SPP1 and plasma cell myeloma: The myeloma-MSC interactions, together with the senescent status of MM-MSCs, further enhance the number of cytokines, chemokines, and soluble factors secreted by MSCs to the BM milieu (e.g., TNFα, SDF-1, BAFF, OPN, HGF, IL-10, IL-8, GDF-15, AREG) which may function as MM growth factors and increase MM proliferation and survival [26,27,29,34].