P100 Soluble IL7Ra isoform modulates IL7 signalling and homeostasis

Abstract

Introduction IL7R (the gene encoding the IL-7 receptor alpha chain) is associated with susceptibility to various autoimmune conditions including Multiple Sclerosis. We confirm and strengthen previous reports that the MS associated allele ( IL7 R∗C) causes increased expression of an alternatively spliced receptor chain resulting in higher plasma levels of a soluble IL7Ra isoform (sIL7Ra), and functionally suggest. Methods To examine sIL7Ra’s role in autoimmunity we expressed the alternatively spliced cDNA clone (lacking exon 6) in the human HEK293 cell line. IL-7 and sIL7Ra levels were determined by ELISA. Binding affinities were measured by Surface Plasmon Resonance. In vitro experiments were carried out on healthy human PBMCs in serum free media, and analysed by flow cytometry. EAE scoring was blinded and performed daily. Results The HEK293 produced sIL7Ra had higher affinity for IL-7 compared to the wild type extra-cellular domain of IL7Ra as determined by Surface Plasmon Resonance, however we did not see any affinity for TSLP. This suggests the sIL7Ra has the potential to impact IL-7 but not TSLP signalling. To better understand the connection between sIL7Ra and autoimmunity we tested its effect on IL-7 signalling and homeostasis. Our results indicate that sIL7Ra prolongs and potentiates the IL-7 signal on T-Cells in vitro by extending CXCR4 up regulation and CD127 down-regulation whilst lowering regulatory CD95 and SOCS expression. In similar fashion, increased lymphocyte proliferation and EAE severity was seen when IL-7 was co administered with sIL7Ra in vivo compared to IL-7 alone. Both in vitro and in vivo, presence of sIL7Ra resulted in slowed IL-7 consumption. Furthermore, in MS patients carrying IL7R *C we saw a twofold increase in plasma IL-7 levels compared to Healthy controls and MS patients with other genotypes. Conclusion Our experiments show a potential for reduced tolerance and increased autoimmunity in the presence of sIL7Ra. This could be of importance in understanding IL-7 biology, both in terms of endogenous homeostasis and clinical administration.