The role of osteopontin in inflammatory processes impaired macrophage accumulation observed in OPN-null mice in response to acute and chronic inflammatory stimuli. In addition to regulating macrophage migration, OPN can also modulate the cytokine production by macro- phages. OPN stimulates production of IL-12 while inhibiting the production of IL-10, thereby promoting Th1 cell mediated responses (Weber et al. 2002; Ashkar et al. 2000). Interestingly, these results were mediated by different receptors. IL-12 production was mediated via an N-terminal fragment interaction with αvβ3 integrin, while IL-10 was inhibited via a C-terminal fragment, possibly via the CD44 receptor. OPN regulation of IL-12 and IL-10 were also demonstrated in vivo in an angiotensin II (AngII)-accelerated model of atherosclerosis. In this model, Bruemmer at al. showed reduced expression of IL-12 and an increased expression of IL-10 in ApoE-/- OPN-/- AngII treated mice compared to ApoE-/-OPN+/+ AngII treated control mice by RT-PCR of whole mouse aortas. These results correlated with less macrophage rich lesions and lower expression of the macrophage marker CD68 (Bruemmer et al. 2003). While OPN is generally classified as a pro-inflammatory cytokine, it also appears to have anti-inflammatory effects. OPN is a potent trans-repressor of inducible nitric oxide synthase (iNOS) expression in macrophages (Rollo et al. 1996). OPN represses inducible nitric oxide synthase (iNOS) by increasing Stat1 ubiquitination and proteasome mediated degradation of Stat1, consequently inhibiting Stat1 mediated iNOS transcription and protein expression (Gao et al. 2007). NO feedback inhibits its own synthesis by increasing transcription of OPN (Guo et al. 2001). OPN inhibition of NO may be particularly relevant for tumor cell evasion of inflammation (Wai et al. 2006; Crawford et al. 1998). Finally, a recent study suggests that OPN may play a role in macrophage differentiation. Using siRNA to stably silence the expression of OPN in RAW 264.7 cells, Nystrom et al. showed that OPN silenced cells displayed an altered pheno- type with monocyte-like characteristics (Nystrom et al. 2007). Further, OPN silenced cells had decreased expression of macrophage scavenger receptor A type 1 (Msr-1), a macrophage differentiation marker. While these studies are intriguing, the phenotype could not be rescued by the addition of exogenous OPN suggesting non-receptor medi- ated effects or iOPN may be involved. Together, these data suggest that OPN may be particularly important in promoting migration and retention of macrophages at sites of acute and chronic inflammation by regulating multiple macrophage func- tions. These studies also emphasize the importance of macrophage-derived OPN in the regulation of OPN’s functions, suggesting that macrophages are both a source and target of OPN. 315 Neutrophils While much is known about the role of OPM in macrophage biology, relatively few studies have ex- plored the function of OPN in neutrophils. Neutrophils express low levels of OPN (Koh et al. 2007). However, OPN is important for the recruitment and migration of neutrophils, as neutrophils from OPN-null mice display reduced chemotaxis toward fMLP and in vivo the recruit- ment of neutrophils to the peritoneal cavity in response to sodium periodate is impaired in OPN-null mice (Koh et al. 2007). OPN-null mice also have impaired neutrophil infiltration into liver when challenged with concanavalin A induced hepatitis (Diao et al. 2004). Despite these defects in migration and chemotaxis, OPN-null neutrophils do not display reduced destructive capacity in terms of phagocy- tosis, the generation of reactive oxygen species, or cytokine production (Koh et al. 2007). Recent reports indicate that polymeric OPN interacts with α9β1 on neutrophils and serves as a potent neutrophil chemoattractant (Nishimichi et al. 2009). T-cells OPN is also known as Eta-1 (early T lymphocyte activation gene 1) for its high expression in activated T cells and it plays an important role in the induction of cell mediated immune responses through the regulation of T cells. Following activation, naïve CD4 T cells can differentiate towards Th1, Th2, or Th17 cells which differ in effector function. The development of Th1 cells leads to cell- mediated immunity while development of Th2 cells provides humoral immunity. Th17 cells are associated with autoimmunity. OPN is not expressed in naïve T-cells but it is strongly upregulated in response to T cell receptor ligation (Shinohara et al. 2005). OPN functions in T cells by mediating migration, adhesion, and co-stimulating T cell proliferation (O’Regan et al. 1999; Patarca et al. 1993). Shinohara et al. have shown that OPN gene expression in T cells is controlled by T-bet, a transcription factor that promotes CD4+ T helper cell lineage commitment to Th1 (Shinohara et al. 2005). Further, T-bet-dependent expres- sion of OPN in T cells is essential for efficient skewing of CD4 T and CD8 T cells toward the Th1 and type 1 CD8 T cell (Tc1) pathway, respectively (Shinohara et al. 2005). In vivo, OPN-null mice display impaired Th1 responses to the intracellular bacterium Listeria monocytogenes and the viral pathogen HSV1 (herpes simplex virus type 1) both of which depend on the induction of IL-12 for protection (Ashkar et al. 2000). Indeed, mice deficient in OPN have decreased IL-12 and IFN-γ production, while IL-10 levels are enhanced (Bruemmer et al. 2003). Further studies have shown that OPN regulates CD3-mediated T cell expression