One mechanism tumors use to evade immune responses is to regulate the abundance of myeloid-derived suppressor cells (MDSC), mainly through secreted factors (Gabrilovich and Nagaraj, 2009; Gabrilovich et al., 2012; Nagaraj and Gabrilovich, 2010). Here we examined whether this is affected by radiation therapy, which recent studies suggest can effectively prime anti-tumor immunity (Formenti and Demaria, 2012; Golden et al., 2012). In contrast, we have shown that radiation very efficiently activates TGFβ which is a suppressor of the adaptive immune response, particularly inhibiting T-cell activity and inducing T-regulatory cells. TGFβ also regulates innate immune response, particularly MDSC, which are a subset of immature myeloid precursors that are potent inhibitors of anti-tumor immunity. Here, we irradiated human breast cancer cell lines that were co-cultured with human monocytic precursors derived from circulating peripheral blood mononuclear cells (PBMC) isolated from healthy blood donors. CD14+ immature myeloid cells (IMC) stimulated with GM-CSF and IL-6 generated CD33+/CD11b+ MDSC with high efficiency (up to 90%). Consistent with the literature, TGFβ significantly promotes the accumulation of MDSC. IMC treated with TGFβ generated twice as many MDSC compared to the positive control of monocytes treated with alone (Mean = 18,509 -/+ 1523 cells (GM-CSF + IL-6 +TGFβ) vs 9010 -/+803.8 cells (GM-CSF + IL-6); N = 11 independent donors). Accordingly, small molecule TβRI inhibitor (LY 2109761) blocked this effect and further reduced by 50% the generation of MDSC compared to the GM-CSF and IL-6 positive control (Mean = 5371 -/+796.8 cells (GM-CSF + IL-6 + LY2109761 vs 9010 -/+803.8 cells (GM-CSF + IL-6); N = 11 independent donors).
As reported (Lechner et al., 2010), tumor cell lines varied in their capacity to elicit MDSC from co-cultured IMC; being MCF7 less efficient than BT549, which are less efficient than MDA-MB-231. MDA-MB-231, which expresses higher levels of GM-CSF and IL-6 mRNA than the other cell lines, was used to test the effect of radiation. IMC were added to MDA-MB-231 monolayers irradiated with 5 Gy and co-cultured for 5 days in complete media (RPMI + 10% FBS). Tumor cell irradiation did not affect MDSC numbers but increased MDSC immunosuppressive capacity, as shown by a significant reduction in autologous CD8+ T-cell proliferation. This effect was TGFβ-dependent, as the immunosuppressive potential of MDSC was significantly reduced when they were generated in a context of TGFβ pathway inhibition. These results suggest that the combination of radiation therapy and inhibition of the TGFβ pathway may have a great therapeutic potential by releasing the suppressive action of MDSC on the anti-tumor immune responses.
* This project has been funded through the Lilly Research Award Program (LRAP)
Citation Format: Alba Gonzalez Junca, Ilenia Pellicciotta, Kyla Driscoll Carroll, David Schaer, Mary Helen Barcellos-Hoff. Radiation-induced TGFβ regulates the myeloid compartment to suppress antitumor immunity. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A151.
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