Novel IL-15 isoforms generated by alternative splicing are expressed in the intestinal epithelium

Counteracting Interleukin-15 to Elucidate Its Modes of Action in Physiology and Pathology

Interleukin (IL)-15 belongs to the common gamma-dependent cytokine family, along with IL-2, IL-4, IL-7, IL-9, and IL-21. IL-15 is crucial for the homeostasis of Natural Killer (NK) and memory CD8 T cells, and to fight against cancer progression. However, dysregulations of IL-15 expression could occur and participate in the emergence of autoimmune inflammatory diseases as well as hematological malignancies. It is therefore important to understand the different modes of action of IL-15 to decrease its harmful action in pathology without affecting its beneficial effects in the immune system. In this review, we present the different approaches used by researchers to inhibit the action of IL-15, from most broad to the most selective. Indeed, it appears that it is important to selectively target the mode of action of the cytokine rather than the cytokine itself as they are involved in numerous biological processes.

Selective Targeting of IL-15Rα Is Sufficient to Reduce Inflammation

Cytokines are crucial molecules for maintaining the proper functioning of the immune system. Nevertheless, a dysregulation of cytokine expression could be involved in the pathogenesis of autoimmune diseases. Interleukin (IL)-15 is a key factor for natural killer cells (NK) and CD8 T cells homeostasis, necessary to fight cancer and infections but could also be considered as a pro-inflammatory cytokine involved in autoimmune inflammatory disease, including rheumatoid arthritis, psoriasis, along with tumor necrosis factor alpha (TNF-α), IL-6, and IL-1β. The molecular mechanisms by which IL-15 exerts its inflammatory function in these diseases are still unclear. In this study, we generated an IL-15-derived molecule called NANTIL-15 (New ANTagonist of IL-15), designed to selectively inhibit the action of IL-15 through the high-affinity trimeric IL-15Rα/IL-2Rβ/γc receptor while leaving IL-15 signaling through the dimeric IL-2Rβ/γc receptor unaffected. Administrating of NANTIL-15 in healthy mice did not affect the IL-15-dependent cell populations such as NK and CD8 T cells. In contrast, we found that NANTIL-15 efficiently reduced signs of inflammation in a collagen-induced arthritis model. These observations demonstrate that the inflammatory properties of IL-15 are linked to its action through the trimeric IL-15Rα/IL-2Rβ/γc receptor, highlighting the interest of selectively targeting this receptor.

Immunomodulatory Effects of IL-2 and IL-15; Implications for Cancer Immunotherapy

The type I cytokine family members interleukin-2 (IL-2) and IL-15 play important roles in the homeostasis of innate and adaptive immunity. Although IL-2 and IL-15 receptor complexes activate similar signal transduction cascades, triggering of these receptors results in different functional activities in lymphocytes. While IL-2 expands regulatory T cells and CD4+ helper T cells, IL-15 supports the development of central memory T cells and NK cells. Recent data have provided evidence that IL-2 and IL-15 differ in their ability to activate T and NK cells to resist various forms of immune suppression. The diverse roles of these two cytokines have on immune cells lead to critical therapeutic implications for cancer treatment. In this review, we discuss the distinct roles of IL-2 and IL-15 in activating various functions in T and NK cells with a particular focus on the signals that participate in the resistance of tumor-derived immune suppressive factors. Furthermore, we summarize current clinical applications of IL-2 and IL-15 in metastatic malignancies, either as monotherapy or in combination with other agents, and highlight the future trends for research on these cytokine-based immunotherapies.

