A heptamethine cyanine dye is a potential diagnostic marker for myeloid-derived suppressor cells

Background Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells with inhibitory effects on T cell proliferation. MDSC are over-amplified in most cancer patients so that cancer cells avoid anticancer immunity. Unlike mouse MDSCs, however, specific surface markers to define human MDSCs is still controversial due its complexity of subsets. Heptamethine cyanine dyes are fluorescent dyes, particularly used for noninvasive in vivo imaging and detection of cancer. MHI-148 is known to be specifically retained by tumor cells but not by normal cells. In this study, we investigated the potential application of MHI-148 as a specific MDSC detection probe. Methods Mice bearing 4T1 breast cancer cells were created in female BALB/c mice. Splenocytes were isolated at 21 days after injection. Cells were stained with anti-Gr-1-FITC, anti-CD11b-PE antibodies for MDSCs or with MHI-148 dye followed by isolating positive cells with cell sorter. To determine whether MHI148-positive cells possess inhibitory effect on T-cell proliferation, EdU-based T cell proliferation assay was performed. Arginase assay and measurement of Nitrite production were also performed for assessing T cell activity inhibition. Results Compared to normal mice, tumor-bearing mice showed tremendous increase of MDSCs (CD11b+/Gr-1+). Over 81% of these MDSCs in tumor bearing mice were reactive to MHI-148 dye. Most sorted cell for MHI-148 fluorescence was also CD11b+/Gr-1+ MDSCs (97.7 %). Notably, lymphocytes and monocytes were not reactive to MHI-148. In addition, MHI-148 dye-positive cells significantly reduced T cell proliferation with increased arginase activity and nitrites concentration, suggesting that MHI-148 reacts to the cells possessing similar function of MDSCs. Conclusions This study demonstrates that MHI-148 reacts to mouse CD11b+/Gr-1+ PBMCs with the function of MDSC characteristics. Further studies have be focused on MHI148 affinity to human MDSC and outcome of which will result in a novel tool to detect MDSC to be utilized to predict cancer patient prognosis. Legal entity responsible for the study The authors. Funding Has not received any funding. Disclosure All authors have declared no conflicts of interest.