Interleukin 15

Expression

Interleukin - 15 (IL-15) was discovered in 1994 by two different laboratories, and characterized as T cell growth factor. Together with interleukin 2 (IL-2), interleukin 4 (IL-4), interleukin 7 (IL-7), interleukin 9 (IL-9), granulocyte colony-stimulating factor (G-CSF), and granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-15 belongs to the four α-helix bundle family of cytokine.

IL-15 is constitutively expressed by a large number of cell types and tissues, including monocytes, macrophages, dendritic cells (DC), keratinocytes, fibroblasts and nerve cells. As a pleiotropic cytokine, it plays an important role in innate and adaptive immunity.

Gene

IL -15 is 14-15 kDa glycoprotein encoded by the 34 kb region 4q31 of chromosome 4, and by the central region of chromosome 8 in mice. The human IL-15 gene comprises nine exons (1 - 8 and 4A) and eight introns, four of which (exons 5 through 8) code for the mature protein (Figure 1).

Two alternatively spliced transcript variants of this gene encoding the same protein have been reported. The originally identified isoform, with long signal peptide of 48 amino acids (IL-15 LSP) consisted of a 316 bp 5’-untranslated region (UTR), 486 bp coding sequence and the C-terminus 400 bp 3’-UTR region. The other isoform (IL-15 SSP) has a short signal peptide of 21 amino acids encoded by exons 4A and 5. Both isoforms shared 11 amino acids between signal sequences of the N-terminus. Although both isoforms produce the same mature protein, they differ in their cellular trafficking. IL-15 LSP isoform was identified in Golgi apparatus [GC], early endosomes and in the endoplasmic reticulum (ER). It exists in two forms, secreted and membrane-bound particularly on dendritic cells. On the other hand, IL-15 SSP isoform is not secreted and it appears to be restricted to the cytoplasm and nucleus where plays an important role in the regulation of cell cycle.

It has been demonstrated that two isoforms of IL-15 mRNA are generated by alternatively splicing in mice. The isoform which had an alternative exon 5 containing another 3’ splicing site, exhibited a high translational efficiency, and the product lack hydrophobic domains in the signal sequence of the N-terminus. This suggests that the protein derived from this isoform is located intracellulary. The other isoform with normal exon 5, which is generated by integral splicing of the alternative exon 5, may be released extracellulary.

Although IL-15 mRNA can be found in many cells and tissues including mast cells, cancer cells or fibroblasts, this cytokine is produced as a mature protein mainly by dendritic cells, monocytes and macrophages. This discrepancy between the wide appearance of IL-15 mRNA and limited production of protein might be explained by the presence of the twelve in humans and five in mice upstream initiating codons, which can repress translation of IL-15 mRNA. Translational inactive mRNA is stored within the cell and can be induced upon specific signal. Expression of IL-15 can be stimulated by cytokine such as GM-CSF, double-strand mRNA, unmethylated CpG oligonucleotides, lipopolysaccharide (LPS) through Toll-like receptors (TLR), interferon gamma (IFN-γ) or after infection of monocytes herpes virus, Mycobacterium tuberculosis and Candida albicans (Figure 2).

Signaling

The prevailing mechanism of IL-15 action seems to be juxtacrine signaling or also determined as cell-to-cell contact. It also includes intracrine and reverse signaling. IL-15 was initially characterized as a soluble molecule. Later it was shown that IL-15 also exists as a membrane-bound form which represents the major form of IL-15 protein. In membrane-bound form it could be bound directly to cellular membrane or presented by IL-15Rα receptor.

The main mechanism of IL-15 signaling is trans-presentation which is mediated by membrane-bound complex IL-15/IL-15Rα (Figure 3). IL-15 bind to IL-15Rα receptor alone with affinity (K_a = 1.10¹¹/M). It can also bind to IL-15Rβγ_c signaling complex with lower affinity (K_a = 1.10⁹/M) (Figure 4).

