CD38 and Antibody Therapy: What Can Basic Science Add?

Human CD38 is a target of different therapeutic antibodies and the results from in vivo applications are favorable. However, the use of CD38 targeting may be improved due to insights still being gleaned from basic research, also covering its paralogue CD157. Myeloma was adopted to confirm in vivo our in vitro models of CD38, which is a receptor and an ectoenzyme. Our hypothesis is that ectoenzymes involved in NAD⁺ metabolism are part of the myeloma escape strategy. Extracellular nucleotides (ATP, NAD⁺), high in tumor environments, also serve as intracellular signal transducers, with their degradation products modulating the communications between myeloma and normal cells. Hypoxia helps reprogram the local metabolism. In the extracellular milieu, nucleotides bind purinergic type P2 receptors or are metabolized by ectoenzymes and converted to adenosine (ADO), an immunosuppressor that may also bind different P1 receptors. The resulting anergic status helps the tumor evade the host immune response by promoting Tregs and MDSC, and depressing NK and T effectors. Another question is whether therapeutic antibodies interfere with the survival mechanisms of myeloma. The unprecedented fact that anti-CD38 antibody therapy targets not only a molecule expressed by the tumor but also by effectors and inhibitory cells is a further source of complication. The direct action of the antibodies on the tumor is being investigated in depth, but little is known about their presentation to the target cells. The steps of Fc domain presentation of the antibodies to the target molecule dissected in vitro using CHO cells expressing 4 distinct human FcRs. Exposure of CD38 to Daratumumab (DARA) bound to CHO/FcRs causes at 37 °C a selective aggregation to one pole of the myeloma cells. Such aggregations (rich in CD38 and bound DARA) are released as microvesicles (MVs) into the culture medium or in vivo into biological fluids. These MVs differ from those spontaneously released simply in the presence of the antibody on their surface. In conclusion, MVs are equipped with an enzymatic network potentially capable of metabolizing both ATP and NAD⁺ to produce ADO. Secondly, they may fuse with neighboring cells and leave the myeloma niche, eventually reaching the blood. The therapeutic IgG on their surface acts as a link which favors the capture by FcRs⁺ cells. This model was confirmed by tracking the migrated MVs which accumulate around NK cells (>30%) and monocytes (>90%). The same MVs then enter the cytoplasm of NK, monocytes and MDSCs, even if the functional effects induced by their internalization remain to be defined. Preliminary results conducted on purified NK cells exposed in vitro to MV from myeloma membranes treated with DARA indicate that they contain gene products of potential relevance. Indeed, a set of genes involved in the immune response and in the regulation of cell death is up-modulated. Instead, another set of genes related to mitosis and cell cycle is down-modulated. Under evaluation is what happens when MVs are taken up by FcR⁺-dendritic cells, which may reveal possible vaccinal effects. The transfer of the results of basic research on CD38 to therapy is just beginning: exploration of its multiple functions are expected to provide new, valuable insights.