This is a prospective, observational, biological, multicenter study to investigate IR profile, ATLG dynamics, main HSCT and disease outcomes in MS. This study will provide a preliminary descriptive evaluation of key study parameters, including ATLG pharmacokinetics and immune reconstitution trends, as well as an initial assessment of variability (e.g., dispersion of immunological biomarkers), to support the interpretation of results and the design of future studies. Since this is an observational study, no clinical decision will be made, and the interim analysis will not have an impact of the study conduction, and it will not be a stopping rule.