Study Evaluating Impact of IL-7 on CD4 Lymphopenia, Risks of Severe Haematological Toxicity and Tumor Progression in Metastatic Breast Cancer Patients

A key secondary objective is to determine if CYT107 treatment enables to reduce the incidence of severe haematological toxicity (any type of haematological toxicity Grade ≥ 3) post-chemotherapy. Other secondary objectives are to assess the impact of CYT107 treatment on the following parameters: * Overall incidence of side effects (any type any grade) * Progression-free survival (PFS) * Compliance to chemotherapy regimen (dose intensity, number of chemotherapy cycles). * CD4 lymphopenia over the study period Exploratory biological markers A series of biomarkers analyses will be performed to evaluate if CYT107 treatment will: * selectively stimulate the proliferation and activation of peripheral immune subsets (analysis of phenotype and activation status of peripheral immune e sub-populations) * selectively improve the functional response of T cells, DC subsets and NK cells. * is able to revert tolerogenic immune burden to increase specific anti-tumor response (measure of antigen specific CD8 response, measure of cytokine plasmatic levels) * enable to increase TCR diversity (analysis of combinatorial diversity).