Abstract 2122: Induction of structural and functional effects of myeloma cells after daratumumab treatment
Authors: Castella Barbara , Faini Angelo C. , Yakymiv Yuliya , Morandi Fabio , Larocca Alessandra , Oliva Stefania , Horenstein Alberto L. , Massaia Massimo , Malavasi Fabio 
Published: August 2018

Abstract
Background CD38 is a pleiotropic cell surface glycoprotein with receptorial and enzymatic functions. The molecule is generally expressed at low levels by different hematological and solid tissues: plasma cells score the highest surface levels of CD38 among mature lymphoid cells. CD38 has become the target of therapeutic antibodies in multiple myeloma (MM). Daratumumab (Dara) has been approved as efficient monotherapy or in combination with other anti-myeloma agents. The results obtained are good in patients refractory to standard myeloma therapies. Dara mediates clinical effects through multiple mechanisms. These include complement- and antibody-dependent cell cytotoxicity, antibody-dependent phagocytosis, programmed cell death and modulation of enzymatic activities. Promising are Dara immunotherapeutic functions in virtue of its ability to induce cytotoxicity exploiting both arms of innate and adaptive immune responses. Recent studies suggest that Dara plays immunomodulatory roles in addition to its direct effects on cytotoxicity.
Results CD38 engagement by Dara on MM cells is followed by a selective polar aggregation of the target molecule in myeloma membranes, with subsequent release of microvesicles (MV) of 100-1,000 nm into the extracellular space. We validated the hypothesis that MV released by MM in the bone marrow (BM) niche may express functional ectoenzymes (CD38, CD39, CD73, and CD203a), potentially capable of metabolizing both ATP and NAD+ and to produce adenosine (ADO). Results indicate that MV obtained after Dara treatment tend to cluster around (and to be internalized in) NK cells, monocytes and MDSC, cells all expressing IgG Fc Receptors (FcR). NK cells, which apparently disappear in patients during Dara treatment: because of this, they were selected for testing MV-mediated effects. Comparative analysis of the genes modulated after exposing NK cells to the MV/Dara complex were followed by functional in vitro experiments. Both sets of results confirmed reduced proliferative ability and enhanced NK cell-mediated killing of MM cells. A further support to the immune modulatory roles exerted by Dara comes from the observation that MV surface represent a clustering of the expected CD38/Dara complex, flanked by a set of ectoenzymes involved in the generation of ADO. Moreover, MV express high amount of CD55 and CD59 molecules, receptors which impair the ability of the complement to exert in situ a cytolytic activity. Another set of observations indicate that PD-L1 tend to accumulate in the surface of MV after Dara treatment, anticipating a role in the modulation of immune checkpoint pathways (PD-1/PD-L1).
Conclusions Results of our observations indicate that MV obtained from MM cells treated with Dara may be a particulate system to influence the BM niche and the successive immune responses elicited by different mechanisms and cell effectors.
Citation Format: Barbara Castella, Angelo C. Faini, Yuliya Yakymiv, Fabio Morandi, Alessandra Larocca, Stefania Oliva, Alberto L. Horenstein, Massimo Massaia, Fabio Malavasi. Induction of structural and functional effects of myeloma cells after daratumumab treatment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2122.