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. In mice, MDSCs are broadly defined by the expression of CD11b and Gr1. We and others have shown that injection of a lethal or sublethal dose of lipopolysaccharide (LPS) into mice could result in the expansion of MDSCs in the bone marrow (BM), spleen and blood. Until now, the molecular mechanisms responsible for this expansion are poorly studied; specifically, the roles of the individual microRNAs (miRNAs) which may be involved remain largely unknown. We performed microarray analysis to compare the miRNA expression profiles of CD11b+ Gr1+ cells sorted from the BM of LPS-injected and phosphate-buffered saline-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 BM chimeric mice led to a noticeable increase in the population of CD11b+ Gr1+ cells, which resulted from reduced cellular apoptosis. Further studies showed that let-7e could 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.