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. 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. 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. 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. 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. Based on a previous report 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, and a complex of IL-15 and soluble IL-15Rα has even more potent effects. 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.
BMC Urol. 2016;16(2) © 2016 BioMed Central, Ltd.