The THP-1 human promonocytic cell line was purchased from the American Type Culture Collection. Cells were cultured in Gibco RPMI (Life Technologies, Burlington, ON, Canada) supplemented with 10% Fetal Bovine Serum (FBS; Hyclone, Logan, UT). Recombinant human IL-27 and IL-6 and recombinant murine IL-27p28 were purchased from R&D Systems (Minneapolis, MN, USA). As outlined on the manufacturer’s product sheets, biological activity of rhIL-27 (50 ng/mL) was measured for antiviral activity using human HepG2 cells infected with encephalomyocarditis virus, biological activity of rhIL-6 was measured for cell proliferation of mouse T1165.85.2.1 cells, and biological activity of rmIL-27p28 was measured for its ability to induce IP-10 secretion by THP-1 cells. Upon analysis of IL-27 and IL-30 dose responses (50–200 ng/mL), we observed that IL-27-induced IP-10 expression was significantly higher than that for IL-30-induced IP-10 expression at all concentrations tested (Figure 1A); therefore, we used doses of 50 ng/mL for each cytokine for our experiments. Neutralizing antibodies (nAbs) for human IL-27, IL-30, and gp130; WSX-1 Fc chimera; and mouse IgG1 and human IgG1 isotype controls were also purchased from R&D systems. Other reagents used include Stattic (Tocris Bioscience, Bristol, UK), Batimastat (BB-94) (EMD Millipore, Burlington, MA, USA), and solvent control DMSO (Bioshop, Burlington, ON, Canada). Recombinant human IL-6Rα was purchased from BioLegend (San Diego, CA, USA).

Whole blood was drawn from healthy volunteers obtained in accordance with the recommendations of Canadian Tri-Council Policy Statement: Ethical Conduct for Research Involving Humans and Health Sciences and Affiliated Teaching Hospitals Research Ethics Board (HSREB) with written informed consent from all subjects. All subjects gave written informed consent in accordance with the Declaration of Helsinki. The protocol was approved by the HSREB. Primary human monocytes were isolated using RosetteSep™ negative selection human monocyte enrichment cocktail (Stem Cell Technologies, Vancouver, BC, Canada) according to manufacturer’s instructions. Briefly, whole blood was supplemented with respective enrichment cocktails and layered over top of the density gradient Lympholyte-H separation medium (Cedarlane, Ballintemple, Ireland) in SepMate 50-mL conical tubes (StemCell Technologies). Buffy coat was extracted after centrifugation, and cells were washed with PBS + 10 mM EDTA + 2% FBS. Cells were incubated in Gibco RPMI 1640 and supplemented with 10% FBS at 37°C and 5% CO₂ and cultured in the presence or absence of IL-30 or IL-27 for the time points described.

Culture supernatants were used to quantify cytokine expression according to manufacturers’ instructions for human TNF-α (Thermo Fisher Scientific, Affymetrix eBioscience, Waltham, MA, USA; Ready-Set-Go kit; sensitivity: 4–500 pg/mL), human CXCL10/IP-10 (R&D Systems, DuoSet kit; sensitivity: 31.2–2,000 pg/mL), and sIL-6Rα (R&D systems, DuoSet kit; sensitivity: 15.6–1,000 pg/mL). All supernatants were either undiluted or diluted 1:2 and 1:5 and were run on the same plate to ensure the readings were comparable. Absorbance was measured with the BioTek ELx800 Microplate Reader (Winooski, VT, USA) at 450 nm. Graphic data present the mean ± SEM, and n values for each experiment are indicated in figure legends.

