tumor mutational burden (TMB) as predictive biomarkers for immunotherapy. Here, we report the PD-L1 expression and TMB status in Chinese BC patients with different subtypes. Using comprehensive molecular analysis, we also characterized the genomic features related to these biomarkers. Methods Tumor samples from 112 Chinese patients with BC were collected and subjected to next-generation sequencing (NGS) and immunohistochemical (IHC) analysis. NGS were performed in a laboratory accredited by College of American Pathologists (CAP) and certified by Clinical Laboratory Improvement Amendments (CLIA) using validated panel targeting 450 cancer genes. Genomic alterations, included including single base substitution, short and long insertions/deletion (Indel), copy number variation, gene fusion, and rearrangement, were assessed. Tumor mutational burden (TMB) was measured by an algorithm developed in-house. Tumor tissues were analyzed for PD-L1 expression by IHC with 22C3 or 28-8 antibodies, respectively. Results The 112 Chinese BC patients consisted of 87 HR+ (77.7%), 19 TNBC (17.0%) and 6 HR-/ HER2 + (5.3%), with a median age of 47.5 years old (range 24-81). Of all patients, 42.0% were positive for PD-L1 expression (CPS≥1), including 30.4% PD-L1 (1≥CPS>10) and 11.6% PD-L1 (CPS≥10). PD-L1 expression were observed in HR+ BCs (41.4%) as well as in TNBC (47.4%) and HR-/ HER2+ (33.3%) subtypes, no significant difference were observed among the three subtypes. 8.9% patients were TMB-High (≥10 muts/Mb) with median TMB of 3.4 muts/Mb (range 0-80.8). TMB was slightly correlated with PD-L1 expression (Kendall tau = 0.241; P =0.01). 17.0% of PD-L1 positive patients and 3.1% of PD-L1 negative patients were TMB-High (p=0.016). Patients with PD-L1 CPS≥10 had significantly higher median TMB (7.1 vs. 3.1 muts/Mb, pTP53 (66%), PIK3CA (36%) and ERBB2 (30%); In TMB-High patients, the most frequently mutated genes was PIK3CA (80%), followed by TP53 (60%) and ERBB2 (40%). Conclusion Our data shows that the PD-L1 expression has no significant difference among HR+, TNBC and HR-/ HER2+ subtypes. These data provide the hypothesis that PD-L1 expression positive in different subtype breast cancers may benefit from immune treatment, including HER2 and Luminal subtypes. Otherwise, there was a correlation between PD-L1 expression and TMB, High TMB was also observed in PD-L1 negative patients, which may enrich the subgroup of patients who could benefit from immunotherapy. The verification of integrated predictive biomarkers for immunotherapy in BC is further needed. Citation Format: Ning Liao, Cao Li, Jundong Wu, Bo Chen, Guochun Zhang, Xueri Li, Liping Guo, Guangnan Wei, Jiali Lin, Yingzi Li, Yuchen Zhang, Hsiaopei Mok, Chongyang Ren, Yulei Wang, Kai Li, Cheukfai Li, Lingzhu Wen, Minghan Jia, Tengjiao Lin, Chunyan Cheng. Pd-l1 expression among different subtypes of chinese breast cancer patients [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS17-44.