Osteopontin enhances phagocytosis through a novel osteopontin receptor, the alphaXbeta2 integrin

Osteopontin (OPN) is a cytokine with multiple functions, including immune defense mechanisms against invading microorganisms. OPN-deficient mice are impaired in clearing intracellular pathogens, suggesting an important role of OPN during phagocytosis, but it remains to be defined how OPN may enhance this innate immune process. Here, we demonstrate that OPN binds to monocytes, but not resting T cells, NK cells, or B cells, and mediates chemoattraction of IL-1-activated human monocytes. Moreover, OPN binds in a specific manner to all known serotypes of the two bacterial species Streptococcus agalactiae and Staphylococcus aureus and opsonizes these bacteria for phagocytosis. We identify the integrin alpha(X)beta(2) (CD11c/CD18), which is highly expressed on the cell surface of monocytes, as a novel OPN receptor. To eliminate the contribution from other molecular interactions between the bacteria and the phagocyte, we show that OPN-coated synthetic beads are phagocytosed in an alpha(X)beta(2) integrin-dependent manner. The ligand recognition does not involve the RGD motif previously reported to support binding of OPN to integrins. Taken together, these data identify the alpha(X)beta(2) integrin as a novel OPN receptor that is required for OPN-mediated phagocytosis, thereby elucidating an important mechanism of an innate immune function of OPN.