Osteopontin: a key cytokine in cell-mediated and granulomatous inflammation

Osteopontin (Opn) is a secreted adhesive, glycosylated phosphoprotein that contains the arginine-glycine-aspartic acid (RGD) cell-binding sequence tha t is found in many extracellular matrix (ECM) proteins (for a review of Opn see References Denhardt & Guo 1993; Patarca et al. 1993; Rittling & Denhar dt 1999). Since its initial description in 1979 as a secreted protein assoc iated with malignant transformation, Opn has been independently discovered by investigators from diverse scientific disciplines, and has been associat ed with a remarkable range of pathologic responses. Opn is an important bon e matrix protein, where it is thought to mediate adhesion of osteoclasts to resorbing bone. However, studies from the past decade have identified an a lternative role for Opn as a key cytokine regulating tissue repair and infl ammation. Recent work by our laboratory and that of others has underlined t he importance of Opn as a pivotal cytokine in the cellular immune response. Despite this Opn is not well known to the immunologist. In this review we will focus on studies that pertain to the role of Opn in cell-mediated and granulomatous inflammation.