Comparative model building of interleukin-7 using interleukin-4 as a template: a structural hypothesis that displays atypical surface chemistry in helix D important for receptor activation

Modulation of Signaling Mediated by TSLP and IL-7 in Inflammation, Autoimmune Diseases, and Cancer

Thymic Stromal Lymphopoietin (TSLP) and Interleukin-7 (IL-7) are widely studied cytokines within distinct branches of immunology. On one hand, TSLP is crucially important for mediating type 2 immunity at barrier surfaces and has been linked to widespread allergic and inflammatory diseases of the airways, skin, and gut. On the other hand, IL-7 operates at the foundations of T-cell and innate lymphoid cell (ILC) development and homeostasis and has been associated with cancer. Yet, TSLP and IL-7 are united by key commonalities in their structure and the structural basis of the receptor assemblies they mediate to initiate cellular signaling, in particular their cross-utilization of IL-7Rα. As therapeutic targeting of TSLP and IL-7 via diverse approaches is reaching advanced stages and in light of the plethora of mechanistic and structural data on receptor signaling mediated by the two cytokines, the time is ripe to provide integrated views of such knowledge. Here, we first discuss the major pathophysiological roles of TSLP and IL-7 in autoimmune diseases, inflammation and cancer. Subsequently, we curate structural and mechanistic knowledge about receptor assemblies mediated by the two cytokines. Finally, we review therapeutic avenues targeting TSLP and IL-7 signaling. We envision that such integrated view of the mechanism, structure, and modulation of signaling assemblies mediated by TSLP and IL-7 will enhance and fine-tune the development of more effective and selective approaches to further interrogate the role of TSLP and IL-7 in physiology and disease.

A combinatorial approach of N-terminus blocking and codon optimization strategies to enhance the soluble expression of recombinant hIL-7 in E. coli fed-batch culture

Human interleukin-7 (hIL-7) is a therapeutically important cytokine involved in lymphocyte development and survival. In previous reports, a uniformly poor expression of hIL-7 has been shown in Escherichia coli host with the problem of inclusion body formation. In this study, the role of codon optimization and N-terminus blocking using various solubility enhancer fusion tags was explored to improve its soluble expression. The use of codon optimization strategy improved its expression to 80 ± 5 mg/L at shake flask level. The utilization of pelB leader sequence resulted in an unprocessed protein in the form of cytoplasmic inclusion bodies with lower expression yields. The N-terminus fusion of small ubiquitin-like modifier (SUMO), thioredoxin (Trx), and NusA tags increased the expression in the range of 90-140 mg/L, where >90 % of the fusion protein was obtained in soluble form. The fed-batch fermentation of SUMO-tagged hIL-7 protein was optimized at bioreactor level, where a high volumetric product concentration of 2.65 g/L was achieved by controlling the plasmid segregation instability using high antibiotic concentration. The specific product yield (Y_P/X) and volumetric product concentration were 1.38 and 2.55-fold higher compared to batch results, respectively. A preparative scale affinity chromatography resulted in a high recovery yield of 50.6 mg/L with ∼90 % purity. The conformational property of purified recombinant hIL-7 from CD spectroscopy showed a typical helical structure with 31.5 % α-helix and 26.43 % β-sheet. The biological activity of purified protein was tested using IL-7-dependent murine immature B lymphocyte (2E8) cell line by 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide salt (MTT) assay, where it showed a similar biological activity as standard control.

Structural insights into the common γ-chain family of cytokines and receptors from the interleukin-7 pathway

Over the past 13 years, numerous crystal structures of complexes of the common γ-chain (γ(c)) cytokine receptors and their cytokines have been solved. Even with the remarkable progress in the structural biology of γ(c) receptors and their cytokines or interleukins, there are valuable lessons to be learned from the structural and biophysical studies of interleukin-7 (IL-7) and its α-receptor (IL-7Rα) and comparisons with other γ(c) family members. The structure of the IL-7/IL-7Rα complex teaches that interfaces between the γ(c) interleukins and their receptors can vary in size, polarity, and specificity, and that significant conformational changes might be necessary for complexes of interleukins and their receptors to bind the shared, activating γ(c) receptor. Binding, kinetic, and thermodynamic studies of IL-7 and IL-7Rα show that glycosylation and electrostatics can be important to interactions between interleukins and their receptor, even where the glycans and charged residues are distant from the interface. The structure of the IL-7Rα homodimer is a reminder that often-ignored non-activating complexes likely perform roles just as important to signaling as activating complexes. And last but not least, the structural and biophysical studies help explain and potentially treat the diseases caused by aberrant IL-7 signaling.

