Expression of functionally active α4β1 integrin by thymic epithelial cells
We have investigated the expression and function of the VLA‐4 heterodimer α4β1, a member of the β1 integrin subfamily, on human thymic epithelial cells (TEC) derived from cortical epithelium. The expression of the α4 integrin chain was studied in four different cloned TEC lines derived from either  fetal or post‐natal human thymus by both flow cytometry and immunoprecipitation techniques with anti‐α4 MoAbs. All different cell lines assayed expressed significant levels of α4, as revealed by their reactivity with MoAbs specific for distinct α4 epitopes. The α4 subunit expressed by TEC was associated to β1 but not to β7 chain, and displayed the characteristic 80/70 kD pattern of proteolytic cleavage. The VLA‐4 integrin in these cells was constitutively active in terms of adhesiveness to both fibronectin and vascular cell adhesion molecule‐1 (VCAM‐1). In addition, this heterodimer localized to punctate regions of the cell in the area of contact with the substratum, named point contacts assessed by staining with the anti‐β1 activation epitope 15/7 MoAb. According to the cortical origin of the TEC lines expressing VLA‐4, human thymus sections stained with different anti‐α4 antibodies revealed the presence of cortical, and in smaller numbers medullary epithelial cells bearing α4 integrin. The expression of α4 in the thymus was also found in both adult and fetal rats, in which epithelial cells were also specifically stained. Altogether, our data show that VLA‐4 is an additional component of the integrin repertoire of TEC, and suggest that it could have an important role in thymus epithelial cell–thymocyte interactions.