Cytokines of the gc-chain family play critical roles in T cell development and differentiation.
We aim ed to understand the mechanism of gc-chain expression during T cell development and activation. In addition to transcriptionally controlled mechanisms, we discovered a novel post-transcriptional pathway of gc-chain expression that leads to the generation of soluble gc-chains. We detected soluble gc proteins in significant amounts in both normal human and mouse serum, and we have investigated its function under both homeostatic and autoimmune conditions. Soluble gc (sgc) is generated by exon skipping and by a frameshift in the open reading frame upon alternative splicing. To assess its role 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 for immune reactivity, sgc Tg mice displayed a significant increase in autoimmune reaction as shown by a faster, stronger and more lasting clinical disease score. The underlying mechanism for such an enhanced reaction turned out to be increased generation of pro-inflammatory Th17 cells, and we are currently in the process of identifying the exact mechanism for this enhanced reactivity. The molecular mechanisms how alternative splicing of the soluble gc chain regulated is currently not known to us. However, we found that immature DP thymocytes and peripheral NK cells express high levels of the alternative splice form so that we consider the involvement of nuclear factors that are highly expressed in these cells in the generation of soluble gc chains. Importantly, the IL-7-specific subunit of the IL-7 receptor also undergoes alternative splicing, and it has been reported alternative splicing results in the generation of soluble IL-7Ra chain proteins. Notably, soluble IL-7Ra proteins are associated with increased risk for multiple sclerosis but how it exacerbates pathology is not clear. Moreover, in the mouse, soluble IL-7Ra proteins have not been reported and there is no molecular evidence for an alternative IL-7Ra splice form. We have set up different systems to detect soluble IL-7R proteins in mice serum but were not successful do far.
We aim to pursue this issue in further studies to investigate the role of soluble IL-7 receptors in a mouse model. Finally, we investigated potential nuclear factors that could control IL-7 receptor expression. We discovered and reported that the zinc finger protein Gfi1 controls IL-7Ra transcription in CD8 lineage T cells. Interestingly, however, absence or overexpression of Gfi1 did not affect IL-7R expression in CD4 lineage cells. These data suggest distinct molecular regulation of IL-7Ra expression in CD4 versus CD8 T cells, which is of interest for understanding the IL-7 in CD4 and CD8 T cells. Notably, we also identified a couple of other zinc finger proteins that control IL-7Ra expression, and we are assessing their effect on IL-7R regulation on both transcriptional and post-transcriptional levels. Along these lines, CD8 T cells are critically dependent on IL-7 for homeostasis and survival while CD4 T cells are rather independent of IL-7. Whether IL-7 receptor regulation is involved in this process is a question that we aim to address using IL-7Ra transgenic and IL-7 transgenic mice. In sum, our studies reveal a complex regulatory network of IL-7R expression, and we consider the understanding of this network important to understand T cell development, differentiation and homeostasis.
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