Generation and Characterization of Microvesicles after Daratumumab Interaction with Myeloma Cells

Daratumumab (DARA) is an anti-CD38 human mAb in phase III clinical trials in myeloma patients. DARA binding induces killing of tumor cells via complement-dependent cytotoxicity (CDC), antibody-dependent cell-mediated cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP) and apoptosis.

This work reports on the results obtained while dissecting the events following in vivo CD38 ligation by DARA. Treatment of myeloma cells with DARA + anti-human IgG at 37 °C influenced myeloma cytoskeleton, with redistribution of the CD38 molecules and a formation of distinct polar aggregates visualized by confocal microscopy. DARA effects are different from those observed using 8 different murine anti-human CD38 mAbs, which tended to internalize. The findings observed after DARA ligation were confirmed by exposing myeloma cells to DARA immobilized on CHO cells modified to express 4 distinct human FcRs. CHO cells adhere to plastic and mimics the events that take place in the myeloma niche. First, the interaction between Dara and the different FcRs was determined according to a biosensor-based approach on an IAsys Plus equipment (Affinity Sensors). Results show that DARA possesses the highest avidity for CD64, due to the higher kinetic stability of the complex. Conversely, the differences in the FcR-DARA recognition phase (k_ass) had only minor effects on final stability of the complex. The results also indicated that NK cells and monocytes are the blood populations with higher k_diss for DARA. The effects observed at 37 °C on myeloma cells in the presence of immobilized DARA were amplified when compared to those with soluble mAb. Results indicate that DARA induces CD38 target to aggregate, polarize and to release microvesicles (MV) from extrusions of the myeloma membrane. MV in culture supernatants and in bone marrow plasma of myeloma patients were characterized for concentration (particles/ml) and size by means of Malvern NanoSight NS300 equipment. DARA treatment was followed by high amounts of MV of different sizes released from myeloma cells. Same experiments repeated using DARA + anti-human IgG labeled with Alexa 488 analyzed by Malvern equipment highlighted the presence of DARA on the surface of MV.

The induction of MV may be relevant for in vivo therapy: MV are outward buds of the membrane, which host molecules clustered in microdomains. The presence of CD38 has been confirmed. MV phenotype was analyzed looking for the ectoenzymes that join CD38 in the regulation of adenosine in the myeloma niche. MV phenotype included not only the presence of the expected CD38, but also of CD203a/PC-1, CD39 and CD73, the ectoenzymatic pathway leading to ADO production.

A first conclusion is that DARA treatment is followed in vivo by a marked release of MV at the tumor site. The fate of the MV bearing DARA on their surface is multiple: on one side, MV may interact locally with different cells and populations of the niche. Another possibility is that MV are released into the blood stream and interact with cell populations therein. The lipid bilayer of MV consents passive movements even through tissues. MV appear as minicellular signals delivering instructions at a distance from their place of origin. Further, the ectoenzymes analyzed are also involved in cell migration or in interaction with countereceptors (e.g., CD31) expressed by endothelial cells. This issue, was investigated by testing FITC-conjugated DARA on a Laboratory-established human myeloma line. MV-DARA-FITC were then exposed to PBMC preparations obtained from normal donors. The results from a cytofluorimetric analysis highlighted the tendency of the labeled MV to cluster around CD16⁺ (NK cells) and CD14⁺ subsets (monocytes). At the moment, it is only possible to conclude that MV are associated with cell membranes, a binding likely mediated by FcRs.

Not known yet whether MV interact with other cell types (e.g., macrophages, dendritic cells or lymphocytes). DARA shows a high affinity to FcRs of immune cell types (NK cells, monocytes, B cells). Given clinical data that indicate a robust increase in T cell counts, activation and clonality following DARA treatment should be expected. MV containing DARA and portions of myeloma cell membranes could help drive antigen presentation and T cell response in some patients. This is being investigated further.