Development of an Economical DNA Delivery System by "Acufection" and its Application to Skin Research

Dysregulation of immune response in skin is associated with numerous human skin disorders. Direct transfer of immune-related genes into skin tissue is a fascinating approach to investigate immune modulation of cutaneous inflammation in mouse models of human diseases. Here we present a cost-effective protocol that delivered naked DNA in mouse skin and leads to transgene expression. The method is coined "acufection", denoting acupuncture-mediated DNA transfection. To perform acufection, mouse skin was first infused with DNA in phosphate-buffered saline (PBS) and then pricked lightly with a bundle of acupuncture needles to facilitate the absorption of DNA and transfection into cells. The plasmid DNA is presumably taken up by the keratinocyte and dendritic cells (DCs) in the skin and expressed into protein. Mechanical prick with the needles per se did not cause skin damage or induce keratinocyte activation. The expression of the transfected genes was detected in the skin at both transcriptional and translational levels following acufection for 2 days and maintained up to 7 days. The primary goal for the development of this acufection method was to investigate a previously undefined isoform of IL-15. Using this method, an alternatively spliced IL-15 isoform with partially deleted exon 7 (IL-15ΔE7) was expressed in the skin and subsequently treated with a Toll-like receptor 7 (TLR7) agonist, imiquimod (IMQ), to induce inflammation. Acufection-delivered IL-15ΔE7 in skin suppressed keratinocyte proliferation, epidermal thickness and neutrophil recruitment in IMQ-induced cutaneous inflammation. With increasing interest in identifying the regulatory mechanisms of cutaneous inflammation, the protocol described here provides a cost effective and versatile alternative to the gene gun system or microseeding for DNA delivery in vivo. It may potentially allow discovery of the function of a novel gene in the skin or for investigating new treatment for cutaneous diseases.

An activation-induced IL-15 isoform is a natural antagonist for IL-15 function

Interleukin 15 (IL-15) expression induces the secretion of inflammatory cytokines, inhibits the apoptosis of activated T cells and prolongs the survival of CD8⁺ memory T cells. Here we identified an IL-15 isoform lacking exon-6, IL-15ΔE6, generated by alternative splicing events of activated immune cells, including macrophages and B cells. In vitro study showed that IL-15ΔE6 could antagonize IL-15-mediated T cell proliferation. The receptor binding assay revealed that IL-15ΔE6 could bind to IL-15Rα and interfere with the binding between IL-15 and IL-15Rα. Over-expression of IL-15ΔE6 in the murine EAE model ameliorated the EAE symptoms of the mice. The clinical scores were significantly lower in the mice expressing IL-15ΔE6 than the control mice and the mice expressing IL-15. The inflammation and demyelination of the EAE mice expressing IL-15ΔE6 were less severe than the control group. Furthermore, flow cytometry analysis demonstrated that IL-15ΔE6 expression reduced the percentages of inflammatory T cells in the spleen and spinal cord, and inhibited the infiltration of macrophages to the CNS. Our results demonstrated that IL-15ΔE6 could be induced during immune activation and function as a negative feedback mechanism to dampen IL-15-mediated inflammatory events.

Interleukin-15, a master piece in the immunological jigsaw of celiac disease

BACKGROUND

The immune response causing celiac disease (CD) depends on the activation of intestinal CD4+ T cells by gluten-derived peptides presented by HLA-DQ2 or HLA-DQ8 molecules, the main genetic risk factor. However, additional factors are necessary to impair immune tolerance to dietary gluten, to stimulate intraepithelial lymphocytes (IEL) and to induce intestinal damage.

KEY MESSAGES

Current data point to a central role of interleukin-15 (IL-15). In situ and ex vivo studies indicate that IL-15 stimulates the accumulation and cytotoxic activation of CD8+ T IEL in active CD, and that of the malignant innate-like IEL in type II refractory CD (RCDII). Other studies show that IL-15 impairs the immunoregulatory control of effector T cells, notably CD8+. Recently, animal models have been designed to investigate the respective role of CD4+ T cells and IL-15 in CD. We discuss more particularly our results in such a model, which shows that IL-15 produced in excess in the intestine can cooperate with CD4+ T cells specific for a dietary antigen to trigger a celiac-like enteropathy. In this mouse model, CD4+ T cells activated by dietary ovalbumin secreted IL-2 which, along with IL-15, stimulated the expansion of noncognate intestinal cytotoxic CD8+ T cells containing large amounts of granzyme B. In the presence of IL-15, the latter cells did not respond to regulatory T cells, and accumulated in the intestine close to epithelial damage.

