Tumor-specific major histocompatibility-II expression predicts pathological complete response to atezolizumab combined to chemotherapy in triple-negative breast cancer

Adding immune checkpoint inhibitors to neoadjuvant chemotherapy improves outcomes in early-stage triple-negative breast cancer (TNBC), but a fraction of patients derive benefit. Tumor-specific MHC-II (tsMHC-II) expression has been shown to be a predictive biomarker of pathological complete response (pCR) to neoadjuvant chemo-immunotherapy in early-stage TNBC. We performed biomarker analysis of the phase III NeoTRIP trial where patients were randomized to neoadjuvant carboplatin and nab-paclitaxel±atezolizumab. Imaging mass cytometry was used to assess tsMHC-II expression in tumor samples. TsMHC-II positivity was predefined as ≥5% of tumor cells expressing MHC-II, and at an 80th percentile exploratory cutoff. TsMHC-II positivity was associated with a higher pCR rate in the atezolizumab arm (OR:2.58; P = 0.016), but not in the chemotherapy-only arm (OR:1.37; P = 0.34) and these results were stronger using the exploratory cutoff. TsMHC-II expression is associated with improved response to neoadjuvant chemo-immunotherapy in early TNBC and could represent a clinically useful predictive biomarker for treatment personalization.