Immune and Gene-expression Profiling in Estrogen Receptor Low and Negative Early Breast Cancer

Background The cut-off of <1% positive cells to define estrogen receptor (ER) negativity by immunohistochemistry (IHC) in breast cancer (BC) is debated. We explored the tumor immune microenvironment and gene-expression profile of patients with early-stage HER2-negative ER-low (ER 1-9%) BC, comparing them to ER-negative (ER <1%) and ER-intermediate (ER 10-50%) tumors. Patients and Methods Among 921 patients with early-stage I-III, ER ≤50%, HER2-negative BCs, tumors were classified as ER-negative (n=712), ER-low (n=128), or ER-intermediate (n=81). Tumor-infiltrating lymphocytes (TILs) were evaluated. CD8+, FOXP3+ cells, and PD-L1 status were assessed by IHC and quantified by digital pathology. We analyzed 776 BC–related genes in 116 samples. All tests were 2-sided at <0.05 significance level. Results ER-low and ER-negative tumors exhibited similar median TILs, significantly higher than ER-intermediate tumors. CD8/FOXP3 ratio and PD-L1 positivity rates were comparable between ER-low and ER-negative groups. These groups showed similar enrichment in Basal-like intrinsic subtypes and comparable expression of immune-related genes. ER-low and ER-intermediate tumors showed significant transcriptomic differences. High TILs (≥30%) were associated with improved relapse-free survival (RFS) in ER-low (5-year RFS 78.6% versus 66.2%, log-rank p=0.033, hazard ratio (HR) 0.37 [95% CI 0.15-0.96]) and ER-negative patients (5-year RFS 85.2% versus 69.8%, log-rank p<0.001, HR 0.41 [95% CI 0.27-0.60]). Conclusions ER-low and ER-negative tumors are similar biological and molecular entities, supporting their comparable clinical outcomes and treatment responses, including to immunotherapy. Our findings contribute to the growing evidence calling for a reevaluation of ER-positive BC classification and management, aligning ER-low and ER-negative tumors more closely.