Extracellular matrix components of the mouse thymic microenvironment. III. Thymic epithelial cells express the VLA6 complex that is involved in laminin-mediated interactions with thymocytes.

We describe herein the expression of the VLA6 complex by murine thymic epithelial cells (TEC). The immunohistochemical distribution revealed that VLA6 is found in both thymic medullary and subcapsullary areas. Moreover, studies by immunoelectron microscopy revealed a membrane labeling of the VLA6 molecule, including at desmosomal sites. By means of immunoblotting, immunoprecipitation, and affinity chromatography of extracts from a mouse TEC line, we further demonstrated that VLA6 is a laminin (LN) receptor in these cells. In keeping with this finding, we showed that TEC adhesion, spreading, and proliferation were enhanced in vitro by LN. The fact that VLA6 is also expressed by the large majority of thymocytes raised the hypothesis that it might be involved in LN-mediated TEC-thymocyte interactions. Interestingly, in vitro experiments showed that there is an increase in the TEC-thymocyte adhesion upon glucocorticoid hormone treatment, a situation in which the expression of VLA6 as well as LN is enhanced. Most importantly, this adhesion can be reversed by pre-treating TEC with an anti-alpha 6 integrin mAb. Additionally, spontaneous in vitro thymocyte release by thymic nurse cell complexes was enhanced by LN and partially blocked by anti-alpha 6 or anti-beta 1 antibodies. Our results suggest that VLA6 is involved in LN-mediated TEC-thymocyte interactions that can be relevant for thymic microenvironmental cell physiology and intrathymic T cell differentiation events.

Identification of nuclear triiodothyronine receptors in the thymic epithelium.

Thymic epithelial cell physiology is known to be under neuroendocrine control. In particular, thyroid hormones modulate thymic hormone secretion by thymic epithelial cells in vivo and in vitro, thus suggesting the existence of specific receptors for those hormones in this component of the thymic microenvironment. Yet, thyroid hormone-binding sites have previously been detected only in crude thymus fractions and lymphocytes. We, thus, decided to search for T3 receptors in the thymic epithelium, by using an antinuclear T3 receptor monoclonal antibody. In situ immunohistochemical analysis of thymic frozen sections showed nuclear labeling of both lymphoid and nonlymphoid cells in the cortex and medulla. Moreover, in vitro studies using thymic epithelial cell lines and the so-called thymic nurse cells revealed a positive reaction in the chromatin, with nucleoli remaining negative. Immunoblot data clearly showed a single protein band of 57K reactive with the antinuclear T3 receptor antibody in murine thymus extracts as well as in the thymic epithelial cell lines. Lastly, in vitro treatment of these cells with T3 resulted in a transient, yet profound, down-modulation of the receptor. In conclusion, our findings provide molecular evidence that the action of thyroid hormones on thymic epithelium occurs via the typical 57K nuclear T3 receptors.