Patients with hormone receptor-positive (HR + )/HER2- breast cancer may benefit from neoadjuvant immune checkpoint inhibitor (ICI) plus chemotherapy. The effect of chemotherapy or ICI run-in before combination therapy in this population is unexplored. In this randomized pilot trial, patients with HR + /HER2- breast cancer received two weeks of neoadjuvant nab-paclitaxel or pembrolizumab, with baseline and post-run-in tumor biopsy, followed by combined nab-paclitaxel/pembrolizumab. The primary endpoint was PD-L1 expression change between biopsies. Tumor whole exome/RNA sequencing were performed. Of 29 patients, 72% were node-positive. Residual cancer burden (RCB) 0-1 rate was 28% (inclusive of patients receiving additional neoadjuvant adriamycin/cyclophosphamide). No significant change in PD-L1 expression occurred following nab-paclitaxel or pembrolizumab run-in, thus the primary endpoint was not met. Other secondary outcome measures included overall response rate of 80% to the neoadjuvant regimen, and 3-year event-free survival of 86% (95% CI 69-100%); there were no unexpected safety signals. In exploratory biomarker analyses, higher baseline PD-L1 expression and inflammatory gene signatures were associated with favorable response (RCB 0-1); higher expression of estrogen response genes, with unfavorable response (RCB 2-3). Clinical Trial Number: NCT02999477.