α4 and α5 Integrins Costimulate the CD3-Dependent Proliferation of Fetal Thymocytes

Although integrin receptors have been shown to function as costimulatory molecules on mature thymocytes and T cells, it is not known whether these receptors can function as costimulatory molecules on immature thymocytes. Previous studies have shown that the expression of α4 and α5 integrins were significantly higher on immature, adult CD4−CD8−thymocytes than on either mature thymocytes or T cells, suggesting that these receptors are involved in early thymocyte development. In this study, we show that day 16 fetal thymocytes express levels of α4 and α5 equivalent to those of adult CD4−CD8−thymocytes. Immobilized fibronectin, a ligand for α4 and α5 integrins, was found to enhance the CD3-dependent proliferation of these fetal thymocytes. In the presence of IL-7, the magnitude of the proliferative response increased with time of incubation, resulting in a dramatic increase in the percentage of γδ thymocytes. The enhancement of proliferation by fibronectin was abrogated by soluble antibodies against α4 and α5, whereas immobilized mAb to α4 and α5 substituted for fibronectin in enhancing CD3-dependent proliferation, demonstrating that α4 and α5 integrins were responsible for the enhanced proliferation by fibronectin. Anti-α4 mAb enhanced proliferation of fetal thymocytes by 100%, whereas anti-α5 mAb and anti-CD28 mAb enhanced proliferation by 25%. Other costimulatory molecules, such as CD2, FcRγ, and Thy-1, had no effect on the CD3-dependent proliferation of day 16 fetal thymocytes. This study demonstrates that α4 and α5 integrins are capable of costimulating fetal thymocytes.