For surface staining, THP-1 cells and primary monocytes were resuspended in FACS buffer (PBS + 0.01% sodium azide + 2% FBS) and incubated with anti-human TLR4 AlexaFluor^® 488 (eBioscience), anti-human WSX-1 FITC (R&D Systems), or anti-human gp130 PE (R&D Systems). Cells incubated with anti-human CD126 (IL-6Rα) antibody (eBioscience) were subsequently stained with anti-mouse PE secondary antibody (BioLegend). Mouse IgG2a κ FITC and PE isotype controls (eBioscience) were used at the same concentration and dye:protein ratio as the TLR4, WSX-1, gp130, and IL-6Rα antibodies. Secondary antibody was added alone as a control. Primary monocytes were stained with CD14 Biotin (eBioscience) and streptavidin AlexaFluor^® 610-R-phycoerythrin conjugate (Invitrogen, Carlsbad, CA, USA) to assess population purity, which was found to be greater than 95%. CD14⁺ primary cells were gated on for analysis of TLR4, WSX-1, gp130, or IL-6Rα. Data were acquired with the Epics XLMCL or CytoFLEX flow cytometer (Beckman Coulter, Pasadena, CA, USA) and analyzed using FlowJo software, version X 10.0.7r2.

Cell pellets were lysed on ice using Nonidet P-40 lysis buffer as previously described (14). Whole-cell extracts were quantified using the Bradford protein assay (Bio-Rad, Hercules, CA, USA), resolved by SDS-PAGE, and transferred to nitrocellulose membrane before analysis by immunoblot. Membranes were blocked in PBS containing 0.5% Tween (PBS-T) and 5% non-fat dry milk before incubation with the following primary antibodies: anti-actin (Cat #MAB1501, Millipore, Billerica, MA, USA), anti-tubulin (B-7, Cat #sc-5286, Santa-Cruz Biotechnology, Dallas, TX, USA) and anti-STAT1-phospho-Tyr701 (Cat #9171), anti-STAT3-phospho-Tyr705 (Cat #9145), anti-STAT1 (Cat #9172), and anti-STAT3 (Cat #4904) all from Cell Signaling Technology (Danvers, MA, USA). After washes in PBS-T, membranes were further incubated with horseradish peroxidase-conjugated secondary antibodies (KPL, Milford, MA, USA, or Jackson Immunoresearch Laboratories, West Grove, PA, USA). Antibodies were diluted in PBS containing 0.5% Tween and either 5% non-fat dry milk or BSA. After washes in PBS-T, immunoreactive bands were visualized by enhanced chemiluminescence (Western Lightning Chemiluminescence Reagent Plus, Perkin-Elmer Life Sciences, Waltham, MA, USA) using a LAS4000mini CCD camera apparatus (GE Healthcare, Little Chalfont, UK).

Statistical analyses were performed with GraphPad Prism 6. For all data sets, p values were calculated using Mann–Whitney U test and Wilcoxon matched-pairs signed-rank test. A p value less than 0.05 was used to define statistical significance. Data are represented as the cumulative mean ± SEM of biological replicates, and n values for each experiment are indicated in the figure legends.

To model IL-27 and IL-30 functions, we focused on human monocytic cells using the THP-1 cell line and primary monocytes as model systems. We previously reported that IL-27 mediates pro-inflammatory cytokine and chemokine production in human monocytes, including IP-10, thus we decided to use this chemokine as a readout for dose–response analysis (8). THP-1 cells were treated with increasing doses of IL-30 or IL-27 for 24 h, and IP-10 expression was assessed. Both cytokines induced significant IP-10 expression at 50 ng/mL (Figure 1A); thus, we chose this dose for subsequent experiments. Previously, our lab demonstrated that IL-27 upregulates TLR4 expression and LPS responsiveness in human monocytes (12). Therefore, to investigate the role of IL-30 in modulating TLR4 expression, we treated THP-1 cells and primary human monocytes with IL-30 (50 ng/mL; 2 nM) or, as a positive control, IL-27 (50 ng/mL; 1 nM) for 24 h and quantified TLR4 expression levels by flow cytometry (Figures 1B,C). IL-30 significantly enhanced TLR4 expression on THP-1 cells (Figure 1B) and primary monocytes (Figure 1C) although to a significantly lesser extent to that of IL-27. To investigate whether IL-30-induced TLR4 is sufficient to prime cells for heightened LPS responses, THP-1 cells and primary human monocytes were pretreated with medium, IL-27, or IL-30 for 16 h and subsequently stimulated with LPS (1 µg/mL) for an additional 4 h. IL-30 pretreatment of THP-1 cells (Figure 1D) and primary human monocytes (Figure 1E) enhanced LPS-induced TNF-α production compared to LPS alone. As expected, IL-27 pretreatment upregulated TLR4 expression and LPS-induced TNF-α expression, but this was to a significantly greater degree than IL-30. Taken together, these results suggest that IL-30 shares functions with IL-27 in human monocytic cells; however, IL-30-mediated effects were not as potent as those with IL-27.

