A novel thymic epithelial adhesion molecule

Interactions between thymocytes and thymic stromal cells are responsible for positive and negative selection of T cell, their differentiation, maturation and proliferation. The signals required for these events to occur often necessitate close contact, and indeed adhesion, between the cell types involved. The identification of specific adhesion molecules in this context, is, therefore, a vital first step in determining the nature of the signal they mediate or facilitate at a given stage of differentiation. In the present work we identify, isolate and partially characterize a ligand present on thymic medullary epithelial cells which selectively binds CD4+ CD8+ thymocytes found primarily in the thymic cortex. This adhesion molecule is composed of two non-covalently associated glycoproteins of 23 kDa and 45 kDa, respectively, both of which are needed to bind to thymocytes. The importance of the finding is that the ligand, in isolated immobilized form, binds the same thymocyte subset as the original epithelial cell line from which it was isolated. The CD4+ CD8+ thymocyte subset is the precursor of single-positive mature T cells; hence the putative biological activity of the ligand in question takes place at a pivotal stage of T cell differentiation.