RESUMO
The EphA receptor tyrosine kinases interact with membrane-bound ligands of the ephrin-A subfamily. Interaction induces EphA receptor oligomerization, tyrosine phosphorylation, and, as a result, EphA receptor signaling. EphA receptors have been shown to regulate cell survival, migration, and cell-cell and cell-matrix interactions. However, their functions in lymphoid cells are only beginning to be described. We show in this study that functional EphA receptors are expressed by murine thymocytes, including CD4(+)CD8(+), CD4(+)CD8(-), and CD4(-)CD8(+) subpopulations. We demonstrate that activation of EphA receptors by the ephrin-A1 ligand inhibits the anti-CD3-induced apoptosis of CD4(+)CD8(+) double-positive thymocytes. Furthermore, ephrin-A1 costimulation suppresses up-regulation of both the IL-2R alpha-chain (CD25) and early activation Ag CD69 and can block IL-2 production by CD4(+) single-positive cells. In agreement, EphA receptor activation in thymocytes also inhibits TCR-induced activation of the Ras-MAPK pathway. Our findings suggest that EphA receptor activation is antithetical to TCR signaling in thymocytes, and that the level of engagement by ephrin-A proteins on thymic APCs regulates thymocyte selection.
Assuntos
Anticorpos/imunologia , Apoptose/imunologia , Complexo CD3/imunologia , Receptores da Família Eph/imunologia , Receptores da Família Eph/metabolismo , Timo/citologia , Timo/imunologia , Animais , Antígenos CD/metabolismo , Antígenos de Diferenciação de Linfócitos T/metabolismo , Apoptose/efeitos dos fármacos , Células Cultivadas , Efrina-A1/farmacologia , Feminino , Regulação da Expressão Gênica , Interleucina-2/metabolismo , Lectinas Tipo C , Sistema de Sinalização das MAP Quinases , Camundongos , Camundongos Endogâmicos BALB C , Proteínas Quinases Ativadas por Mitógeno/metabolismo , Receptores de Antígenos de Linfócitos T/metabolismo , Receptores de Interleucina-2/metabolismo , Timo/efeitos dos fármacos , Timo/metabolismo , Proteínas ras/metabolismoRESUMO
EphB6 is the most recently identified member of the Eph receptor tyrosine kinase family. EphB6 is primarily expressed in thymocytes and a subpopulation of T cells, suggesting that it may be involved in regulation of T lymphocyte differentiation and functions. We show here that overexpression of EphB6 in Jurkat T cells and stimulation with the EphB6 ligand, ephrin-B1, results in the selective inhibition of TCR-mediated activation of JNK but not the MAPK pathway. EphB6 appears to suppress the JNK pathway by preventing T cell receptor (TCR)-induced activation of the small GTPase Rac1, a critical event in initiating the JNK cascade. Furthermore, EphB6 blocked anti-CD3-induced secretion of IL-2 and CD25 expression in a ligand-dependent manner. Dominant negative EphB6 suppressed the inhibitory activity of the endogenous receptor and enhanced anti-CD3-induced JNK activation, CD25 expression, and IL-2 secretion, confirming the requirement for EphB6-specific signaling. Activation of the JNK pathway and the establishment of an IL-2/IL-2R autocrine loop have been shown to play a role in the negative selection of CD4(+)CD8(+) self-reacting thymocytes. In agreement, stimulation of murine thymocytes with ephrin-B1 not only blocked anti-CD3-induced CD25 up-regulation and IL-2 production, but also inhibited TCR-mediated apoptosis. Thus, EphB6 may play an important role in regulating thymocyte differentiation and modulating responses of mature T cells.