Thymic epithelium in vitro. V. Binding of thymocytes to cultured thymic epithelial cells

Direct contact between thymocytes and thymic stromal elements may be one of the mechanisms involved in thymocyte differentiation. Thymic lymphoepithelial complexes have been isolated in which thymocytes appear to be in direct association with cortical epithelial cells. We have previously reported the isolation and successful culture of two morphologically distinct types of murine thymic epithelial cells. We have utilized these to study the interactions of lymphoid and epithelial cells by means of an in vitro assay of the binding of radiolabeled thymocytes to monolayers of these cultured thymic epithelial cells. The percentage of bound cells increased rapidly during the first hour of incubation, reaching approximately 40% binding. Binding continued to increase slowly until plateau levels were reached at approximately 5 hr. Thymocyte binding to thymic epithelium, but not fibroblast monolayers, was trypsin-sensitive, suggesting that specific protein interactions may be involved. Binding of thymocytes to epithelium was temperature-dependent, involved formation of cytoplasmic projections, and was inhibited by cytochalasin B. We also found that cortical thymocytes (peanut agglutinin-positive (PNA+)cells) bound to cultured epithelium to a greater degree than medullary thymocytes (PNA- cells). This correlates with in vivo studies by others in which thymocytes associated with lymphoepithelial complexes have been found to have immature phenotypes. This system provides a means for a quantitative study of the role of cell to cell contact in the process of thymocyte selection and differentiation.