SPP1 and myeloid sarcoma: The aim of our research was to recapitulate, in vitro, the OPN effects on human cells with a particular focus on processes involved in MS relapse, that is, T cell and monocyte activation, T cell apoptosis, lymphocytes, and endothelial cell migration and adhesion, by using recombinant forms of OPN-FL, OPN-N, and OPN-C produced in an eukaryotic system in order to ensure their posttranslation modifications.