Thymocyte maturation and thymic architecture is dependent upon precise intrathymic cell migration. The orchestration of thymocyte trafficking, however, is not well understood at the molecular level. Recently, we described the highly-regulated plexinD1 expression on CD4+CD8+ double positive (DP) thymocytes which is further modified by T cell receptor/co-receptor engagement. Activation of plexinD1 via its ligand, semaphorin 3E (sema3E), represses CXCR4/CXCL12 signaling in pre-selection DP thymocytes and CCR9/CCL25 signaling in CD69+ post-selection DP thymocytes. Using Plxnd1-deficient fetal liver cell-transplanted mice, loss of plexinD1 causes CD69+ thymocytes to remain in the cortex, maturing in situ to form ectopic single positive (SP) thymocyte clusters. As a consequence, the boundary between DP and SP thymocytes at corticomedullary junctions is disrupted and medullary structures form under the thymic capsule. These results demonstrate the importance of plexinD1 in directing migration of maturing thymocytes via modulation of biological responses to chemokine gradients. The dysmorphic corticomedullary structures observed in Plxnd1-/- and Sema3e-/- knockout mice are further consistent with this view. To address the cellular interactions and functional consequences of Plxnd1 deficiency in thymic development, the present proposal will pursue three major aims. First, we will focus on the role of plexinD1 in regulation of CCR9/CCL25 and CXCR4/CXCL12 pathways and the directed migration of DP thymocytes using Ccr9-/- and Cxcr4 conditional knockout mice. Specifically, the links between these chemokine signaling pathways and Plxnd1 deficiency as well as the associated retarded thymocyte migration towards the medulla of pre-selected (CD69-) and post-selected (CD69+) DP thymocytes will be assessed. Second, we will analyze by multi-photon excitation microscopy the dynamics of GFP- expressing (green) Plxnd1-/- thymocytes in fetal liver-transplanted mice interacting with tdTomato (red) cortical thymic epithelial cells within renal subcapsular fetal thymic implants, using CFP-expressing (blue) wild type cells as an internal control. Third, we will analyze the impact of Plxnd1 deficiency on negative selection during T cell development using OT-I/OT-II TCR and RIP-OVA transgenic mice and fetal liver cell transplantation, in particular screening mice transplanted with Plxnd1-/- fetal liver cells for evidence of autoimmune phenomena. Collectively, our studies will offer insight into the molecular basis of plexinD1-mediated regulation of DP thymocyte movements within the thymus with consequential effects upon thymocyte differentiation and selection that may lead to autoimmune pathology in the mature immune system.
T cell function is critical for mammalian host protection against a wide variety of infectious disorders and cancers. T cells emanate from the thymus. Hence, understanding of the processes required for normal thymic architecture, differentiation and maturation will contribute to generation of more effective immunity.
|Choi, Young I; Duke-Cohan, Jonathan S; Chen, Wei et al. (2014) Dynamic control of ?1 integrin adhesion by the plexinD1-sema3E axis. Proc Natl Acad Sci U S A 111:379-84|