Drug-induced senescent Multiple Myeloma cells elicit NK cell proliferation by direct or exosome-mediated IL-15 trans-presentation

Treatment of multiple myeloma (MM) cells with sub-lethal doses of genotoxic drugs leads to senescence and results in increased NK cell recognition and effector functions. Herein we demonstrated that doxorubicin- and melphalan-treated senescent cells display increased expression of IL15, a cytokine involved in NK cell activation, proliferation, and maturation. IL15 up-regulation was evident at the mRNA and protein level, both in MM cell lines and malignant plasma cells (PCs) from patients' bone marrow (BM) aspirates. However, IL15 was detectable as a soluble cytokine only in vivo, thus, indicating a functional role of IL15 in the BM tumor microenvironment. The increased IL15 was accompanied by enhanced expression of the IL15/IL15RA complex on the membrane of senescent myeloma cells, allowing the functional trans-presentation of this cytokine to neighboring NK cells, which consequently underwent activation and proliferation. We demonstrated that MM cell-derived exosomes, the release of which was augmented by melphalan (MEL) treatment in senescent cells, also expressed IL15RA and IL15, and their interaction with NK cells in the presence of exogenous IL15 resulted in increased proliferation. Altogether, our data demonstrated that low doses of chemotherapeutic drugs, by inducing tumor cell senescence and a senescence-associated secretory phenotype (SASP), promoted IL15 trans-presentation to NK cells and, in turn, their activation and proliferation, thus, enhancing NK cell-tumor immune surveillance and providing new insights for the exploitation of senescence-based cancer therapies.