Purpose: We study CD38 levels in immunosuppressive CD4⁺CD25^highFoxp3⁺ regulatory T cells (Treg) and further define immunomodulating effects of a therapeutic CD38 mAb isatuximab/SAR650984 in multiple myeloma.

Experimental Design: We evaluated percentages of CD38-expressing subsets in Tregs from normal donors and multiple myeloma patients. Peripheral blood mononuclear cells (PBMC) were then treated with isatuximab with or without lenalidomide or pomalidomide to identify their impact on the percentage and immunosuppressive activity of Tregs on CD4⁺CD25⁻ T cells (Tcons). We investigated the mechanism of increased Tregs in multiple myeloma patients in ex vivo cocultures of multiple myeloma cells with PBMCs or Tcons.

Results: CD38 expression is higher on Tregs than Tcons from multiple myeloma patients versus normal donors. CD38 levels and the percentages of CD38^high Tregs are increased by lenalidomide and pomalidomide. Isatuximab preferentially decreases Treg and increases Tcon frequencies, which is enhanced by pomalidomide/lenalidomide. Isatuximab reduces Foxp3 and IL10 in Tregs and restores proliferation and function of Tcons. It augments multiple myeloma cell lysis by CD8⁺ T and natural killer cells. Coculture of multiple myeloma cells with Tcons significantly induces Tregs (iTregs), which express even higher CD38, CD25, and FoxP3 than natural Tregs. This is associated with elevated circulating CD38⁺ Tregs in multiple myeloma patients versus normal donors. Conversely, isatuximab decreases multiple myeloma cell- and bone marrow stromal cell–induced iTreg by inhibiting both cell–cell contact and TGFβ/IL10. Finally, CD38 levels correlate with differential inhibition by isatuximab of Tregs from multiple myeloma versus normal donors.

Conclusions: Targeting CD38 by isatuximab can preferentially block immunosuppressive Tregs and thereby restore immune effector function against multiple myeloma. Clin Cancer Res; 23(15); 4290–300. ©2017 AACR.