STAT5 tetramerization in autoimmune-mediated neuroinflammation

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Project Summary/Abstract Multiple sclerosis (MS) is an immune-mediated disease that impacts approximately 2.3 million people world- wide. MS is caused by the activation and the complex interactions between different immune cell types that cause inflammation in the central nervous system (CNS), which leads to demyelination and axonal degeneration. A validated animal model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE), has been utilized to demonstrate that the cognate interactions between CD4+ T cells and monocyte-derived cells (MDCs), defined here a mixed population of dendritic cells and macrophages differentiated from monocytes during the autoimmune-driven neuroinflammation, are required for the pathogenesis of EAE. The overall goal of this proposal is to identify signaling pathways and their downstream effector mediators that regulate the interactions of CD4+ T cells and MDCs within CNS that promote the pathogenesis of EAE. Signal transducers and activators of transcription 5A and 5B (STAT5A and STAT5B) play a critical role in mediating cellular responses following stimulation of cytokines, interferons, growth factors. The activated STAT5 proteins can form dimers and tetramers. The biological functions of STAT5 tetramers are not fully understood. Using a Stat5a-Stat5b N-domain double knock-in (DKI) mouse strain, in which STAT5 tetramers cannot be formed but STAT5 dimers are unaffected, we found that STAT5 tetramers promote the pathogenesis of EAE. The mild EAE phenotype in DKI mice correlates with reduced interactions of CD4+ T cells and monocytes/MDCs in the spinal cord meninges during EAE. We further demonstrated that STAT5 tetramer activation by cytokine GM-CSF promotes monocyte differentiation into dendritic cells and the expression of chemokine CCL17. Correspondingly, the expression of CCL17 in the spinal cords is significantly reduced in DKI mice during EAE. Importantly, mice receiving STAT5 tetramer-deficient Th17 cells treated with CCL17 developed more severe EAE with earlier onset of the disease compared with mice receiving cells without CCL17 treatment. Our central hypothesis is that GM-CSF-mediated STAT5 tetramerization is critical for facilitating the interactions of pathogenic Th17 cells and monocytes/MDCs in the spinal cord meninges via a CCL17-dependent mechanism, and these cell interactions are required to promote the pathogenesis of EAE. We will test our hypothesis in three specific aims. In Aim 1, we will utilize different EAE induction strategies to determine the pathogenic role of STAT5 tetramers in CD4+ T cells and monocytes/MDCs during EAE. In Aim 2, we will test our hypothesis that STAT5 tetramer signaling promotes monocyte differentiation and Th17 cell-MDC interactions at the spinal cord meninges during EAE using intravital imaging technique. In Aim 3, we will investigate the mechanism by which CCL17 promotes the pathogenesis of EAE. Specifically, we will test our hypothesis that CCL17 increases the affinity of integrin LFA-1 on Th17 cells, thereby facilitates Th17 cell-MDC interactions. Targeting the pathways and mediators that control the interactions between CD4+ T cells and MDCs within CNS may become a novel strategy for the treatments of MS.