To investigate the signaling mechanisms involved in the function of IL-30 and IL-27 in human monocytes, we further assessed IP-10 production in response to IL-30 and IL-27. Initially, THP-1 cells were cultured in the presence or absence of IL-27 (50 ng/mL; 1 nM) or IL-30 (50 ng/mL; 2 nM) for 4–24 h, and IP-10 secretion was quantified in cell-free supernatants by ELISA (Figure 2A). Both IL-27 and IL-30 induced IP-10 secretion; however, IL-30 produced significantly less IP-10 compared to IL-27 at all time points measured. Of note, IL-30 stimulation did not induce TNF-α and IL-6 production in THP-1 cells (data not shown), two cytokines previously demonstrated to be produced in response to IL-27 in primary monocytes (8). We then compared IL-30- and IL-27-mediated IP-10 production from THP-1 cells and primary human monocytes after 24 h. Both IL-30 and IL-27 induced IP-10 secretion; however, we observed a range in responsiveness to each cytokine, notably in primary human monocytes where some donors exhibited very low responses to IL-30 (Figure 2B). Box and whisker plots including all outliers depict the range of IL-27- and IL-30-induced IP-10 expression of at least 10 independent experiments for both THP-1 cells and primary human monocytes. Basal IP-10 expression levels in THP-1 cells ranged from undetectable to 35.9 pg/mL, while in primary cells, basal expression was slightly higher, ranging from undetectable to 390.0 pg/mL. The detected concentrations of IL-30-induced IP-10 ranged from 30.9 to 1180.7 pg/mL in primary monocytes and 219.0 to 1798.8 pg/mL in THP-1 cells. On the other hand, IL-27 consistently induced IP-10 at significantly higher concentrations in each cell type, ranging from 693.0 to 2112.4 pg/mL in primary monocytes and 452.8 to 2277.6 pg/mL in THP-1 cells. These results confirm our observation from Figure 1 that IL-30-mediated functions in human monocytic cells are significantly lower than those induced by IL-27.

Since IL-30 is a constituent of IL-27 as the IL-27p28 subunit, we sought to determine if this subunit is the key component required for the induction of IL-27-mediated responses. Therefore, to determine if IL-30 and IL-27 function independently, we used specific nAbs to IL-27 and IL-30. Cells were cultured in medium alone, IL-27 (50 ng/mL), or IL-30 (50 ng/mL) in the presence or absence of IL-27 nAb (1–5 µg/mL) or IL-30 nAb (1–5 µg/mL) for 24 h (Figures 2C–E). We observed a dose-dependent inhibition of IL-27-induced IP-10 production by the IL-27 nAb but not by the IL-30 nAb in THP-1 cells (Figure 2C). As for IL-30, while its function was abrogated by IL-30 neutralization, it was not significantly inhibited by the IL-27 nAb in THP-1 cells (Figure 2C). This trend was recapitulated in primary human monocytes using the highest doses of the IL-27 nAb and IL-30 nAb (Figure 2D). This indicates that even though IL-27p28 is a component of IL-27, the IL-27p28 (IL-30) nAb is specific to epitopes on IL-30 and not IL-27. Furthermore, the two nAbs act as controls for one another, since they are polyclonal goat IgG; the IL-30 nAb does not impact IL-27-induced IP-10 and the IL-27 nAb does not impact IL-30-induced IP-10. In addition, isotype control goat IgG antibody did not have an effect on IL-27- or IL-30-induced IP-10 production in monocytes. To further confirm that neutralization of the IL-27p28 subunit does not impact IL-27-mediated functions, we stimulated primary human monocytes with IL-27 or IL-30 in the presence of each of the two nAb. The data show that IL-27- and IL-30-mediated TLR4 upregulation is abolished by the IL-27 nAb and IL-30 nAb, respectively (Figure 2E), confirming our above observations in Figures 2C,D. This demonstrates that IL-30 functions independently of IL-27 with respect to pro-inflammatory responses in human monocytes.

