SPP1 and autoimmune disease: The extracellular matrix protein osteopontin (OPN) is a mediator of systemic inflammation and has multiple biological functions.16 Local production and elevated circulating levels of OPN have been observed in several autoimmune diseases, including SLE.17,18 Overexpression of OPN in lupus-prone mice induces B-cell activation and subsequent production of anti-double-stranded (ds) DNA antibodies, a distinctive laboratory finding in subjects with SLE.