CONCLUSION

On the basis of these data, we propose that, in CD, gluten-specific CD4+ T cells synthesize cytokines that synergize with IL-15 to license the expansion and activation of cytotoxic IEL, which drive tissue damage. We suggest that IL-15 is a meaningful therapeutic target, notably in patients with RCDII in which malignant IEL can respond to IL-15 independently of signals provided by CD4+ T cells.

IL-15: a central regulator of celiac disease immunopathology

Interleukin-15 (IL-15) exerts many biological functions essential for the maintenance and function of multiple cell types. Although its expression is tightly regulated, IL-15 upregulation has been reported in many organ-specific autoimmune disorders. In celiac disease, an intestinal inflammatory disorder driven by gluten exposure, the upregulation of IL-15 expression in the intestinal mucosa has become a hallmark of the disease. Interestingly, because it is overexpressed both in the gut epithelium and in the lamina propria, IL-15 acts on distinct cell types and impacts distinct immune components and pathways to disrupt intestinal immune homeostasis. In this article, we review our current knowledge of the multifaceted roles of IL-15 with regard to the main immunological processes involved in the pathogenesis of celiac disease.

An alternatively spliced IL-15 isoform modulates abrasion-induced keratinocyte activation

In a routine phenotype-driven screen, we identified a point mutation in exon 7 of the IL-15 gene in Pedigree 191 (deficient memory (DM)) of N-ethyl-N-nitrosourea mutagenized mice. The DM epidermis expressed an alternatively spliced IL-15 mRNA isoform, IL-15ΔE7, and a wild-type (WT) IL-15 isoform at comparable levels. Mechanical stimulation of DM skin or DM skin graft transplanted onto the WT host resulted in reduced keratinocyte activation and inhibition of neutrophil infiltration into the dermis, demonstrating that DM keratinocytes produced less inflammatory response to external stimulation. Ectopic expression of IL-15ΔE7 in WT skin prevented abrasion-induced epidermal thickening, blocked the accumulation of nuclear antigen Ki67(+) cells in the basal and the suprabasal cell layers, increased loricrin expression, and also increased keratinocyte CXCL1 and G-CSF production. IL-15ΔE7 also profoundly blocked neutrophil infiltration in SDS- or immiquimod (IMQ)-treated WT skin. Recombinant IL-15ΔE7 failed to activate STAT-5 and its downstream target bcl-2 expression. Our study points to IL-15ΔE7 as a potential therapeutic agent for treating neutrophilia-associated inflammatory skin disorders.

IL-15: a central regulator of celiac disease immunopathology

Interleukin-15 (IL-15) exerts many biological functions essential for the maintenance and function of multiple cell types. Although its expression is tightly regulated, IL-15 upregulation has been reported in many organ-specific autoimmune disorders. In celiac disease, an intestinal inflammatory disorder driven by gluten exposure, the upregulation of IL-15 expression in the intestinal mucosa has become a hallmark of the disease. Interestingly, because it is overexpressed both in the gut epithelium and in the lamina propria, IL-15 acts on distinct cell types and impacts distinct immune components and pathways to disrupt intestinal immune homeostasis. In this article, we review our current knowledge of the multifaceted roles of IL-15 with regard to the main immunological processes involved in the pathogenesis of celiac disease.

Dendritic cells in alcoholic liver injury and fibrosis

Alcohol consumption impairs the development of innate and adaptive immune responses, however the exact mechanism by which alcohol leads to immune defects remains to be established. Dendritic cells (DCs) form a heterogeneous population of hematopoietic cells that are present in all tissues including the liver. DC are initially described playing a key role in the induction of innate and adaptive immune response against specific antigens. In our presentation, we discussed few new aspects of DC development, critical assessment of DC in non-lymphoid organs and the impact of alcohol consumption on DC function. Understanding the mechanism by which DC modulate liver function after alcohol consumption may help uncover novel therapeutic strategies for the treatment of these conditions.

