Identification of the subpopulations of myeloid-derived suppressor cells and the function study in elderly tumor-bearing mice

Abstract

Objective To investigate the level of subpopulations of myeloid-derived suppressor cells(MDSCs) in elderly tumor-bearing mice versus elderly tumor-free mice, and to study the difference in immune suppressive functions between different subpopulations and their mechanisms. Methods A total of 20 healthy C57BL/6 elderly mice(aged 18-20 months) were randomly chosen to establish Lewis lung cancer models.The amount of monocytic-MDSCs(MO-MDSCs) and polymorphonuclear granulocytic-MDSCs(PMN-MDSCs) in tumor-free and tumor-bearing elderly mice was evaluated by using flow cytometry.MO-MDSCs and PMN-MDSCs were separated with Magnetic-Activated Cell Sorting(MACS) MicroBeads and their morphological characteristics were observed after May-Grunwald-Giemsa staining.The effects of MO-MDSCs and PMN-MDSCs on the proliferation of T cells were determined by Brdu-enzyme-linked immunosorbent assay(ELISA). And the immune suppressive mediators secreted by the subpopulations were detected by Real-time polymerase chain reaction(PCR). Results Compared to the tumor-free group, the proportion of MO-MDSCs in the spleen of tumor-bearing group were increased [(12.44±1.20) % vs.(38.42±3.66) %, t=5.67, P<0.001], while PMN-MDSCs were not [(10.34±0.68) % vs.(12.18±1.27) %, t=2.21, P=0.09]. The result of Brdu-ELISA showed that MO-MDSCs could suppress the proliferation of T cells [(0.30±0.18) vs.(3.38±0.96), t=8.33, P<0.001], while PMN-MDSCs could not [(2.69±0.45)vs.(3.38±0.96), t=1.72, P=0.11]. The result of PCR showed that as compared with PMN-MDSCs, Mo-MDSCs had the increased expression levels of arginase-1(ARG-1), inducible nitric oxide synthase(iNOS), interleukin-10(IL-10), interferon-γ(IFN-γ)(t=4.31, 8.89, 1.70, 3.13, respectively, P<0.01 or 0.05), while the expression levels of interleukin-13(IL-13), transforming growth factor-β(TGF-β) had no differences(t=4.94 and 2.75, P=0.39 and0.47). Conclusions MO-MDSCs are significantly increased in elderly Lewis lung cancer mice models.MO-MDSCs could mediate lung tumor evasion by suppressing the proliferation of T cells through highly expressing ARG-1, iNOS, IL-10 and IFN-γ. Key words: Lung neoplasms; Immunity; Tumor escape