Objectives: Intrathymic T cell differentiation is driven by the thymic microenvironment, a tridimensional network of cells and extracellular matrix (ECM). Previous data showed that lymphoid and microenvironmental compartments are under the control of hormones and growth factors. We then attempted to defi ne if insulin-like growth factor-II (IGF-II) was also involved in such a control. Methods: We used IGF-II transgenic (Tg) mice and studied their thymic microenvironment by immunohistochemistry. Moreover, we evaluated thymocytes in terms of their ability to adhere to thymic epithelial cells and to migrate through epithelial cells and ECM. Results: Transgenic IGF-II expression results in abnormalities of the thymic epithelium. Terminal differentiation of thymic epithelial cells (TEC) is modified, with the appearance of large clusters of cells immunoreactive to the monoclonal antibody KL1, which specifi cally recognizes highly differentiated TEC. Accordingly, treatment of cultured TEC with exogenous IGF-II induces the appearance of KL1+ cells and increases TEC proliferation. IGF-II Tg animals exhibit increased serum levels of the TEC-derived hormone thymulin. These effects were seen even when the IGF-II transgene was inserted in dwarf mice. Moreover, deposition of fi bronectin and laminin is also enhanced in IGF-II Tg mouse thymus and in IGF-II-treated TEC cultures. Furthermore, ECM-mediated interactions between thymocytes and TEC are affected by exogenous IGF-II, as exemplifi ed by the enhancement of thymocyte adhesion to TEC monolayers and thymocyte migration in thymic nurse cell complexes. Conclusions: Our data indicate that IGF-II pleiotropically affects the thymic epithelium, both in vivo and in vitro, and that some of these changes may have consequences on thymocyte/TEC interactions.2025-01-01