SPP1 and autoimmune disease: Opn plays a key role in the formation, maturation, differentiation, and activation of T cells: secreted Opn (sOpn) is involved in the formation of T helper type 1 (Th1) and Th17, pathogenic T cells in various autoimmune diseases, whereas intracellular Opn (iOpn) is required for the development of Th17 cells as well as the production of IFN-α [3] and IFN-β by plasmacytoid dendritic cells DCs (pDCs) [4].