Pb1795: mesenchymal stromal cell-dependent adhesion pathways protect aml cells from venetoclax-induced apoptosis

Topic: 3. Acute myeloid leukemia - Biology & Translational Research Background: Acute myeloid leukemia (AML) patients have a poor prognosis with standard treatments due to a high relapse rate. Recently, Venetoclax (VEN), the first B-cell lymphoma 2 (BCL-2) selective inhibitor to enter the clinic, has shown a breaking-through effect in combination with demethylating agents in older patients with treatment-naïve AML. However, resistance mechanisms were described, hampering the complete remission of VEN-treated patients. It has been proposed that the bone marrow microenvironment in general and Mesenchymal stromal cells (MSCs), in particular, favor a protective niche that sustains leukemic cell viability and plays a crucial role in determining drug sensitivity in several hematological disorders, including AML. A previous study demonstrated that VEN, as well as other drugs commonly used in AML therapy, showed a reduced effect in the presence of an MSC-conditioned medium, suggesting that MSC-dependent release of inflammatory signals and other soluble factors could be involved in drug resistance. Aims: Our study aims to investigate the mechanisms of MSC-mediated VEN resistance in AML. Methods: We performed MSC/AML cell co-culture experiments using the VEN-sensitive AML cell line MV4-11. We optimized an MSC/leukemic cell co-culture model able to separate in vitro VEN-sensitive from VEN-resistant leukemic cells. We sorted VEN-sensitive from VEN-resistant leukemic cells and analyzed and compared Gene Expression Profiling (GEP). Results: We found that MSCs significantly reduced MV4-11 apoptosis after VEN exposure. The addiction of transwells to the co-culture system abrogated the MSC-protective effect on VEN-induced apoptosis, suggesting the establishment in our setting of a complex interplay between AML cells and MSCs involving direct cell contact. Furthermore, when we cultured MV4-11 cells in direct contact with MSCs, we were able to distinguish, among leukemic cells, a cell population that remained floating in the supernatant and a cell population that kept adherent to MSCs and could be collected by trypsin detachment only. Interestingly, MSCs protected the adherent but not the floating cells from VEN-induced apoptosis, suggesting a central role of a close adhesion in the MSC-driven resistance to VEN. GEP in adherent vs. floating MV4-11 cells revealed 663 differential expressed genes (DEGs) (FC| ≥ 2 and P ≤ 0.05, according to Transcriptome Analysis Console software guidelines). Interestingly, the principal component analysis suggested that the floating cell GEP was more similar to the GEP of MV4-11 cells cultured without MSCs compared to adherent cell GEP. Enrichment analysis showed that DEGs were involved in cell adhesion migration and proliferation pathways. Among DEGs, we found that CD90/Thy-1, an adhesion molecule whose expression correlated with unfavorable karyotypes and shorter survival in AML, was significantly up-regulated in adherent MV4-11 cells. Flow cytometry analysis confirmed that the adherent compared to the floating MV4-11 cells showed a higher expression of CD90. Summary/Conclusion: Our study demonstrated that MSCs protect AML cells from VEN-induced apoptosis through a complex mechanism involving tight adhesion-mediated pathways. Further clarification of these interactions can shed light on putative targets to counteract MSC-mediated drug resistance to VEN treatment. Keywords: Acute myeloid leukemia, Drug resistance, Stromal cell