SPP1 and bacterial infectious disease: SPP1, a secreted arginine-glycine-aspartate (RGD)-containing phosphoprotein 1 (previous names: osteopontin, bone sialoprotein I), enhances IFN-γ and IL-12 production and contributes to host defense, bone formation, and wound healing by stimulating macrophage migration as well as protecting against viral and bacterial infections through its pro-Th1 effect [54].