Thrombin cleavage site (αvβ3, αvβ5, αvβ1, α8β1) (CD44) P P P P P P P P P PP NH2 1 Ca2+ binding domain RGD SVVYGLR (α9β1, α4β1, α4β7) 169 Ca2+ binding domain 314 Fig. 1 OPN structural features. The cell adhesive domains are indicated in color. The known specific integrin receptors for each adhesion domain are also shown in color. The calcium (Ca2+) binding domains are shown in yellow and other ma- trix binding domains are in black. Also shown are known phosphorylation sites (P). Arrows indicate known cleavage sites for thrombin and MMPs. Thrombin cleavage site in blue and MMPs cleavage site in green Macrophages OPN is not expressed in circulating monocytes, but is dramatically upregulated during macrophage differentiation and constitutes one of the major macrophage products (Krause et al. 1996). OPN is known to be induced in macrophages by several inflammatory cytokines, including TNF-α, IL-1β, IFN-γ, and IL-6, and other factors including angiotensin-II, oxidizedLDL, and phorbol-ester are known inducers of OPN in macrophages (Nakamachi et al. 2007; Ogawa et al. 2005; Bruemmer et al. 2003). More recently, Liver X Receptor and Peroxisome proliferator- activated receptor α antagonists have been shown to suppress OPN expression (Nakamachi et al. 2007; Ogawa et al. 2005). Functionally OPN plays a key role in macrophage biology by regulating migration, survival, phagocytosis, and pro-inflammatory cytokine production (Bruemmer et al. 2003; Nyström et al. 2007). We and others have shown that OPN serves as a potent chemoattractant for macrophages (Bruemmer et al. 2003; Persy et al. 2003; Giachelli et al. 1998; Panzer et al. 2001). Functional inhibition of OPN and genetic ablation of OPN in mice greatly impair macrophage recruitment in several models of acute inflammation. In OPN-null mice acute macrophage infiltration was greatly diminished com- pared to wild-type mice in an obstructed kidneys model (Ophascharoensuk et al. 1999), and in a thioglycollate induced peritonitis model (Bruemmer et al. 2003). Further, purified OPN induced macrophage accumulation when injected in rat dermis and following intradermal injection of N-formyl-met-leu-phe (FMLP), a potent macrophage chemotactic peptide. The OPN effect was neutralized by an anti-OPN blocking antibody (Giachelli et al. 1998). In a rat model of heart injury OPN was highly expressed in the granulation tissue associated macrophages, and it was downregulated with healing progression and formation of the fibrotic scar (Murry et al. 1994). Wound healing studies in mice also indicate that OPN is expressed during the S.A. Lund et al. COOH 300 MMP cleavage site acute inflammatory phase at very high levels in infiltrating leukocytes and other cell types where it appears to regulate leukocyte infiltration and activation as well as proper matrix organization (Liaw et al. 1998). Interestingly, downregulation of OPN at the wound site with antisense mRNA diminished macrophage infiltration and accelerated wound healing (Mori et al. 2008). OPN also modifies chronic inflammatory responses. Chronic inflammation is characterized by the persistence of macrophages at sites of injury and disease. Deficits in macrophage accumulation have been noted in OPN-null mice when challenged with chronic inflammatory condi- tions, including atherosclerosis, delayed-type hypersensi- tivity (Yu et al. 1998; Ashkar et al. 2000), granulomatous disease (Weber et al. 2002; Nau et al. 1999), and biomaterial implantation (Steitz et al. 2002; Tsai et al. 2005). These data suggest that OPN may be particularly important in promoting migration and retention of macro- phages at sites of acute and chronic inflammation. We have also shown that OPN regulates foreign body giant cell (FBGC) formation in vitro and in vivo. In a recent paper we described that despite the defect in macrophage recruitment, OPN-null mice formed more FBGCs on the surface of the implant. In vitro, OPN inhibited macrophage fusion to form FBGCs in a dose dependent manner (Tsai et al. 2005). In vitro, OPN-null macrophages exhibit reduced basal migration and impaired migration towards MCP-1, despite the fact that wild type and OPN-null macrophages express comparable levels of CCR-2, the MCP-1 receptor. This may be a consequence of a lack of a permissive pro-migratory substrate, and the reduced expression of CD44 observed in OPN-null macrophages. CD44 is well known to be essential for macrophage migration (Marcondes et al. 2008), and its expression is upregulated by OPN in macrophages (Chellaiah et al. 2003). Macrophages from OPN-null mice are also more susceptible to programmed cell death (Bruemmer et al. 2003). Together with impaired migration, macrophage apoptosis may further contribute the