Alpha 6 integrins participate in pro-T cell homing to the thymus

During embryogenesis, colonization of the thymic rudiment by hemopoietic progenitor cells depends on the adhesion of these cells to the jugular endothelium. Previously, we showed that progenitor T cells (pro-T cells) interact with alpha 6 integrins present on vascular endothelium. Here, we demonstrate that anti-alpha 6 integrin antibodies reduced the number of thymocytes up to 80% in a congenic mouse model for thymus colonization by pro-T cells. In organotypic thymus cultures, the anti-alpha 6 integrin antibodies did not influence T cell development and proliferation. From this, we conclude that alpha 6 integrin participates in thymus homing. During mouse thymus ontogeny, alpha 6 integrin mRNA and protein expression was found as early as day 10 of development; at day 11, perithymic endothelial cells were alpha 6 integrin positive. Two alpha 6 integrin mRNA exist which are produced by alternative exon usage. The longer form, alpha 6 integrin, predominates during early embryonic stages, while the shorter alpha 6A form was present later during development. Although alpha 6 integrins can be displayed by immature thymocytes, strongest expression was found on intra- and perithymic vascular endothelium. These data suggest that alpha 6 integrins are involved in the homing of pro-T cells to the developing thymus by mediating adhesion of pro-T cells to the vascular endothelium.