Murine models suggest that IL-30 solely signals through the gp130-STAT3 axis (11), whereas IL-27 induces the phosphorylation of STAT3 and STAT1 in human monocytes (8); therefore, we investigated whether IL-30 engages STAT3 as well as STAT1 in human monocytic cells. We stimulated THP-1 cells and primary human monocytes with IL-27 or IL-30 for 5–30 min and examined STAT3 and STAT1 phosphorylation by immunoblot (Figure 3A). As expected, IL-27 highly activated STAT3 and STAT1 within 5 min and increased over time, peaking at 30 min. In contrast, IL-30 induced only minimal STAT1 phosphorylation compared to IL-27, and IL-30-induced STAT3 phosphorylation was barely detectable after 30 min in monocytes and THP-1 cells.

Given that much of the literature regarding IL-30 signaling indicates the involvement of the gp130-STAT3 signaling axis (5, 11), we investigated whether inhibition of STAT3 activation could modulate IL-30 responses in human monocytes. Therefore, we used Stattic, a small molecule that blocks STAT3 activation and dimerization via inhibition of tyrosine-phosphorylated binding motifs from binding the STAT3 Src homology 2 domains (15). We cultured THP-1 cells and primary human monocytes in medium alone or in the presence of IL-27 or IL-30 with or without Stattic (1–5 µM) or DMSO (solvent control) for 24 h and measured IP-10 production by ELISA (Figures 3B,C) and STAT3 and STAT1 phosphorylation after 30 min by immunoblot (Figure 3D). Interestingly, Stattic inhibited both IL-27- and IL-30-induced IP-10 production in a dose-dependent manner, with statistically significantly reduction in THP-1 cells and complete abrogation of IP-10 production in primary monocytes at 5 µM of Stattic compared to DMSO solvent controls (Figures 3B,C). Immunoblot analysis showed that Stattic inhibited IL-27-induced STAT3 phosphorylation and not STAT1 phosphorylation in THP-1 cells, while in primary monocytes, Stattic inhibited IL-27-induced STAT3 phosphorylation and partially affected that of STAT1 (Figure 3D). The partial STAT1 inhibition may be due to non-specific effects of Stattic on STAT1 as others have documented (16). Stattic did not notably reduce IL-30-induced STAT phosphorylation in either cell type. Overall, the data indicate a requirement for STAT3 activation in IL-30-induced IP-10 production after 24 h.

