The conveyor belt hypothesis for thymocyte migration: participation of adhesion and de-adhesion molecules

Thymocyte differentiation is the process by which bone marrow-derived precu rsors enter the thymus, proliferate, rearrange the genes and express the co rresponding T cell receptors, and undergo positive and/or negative selectio n, ultimately yielding mature T cells that will represent the so-called T c ell repertoire. This process occurs in the context of cell migration, whose cellular and molecular basis is still poorly understood. Kinetic studies f avor the idea that these cells leave the organ in an ordered pattern, as if they were moving on a conveyor belt. We have recently proposed that extrac ellular matrix glycoproteins, such as fibronectin, laminin and type IV coll agen, among others, produced by non-lymphoid cells both in the cortex and i n the medulla, would constitute a macromolecular arrangement allowing diffe rentiating thymocytes to migrate. Here we discuss the participation of both molecules with adhesive and de-adhesive properties in the intrathymic T ce ll migration. Functional experiments demonstrated that galectin-3, a solubl e beta-galactoside-binding lectin secreted by thymic microenvironmental cel ls, is a likely candidate for de-adhesion proteins by decreasing thymocyte interaction with the thymic microenvironment.