Signaling and Function of Interleukin-2 in T Lymphocytes

Abstract

The discovery of interleukin-2 (IL-2) changed the molecular understanding of how the immune system is controlled. IL-2 is a pleiotropic cytokine, and dissecting the signaling pathways that allow IL-2 to control the differentiation and homeostasis of both pro- and anti-inflammatory T cells is fundamental to determining the molecular details of immune regulation. The IL-2 receptor couples to JAK tyrosine kinases and activates the STAT5 transcription factors. However, IL-2 does much more than control transcriptional programs; it is a key regulator of T cell metabolic programs. The development of global phosphoproteomic approaches has expanded the understanding of IL-2 signaling further, revealing the diversity of phosphoproteins that may be influenced by IL-2 in T cells. However, it is increasingly clear that within each T cell subset, IL-2 will signal within a framework of other signal transduction networks that together will shape the transcriptional and metabolic programs that determine T cell fate.

Keywords: JAK1/3; PI3K; cytokine signaling; interleukin-2.

Figure 1

IL-2 biology: IL-2 positively influences the homeostasis and development of a number of T cell lineages. Expression of IL-2R is restricted to nTregs (a) and antigen-activated T cells (b,c), thereby guaranteeing the specificity of IL-2 activities. IL-2-responsive T cell subsets have extremely diverse characteristics and proinflammatory and anti-inflammatory biological roles. Moreover, IL-2 inhibits the differentiation of Th17 and Tfh cells (b), thus making IL-2 an important regulator of T cell lineage commitment. In addition, the levels of IL-2 signaling can define the effector/memory fates of CD4⁺ and CD8⁺ T cells: High levels of IL-2 favor the development of short-lived effector cells (b), while low levels of IL-2 signaling promote the differentiation of memory T cells (c). IL-2 stimulation of T cell function is depicted by arrows, IL-2 inhibition of lineage development is represented by inhibitory lines, and no response to IL-2 is indicated by a line with a circle at the end. Abbreviations: IFN-γ, interferon gamma; IL-2R, interleukin-2 receptor; LT-α, lymphotoxin alpha; nTreg, thymically derived regulatory T cell; Tfh, follicular helper T; TNF-α, tumor necrosis factor alpha.

Figure 2

Signaling downstream of the IL-2 receptor (IL-2R): (a) Janus kinase-signal transducer and activator of transcription (JAK-STAT) signal transduction. The IL-2Rβ chain couples to JAK1, and the γ chain (γ_c) couples to JAK3. IL-2R occupancy results in the activation of the JAKs and the tyrosine phosphorylation of IL-2Rβ and γ_c (yellow circles). The tyrosine phosphorylations in IL-2Rβ are best characterized in terms of the signaling complexes they coordinate; the phosphorylation of Y395 and Y498 (in murine IL-2Rβ) permits the recruitment of STAT5A, STAT5B, and STAT3, while the phosphorylation of Y341 allows recruitment of SHC1. Following their recruitment to the receptor, STAT5 proteins and SHC1 are tyrosine phosphorylated by JAKs. Tyrosine phosphorylation of STAT5 permits dimerization, nuclear translation, and STAT5-mediated transcription. Tyrosine phosphorylation of SHC1 allows the recruitment of GRB2 and SOS to facilitate GTP loading of Ras and activation of the classical Raf-ERK mitogen-activated protein kinase (MAP kinase) cascade. STAT proteins are also phosphorylated on serine residues (peach circles) in response to IL-2 signaling. While the STAT5 serine kinase is unknown, the STAT3 proteins are phosphorylated in response to MAP kinase signaling. The sites of IL-2-regulated phosphorylation in (murine) IL-2R, SHC1 (isoform 2), STAT5A, and STAT5B are annotated where positions have been mapped. (b) Metabolic pathways activated downstream of IL-2R. IL-2 sustains the expression of MYC and activation of the mammalian target of rapamycin complex 1 (mTORC1) and the hypoxia inducible factor 1 transcriptional complex (HIF1α/HIF1β) to maintain the uptake of nutrients such as iron and glucose. IL-2 also sustains the expression of the amino acid transporter SLC7A5, which transports many of the essential amino acids, such as leucine, into cells. Amino acids are required to sustain mTORC1 activity and fuel translation. Glucose metabolism, via glycolysis and oxidative phosphorylation (OxPhos), sustains ATP generation, and high ATP:ADP and ATP:AMP ratios, thus preventing the activation of AMP-activated protein kinase (AMPK), which inhibits mTORC1. Protein synthesis is essential for maintaining the expression of proteins, such as MYC, with high turnover rates. Through the mTORC1 pathway, IL-2 can also sustain glycolytic metabolism and other biosynthesis in cytotoxic T lymphocytes to support cell proliferation, growth, and the expression of effector molecules.

Figure 3

The phosphorylation networks of IL-2 signaling in effector T lymphocytes. Global phosphoproteomic studies have revealed the range of tyrosine (yellow circles) and serine/threonine phosphorylations (peach circles) regulated following IL-2 stimulation of cytotoxic T lymphocytes. These phosphorylations are on diverse proteins involved in multiple aspects of T cell biology and include scaffold proteins, proteins that can direct gene expression, and cytoskeletal regulators. These comprehensive phosphoproteomic studies highlight that IL-2 signaling involves much more than JAK1/3-STAT5 and reinforce the importance of serine/threonine kinase pathways in IL-2 responses.

Figure 4

Revised model for IL-2 signal transduction: IL-2 signaling outputs are likely to be modulated by other cellular signals, such as “preorganized” phosphorylation networks that do not depend on IL-2. Constitutive LCK/FYN activity has the potential to influence the phosphorylation of a number of signaling molecules in T cells and may be a key controller of the accumulation of PI(3,4,5)P₃ in IL-2-dependent T cells. Levels of PI(3,4,5)P₃ in cells depend on the balance in activity of phosphatidylinositol 3-kinase δ isoform (PI3Kδ or p110δ), which phosphorylates PI(4,5)P₂ to generate PI(3,4,5)P₃, and the PI(3,4,5)P₃ phosphatases, PTEN and SHIP1/2. PI(3,4,5)P₃ can coordinate the localization of a number of proteins, including the kinases phosphoinositide-dependent kinase 1 (PDK1) and AKT. Phosphorylation of T308 in the activation loop of AKT is induced by PDK1 and is coordinated by PI(3,4,5)P₃ and mTORC2-mediated phosphorylation of AKT S473. Activated AKT may then go on to phosphorylate and inhibit FOXO family transcription factors, to influence transcriptional processes in T cells. However, PI3K-AKT activity in cytotoxic T lymphocytes is not a prerequisite for mTORC1 activity, and sustained activation of this complex is dependent on glucose metabolism and amino acid levels that can be regulated by IL-2 and environmental nutrient availability.