Signaling pathway of IL-15 begins with binding to IL-15Rα receptor, with subsequent presentation to surrounding cells bearing IL-15Rβγc complex on their cell surface. Upon binding IL-15β subunit activates Janus kinase 1 (Jak1) and γc subunit Janus kinase 3 (Jak3), which leads to phosphorylation and activation of signal transducer and activator of transcription 3 (STAT3) and STAT5. Due to sharing of receptor subunits between IL-2 and IL-15, both of these cytokines have similar downstream effects including the induction of B-cell lymphoma (Bcl-2), MAP (mitogen-activated protein kinase) kinase pathway and the phosphorylation of Lck (lymphocyte-activated protein tyrosine kinase) and Syk (spleen tyrosine kinase) kinases, which leads to cell proliferation and maturation (Figure 5).

In mast cells, the IL-15R signaling pathway has been found to include Jak2 and STAT5 instead Jak1/3 and STAT3/5. Phosphorylation STATs form transcription factors and activate transcription of appropriate genes. The β chain of IL-15R recruits and also activates protein tyrosine kinases of the Src family including Lck, Fyn and Lyn kinase. It also activates phosphatidylinositol 3-kinase (PI3K) and AKT signaling pathway and induce expression of transcription factors including c-Fos, c-Jun, c-Myc and NF-κB.

IL-15 is also able to bind to the 15Rβγc signaling complex with intermediate affinity without requirement for IL-15Rα rreceptor. Upon binding IL-15 to signaling complex activates kinases of the Src family including Lck and Fyn, which subsequently activates PI3K and MAPK signaling pathway. The second mechanism of IL-15 action is cis-presentation, when il-15 is presented by IL-15Rα to 15Rβγc signaling complex on the same cell. This mechanism is mediated by the C-terminus flexibility which is mediated by 32 amino acids linker and/or 74 amino acids long PT region (Figure 6).

Function

Interleukin 15 (IL-15) regulates T and natural killer (NK) cell activation and proliferation. Survival signals that maintain memory T cells in the absence of antigen are provided by IL-15. This cytokine is also implicated in NK cell development. In rodent lymphocytes, IL-15 prevents apoptosis by inducing an apoptosis inhibitor, BCL2L1/BCL-x(L). In humans with celiac disease IL-15 similarly suppresses apoptosis in T-lymphocytes by inducing Bcl-2 and/or Bcl-xL.

A hematopoietin receptor, the IL-15 receptor, that binds IL-15 propagates its function. Some subunits of the IL-15 receptor are shared in common with the receptor for a structurally related cytokine called interleukin 2 (IL-2) allowing both cytokines to compete for and negatively regulate each other's activity. CD8+ memory T cell number is controlled by a balance between IL-15 and IL-2. When IL-15 binds its receptor, JAK kinase, STAT3, STAT5, and STAT6 transcription factors are activated to elicit downstream signaling events.

As a myokine, interleukin-15 appears to play a significant role in the reduction of visceral (intra-abdominal or interstitial) fat.

Epstein-Barr virus

In humans with history of acute infectious mononucleosis (the syndrome associated with primary Epstein-Barr virus infection), IL-15R expressing lymphocytes are not detected—even 14 years after infection.

Celiac disease

There have been recent studies suggesting that suppression of IL-15 may be a potential treatment for celiac disease and even presents the possibility of preventing its development. In one study with mice blocking IL-15 with an antibody led to the reversal of autoimmune intestinal damage. In another study mice used were able to eat gluten without developing symptoms.

Metastatic cancer

IL-15 has been shown to enhance the anti-tumor immunity of CD8+ T cells in pre-clinical models. A phase I clinical trial to evaluate the safety, dosing, and anti-tumor efficacy of IL-15 in patients with metastatic melanoma and renal cell carcinoma (kidney cancer) has begun to enroll patients at the National Institutes of Health.

ALT-803

A combined IL-15N72D mutant and the soluble domain of IL-15Rα, known as ALT-803, As of 2014 INDs have been submitted for clinical trials for 4 indications : metastatic melanoma, relapse of hematological malignancies after allogeneic stem cell transplantation, refractory multiple myeloma, and BCG-naïve non-muscle invasive bladder cancer in combination with BCG. In 2016 it started a clinical trial for advanced or metastatic non-small cell lung cancer (NSCLC).