A ligand of human thy-1 is localized on polymorphonuclear leukocytes and monocytes and mediates the binding to activated thy-1-positive microvascular endothelial 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 Thy-1 expression was found on activated endothelium. In this study we were interested 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 of bullous pemphigoid skin a Thy-1 ligand was found on inflammatory cells, whereas the Thy-1 antigen was expressed on the endothelial cells and fibroblasts. By flow cytometry we could show the expression of a Thy-1 ligand on polymorphonuclear leukocytes and monocytes, whereas lymphocytes did not express this Thy-1 ligand. To study whether Thy-1 is involved in cell-cell adhesion we separated Thy-1-positive and Thy-1-negative cells by magnetic cell separation using the monoclonal antibody AS02. Cell adhesion assays and blocking experiments revealed a direct involvement of the Thy-1/Thy-1 ligand interaction in the binding of monocytes and polymorphonuclear leukocytes to Thy-1-positive activated endothelial cells and fibroblasts.