The cell adhesion molecule Thy-1 (CD90) mediates the adhesion of melanoma cells to activated human endothelial cells (EC) via the interaction with the αvβ3-integrin on the tumor cells in vitro. Here, we report a strong expression of Thy-1 on both blood vessel and lymphatic EC in melanoma and melanoma metastases. Vascular endothelial growth factor and tumor necrosis factor-α were identified as inducers of Thy-1 expression on EC in vitro. The physiological role of Thy-1 for lymphogenic and hematogenic metastasis of melanoma cells was substantiated in an experimental metastasis model using B16/F10 melanoma cells. Mice lacking Thy-1 showed markedly diminished experimental lung metastasis after injection of B16/F10 melanoma cells compared to wild-type littermate controls. In addition, on generation of a primary subcutaneous tumor, metastasis to regional lymph nodes was clearly reduced in Thy-1(-/-) mice. However, Thy-1 deletion did not affect subcutaneous primary tumor growth, tumor-induced recruitment of inflammatory cells or T cells, angiogenesis, or T-cell activation. In conclusion, Thy-1 contributes to metastasis of melanoma cells by mechanisms likely involving a Thy-1-mediated adhesion of melanoma cells to EC.

Human Thy-1 (CD90) has been shown to mediate adhesion of inflammatory cells to activated microvascular endothelial cells via interaction with Mac-1 (CD11b/CD18) in vitro. Since there are no data showing the physiological relevance of Thy-1 for the recruitment of inflammatory cells in vivo, different inflammation models were investigated in Thy-1-deficient mice and littermate controls. In thioglycollate-induced peritonitis, the number of neutrophils and monocytes was significantly diminished in Thy-1-deficient mice. During acute lung inflammation, the extravasation of eosinophils and monocytes into the lung was significantly reduced in Thy-1-deficient mice. Moreover, during chronic lung inflammation, the influx of eosinophils and monocytes was also strongly decreased. These effects were independent of Thy-1 expression on T cells, as shown by the transplantation of WT BM into the Thy-1-deficient mice. In spite of the strong Thy-1 expression on T cells in the chimeric mice, the extravasation of the inflammatory cells in these mice was significantly diminished, compared to control mice. Finally, the altered number and composition of infiltrating leukocytes in Thy-1-deficient mice modified the chemokine/cytokine and protease expression at the site of inflammation. In conclusion, Thy-1 is involved in the control of inflammatory cell recruitment and, thus, also in conditioning the inflammatory microenvironment.Key words: Extravasation . Inflammation . Thy-1 Supporting Information available online IntroductionThe recruitment of inflammatory cells to sites of inflammation plays an important role in the pathogenesis of several inflammatory diseases. Leukocyte adhesion to endothelial cells (ECs) follows a multistep process, including the capture of free leukocytes out of the blood stream, rolling, firm adhesion, and transendothelial diapedesis. The importance of several adhesion molecules in this series of events has been described previously [1]. In ICAM-1-deficient mice, neutrophil recruitment was significantly reduced, but it was not completely blocked in a chemical peritonitis model or in a lipopolysaccharide 645(LPS)-induced airway inflammation model, indicating the involvement of additional adhesion molecules [2,3]. Furthermore, leukocyte recruitment in experimental colitis was not affected by blocking ICAM-1 or MadCAM, whereas the blocking of VCAM-1 resulted in a significant attenuation of colitis [4]. Thus, under specific inflammatory conditions, certain adhesion molecules mediate adhesion and transmigration of leukocytes into the perivascular tissue.Recently, human Thy-1 expressed on ECs was identified as an adhesion molecule mediating the binding of neutrophils and monocytes to activated microvascular ECs [5]. Thy-1 is a highly glycosylated GPI-anchored surface protein and a member of the immunoglobulin superfamily [6,7,8]. In humans, Thy-1 is expressed on ECs at sites of inflammation or in tumours whereas ECs do not express Thy-1 in healthy tissue [5,9]. Thy-1 is also expressed on fibroblasts, neurons, and a su...