Background
Renal cell carcinoma (RCC) is the most common primary malignancy of the renal parenchyma, comprising 3 % of all adult malignancies, and its incidence has been increasing [
1,
2]. Although early RCC can be cured by surgery, one-third of RCC patients exhibit metastasis at diagnosis. Metastatic RCC has poor prognosis, with a 5-year survival rate of only 10 % [
3], and approximately 20-25 % of patients with metastatic RCC do not respond to treatment and symptoms progress rapidly [
4]. Sorafenib is one of target drugs against RCC that prolongs patient survival, but rarely leads to complete remission [
5‐
7]; moreover, long-term sorafenib treatment can exacerbate RCC by creating ischemic conditions [
8,
9].
RCC is considered as an immunogenic tumor owing to its spontaneous regression, variable growth, late metastasis, high degree of T cell infiltration, and high incidence in immunosuppressed patients. However, RCC can also suppress the anti-tumor immunity of naïve and memory CD4
+ T, natural killer (NK), and dendritic cells [
10], and evade the cytotoxic effect of NK cells [
11,
12]. Therefore, a drug that potentiates immune response may be effective in the treatment of RCC. Indeed, high doses of interleukin (IL)-2 have been shown to suppress RCC progression without inducing tumor ischemia, leading to complete remission in 10–20 % of patients [
13,
14]. Blockade of CTLA4, a T-cell inhibitory receptor with ipilimumab, and increasing T-cell proliferation and cytotoxic effects with PD-1/PD-L1 axis inhibition also induced regression of renal cell carcinoma in some patients [
15,
16]. However, high-dose IL-2 therapy also induces systemic inflammatory responses, including capillary leak syndrome, heart failure, and pulmonary edema, thereby hindering the broad application of high-dose IL-2 therapy in the treatment of advanced RCC [
17,
18].
Recently, immune complexes (IL-2C) composed of with low-dose IL-2 and stimulating anti-IL-2 antibody (S4B6) have been shown to enhance immune responses via selective structural interactions [
19‐
23]. Stimulating IL-2C can preferentially expand memory CD8
+ T and NK cells—while more weakly affecting regulatory T cells—via the interaction of anti-IL-2 antibodies (S4B6) and CD25 binding region of IL-2, leading to inhibition of both leukemia and melanoma [
19,
23]. Interestingly, the half-life of IL-2 is increased in IL-2C; as such, low-dose IL-2C has immune enhancing effects that are comparable to those of high-dose IL-2 therapy without accompanying serious side effects such as capillary leak syndrome [
19,
23]. Low-dose IL-2C therapy is therefore expected to be an effective and safe treatment for immunogenic tumors.
Here, we investigated the efficacy and safety of low-dose IL-2C treatment for RCC in a syngeneic murine model. We found that IL-2C treatment enhanced anti-tumor immunity against RCC without causing pulmonary edema, although it did not have sufficient potency to suppress tumor growth.
Methods
Cells and mice
The RENCA, a murine RCC cell line from a BALB/c mouse background was purchased from Korean Cell line Bank (Seoul, Korea), and cultured in Eagle’s Minimum Essential Medium (Gibco/Invitrogen, Grand Island, NY, USA) containing 10 % fetal bovine serum (Gibco/Invitrogen) at 37 °C and 5 % CO2. BALB/c mice were purchased from Orient Bio Inc. (Seongnam, Korea) and maintained at the Biomedical Research Institute of Seoul National University Hospital. Mouse experimental protocols were approved by the Animal Ethics Committee of Seoul National University College of Medicine.
Preparation of IL-2/anti-IL-2 antibody complex
Recombinant murine IL-2 was purchased from eBioscience (San Diego, CA, USA) and the S4B6 anti-mouse IL-2 monoclonal antibodies was provided by Dr. Charles D. Surh (La Jolla Institute for Allergy and Immunology, La Jolla, CA, USA). S4B6 (7.5 μg) was mixed with IL-2 (1.5 μg, equivalent to 8555 IU) and incubated at 37 °C for 30 min before use. To evaluate the immune-enhancing effects of IL-2 under normal conditions, IL-2C or phosphate-buffered saline (PBS) was administered daily to mice by intraperitoneal injection for 5 days, before the spleen was harvested for immune cell analysis.
