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Abstract

Osteopontin (Opn) is a secreted adhesive, glycosylated phosphoprotein that contains the arginine-glycine-aspartic acid (RGD) cell-binding sequence that is found in many extracellular matrix (ECM) proteins (for a review of Opn see References Denhardt & Guo 1993; Patarca et al. 1993; Rittling & Denhardt 1999). Since its initial description in 1979 as a secreted protein associated with malignant transformation, Opn has been independently discovered by investigators from diverse scientific disciplines, and has been associated with a remarkable range of pathologic responses. Opn is an important bone matrix protein, where it is thought to mediate adhesion of osteoclasts to resorbing bone. However, studies from the past decade have identified an alternative role for Opn as a key cytokine regulating tissue repair and inflammation. Recent work by our laboratory and that of others has underlined the 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.

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Development of TH1 CD4+ T cells through IL-12 produced by Listeria-induced macrophages

C C Hsieh, S Macatonia, Adam C Tripp … (1993)

Development of the appropriate CD4+ T helper (TH) subset during an immune response is important for disease resolution. With the use of naïve, ovalbumin-specific alpha beta T cell receptor transgenic T cell, it was found that heat-killed Listeria monocytogenes induced TH1 development in vitro through macrophage production of interleukin-12 (IL-12). Moreover, inhibition of macrophage production of IL-12 may explain the ability of IL-10 to suppress TH1 development. Murine immune responses to L. monocytogenes in vivo are of the appropriate TH1 phenotype. Therefore, this regulatory pathway may have evolved to enable innate immune cells, through interactions with microbial pathogens, to direct development of specific immunity toward the appropriate TH phenotype.

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Eta-1 (osteopontin): an early component of type-1 (cell-mediated) immunity.

S Ashkar, G F Weber, V Panoutsakopoulou … (2000)

Cell-mediated (type-1) immunity is necessary for immune protection against most intracellular pathogens and, when excessive, can mediate organ-specific autoimmune destruction. Mice deficient in Eta-1 (also called osteopontin) gene expression have severely impaired type-1 immunity to viral infection [herpes simplex virus-type 1 (KOS strain)] and bacterial infection (Listeria monocytogenes) and do not develop sarcoid-type granulomas. Interleukin-12 (IL-12) and interferon-gamma production is diminished, and IL-10 production is increased. A phosphorylation-dependent interaction between the amino-terminal portion of Eta-1 and its integrin receptor stimulated IL-12 expression, whereas a phosphorylation-independent interaction with CD44 inhibited IL-10 expression. These findings identify Eta-1 as a key cytokine that sets the stage for efficient type-1 immune responses through differential regulation of macrophage IL-12 and IL-10 cytokine expression.

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Isolation and characterization of two sialoproteins present only in bone calcified matrix.

D Heinegård, Carrie A. Franzen (1985)

Two different sialoproteins were isolated from the mineralized matrix of bovine bone by using extraction with guanidinium chloride first without and then with EDTA. The sialoproteins were purified by chromatography on DEAE-cellulose eluted with a sodium acetate gradient in 7 M-urea, pH 6. Two sialoproteins (I and II) were then separated by chromatography on DEAE-cellulose eluted with a sodium chloride gradient in 7 M-urea, pH 4. The ratio between recovered sialoprotein I and II was 1:5. The chemical analysis of the two sialoproteins showed that they differed. Both, however, had very high contents of aspartic acid/asparagine and glutamic acid/glutamine though they differed markedly in contents of leucine and glycine. Both sialoproteins contained phosphate, sialoprotein I more than sialoprotein II. Content of sialic acid was substantially higher in the more prominent sialoprotein II (13.4% of dry weight) than in sialoprotein I (4.8% of dry weight). The peptide patterns produced by trypsin digests of [125I]iodinated sialoproteins I and II showed both structural similarities and structural differences. Sialoprotein II, being the major component, was characterized further. Its molecular mass was 57300 Da determined by sedimentation-equilibrium centrifugation in 6 M-guanidinium chloride, and its sedimentation coefficient (S0(20),w) was 2.53 S. Upon rotary shadowing, sialoprotein II appeared as an extended rod, having a core with an average length of 40 nm. Two types of oligosaccharides, N-glycosidically and O-glycosidically linked to the core protein, were isolated from sialoprotein II. Contents of mannose and sialic acid in the O-linked oligosaccharide were surprisingly high. Antibodies against sialoprotein II were raised in rabbits and an enzyme-linked immunosorbent assay was developed. Antigenicity of sialoprotein II was not affected by reduction and alkylation, was only partially lost upon trypsin digestion and was completely lost upon fragmentation of the core protein by alkaline-borohydride treatment, indicating that all antigenic sites were located in the protein portion. Sialoprotein I expectedly showed only partial immunological cross-reactivity with sialoprotein II. The quantity of sialoprotein II in bone extracts was found to be about 1.5 mg/g wet wt. of bone, but the protein was not detected in extracts of a number of other bovine tissues i.e. aorta, cartilage, dentine, kidney, liver, muscle, sclera, skin and tendon.