A Study Of Changes In PD-L1 Expression During Preoperative Treatment With Nab-Paclitaxel And Pembrolizumab In Hormone Receptor-Positive Breast Cancer

Inclusion Criteria: * Participants must have histologically or cytologically confirmed invasive breast cancer. * Participants must have operable breast cancer, with tumors greater than or equal to 2 cm in size; Participants must not have any evidence of distant metastatic disease. Inflammatory breast cancer is permitted. * All confirmed invasive disease must have been tested for ER, PR, and HER2 and participants must have hormone receptor-positive, HER2-negative breast cancer (ER\>1% or PR\>1%, AND HER2-negative per ASCO CAP guidelines, 2013). * Participants with multicentric, multifocal, and/or contralateral cancers are allowed as long as one lesion meets eligibility and no biopsied tumor is HER2+. * Prior systemic therapy: No prior chemotherapy, biologic therapy, hormonal therapy or investigational therapy for this operable breast cancer. * Prior radiation therapy: No prior radiation to the ipsilateral breast. * The participant is ≥18 years old * The participant has an Eastern Cooperative Oncology Group (ECOG) performance status ≤1 (see Appendix A) * Participants must have normal organ and marrow function as defined below: * Absolute neutrophil count ≥1500/mm3 * Platelets ≥100,000/mm3 * Hemoglobin ≥9 g/dL * Total Bilirubin ≤1.5 mg/dL (\< 2.0 in participants with known Gilbert's syndrome) * Serum creatinine ≤1.5 mg/dL OR calculated GFR ≥60mL/min * Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 2.5 times the upper limit of normal. * International normalized ratio (INR) or Prothrombin Time (PT) \<1.5 times the upper limit of normal unless subject is receiving anticoagulant therapy, as long as PT or PTT is within therapeutic range of intended use of anticoagulants. * Activated Partial Thromboplastin Time (aPTT) \<1.5 times the upper limit of normal unless subject is receiving anticoagulant therapy, as long as PT or PTT is within therapeutic range of intended use of anticoagulants. * The participant is capable of understanding and complying with the protocol and has signed the informed consent document. * The participant must be willing to undergo the three required research biopsies over the course of protocol therapy. Participants who undergo an attempted research biopsy procedure for the purpose of this protocol, and in whom inadequate tissue is obtained, are not required to undergo a repeat biopsy in order to continue on protocol. * The effects of pembrolizumab on the developing human fetus are unknown. For this reason, both women and men of child-bearing potential must agree to use adequate contraception (Section 5.5.2) starting with the first dose of study therapy and for the duration of study participation, through 120 days after the last dose of study medication. Note: Abstinence is acceptable if this is the usual lifestyle and preferred contraception for the subject. Should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately. While on the study, women may not breast-feed. Women of childbearing potential are defined as those who have not been surgically sterilized or have not been free from menses for \> 1 year. * Female subject of childbearing potential should have a negative urine or serum pregnancy test within 72 hours prior to receiving the first dose of study medication. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. * Participants on bisphosphonates may continue receiving bisphosphonate therapy during study treatment. Exclusion Criteria: * The participant has received prior pembrolizumab or any other anti-PD-1, anti-PD-L1, or anti-PD-L2 therapy, or has participated in any prior studies involving pembrolizumab * Hypersensitivity to pembrolizumab or any of its excipients. * The participant has any history or evidence of active, non-infectious pneumonitis or interstitial lung disease. * The participant has an uncontrolled intercurrent illness including, but not limited to, uncontrolled hypertension, unstable angina pectoris, uncontrolled cardiac arrhythmia, congestive heart failure (New York Heart Association Class III or IV; see Appendix B), active ischemic heart disease, myocardial infarction within the previous six months, uncontrolled diabetes mellitus, chronic liver or renal disease, or severe malnutrition. * Concurrent use of potent CYP3A4 inhibitors (see Appendix C), such as ketoconazole and erythromycin, should be avoided during the study treatment with nab-paclitaxel. * Pregnant women are excluded from this study because pembrolizumab has the potential for teratogenic or abortifacient effects. Because there is an unknown but potential risk for adverse events in nursing infants secondary to treatment of the mother with pembrolizumab, breastfeeding should be discontinued if the mother is treated with pembrolizumab. * Active infection requiring intravenous antibiotics at week 1 day 1. * Individuals with a history of a second malignancy are ineligible except for the following circumstances. Individuals with a history of other malignancies are eligible if they have been disease-free for at least 5 years and are deemed by the investigator to be at low risk for recurrence of that malignancy. Individuals with the following cancers are eligible if diagnosed and treated within the past 5 years: cervical cancer in situ, and non-melanoma cancer of the skin. Participants with other cancers diagnosed within the past 5 years and felt to be at low risk of recurrence should be discussed with the study sponsor to determine eligibility. * The participant has a medical condition that requires chronic systemic steroid therapy or any other form of immunosuppressive medication including disease modifying agents, or has required such therapy in the last 2 years. Replacement therapy (eg., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a form of systemic treatment. * The participant has an active autoimmune disease or a documented history of autoimmune disease or syndrome that requires systemic steroids or immunosuppressive agents. * The participant is known to be positive for Hepatitis B surface antigen, or Hepatitis C RNA. Testing for screening is not required. * Known HIV-positive participants.HIV-positive patients on combination antiretroviral therapy are ineligible because of the potential for pharmacokinetic interactions with pembrolizumab. In addition, these participants are at increased risk of lethal infections with bone marrow suppressive therapy, i.e. nab-paclitaxel. Appropriate studies will be undertaken in participants receiving combination antiretroviral therapy when indicated. Testing for screening is not required. * The participant has received a live vaccine within 28 days of planned start of study therapy. * Seasonal influenza vaccines for infection are generally inactivated flu vaccines and are allowed; however intranasal influenza vaccines (i.e. Flu-Mist ®) are live attenuated vaccines, and are not allowed.