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Impact of mesenchymal stromal cell-derived vesicular cargo on B-cell acute lymphoblastic leukemia progression.

Leukemia cells reciprocally interact with their surrounding bone marrow microenvironment (BMM), rendering it hospitable to leukemia cell survival, for instance by release of small extracellular vesicles (sEV). In converse, we show here, that BMM-deficiency of pleckstrin homology domain family M member 1 (PLEKHM1), which serves as a hub between fusion and secretion of intracellular vesicles and is important for vesicular secretion in osteoclasts, accelerates murine BCR-ABL1+ B-cell acute lymphoblastic leukemia (B-ALL) via regulation of the cargo of sEV released by BMM-derived mesenchymal stromal cells (MSC). PLEKHM1-deficient MSC and their sEV carry increased amounts of syntenin and syndecan-1, resulting in a more immature B cell phenotype and increased number/function of leukemia-initiating cells (LIC) via focal adhesion kinase (FAK) and AKT signaling in B-ALL cells. Ex vivo pretreatment of LIC with sEV derived from PLEKHM1-deficient MSC led to a strong trend towards acceleration of murine and human BCR-ABL1+ B-ALL. In turn, inflammatory mediators such as recombinant or B-ALL-cell derived tumor necrosis factor (TNF) α or interleukin (IL)-1β condition murine and human MSC in vitro, decreasing PLEKHM1, while increasing syntenin and syndecan-1 in MSC, thereby perpetuating the sEV-associated circuit. Consistently, human trephine biopsies of patients with B-ALL showed a reduced percentage of PLEKHM1+ MSC. In summary, our data reveal an important role of BMM-derived sEV for driving specifically BCR-ABL1+ B-ALL, possibly contributing to its worse prognosis compared to BCR-ABL1- B-ALL, and suggest that secretion of inflammatory cytokines by cancer cells in general may similarly modulate the tumor microenvironment.