A ligand of human Thy-1 is localized on polymorphonuclear leukocytes and monocytes and mediates the binding to activated Thy-1-positive microvascularendothelial cells and fibroblasts

Thy-1 is known to be expressed on fibroblasts, nerve cells, and blood stem cells. Previous studies have shown the induction of Thy-1 on phorbol ester stimulated human dermal microvascular endothelial cells in vitro. In situ T hy-1 expression was found on activated endothelium. In this study we were i nterested in the localization of a Thy-1 ligand and the characterization of the function of Thy-1 on human dermal microvascular endothelial cells and fibroblasts. Human Thy-1 purified from fibroblast extracts was labeled and used as a probe for the detection of a Thy-1 ligand. In cryostat sections o f bullous pemphigoid skin a Thy-1 ligand was found on inflammatory cells, w hereas the Thy-1 antigen was expressed on the endothelial cells and fibrobl asts. By flow cytometry we could show the expression of a Thy-1 ligand on p olymorphonuclear leukocytes and monocytes, whereas lymphocytes did not expr ess this Thy-1 ligand. To study whether Thy-1 is involved in cell-cell adhe sion we separated Thy-1-positive and Thy-1-negative cells by magnetic cell separation using the monoclonal antibody AS02. Cell adhesion assays and blo cking experiments revealed a direct involvement of the Thy-1/Thy-1 ligand i nteraction in the binding of monocytes and polymorphonuclear leukocytes to Thy-1-positive activated endothelial cells and fibroblasts. .