The Regulation Of MDSCs Function In Tumor Acidification Microenvironment

Myeloid-derived suppressor cells(MDSCs), a heterogeneous population of cells that consists of immature macrophages, granulocytes, dendritic cells(DCs) and other myeloid cells at early stages of differentiation, is a key factor for the failure of tumor immunotherapy. Therefore, people tried to reduce the immunosuppressive status through elimination of MDSCs or inhibition of MDSCs.Acid-sensing ion channels(ASICs) is known as the H-gated cation channels, who can perceive H+ gradient of the cell through the channel. So far it is not clear if MDSCs can express ASICs,if the acidification of tumor microenvironment can activate ASICs,and if ASICs participate in the immunosuppression function of MDSCs?We proposed that ASICs acted as sensors for extracellular acidosis and regulate the responses of MDSCs to acidosis in tumor environment.In this study,the expression of ASICs in mouse MDSCs and the effect of ASICs in MDSCs function in acidosis were explored. The results were as follows: 1?MDSCs express more ASICs in tumor-bearing mouseA mouse of tumormodel was established, then normal and tumor-bearing MDSCs were separated respectively, RT-PCR and Western blotting were applied to detect the expression of ASICs at mRNA and protein levels: Results showed that, mRNAs and proteins for ASIC1,ASIC2 and ASIC3 were detected in all MDSCs, while, tumor-bearing MDSCs expressed more than normal mouse. Immunocytochemistry was usedto determine subcellular distributions of ASICs in MDSCs and the result showed that ASIC1 and ASIC3 were distributed in cytoplasm,while ASIC2 located in cell membrane. 2? The effect of acidification oftumor microenvironment onMDSCs function.(1)Acidification could promote ROS secretion.When the pH was 5.5, the production of ROS in MDSCs increased about 2.6 folds than in normal mice(p<0.001). However, it had no effect to NO production.It indicated that PMN-MDSC was activated to increase the production of ROS, which could enhance its immunosuppressive activity, whileMo-MDSC was not activated obviously,who secreted NO.(2)Acidification could promote MDSCs to express TGF-?1,When the pH was 5.5, TGF-?1 expression in tumor-bearing mice was significantly higher than in normal mice(>3 times),When amiloride, aASIC blocking reagent was used,the express of TGF-?1 was reduced(p<0.001), but the low p H had no influence on IL-10, Which suggested that ASICs was involved in the immunosuppression of MDSCsbymodulating the secretion of TGF-?1.(3)Acidification could promote MDSCs to inhibit T cell proliferation: When T cell was co-cultured with MDSCs in the medium of pH5.5, the proliferation rate of T cell dropped from 32.6% to 20.4%(p<0.01), which showed that MDSCs could inhibit T cell proliferation in tumor acidification microenvironment. 3?The cell signaling pathway by which theacidification microenvironmentin tumor affects MDSCs.Western blotting was used to detect the protein phosphorylation in p38 MAPK and NF-?B signaling pathway.Results showed thatacidification could induce p38 phosphorylation and then activate p65 phosphorylation in the cell nucleus in MDSCs to inhibit immune function. This effect could be blocked by amiloride, a ASIC blocking reagent.It suggested that the acidification of tumor microenvironment could enhanceimmune suppressionactivity of MDSCs through p38 MAPK and NF-?B signaling pathway.Conclusions:1. MDSCs can express the mRNA and protein of ASIC1, ASIC2 and ASIC3, ASIC1 and ASIC3 were located in cytoplasm,while ASIC2 located in cell membrane. Tumor-bearing MDSCs express higher ASICs than normal mouse cells.2. Tumor acidification microenvironment could promote MDSCs to secrete ROS and TGF-?1, and inhibit T cell proliferation.3. MDSCs express more ASICs in tumor-bearing mouse, the acidification signals pass through ASICs in tumor microenvironment, it activates p38 phosphorylation, and then activates p65 phosphorylation in the cell nucleus, to regulate the expression of related genes in MDSCs, which resulted in the immunosuppressive function.