Structural and biophysical studies of the human IL-7/IL-7Ralpha complex

IL-7 and IL-7Ralpha bind the gamma(c) receptor, forming a complex crucial to several signaling cascades leading to the development and homeostasis of T and B cells. We report that the IL-7Ralpha ectodomain uses glycosylation to modulate its binding constants to IL-7, unlike the other receptors in the gamma(c) family. IL-7 binds glycosylated IL-7Ralpha 300-fold more tightly than unglycosylated IL-7Ralpha, and the enhanced affinity is attributed primarily to an accelerated on rate. Structural comparison of IL-7 in complex to both forms of IL-7Ralpha reveals that glycosylation does not participate directly in the binding interface. The SCID mutations of IL-7Ralpha locate outside the binding interface with IL-7, suggesting that the expressed mutations cause protein folding defects in IL-7Ralpha. The IL-7/IL-7Ralpha structures provide a window into the molecular recognition events of the IL-7 signaling cascade and provide sites to target for designing new therapeutics to treat IL-7-related diseases.

Full-length and internally deleted forms of interleukin-7 are present in horse (Equus caballus) lymph node tissue

Horse IL-7 (HIL-7) cDNA was isolated from adult lymph node tissue by reverse transcription polymerase chain reaction (RT-PCR) using oligonucleotide primers based on horse genomic sequences (The Broad Institute). In addition, to the full-length (FL) 531bp reading frame encoding 176 amino acids, shorter open-reading frames of 477, 396 and 264bp were also amplified. Nucleotide sequence analysis of these RT-PCR products demonstrated they were homologous except the shorter species were missing internal sequences consistent with multiple RNA splicing events. Consequently, the shorter open-reading frames were re-named splice variant (SV) 1 (477bp), 2 (396bp) and 3 (264bp). Organization of the horse IL-7 is predicted to be similar to that in humans with exon 5 deleted from SV1, exons 3, 5 deleted from SV2 and exons 3, 4, and 5 missing from SV3. Each of these open-reading frames has the potential to be stably expressed as demonstrated using a polyclonal antiserum against human IL-7 to visualize the protein products produced when the FL HIL-7 and each SV were molecularly cloned into pCI and transfected in brefeldin A treated HEK 293 cells. Furthermore, addition of supernatants to horse PBMC from HEK cells transfected (without brefeldin A treatment) with pCI HIL-7 FL, pCI HIL-7SV1, pCI HIL-7SV2 and pCI IL-7SV3 all induced significant incorporation of (3)H-thymidine in the presence of sub-stimulatory amounts of concanavalin A compared to supernatants from mock-transfected cells. Therefore, all isoforms of horse IL-7 described in this report have the ability to stimulate proliferative responses in ex vivo horse PBMC cultures.

Crystallization and preliminary X-ray diffraction of human interleukin-7 bound to unglycosylated and glycosylated forms of its alpha-receptor

The interleukin-7 (IL-7) signaling pathway plays an essential role in the development, proliferation and homeostasis of T and B cells in cell-mediated immunity. Understimulation and overstimulation of the IL-7 signaling pathway leads to severe combined immunodeficiency, autoimmune reactions, heart disease and cancers. Stimulation of the IL-7 pathway begins with IL-7 binding to its alpha-receptor, IL-7R alpha. Protein crystals of unglycosylated and glycosylated complexes of human IL-7-IL-7R alpha extracellular domain (ECD) obtained using a surface entropy-reduction approach diffract to 2.7 and 3.0 A, respectively. Anomalous dispersion methods will be used to solve the unglycosylated IL-7-IL-7R alpha ECD complex structure and this unglycosylated structure will then serve as a model in molecular-replacement attempts to solve the structure of the glycosylated IL-7-alpha-receptor complex.

