Intracellular Interaction of Interleukin-15 with Its Receptor α during Production Leads to Mutual Stabilization and Increased Bioactivity

Cristina Bergamaschi,Margherita Rosati,Rashmi Jalah,Antonio Valentin,Viraj Kulkarni,Candido Alicea,Gen-Mu Zhang,Vainav Patel,Barbara K Felber,George N Pavlakis

doi:10.1074/jbc.m705725200

Abstract

We show that co-expression of interleukin 15 (IL-15) and IL-15 receptor alpha (IL-15Ralpha) in the same cell allows for the intracellular interaction of the two proteins early after translation, resulting in increased stability and secretion of both molecules as a complex. In the absence of co-expressed IL-15Ralpha, a large portion of the produced IL-15 is rapidly degraded immediately after synthesis. Co-injection into mice of IL-15 and IL-15Ralpha expression plasmids led to significantly increased levels of the cytokine in serum as well as increased biological activity of IL-15. Examination of natural killer cells and T lymphocytes in mouse organs showed a great expansion of both cell types in the lung, liver, and spleen. The presence of IL-15Ralpha also increased the number of CD44(high) memory cells with effector phenotype (CD44(high)CD62L-). Thus, mutual stabilization of IL-15 and IL-15Ralpha leads to remarkable increases in production, stability, and tissue availability of bioactive IL-15 in vivo. The in vivo data show that the most potent form of IL-15 is as part of a complex with its receptor alpha either on the surface of the producing cells or as a soluble extracellular complex. These results explain the reason for coordinate expression of IL-15 and IL-15Ralpha in the same cell and suggest that the IL-15Ralpha is part of the active IL-15 cytokine rather than part of the receptor.

Highlights

Interleukin-15 (IL-15)2 is a pleiotropic cytokine produced in many tissues
It has been reported that IL-15R␣ is cleaved by TACE/ADAM17 together with transpresented IL-15 [22], and the soluble IL-151⁄7IL-15sR␣ complexes may trigger signaling upon binding to target cells expressing intermediate/low affinity receptor
We present data demonstrating that co-expression of IL-15 and IL-15R␣ results in intracellular complex formation and sta

Summary

Introduction

Interleukin-15 (IL-15) is a pleiotropic cytokine produced in many tissues. It is a member of the four ␣-helix bundle family of cytokines and was initially described as a T cell proliferation factor [1, 2]. Addition of IL-15sR␣ in vitro was reported to block the response of cell lines to IL-15 [10, 11] Despite these findings, more recent reports show that a soluble sushi domain of IL-15R␣ or IL-15sR␣ linked to an Fc fragment can enhance IL-15 activity both in vitro and in vivo [12,13,14]. The levels of secreted cytokine are increased, and in vivo experiments demonstrate that this leads to greatly enhanced biological function. These results explain several puzzling observations about IL-15 function and interactions and suggest methods to use the improved IL-15 cytokine expression plasmids for the optimal induction of the immune system

Methods

Results

Conclusion