MM-324 Effects of Soluble and Microvesicle-Bound Therapeutic Anti-CD38 Antibodies on Immune Effector Cells in Multiple Myeloma

Abstract

CD38 is a pleiotropic cell surface molecule with a short cytoplasmic tail. This molecule is highly expressed by multiple myeloma (MM) cells, and it was chosen as the target of therapeutic monoclonal antibodies, such as daratumumab (Dara). This work aims at investigating how Dara interacts with target and immune cells and what effects are induced on natural killer (NK) cells and myeloid-derived suppressor cells (MDSCs). Experiments were conducted on MM-derived cell lines stabilized in our laboratory and fresh human myeloma cells. Microvesicles (MVs) were collected through serial centrifugations. MVs' surface phenotype and internalization were evaluated by flow cytometry. Functional experiments were performed on cells purified from peripheral blood or bone marrow mononuclear cells and analyzed by flow cytometry. As previously reported, CD38 engagement by Dara on MM cells in conditions of membrane fluidity is followed by the release of MVs with peculiar phenotype and cargo. To extend these observations, the activity of both soluble and insolubilized Dara was assessed by exposing NK cells and MDSC to Dara. The results indicate that Dara treatment of NK cells is followed by increased expression of CD69 (activation marker) and CD107 (degranulation marker) and decreased IFNγ production by the latter cells. NK proliferation was reduced by Dara treatment, thus confirming in-vivo observations. On MDSCs, Dara treatment was followed by increased nitrite release and iNos expression. Cell viability was not altered by Dara treatment. MVs were also incubated with dendritic cells, and a high internalization rate was observed, the consequences of which are under investigation. The results of this work indicate that MVs obtained from Dara-treated MM cells may be a particulate system to modulate the immune response, either locally in the bone marrow niche or at distance. Moreover, these observations provide evidence that Dara enhances NK degranulation and activation while paradoxically decreasing their proliferation. These phenomena were observed with both the soluble and insolubilized forms of the antibody. Our data may describe a mechanism underlying Dara effectiveness and may also partially explain the onset of Dara resistance. In conclusion, our results may pave the way for new strategies to improve treatment efficacy and avoid the onset of resistance.