Molecular and functional evidence for activity of murine IL-7 on human lymphocytes

Although interleukin-7 (IL-7) is essential for human and murine lymphopoiesis and homeostasis, clear disparities between these species regarding the role of IL-7 during B-cell development suggest that other, subtler differences may exist. One basic unsolved issue of IL-7 biology concerns cross-species activity, because in contrast to the human ortholog, the ability of murine (m)IL-7 to stimulate human cells remains unresolved. Establishing whether two-way cross-species reactivity occurs is fundamental for evaluating the role of IL-7 in chimeric human-mouse models, which are the most versatile tools for studying human lymphoid development and disease in vivo. Here, we show that mIL-7 triggers the same signaling pathways as human (h)IL-7 in human T cells, promoting similar changes in viability, proliferation, size, and immunophenotype, even at low concentrations. This ability is not confined to T cells, because mIL-7 mediates cell growth and protects human B-cell precursors from dexamethasone-induced apoptosis. Importantly, endogenous mIL-7 produced in the mouse thymic microenvironment stimulates human T cells, because their expansion in chimeric fetal thymic organ cultures is inhibited by a mIL-7-specific neutralizing antibody. Our results demonstrate that mIL-7 affects human lymphocytes and indicate that mouse models of human lymphoid development and disease must integrate the biological effects of endogenous IL-7 on human cells.

T cell development, ageing and Interleukin-7

Interleukin-7 (IL-7) is a cytokine with a central role in the development and maintenance of the peripheral T cell pool. In the mouse, expression of the IL-7 gene in the thymus has been carefully followed from gestation onwards throughout the lifespan. One of the features of its expression in the thymus is that it changes with time, declining measurably as the animal ages. This reduction is associated with a decrease in thymic size, cellularity and output. Analysis of transgenic animals carrying either IL-7 or IL-7 receptor transgenes reveals that the intrathymic level of IL-7 has a critical effect on the production of T cells, and that this may not be a linear relationship. This is an important consideration for therapy involving treatment of old animals with IL-7 of which there are reports indicating some rejuvenation of the thymus following IL-7 treatment, which is never complete. The thymus does not appear to return to the size and cellularity seen in youth. Several possible scenarios could account for this, including the inability to maintain IL-7 within defined limits in the thymus during the therapy.

Murine and Human IL-7 Activate STAT5 and Induce Proliferation of Normal Human Pro-B Cells

The role of IL-7 in lymphoid development and T cell homeostasis has been extensively documented. However, the role of IL-7 in human B cell development remains unclear. We used a xenogeneic human cord blood stem cell/murine stromal cell culture to study the development of CD19+ B-lineage cells expressing the IL-7R. CD34+ cord blood stem cells were cultured on the MS-5 murine stromal cell line supplemented with human G-CSF and stem cell factor. Following an initial expansion of myeloid/monocytoid cells within the initial 2 wk, CD19+/pre-BCR- pro-B cells emerged, of which 25-50% expressed the IL-7R. FACS-purified CD19+/IL-7R+ cells were larger and, when replated on MS-5, underwent a dose-dependent proliferative response to exogenous human IL-7 (0.01-10.0 ng/ml). Furthermore, STAT5 phosphorylation was induced by the same concentrations of human IL-7. CD19+/IL-7R- cells were smaller and did not proliferate on MS-5 after stimulation with IL-7. In a search for cytokines that promote human B cell development in the cord blood stem cell/MS-5 culture, we made the unexpected finding that murine IL-7 plays a role. Murine IL-7 was detected in MS-5 supernatants by ELISA, recombinant murine IL-7 induced STAT5 phosphorylation in CD19+/IL-7R+ pro-B cells and human B-lineage acute lymphoblastic leukemias, and neutralizing anti-murine IL-7 inhibited development of CD19+ cells in the cord blood stem cell/MS-5 culture. Our results support a model wherein IL-7 transduces a replicative signal to normal human B-lineage cells that is complemented by additional stromal cell-derived signals essential for normal human B cell development.

