Pembrolizumab and Chemotherapy Treatment or no Treatment Guided by the Level of TILs in Resected Early-stage TNBC

Inclusion Criteria: 1. Understand, sign, and date the written informed consent form prior to any protocol- specific procedures performed, 2. Men and women aged ≥ 18 years, 3. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1, 4. Histologically confirmed and radically removed pT1b/c N0M0 TNBC as defined according to AJCC TNM stage-8th version, * Histologically documented TNBC (negative HER2, ER, and PgR status). HER2 negativity is defined by local laboratory assessment using in situ hybridization and immunohistochemistry assays as per ASCO/CAP criteria and ER/PgR negativity is defined by local laboratory assessment \< 10% using immunohistochemistry assays, * Bilateral and/or multifocal primary tumor is allowed and the tumor with the most advanced T stage should be used to asses for eligibility. If multifocal tumor, a pathologic confirmation of TNBC is required for each focus, 5. Adequately excised breast cancer: subjects must have undergone either breast- conserving surgery or mastectomy/nipple- or skin-sparing mastectomy. * For subjects who undergo breast-conserving surgery, the margins of the resected specimen must be histologically free of invasive tumor and ductal carcinoma in situ (DCIS) as determined by the local pathologist. Reresections to ensure no ink on tumor margins are allowed. Subjects with margins positive for lobular carcinoma in situ (LCIS) are eligible without additional resection. * For subjects who undergo mastectomy/nipple- or skin-sparing mastectomy, margins must be free of gross residual tumor. It is recommended that subjects should have a negative microscopic margin in accordance with local pathology protocol, 6. Have had sentinel lymph node biopsy (SLNB) and/or axillary lymph node dissection (ALND) for evaluation of pathologic nodal status. Axillary nodal dissection(s) should yield a total of at least six nodes (including the axillary lymph nodes resected at the SLNB plus the lymph nodes collected at the axillary nodal dissection), 7. At least 4 weeks but no more than 12 weeks between definitive breast surgery (or the last surgery with curative intent if additional resection is required for breast cancer) and treatment initiation for cohort 1 and no more than 12 weeks for cohort 2, 8. Centrally assessed TILs score from surgical formalin-fixed paraffin embedded (FFPE) tumor sample, using an H\&E stained diagnostic digital slide, according to the most recent International TILs Working Group guidelines, * Cohort 1 will include patients aged \> 40 years with 30% ≤ sTILs \< 50% and those aged * 40 years with 30% ≤ sTILs \< 75% * Cohort 2 will include patients aged \> 40 years with sTILs ≥ 50% and those aged ≤ 40 years with sTILs ≥ 75% 9. Women of childbearing potential have a negative serum pregnancy test within 72 hours prior to receiving the first dose of study medication for cohort 1 and within 7 days of inclusion for cohort 2, 10. Women of childbearing potential must agree to use protocol-specified method(s) of contraception for 3 years after patient inclusion. Men subjects who engage in heterosexual intercourse must agree to use protocol-specified method(s) of contraception during trial treatments and for at least 6 months after the last dose of trial treatments. Females of childbearing potential are those who have not been surgically sterilized or have not been free from menses for \> 1 year, 11. Patients affiliated to the social security system (or equivalent)- France only, 12. Patient is willing and able to comply with the protocol for the duration of the trial including undergoing treatment and scheduled visits, and examinations including follow-up. Additional inclusion criteria for subjects of cohort 1: 13. Left ventricular ejection fraction (LVEF) of ≥ 50% as assessed by echocardiogram or cardiac scintigraphy, 14. Demonstrate adequate organ function within 7 days of inclusion * Absolute Neutrophil Count (ANC) ≥ 1,500 /µL * Platelets ≥ 100,000 /µL * Hemoglobin ≥ 9 g/dL * Creatinine clearance ≥ 30 mL/min for subject with creatinine levels \> 1.5 x institutional upper limit of normal (ULN) * Total bilirubin ≤ 1.5 x ULN or direct bilirubin ≤ ULN for subjects with total bilirubin levels \> 1.5 ULN * Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5 x ULN * Albumin ≥ 3.0 g/dL * Lactate dehydrogenase (LDH) \< 2.5 X ULN * International normalized ratio/partial thromboplastin time (INR/PTT) ≤ 1.5 x ULN (unless subject is receiving anticoagulant therapy as long as prothrombin time (PT) or PTT is within therapeutic range of intended use of anticoagulants) * Thyroid stimulating hormone (TSH), free T4 (FT4), and free T3 (FT3) within normal ranges * Cortisol at 8 AM within normal ranges * Lipase and amylase \< 3 ULN * Fasting plasma glucose ≤ 120 mg/dl or 6.7 mmol/L * Troponin within normal range Exclusion Criteria: 1. History of invasive malignancy ≤ 3 years prior to signing informed consent except for adequately treated basal cell or squamous cell skin cancer, 2. Having received prior chemotherapy or targeted therapy within the past 12 months, 3. Has a prior history of DCIS and/or LCIS that was treated with any form of systemic, hormonal therapy, or radiotherapy to the ipsilateral breast; subjects who had their DCIS/LCIS treated only with surgery and/or contralateral DCIS treated with radiotherapy are allowed to enter the study, 4. Having received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agents or with an agent directed to another co-inhibitory T-cell receptor (e.g., CTLA-4, OX-40, CD137), 5. Treatment with systemic immunostimulatory agents (including, but not limited to, interferons, interleukin-2) within 4 weeks or 5 half-lives of the drug, whichever is longer, prior to inclusion, 6. Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive medications (including prednisone, cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-tumor necrosis factor \[anti-TNF\] alpha agents) within 7 days prior to inclusion: * Subjects who have received acute, low-dose, systemic immunosuppressant medications (e.g., a one-time dose of dexamethasone for nausea) may be enrolled in the study * The use of inhaled corticosteroids and mineralocorticoids is allowed, 7. Has an active autoimmune disease that has required systemic treatment in the past 2 years (i.e., with use of disease modifying agents, corticosteroids or immunosuppressive drugs). Replacement therapy (e.g., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency) is not considered a form of systemic treatment; subjects with eczema, psoriasis, lichen simplex chronicus, or vitiligo with dermatologic manifestations only are eligible if: * Rash must covers \<10% of body surface area. * Disease is well controlled at baseline and requires only low-potency topical Corticosteroids and no acute exacerbations requiring psoralen plus ultraviolet A radiation, methotrexate, retinoids, biologic agents, oral calcineurin inhibitors, or oral corticosteroids occurred within the previous 12 months, 8. Has a known history of Human Immunodeficiency Virus (HIV), 9. Prior allogeneic stem cell or solid organ transplant, 10. Has a known history of active Bacillus Tuberculosis, 11. Patients with any other disease or illness which requires hospitalisation or is incompatible with the trial treatment are not eligible, 12. Pregnant women or breastfeeding or expecting to conceive within the projected duration of the study, from the inclusion visit until the end of the 3 years follow up. Men subjects who engage in heterosexual intercourse and refuse to use protocol-specified method(s) of contraception during trial treatments and for at least 6 months after