LPS expands MDSCs by inhibiting apoptosis through the regulation of the GATA2/let-7e axis

Myeloid-derived suppressor cells (MDSCs) represent a group of immature myeloid cells composed of myeloid progenitor cells and immature myeloid cells that can negatively regulate immune responses by inhibiting T cell function. Our lab and other groups have shown that injection of a lethal or sublethal dose of LPS into mice can result in the expansion of MDSCs in the bone marrow, spleen and blood. Until now, the molecular mechanisms responsible for this expansion were poorly studied. Specifically, the role of the individual microRNAs (miRNAs) involved remained largely unknown. We performed microarray analysis to compare the miRNA expression profiles of Gr-1+CD11b+ cells sorted from the bone marrow of LPS-injected and PBS-injected mice. We identified let-7e, which was highly upregulated in the LPS-treated group, as a potent regulator of LPS-induced MDSC expansion. Furthermore, let-7e overexpression in bone marrow chimeric mice leads to a noticeable increase in the population of Gr-1+CD11b+ cells, which results from reduced cellular apoptosis. Further studies showed that let-7e can directly target caspase-3 to inhibit cell apoptosis, and upregulation of let-7e in LPS-stimulated MDSCs could be due to the relieved repression of let-7e transcription exerted by downregulated GATA2. Our findings suggest that LPS expands MDSCs by inhibiting apoptosis through the regulation of the GATA2/let-7e axis. Mice were injected (ip) with 100ng LPS(L3024-10MG, Sigma) dissolved in PBS, with mice injected with PBS as control group. Thirty hours later, mice were sacrificed and Gr-1+CD11b+ cells were sorted from bone marrow by flow cytometer.Total RNA was harvested using TRIzol (Invitrogen) and miRNeasy mini kit (QIAGEN) according to manufacturer’s instructions. After having passed RNA quantity measurement using the NanoDrop 1000, these two samples were labeled using the miRCURY™ Hy3™/Hy5™ Power labeling kit and hybridized on the miRCURY™ LNA Array (v.18.0). Following the washing steps the slides were scanned by the Agilent Scanner G2505C.