The indication that IL-30 may follow a differential signaling pathway from IL-27, as demonstrated in Figure 3A, led us to investigate the IL-30 receptor requirements. Given that IL-27 signaling requires both WSX-1 and gp130 (2), and IL-30 signaling appears to require gp130 (5, 11), we investigated these receptors. We inhibited IL-27 and IL-30 binding to either gp130 or WSX-1 receptor chains using gp130 nAb or a WSX-1 Fc chimera, respectively. THP-1 cells and primary human monocytes were cultured in medium, IL-27, or IL-30 in the presence or absence of gp130 nAb (1–5 µg/mL), WSX-1 Fc chimera (1–5 µg/mL) or appropriate isotype controls. To demonstrate receptor requirements for signaling, STAT3 and STAT1 phosphorylation were measured by immunoblot after 30 min (Figure 4A). Phosphorylation of STAT1 following IL-27 and IL-30 treatment as well as IL-27-induced STAT3 phosphorylation were inhibited by WSX-1 Fc chimera (1 µg/mL) compared to the human IgG isotype control. This suggests that binding to WSX-1 is required for IL-30-induced JAK/STAT signaling. Surprisingly, neutralization of gp130 did not reduce IL-27- or IL-30-induced STAT3 or STAT1 phosphorylation. To confirm the specificity of the gp130 nAb and WSX-1 Fc chimera, we stimulated THP-1 cells in the presence or absence of the gp130 nAb or the WSX-1 Fc chimera with recombinant human IL-6 (10 ng/mL), which engages gp130 and IL-6Rα, but not WSX-1, to induce STAT3 phosphorylation (Figure 4B) (17). These results showed clear IL-6-induced STAT3 phosphorylation as well as inhibition of this STAT3 phosphorylation by gp130 nAb treatment compared to mIgG1 isotype control. As expected, the WSX-1 Fc chimera did not interfere with IL-6-induced STAT3 phosphorylation.

To further determine the receptor requirements for IL-27- and IL-30-induced functions, we examined IP-10 expression at 24 h under the same conditions as above. IL-27-induced IP-10 expression was significantly but only partially affected by gp130 nAb at all doses, while the WSX-1 Fc chimera had a dose-dependent inhibition of IL-27-induced IP-10 expression in THP-1 cells (Figure 4C). IL-30-induced IP-10 expression was significantly but also partially inhibited at all three doses of the gp130 nAb, while the WSX-1 Fc chimera resulted in complete abrogation of IL-30-induced IP-10 at all doses tested (Figure 4C). In primary monocytes, the gp130 nAb and the WSX-1 Fc chimera had a partial effect on IL-27-induced IP-10 production, but the effect of the gp130 nAb did not reach statistical significance (Figure 4D). Similarly, the gp130 nAb partially reduced IL-30-induced IP-10 production, whereas the WSX-1 Fc chimera completely eliminated IL-30-mediated chemokine production although the effect of the gp130 nAb did reach statistical significance in primary monocytes (Figure 4D). These findings suggest that IL-30 signaling requires engagement of WSX-1 to induce cytokine production and gp130 is partially required.

Since we observed that IL-30-induced IP-10 expression after 24 h was dependent on STAT3 (Figures 3B,C), we tested if IL-30 stimulation resulted in STAT3 phosphorylation at later time points. Thus, we stimulated THP-1 cells and primary monocytes with IL-30 or IL-27 for 0.5, 1, 4, 8, 16, or 24 h and measured phosphorylation of STAT3 and STAT1 (Figure 5). As previously shown, IL-27 significantly activated both STAT3 and STAT1 after 30 min, which decreased by 4-h stimulation in both cell types. Interestingly, IL-27 also induced a secondary wave of STAT signaling initiated at 8 h, peaking at 16 h, and decreasing at 24 h in both cell types. As well, stimulation with IL-27 increased total STAT1 expression in both cell types. As in Figure 3A, IL-30 did not significantly phosphorylate STAT3 at early time points, but after 16–24 h, STAT3 and STAT1 phosphorylation was observed in IL-30-treated THP-1 cells. In primary monocytes, IL-30-induced STAT3 phosphorylation was barely detectable at all time points, while STAT1 activation peaked after 8 h. Like IL-27, IL-30 increased total STAT1 expression at 8–24 h in both cell types, with a weaker IL-30-induced STAT1 expression observed in primary monocytes compared to the more robust response in THP-1 cells. Taken together, the data indicate that both IL-30 and IL-27 modulate STAT1 expression and suggest the existence of an intermediary interaction/protein required for downstream STAT1/3 activation and IP-10 production.

