Acute Myeloid Leukemia Cells Export c-Myc in Extracellular Vesicles Driving a Proliferation of Immune-Suppressive Myeloid-Derived Suppressor Cells

Myeloid-derived suppressor cells (MDSCs) play a critical role in promoting immune tolerance and disease growth. We have previously shown that MDSCs are expanded in patients with AML and can be induced from healthy donor PBMCs by co-culture with leukemic cells; a mechanism dependent on expression of the MUC1-C oncoprotein. We sought to elucidate the precise mechanism by which MUC1-C signaling mediates the expansion of this immune suppressive population of immature myeloid cells.

We have previously demonstrated that AML cells release membrane bound extracellular vesicles, which traffic to co-cultured cells. We hypothesized that AML EVs may mediate the expansion of MDSCs. MOLM-14 and THP-1 AML EVs were isolated using the ExoQuick precipitation technique, and analyzed by flow cytometry, and compared to size standardized beads, demonstrating particles between 200-300nM in diameter. Furthermoreisolated AML EVs were visualised using Transmission Electron Microscopy demonstrating multiple rounded structures measuring 100-200nM in diameter and bound by darkly staining membrane. Subsequently, healthy donor PBMCs were cultured for three days with GFP tagged AML EVs and then quantified for CD33+/HLADR-/CD11b+ MDSCs and HLADR+/CD11c+ antigen presenting myeloid cells by flow cytometry. In the PBMCs co-cultured with EVs, the proportion of MDSCs increased 8-fold, whilst the proportion of HLADR+/CD11c+ antigen presenting myeloid cells decreased by 10 fold (n=3, p<0.05).

We subsequently investigated how MUC1-C signaling, necessary for the expansion of MDSCs, might alter AML extracellular vesicles composition. We evaluated AML EVs for the presence of the pro-proliferative oncoprotein c-Myc by immune-blotting, demonstrating that AML cells secrete EVs containing c-Myc, which is abrogated by downregulation of MUC1-C. Furthermore, EVs containing MUC1 and c-myc led to an up-regulation of the c-Myc downstream targets cyclin D2 and cyclin E1 in co-cultured MDSCs, indicating that c-Myc containing EVs may drive MDSC proliferation. Critically, EVs from MUC1-C silenced AML cells failed to elicit this increase in c-Myc and cyclin D2 and E1 expression in EV exposed MDSCs. Interestingly, exposure of MDSCs to AML EVs lead to an increased expression of PD-L1, which was abrogated in EVs from MUC1-C silenced AML cells.

We then sought to determine how MUC1 signaling promotes c-Myc signaling in AML. MUC1-C silencing did not alter c-Myc mRNA levels suggesting a post-transcriptional level of regulation. Micro RNAs are small non-encoding RNA molecules involved in post-translational regulation of gene expression. MiR34a, a known p53 inhibitor, has been implicated in regulating the expansion of MDSCs and it is known that tumor cells suppress MiR34a expression as part of their self-protective armoury. Furthermore, MiR34a is a predicted negative regulator of c-Myc, due to a complementary sequence for MiR34a in the c-Myc promoter region. Using qPCR, we have demonstrated that MUC1-C silencing results in increased expression of MiRNA34a. Furthermore, over-expression of MiR34a in AML cells led to a dramatic down-regulation of c-Myc protein expression, and conversely silencing of MiR34a led to a significant upregulation of c-Myc expression, confirming that MiR34a regulates c-Myc expression in AML. To confirm MiR34a as a critical negative regulator of MDSC expansion, MiR34a altered cells were interrogated for their ability to elicit an expansion of MDSCs in co-cultured PBMCs. Overexpression of MiR34a in AML cells partially abrogated their ability to induce MDSCs from co-cultured donor PBMCs. In concert, silencing of MiR34a in MUC1-C silenced AML cells, recapitulated their ability to induce MDSCs in this model.

Taken together, this study illustrates a novel role of the MUC1-C and c-Myc oncoproteins in driving MDSC proliferation and MDSC PD-L1 expression. We have demonstrated that AML EVs alter the tumor microenvironment away from antigen presentation capable myeloid cells and towards immature immune suppressive MDSCs.