Production and purification of refolded recombinant human IL-7 from inclusion bodies

A recombinant form of human rhIL-7 was overexpressed in Escherichia coli HMS174 (DE3) pLysS under the control of a T7 promoter. The resulting insoluble inclusion bodies were separated from cellular debris by cross-flow filtration and solubilized by homogenization with 6 M guanidine HCl. Attempts at refolding rhIL-7 from solubilized inclusion bodies without prior purification of monomeric, denatured rhIL-7 were not successful. Denatured, monomeric rhIL-7 was therefore initially purified by size-exclusion chromatography using Prep-Grade Pharmacia Superdex 200. Correctly folded rhIL-7 monomer was generated by statically refolding the denatured protein at a final protein concentration of 80-100 microg/ml in 100 mM Tris, 2mM EDTA, 500 mM L-arginine, pH 9.0, buffer with 0.55 g/l oxidized glutathione at 2-8 degrees C for at least 48 h. The refolded rhIL-7 was subsequently purified by low-pressure liquid chromatography, using a combination of hydrophobic interaction, cation-exchange, and size-exclusion chromatography. The purified final product was >95% pure by SDS-PAGE stained with Coomassie brilliant blue, high-pressure size-exclusion chromatography (SEC-HPLC), and reverse-phase HPLC. The endotoxin level was <0.05 EU/mg. The final purified product was biologically active in a validated IL-7 dependent pre-B-cell bioassay. In anticipation of human clinical trials, this material is currently being evaluated for safety and efficacy in non-human primate toxicology studies.

Altered biological activity associated with C-terminal modifications of IL-7

Interleukin 7 (IL-7) is a pleiotropic cytokine which plays a role in both T and B cell function as well as in establishment and maintenance of immunological barriers in epithelial tissues. The heterodimeric IL-7 receptor (IL-7R) consists of the p76 IL-7Ralpha subunit and the p64 common gamma (gammac) subunit. Ligand-binding induces signal transduction through tyrosine phosphorylation of the janus (Jak) and src-related kinases as well as by activation of phosphatidinositol-3 kinase (P13-kinase). In an effort to further define the requirements for ligand-receptor interactions and to subsequently develop candidate receptor binding antagonists with selective biological activities, we examined a series of IL-7 mutants in which the carboxy terminal hydrophobic residues were substituted with aliphatic amino acids. In this study we describe abrogation of IL-7 driven proliferation and attenuated phosphotyrosine signaling by IL-7(143) (Trp-Ala) and IL-7(143) (Trp-His) in IL-7R expressing T and B leukemia cells. Decreased phosphorylation of Jak3 kinase by IL-7W143A, IL-7W143P and IL-7W143H suggest that alterations in this region of the carboxyterminal region of IL-7 affects its interaction with the gammac subunit of the IL-7R.

Structure function analysis of interleukin 7: requirement for an aromatic ring at position 143 of helix D

The residues located at the carboxyl terminus of helix D in interleukin-7 (IL-7) have previously been targeted as important for recruitment and binding to the gamma chain component of the IL-7 receptor (IL-7R). In this study, Trp 143 of helix D was mutated to His, Phe, Tyr and Pro and these mutants, along with a W143A mutant previously described, were studied to determine the effects on activation of DNA synthesis and binding affinity to IL-7R positive 2E8 cells. The W143F and W143Y mutants were similar to wild type IL-7 in their binding properties and retained 85% and 74% of their activating properties, respectively. In contrast, the W143H mutant possessed a lower binding affinity and a corresponding decrease in activation, the W143A mutant possessed an over 100-fold decreased binding affinity and some residual activation activity and the W143P mutant possessed a greatly decreased binding affinity and did not activate. These results strongly suggest an aromatic residue is required at position 143 for IL-7R binding and subsequent signal transduction.