Previous work revealed the existence of a severe thymic atrophy with massive loss of immature CD4+CD8+ thymocytes in animals developing insulin-dependent diabetes, chemically induced by alloxan. Furthermore, the intrathymic expression of chemokines, such as CXCL12, is changed in these animals, suggesting that cell migrationrelated patterns may be altered. One molecular interaction involved in normal thymocyte migration is that mediated by soluble semaphorin-3A and its cognate receptor neuropilin- 1. Objectives: We investigated herein the expression and role of semaphorin-3A in the migratory responses of thymocytes from alloxan-induced diabetic mice. We characterized semaphorin-3A and its receptor, neuropilin-1, in thymuses from control and diabetic mice as well as semaphorin-3Adependent migration of developing thymocytes in both control and diabetic animals. Methods: Diabetes was chemically induced after a single injection of alloxan in young adult BALB/c mice. Thymocytes were excised from control and diabetic individuals and subjected to cytofluorometry for simultaneous detection of semaphorin-3A or neuropilin- 1 in CD4/CD8-defined subsets. Cell migration in response to semaphorin-3A was performed using cell migration transwell chambers. Results: Confirming previous data, we observed a severe decrease in the total numbers of thymocytes in diabetic mice, which comprised alterations in both immature (double-negative subpopulations) and mature CD4/ CD8-defined thymocyte subsets. These were accompanied by a decrease in the absolute numbers of semaphorin-3Abearing thymocytes, comprising CD4–CD8–, CD4+CD8+, and CD4–CD8+ cells. Additionally, immature CD4–CD8– and CD4+CD8+ developing T cells exhibited a decrease in the membrane density of semaphorin-3A. The relative and absolute numbers of neuropilin-1-positive thymocytes were also decreased in diabetic mouse thymocytes compared to controls, as seen in CD4–CD8–, CD4+CD8+, and CD4–CD8+ cell subpopulations. Functionally, we observed a decrease in the SEMA-3A: Reduction of Thymocyte Migration in Chemically Induced Diabetic Mice Neuroimmunomodulation 2020;27:28–37 29 DOI: 10.1159/000506054 chemorepulsive role of semaphorin-3A, as revealed by transwell migration chambers. Such an effect was seen in all immature and mature thymocyte subsets..