Interleukin-30 binds the sIL-6Rα subunit to signal via gp130 homodimers in murine cells (3, 11). In turn, we hypothesized that IL-30 may require sIL-6Rα for signaling in human cells. Increasing levels of sIL-6Rα were observed in supernatants upon culturing THP-1 cells for 4–24 h, indicating that culture conditions alone are sufficient to induce sIL-6Rα production in monocytic cells (Figure 6A). Next, we examined whether stimulation with IL-30 or IL-27 could induce upregulation of sIL-6Rα. Upon examination of time course stimulations for 4, 8, or 16 h, neither IL-30 nor IL-27 modulated sIL-6Rα levels (data not shown). Likewise, we found that there was no significant difference in sIL-6Rα production among medium-, IL-30-, or IL-27-treated monocytic cells after 24 h (Figures 6B,C). To determine if cleaved sIL-6Rα is involved in IL-30- and IL-27-induced IP-10 production, the matrix metalloproteinase inhibitor, BB-94 (batimastat), was added to the culture conditions to prevent cleavage of membrane bound IL-6Rα. Cells were treated with BB-94 for 30 min prior to IL-30 or IL-27 treatment for 24 h. In primary monocytes and THP-1 cells, BB-94 significantly reduced the formation of sIL-6Rα after 24 h compared to DMSO solvent controls (Figures 6D,F). Interestingly, THP-1 cells exhibited a greater decrease in sIL-6Rα expression in response to BB-94 compared to primary monocytes. This was recapitulated in the levels of IP-10 production induced by IL-30 and IL-27 in THP-1 cells and primary monocytes treated with BB-94 (Figures 6E,G). In light of the more prominent BB-94-mediated inhibition of sIL-6Rα secretion in THP-1 cells compared to primary monocytes, further investigation utilizing exogenous addition of recombinant sIL-6Rα was performed in THP-1 cells. For these experiments, IL-30 and IL-27 were preincubated for 2 h with recombinant sIL-6Rα at 2.5 ng/mL, as this concentration is in the range detected by ELISA in THP-1 culture medium (Figure 6F). The cells were cultured with either BB-94 or DMSO in the presence or absence of IL-30 or IL-27 alone or IL-30 + sIL-6Rα or IL-27 + sIL-6Rα for 24 h. Exogenous addition of recombinant sIL-6Rα following BB-94 or DMSO treatment resulted in statistically significantly increased detection of sIL-6Rα in all conditions, except in IL-27-stimulated DMSO-treated cells, which exhibited a minor increase that did not reach statistical significance (p = 0.1309) (Figure 6F). Addition of IL-30 + sIL-6Rα or IL-27 + sIL-6Rα to DMSO-treated control cells did not impact IP-10 expression. However, we observed a moderate, yet significant, increase in IP-10 production upon addition of complexed IL-30 + sIL-6Rα or IL-27 + sIL-6Rα compared to IL-30 or IL-27 alone in BB-94 treated cells (Figure 6G).

Interleukin (IL)-6 can signal in trans with sIL-6Rα and membrane-bound gp130 (17). To quench endogenous sIL-6Rα, we decided to test if delivery of recombinant IL-6 could modulate IL-30 or IL-27 responsiveness. Thus, we pretreated THP-1 cells and primary human monocytes with IL-6 for 30 min prior to the addition of IL-30 or IL-27 and measured IP-10 production after 24 h in cell-free supernatants (Figures 7A,B). IL-6 pretreated cells produced significantly less IL-27- and IL-30-induced IP-10 relative to cells that were not exposed to IL-6. Furthermore, we investigated the effect of IL-6 on membrane IL-6Rα, gp130, and WSX-1 in THP-1 cells and primary human monocytes after 30 min stimulation. IL-6 did not significantly alter WSX-1 expression in either cell type (Figure 7C). Alternatively, IL-6 significantly downregulated membrane-bound gp130 (Figure 7D) and IL-6Rα levels (Figure 7E) in both THP-1 cells and primary monocytes. Taken together, the data suggest that IL-30- and IL-27-induced IP-10 production is blocked by IL-6 as a result of internalization of IL-6Rα and/or gp130.