Inclusion Criteria:  Male or female, aged ≥18 years, Performance status Eastern Cooperative Oncology Group World Health Organization (ECOG-WHO) ≤1, Histologically proven metastatic squamous cell carcinoma of anus (SCCA) Presence of a evaluable lesion on CT-scan/MRI assessed by RECIST v1.1 criteria, Patient eligible to the mDCF regimen CT scan performed within 30 days prior inclusion, PET scan performed within 30 days prior inclusion Life expectancy ≥12 months,  Adequate organ and marrow function, based upon meeting all of the following laboratory criteria within 14 days before first dose of study treatment:  Absolute neutrophil count (ANC) ≥ 1500/mm3 (≥ 1.5 GI/L) without granulocyte colony-stimulating factor support. White blood cell count ≥ 2500/mm3 (≥ 2.5 GI/L). Platelets ≥ 100,000/mm3 (≥ 100 GI/L) without transfusion. Hemoglobin ≥ 9 g/dL (≥ 90 g/L). Alanine aminotransferase (ALT), aspartate aminotransferase (AST) ≤ 3 x upper limit of normal (ULN), or ≤ 5 x ULN with documented liver metastases. Total bilirubin ≤ 1.5 x ULN (for subjects with Gilbert's disease ≤ 3 x ULN). Serum albumin ≥ 2.8 g/dl. Calculated creatinine clearance ≥ 60 mL/min (using the MDRD formula): Urine protein/creatinine ratio (UPCR) ≤ 1 g/g Signed and dated informed consent, to participate indicating that the subject has understood the purpose and the procedures required by the study and that he agrees to participate in the study and to comply with the requirements and restrictions inherent in this study Patient affiliated to or beneficiary of French social security system Ability to comply with the study protocol, in the Investigator's judgment  Exclusion Criteria:  HIV positive patient , CD4 count < 400 cells/mm3 (HIV test mandatory before inclusion) Diagnosis of additional malignancy within 2 years prior to the inclusion with the exception for superficial skin cancers, or localized, low grade tumors deemed cured and not treated with systemic therapy, Any medical or psychiatric condition or disease, which would make the patient inappropriate for entry into this study, Current participation in a study of an investigational agent or in the period of exclusion, Receipt of any type of cytotoxic, biologic or other systemic anticancer therapy (including investigational) within 4 weeks before first dose of study treatment, Radiation therapy for bone metastasis within 2 weeks, any other radiation therapy within 4 weeks before first dose of study treatment. Systemic treatment with radionuclides within 6 weeks before the first dose of study treatment. Subjects with clinically relevant ongoing complications from prior radiation therapy are not eligible, Pregnancy, breast-feeding or absence/refusal of adequate contraception for fertile patients during the period of treatment and for 6 months from the last treatment administration, Patient under guardianship, curatorship or under the protection of justice. Previously received immunotherapy Previously received chemotherapy Local or locoregional recurrence Untreated or symptomatic central nervous system (CNS) lesion. However, patients are eligible if: a) all known CNS lesions have been treated with radiotherapy or surgery and b) patient remained without evidence of CNS disease progression ≥ 4 weeks after treatment and c) patients must be off corticosteroid therapy for ≥ 2 weeks Use of hematopoietic colony-stimulating growth factors (e.g. G-CSF, GMCSF, M-CSF), thrombopoietin mimetics or erythroid stimulating agents ≤ 2 weeks prior start of study treatment. If erythroid stimulating agents were initiated more than 2 weeks prior to the first dose of study treatment and the patient is on a stable dose, they can be maintained. Use of any live vaccines against infectious diseases within 4 weeks of initiation of study treatment Elevated Cardiac troponin T (cTnT) or cardiac troponin I (cTnI) elevation > 2x ULN  Systemic chronic steroid therapy (> 10mg/day prednisone or equivalent) or any immunosuppressive therapy 7 days prior to planned date of first dose of study treatment.  Note: Topical, inhaled, nasal and ophthalmic steroids are allowed. For patients with adrenal insufficiency, replacement dose of prednisone > 10 mg/ day or equivalent are permitted  Active, known or suspected autoimmune disease or a documented history of autoimmune disease Note: Patients with vitiligo, controlled type I diabetes mellitus on stable insulin dose, residual autoimmune-related hypothyroidism only requiring hormone replacement or psoriasis not requiring systemic treatment are permitted. Allogenic bone marrow or solid organ transplant History of severe hypersensitivity reactions to other monoclonal antibodies, which in the opinion of the investigator may pose an increased risk of serious infusion reaction History or current interstitial lung disease or non-infectious pneumonitis Active Hepatitis B infection (HBsAg positive) Active hepatitis C (HCV RNA positive) Pregnant or nursing (lactating) women confirmed by a positive hCG laboratory test within 72 hours prior to initiating study treatment. Women of child-bearing potential, defined as all women physiologically capable of becoming pregnant, unless they are using highly effective methods of contraception during dosing and for 150-days after stopping treatment with Spartalizumab.