Trans-presentation: a novel mechanism regulating IL-15 delivery and responses

Interleukin (IL)-15 is a cytokine that acts on a wide range of cell types but is most crucial for the development, homeostasis, and function of a specific group of immune cells that includes CD8 T cells, NK cells, NKT cells, and CD8 alpha alpha intraepithelial lymphocytes. IL-15 signals are transmitted through the IL-2/15R beta and common gamma (gamma C) chains; however, it is the delivery of IL-15 to these signaling components that is quite unique. As opposed to other cytokines that are secreted, IL-15 primarily exists bound to the high affinity IL-15R alpha. When IL-15/IL-15R alpha complexes are shuttled to the cell surface, they can stimulate opposing cells through the beta/gamma C receptor complex. This novel mechanism of IL-15 delivery has been called trans-presentation. This review discusses how the theory of trans-presentation came to be, evidence that it is the major mechanism of action, the current understanding of the cell types thought to mediate trans-presentation, and possible alternatives for IL-15 delivery.

Expression analysis and functional activity of interleukin-7 splice variants

Alternative splicing results in multiple protein isoforms derived from a single gene. The magnitude of this process ranges from a complete loss of function to gain of new function. We examined, as a paradigm, alternative splicing of the non-redundant human cytokine, interleukin-7 (IL-7). We show that extensive IL-7 splicing in human tissues of different histology, including MTB+ granuloma lesions, transformed tissue and tumor cell lines. IL-7 splice variants were expressed as recombinant proteins. A differentially spliced IL-7 isoform, lacking exon 5, leads to STAT-5 phosphorylation in CD4+ and CD8+ T cells, promotes thymocyte maturation and T-cell survival. Human tumor lesions show aberrant IL-7 isoform expression, as compared with the autologous, non-transformed tissue. Alternatively spliced cytokines, such as IL-7, represent candidates for diagnostics and therapeutic interventions.

Identification of novel viral interleukin-10 isoforms of human cytomegalovirus AD169

Two products of human cytomegalovirus (HCMV) UL111a gene have been previously identified to resemble human IL-10 (hIL-10). These viral IL-10s (vIL-10s) are able to induce signal transduction events and biological activities in a variety of cells. In this study, five novel vIL-10 transcripts were identified from HCMV AD169 infected MRC-5 cells. Some vIL-10 isoforms were post-translationally glycosylated, depending on the existence of a predicted N-linked glycosylation site. Similar to hIL-10, four of the vIL-10 isoforms apparently formed putative dimers. Among the different vIL-10 isoforms, vIL-10A significantly induced the phosphorylation of transcription factor STAT3 in THP-1 cells. All identified vIL-10 isoforms were able to form complexes with hIL-10, and enhanced hIL-10-induced STAT3 phosphorylation in different degrees. Identification of diverse forms of vIL-10 suggests that HCMV has developed a sophisticated mechanism to interfere with hIL-10 signaling pathway.

Thymic differentiation of TCRαβ+CD8αα+IELs

Intraepithelial lymphocytes (IELs) contain several subsets, but the origin of the T-cell receptor (TCR)alphabeta(+) CD8 alpha alpha(+) IELs has been particularly controversial. Here we provide a synthesis, based on recent work, that attempts to unify the divergent views. The intestine has a primordial function in lymphopoiesis, and precursors with the potential to differentiate into T cells are found both in the epithelium and underlying lamina propria. Moreover, the thymus has been reported to export cells to the intestine that are not fully differentiated. TCR alpha beta(+) CD8 alpha alpha(+) IELs can differentiate in the intestine from each of these sources, but in normal euthymic mice, the thymus appears to be the major source for TCR alpha beta(+) CD8 alpha alpha(+) IELs. This unique IEL subset is a self-reactive population that requires exposure to self-agonists for selection in the thymus, similar to other regulatory T-cell populations. IELs transition through a double-positive (DP) intermediate in the thymus, but they originate from a subset of the DP cells that can be identified by its expression of CD8 alpha alpha homodimers. The agonist-selected cells in the thymus are TCRbeta(+) but CD4 and CD8 double negative. The evidence suggests that reacquired expression of CD8 alpha alpha and downregulation of CD5 occur after thymus export, perhaps in the intestine under the influence of interleukin-15. As a result of agonist exposure, a new gene expression program is activated. Therefore, the increased understanding of the developmental origin of TCR alpha beta(+) CD8 alpha alpha(+) IELs may help us to understand how they participate in immune regulation and protection in the intestine.