For more information regarding Bristol-Myers Squibb Clinical Trial participation, please visit www.BMSStudyConnect.com. Inclusion Criteria: - Participants must have untreated, unilateral, histologically confirmed ER+, HER2- invasive breast cancer with primary tumor ≥2 cm in largest diameter (cT1-3) in one dimension by clinical or radiographic exam, for whom neoadjuvant endocrine monotherapy deemed to be a suitable therapy. - Participants must be deemed eligible for surgery and must agree to undergo surgery after completion of neoadjuvant therapy and agree to provide tumor tissue at baseline, on-treatment, and at surgery. - Women must have documented proof that they are not of childbearing potential. - ECOG <=1. Exclusion Criteria: - Participants who may have had any treatment, including radiotherapy, chemotherapy, and/or targeted therapy administered for the currently diagnosed breast cancer prior to enrollment or upfront chemotherapy is clinically judged appropriate as optimal neoadjuvant treatment. - Participants who have a history of or active, known or suspected autoimmune disease, or other syndrome that requires systemic steroids above physiological replacement dose or autoimmune agents for the past 2 years. - Prior treatment with either ET or CDK4/6 inhibitors for Breast Cancer or an anti-PD-1, anti-PD-L1, anti-PD-L2, or anti-CTLA-4 antibody, or any other antibody or drug specifically targeting T-cell co-stimulation or checkpoint pathways, or history of allergy, or hypersensitivity to study drug components - Prior Malignancy active within the previous 3 years as well as or participants with serious or uncontrolled medical disorders. - Personal history of any of the following conditions: syncope of either unexplained or cardiovascular etiology, ventricular arrhythmia (including but not limited to ventricular tachycardia and ventricular fibrillation), long or short QT syndrome, Brugada syndrome, or known history of corrected QT prolongation, Torsade de Pointes, or sudden cardiac arrest. Other inclusion/exclusion criteria apply.