Osteopontin (OPN) is a glycoprotein expressed by various tissues and cells. The existence of variant forms of OPN as a secreted (sOPN) and intracellular (iOPN) protein and its modification through post-translational modification and proteolytic cleavage explain its broad range of functions. There is increasing knowledge which receptors OPN isoforms can bind to and which signaling pathways are activated to mediate different OPN functions. sOPN interacts with integrins and CD44, mediates cell adhesion, migration and tumor invasion, and has T helper 1 (Th1) cytokine functions and anti-apoptotic effects. iOPN has been described to regulate macrophage migration and interferon-a secretion in plasmacytoid dendritic cells. Both sOPN and iOPN, through complex functions for different dendritic cell subsets, participate in the regulation of Th cell lineages, among them Th17 cells. For skin disease, OPN from immune cells and tumor cells is of pathophysiological relevance. OPN is secreted in autoimmune diseases such as lupus erythematosus, and influences inflammation of immediate and delayed type allergies and granuloma formation. We describe that OPN is overexpressed in psoriasis and propose a model to study OPN function in psoriatic inflammation. Through cytokine functions, OPN supports immune responses against Mycobacteria and viruses such as herpes simplex virus. OPN is also implicated in skin tumor progression. Overexpression of OPN influences invasion and metastasis of melanoma and squamous cell carcinoma cells, and OPN expression in melanoma is a possible prognostic marker. As OPN protein preparations and anti-OPN antibodies may be available in the near future, in-depth knowledge of OPN functions may open new therapeutic approaches for skin diseases.Key words: allergy -autoimmune disease -melanomaosteopontin -psoriasis -skin disease Please cite this paper as: Osteopontin and the skin: multiple emerging roles in cutaneous biology and pathology. Experimental Dermatology 2009; 18: 750-759.

Dysfunctional Tregs have been identified in individuals with psoriasis. However, their role in the pathogenesis of the disease remains unclear. Here we explored the effect of diminished CD18 (β 2 integrin) expression on the function of CD4 + CD25 + CD127 -Tregs using the Cd18 hypomorphic (Cd18 hypo ) PL/J mouse model of psoriasis that closely resembles the human disease. We found that reduced CD18 expression impaired cell-cell contact between Tregs and DCs. This led to dysfunctional Tregs, which both failed to suppress the pathogenic T cells and promoted the onset and severity of the disease. This failure was TGF-β-dependent, as Tregs derived from Cd18 hypo PL/J mice had diminished TGF-β1 expression. Adoptive transfer of Tregs expressing wild-type levels of CD18 into affected Cd18 hypo PL/J mice resulted in a substantial improvement of the psoriasiform skin disease, which did not occur upon coinjection of the cells with TGF-β-specific neutralizing antibody. Our data indicate a primary dysfunction of Cd18 hypo Tregs, allowing subsequent hyperproliferation of pathogenic T cells in the Cd18 hypo PL/J mouse model of psoriasis. This study may provide a step forward in our understanding of the unique role of CD18 expression levels in avoiding autoimmunity.

Smoking-related interstitial lung diseases are characterized by the accumulation of macrophages and Langerhans cells, and fibrotic remodeling, which are linked to osteopontin (OPN) expression. Therefore, OPN levels were investigated in bronchoalveolar lavage (BAL) cells in 11 patients with pulmonary Langerhans cell histiocytosis (PLCH), 15 patients with desquamative interstitial pneumonitis (DIP), 10 patients with idiopathic pulmonary fibrosis, 5 patients with sarcoidosis, 13 otherwise healthy smokers, and 19 non-smoking controls. Furthermore, OPN overexpression was examined in rat lungs using adenoviral gene transfer. We found that BAL cells from patients with either PLCH or DIP spontaneously produced abundant amounts of OPN. BAL cells from healthy smokers produced 15-fold less OPN, and those cells from non-smoking healthy volunteers produced no OPN. BAL cells from patients with either idiopathic pulmonary fibrosis or sarcoidosis produced significantly less OPN, as compared with patients with PLCH. These data were confirmed by immunochemistry. Nicotine stimulation increased production of both OPN and granulocyte-macrophage colony stimulating factor by alveolar macrophages from smokers. Nicotinic acetylcholine receptor expression resembled the pattern of spontaneous OPN production and was dramatically increased in both PLCH and

Allergic contact dermatitis is a T cell-mediated immune response , which in its relapsing chronic form is of high socioeconomic impact. The phosphoglycoprotein osteopontin (OPN) has chemotactic and Th1 cytokine functions and in various models is essential for robust T cell-mediated immunity. Here we demonstrate that OPN is abundantly expressed by both effector T cells and keratinocytes in allergic contact dermatitis lesions. T cells from nickel-allergic donors secrete high levels of OPN following antigen-specific stimulation. OPN may substitute for missing IFN-␥ secretion in T effector cells because low IFN-␥-producing T cell clones secrete high levels of OPN , and OPN down-modulates their interleukin-4 expression. Furthermore , interferon-␥ from T effector cells augments OPN in allergic contact dermatitis by inducing OPN in keratinocytes , which in turn polarizes dendritic cells and attracts inflammatory cells. In the murine contact hypersensitivity (CHS) model for allergic contact dermatitis , OPN is strongly induced in antigen-specific proliferating T cells , and OPN null mice display a reduced chronic CHS inflammatory response due to a decreased influx of effector T cells. Importantly, because of its function for chronic allergic contact dermatitis , OPN may well be a therapeutic target , because anti-OPN antibody treatment in part suppresses established chronic CHS.

TO THE EDITOR Osteopontin is a secreted arginine-glycine-aspartate (RGD)-containing phosphoglycoprotein that interacts with different cell-surface receptors, including αvβ3, αvβ5, α4β1, α9β1, and certain CD44 isoforms (Denhardt et al., 2001; Sodek et al., 2006; Scatena et al., 2007). Osteopontin (OPN) has been implicated to have important cytokine and chemokine functions in Th1/Tc1-mediated immunity against viral and bacterial pathogens and in type-1-mediated diseases such as rheumatoid arthritis, autoimmune encephalitis, and granuloma formation