In vivo tumor model
Eight-week old BALB/c mice were subcutaneously injected with RENCA cells (1 × 105) in 0.1 ml of 1× PBS to induce syngeneic RCC formation. IL-2C (treatment group) or PBS (control group) was intraperitoneally administered to mice every other day from day 0 to 28. Tumor size (length × width) was measured every other day using calipers. IL-2C with S4B6 (7.5 μg) and IL-2 (1.5 μg) or phosphate-buffered saline (PBS) was administered every 2 days to mice by intraperitoneal injection until 28 days. In high-dose IL-2 group, higher dose of IL-2 (35 μg, 200,000 IU) was administered to mice with the same schedule. Spleen, lung and tumor tissues were harvested 28 days after injection of RENCA cells. Tumor weight was measured after harvest. Pulmonary edema was assessed by lung weight, which was calculated by subtracting the dry weight from the wet weight.
Flow cytometry
Splenocytes were labeled with the following antibodies: anti-CD4-allophycocyanin (APC), anti-CD8-fluorescein isothiocyanate (FITC), anti-CD44-APC, anti-CD45-FITC, anti-CD49-phycoerythrin (PE), and anti-F4/80-PE and the vital dye 7-aminoactinomycin D (7-AAD) (BD Biosciences, San Jose, CA, USA). Forkhead homeobox protein 3 (Foxp3) was labeled using the anti-mouse Foxp3-FITC staining kit (eBioscience) according to the manufacturer’s instructions. For analysis of tumor-infiltrating cells, tumors were dissociated with 200U/ml collagenase IV at 37 °C for 30 min. Flow cytometric analysis was carried out on a Canto II Instrument (BD Biosciences).
Enzyme-linked immunoSPOT (ELISPOT) assay
Interferon (IFN)-γ- or IL-10-producing T cells were detected with the ELISPOT assay. Spleens were harvested 28 days after mice were injected with RENCA cells. A 96-well plate was coated with anti-IFN-γ or -IL-10 capture antibodies using ELISPOT mouse IFN-γ or mouse IL-10 kits (BD biosciences). For IFN-γ ELISPOT, splenocytes (1 × 105/well) were incubated with 5 ng/ml phorbol 12-myristate 13-acetate (Sigma, St. Louis, MO, USA) and 500 ng/ml of inomycin (Sigma) at 37 °C for 8 h. For IL-10 ELISPOT, splenocytes (5 x 105/well) were incubated with 1 μg/ml of lipopolysaccharide (Sigma) for 24 h. Detection antibodies were then added, along with horseradish peroxidase (HRP)-streptavidin (BD Biosciences). After adding 3'-amino-9-ethylcarbazole substrate (BD Biosciences) for development, colored spots were measured with an ELSPOT reader (Cellular- Technology, Cleveland, OH, USA).
Immunohistochemistry
Tumor tissue with overlying skin was harvested on day 28. Anti-CD4, anti-CD8, anti-CD49b and anti-F4/80 antibodies (eBioscience) were incubated with tissue sections at 4 °C overnight. Sections were then treated sequentially with secondary antibody (ZytoChem Plus HRP One-Step Polymer anti-mouse; Zytomed, Berlin, Germany) and substrate solution (ImmPACT NovaRED Peroxidase Substrate Kit; Vector, Burlingame, CA, USA). Pulmonary edema was assessed by hematoxylin and eosin staining.
Statistical analysis
Continuous variables were compared between the IL-2C and the PBS groups using the Student’s t-test. RCC growth over 4 weeks was compared between the two groups with the linear mixed model. A P value < 0.050 was considered statistically significant. Analyses were carried out using SPSS v.22.0 software (SPSS Inc., Chicago, IL, USA).
Discussion
The present study investigated for the first time the anti-tumorigenic effects of IL-2C against RCC in vivo. We found that stimulating IL-2C induced the expansion of CD8+ memory T and NK cell populations, shifted the Th1/Th2 balance in favor of Th1, and increased immune cell infiltration into tumor tissue in mice with RCC, all without inducing serious side effects such as pulmonary edema. However, the enhancement of anti-tumor immunity by IL-2C was not sufficient to inhibit RCC growth significantly.
