• Patient should understand, sign, and date the written informed consent form (including the consent to collect tissue, blood and stool samples, as specified by the protocol) prior to any protocol-specific procedures performed. Patient should be able and willing to comply with study visits and procedures as per protocol.

• Female patients aged 18 years or older

• Histologically confirmed untreated invasive carcinoma of the breast (ER \< 10%, PR \< 10%, HER2 negative) as locally determined

• Tumor infiltrating lymphocytes (TILs) ≥ 5% in breast tumor biopsy as locally determined

• Breast cancer clinical TNM stage I (cT1N0 as measured by radiological imaging). Bilateral, multicentric and multifocal tumors are allowed, assuming tumor evaluations and pre- and post-treatment biopsies are performed in the same target lesion. Only the largest tumor will be measured to determine the study eligibility.

• No evidence of metastatic disease or confirmed lymph node involvement

• Eastern Cooperative Oncology Group (ECOG) performance status 0/1

• Patients of child-bearing potential are eligible, provided they have a negative serum β-hCG pregnancy test within 2 weeks or urine pregnancy test within 48 hours prior to the first dose of study treatment, and agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive methods with a failure rate of \< 1% per year during the treatment period and for 3 months after the last dose of durvalumab Note: A woman is considered of childbearing potential following menarche and until becoming post-menopausal (≥ 12 months of non-therapy-induced amenorrhea) unless permanently sterile. Permanent sterilization methods include hysterectomy, bilateral oophorectomy and bilateral salpingectomy.

• Sexually active women of childbearing potential must agree to use a highly effective method of contraception supplemented by a barrier method, or to abstain from sexual activity during the study and for at least 3 months after the last study treatment administration. Female subjects should also refrain from breastfeeding throughout this period.

⁃ Note: A highly effective birth control method is a one, which can achieve a failure rate of less than 1% per year when used consistently and correctly. Such methods include: combined (estrogen and progestogen containing) hormonal contraception; progestogen-only hormonal contraception associated with inhibition of ovulation; intrauterine device (IUD); intrauterine hormone-releasing system (IUS); bilateral tubal occlusion; vasectomized partner (on the understanding that this is the only one partner during the whole study duration), and sexual abstinence during the entire period of risk associated with study treatment. To prevent the risk of interaction between the study drug and hormonal contraceptives, hormonal contraceptives should be supplemented with a barrier method (preferably male condom). Following methods are considered as unacceptable methods (non-exhaustive list): periodic abstinence (calendar, symptothermal, post-ovulation methods) and withdrawal (coitus interruptus).

⁃ \- Blood tests demonstrating: Creatinine ≤ 1.5 x ULN Bilirubin ≤ 1.5 x ULN, AST or ALT \< 3 x ULN, ALP \< 2.5 x ULN (patients with known Gilbert disease who have serum bilirubin level ≤ 3 × the institutional ULN may be enrolled) For patients not receiving therapeutic anticoagulation: INR or aPTT ≤ 1.5 x ULN. For patients receiving therapeutic anticoagulation: stable anticoagulant regimen Adequate bone marrow function as shown by: ANC ≥ 1.5 x 109/L, Platelets ≥ 100 x 109/L, Hb \> 9 g/dL, Serum albumin \> 2.5 g/dL Fasting Serum amylase ≤ 2 × ULN Fasting Serum lipase ≤ ULN

⁃ \- Patients must be affiliated to a social security system or beneficiary of the same.