IL-7 signaling is critically dependent on gc cytokine receptor expression but the molecular mechanism of gc expression during T cell development and differentiation are unknown. In course of our studies of gc mRNA expression, we discovered a novel post-transcriptional pathway of gc-chain expression that results in the generation of soluble and secreted gc-chain proteins. We detected such soluble gc (sgc) proteins in significant amounts in both normal human and mouse serum, and we subsequently investigated their function under both homeostatic and autoimmune conditions. To assess the role of sgc in vivo, we generated sgc transgenic mice that overexpress soluble gc in T cells. These sgc Tg mice expressed high levels of sgc in serum, and their T cells showed an increased percentage of activated memory phenotype. Importantly, when sgc Tg mice were challenged in an experimental autoimmune encephalomyelitis (EAE) model, sgc Tg mice displayed a significant increase of autoimmune reaction as shown by faster and more severe clinical disease scores. On the other hand, genetically engineered mice that are unable to produce soluble gc were protected from inflammatory autoimmune disease, suggesting that soluble gc determines the disease severity of autoimmune inflammation. Since T cell activation induces soluble production and because soluble gc in turn induces inflammatory cytokine expression, we propose that soluble gc production is a mechanism to reinforce pro-inflammatory immune responses downstream of T cell activation. In agreement with this perspective, soluble gc is highly expressed under inflammatory conditions in both humans and mice;soluble gc was abundantly expressed in synovial fluid of patients with rheumatoid arthritis and also highly induced in mice challenged with parasites. Because sgc is pro-inflammatory, it is interesting to speculate if neutralization of serum sgc expression would ameliorate inflammatory disease and dampen destructive T cell responses. Generation of sgc-specific monoclonal antibodies and assessing their effects on pro-inflammatory responses in vivo are interesting issues that we plan to pursuit, and we acknowledge the potential to utilize such antibodies as therapeutics to dampen inflammation and ameliorate disease. In parallel to soluble gc receptors, we also observed that IL-7Ra chains were present in serum of human and mice. In humans, the IL-7Ra undergoes alternative splicing, and it has been reported that alternative splicing induces the generation of soluble IL-7Ra chain proteins. This post-transcriptional event explains the presence of soluble IL-7Ra in human serum. In contrast, in the mouse, soluble IL-7Ra proteins have not been reported and there is no molecular evidence for an alternative IL-7Ra splice isoform. Thus, the origin of soluble IL-7Ra proteins as we discovered in mouse serum remains unaccounted for.
We aim to pursue this issue in further studies to investigate the role of soluble IL-7 receptors in a mouse model. Finally, in an attempt to understand the regulatory mechanisms of IL-7Ra and gc expression at transcriptional level, we conducted a series of study addressing the role of transcription factors in IL-7 receptor expression. We previously reported that the zinc finger protein Gfi1 controls IL-7Ra transcription specifically in CD8 lineage T cells but not in CD4 lineage cells. In the current study, we have identified a number of other transcription factors that control IL-7Ra and gc cytokine receptor expression in a stage and cell-specific manner. Characterization of their role and molecular mechanism of regulation is a major goal of this project.
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