IL-2C can enhance or suppress immunity depending on the type of anti-IL-2 monoclonal antibody. For example, the monoclonal antibody JES6-1 binds to the IL-2 epitope, and hinders binding to IL-2 receptor (R)-β while enabling binding to IL-2R-α. Since both CD8
+ memory T and NK cells constitutively express IL-2R-β, and regulatory T cells constitutively express both IL-2R-β and IL-2R-α, an IL-2C comprising JES6-1 preferentially induced the expansion of regulatory T cells [
24]. In contrast, S4B6 binds to an epitope of IL-2 such that binding to IL-2R-α is blocked in favor of IL-2R-β binding [
23]. Therefore, IL-2C comprising S4B6 induces the expansion of CD8
+ memory T and NK cells over regulatory T cells.
Immune complexes consisting of low-dose IL-2 and the S4B6 clone of the anti-IL-2 antibody was found to inhibit metastasis of melanoma and leukemia in a mouse model by inducing the expansion of CD8
+ T and NK cell populations [
19,
23]. In accordance with these findings, we also found that S4B6-containing IL-2C increased CD8
+ T and NK cell number as well as their infiltration into RCC lesion, although the growth of RCC was not significantly affected in a syngeneic RCC mice model.
There are a few possible explanations for the insufficient effects of IL-2C on RCC growth. Firstly, immunosuppression by RCC is strong enough to counter immune-potentiating effects of IL-2C, which promotes RCC proliferation and survival [
10‐
12]. For instance, RCC exhibits resistance to NK cell-mediated lysis, despite IL-2C-induced NK cell expansion and infiltration into RCC lesions [
11,
12]. Secondly, the immunogenicity of RCC may be lower than that of malignant melanoma. Tumor-associated antigens are required for immune cell infiltration into tumors [
25,
26]; however, there are fewer RCC-associated antigens than tumor-associated antigens that have been found in melanoma [
27]. Therefore, a relative lack of targeting antigens may be a reason why adoptive therapy with CD8
+ tumor-infiltrating lymphocytes has not been clinically effective for RCC treatment [
28]. Third, lack of kidney-specific microenvironment might have influenced the results. However, when we injected RENCA cells into the renal subcapsular space, the results were the same as those in the subcutaneous RCC model (data not shown).
The amount of IL-2 that was used in IL-2C therapy was 23 times lower than the amount of IL-2 in high-dose IL-2 therapy [
23]. Based on a previous report [
23] and our results, low-doses of IL-2C do not cause significant adverse reactions such as pulmonary edema, and is therefore safe for clinical application. However, because even high-dose IL-2 therapy in the present study did not increase lung weight significantly, further studies using higher dose of IL-2C and IL-2 are needed to confirm safety as well as insufficient efficacy of IL-2C in comparison to high-dose IL-2.
Since IL-2C alone cannot suppress RCC growth, additional studies are needed to determine the impacts of other therapies used in combination with IL-2C on RCC. For example, IL-15 can also induce the expansion of NK and CD8
+ T cell populations and thereby suppress the growth of malignant melanoma [
29], and a complex of IL-15 and soluble IL-15Rα has even more potent effects [
30]. Therefore, it is worth investigating whether IL-2C used in conjunction with an IL-15 complex has greater effectiveness in suppressing RCC growth. We may also try to combine IL-2C with the current target agents such as sorafenib to obtain additive effects.
Acknowledgements
This work was supported by a grant from the Korean Health Technology R&D Project, Ministry of Health & Welfare, Republic of Korea (A111355), and by a grant (0320140430, 2014-1033) from the SNUH Research Fund. We thank Charles D Surh for providing anti-IL-2 antibody.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
KHH and KWK performed the design of the study, experiments, data analysis, and drafted the manuscript. JJY and JGL contributed to the design of study, experiments and data analysis. EML and MH helped the experiments and data analysis. EJC, SSK, HJL and TYK contributed the experiments. CA provided intellectual advice to the study. JY conceived of and designed the study and supervised the work. All authors read and approved the final manuscript.