Targeting CD38 Suppresses Induction and Function of T Regulatory Cells to Reverse Immunosuppression in Multiple Myeloma

We here study targeting CD38 to overcome immunosuppression by CD4+CD25^highFoxp3+ T regulatory cells (Tregs) in multiple myeloma (MM). CD38 is differentially expressed on T cell subsets with higher levels on Tregs than CD4+CD25- conventional T cells (Tcons) from MM patients vs. normal donors. CD38 levels and the percentages of CD38^high Tregs are further increased by low doses of Pomalidomide (Pom) or Lenalidomide (Len), which could confer further sensitivity to CD38 targeting. This result further support combined targeting CD38 with immunomodulatory drugs (IMiDs) to mitigate tumor-related immunosuppression. Importantly, anti-CD38 mAb SAR650984 (SAR) preferentially decreases Treg while increases Tcon frequencies, which is enhanced by Pom/Len. SAR induces apoptosis and inhibits proliferation of Tregs in Fc-independent manner. It further reduces Foxp3 and IL10 in Tregs, blocks migration of Tregs, and restores proliferation and function of Tcons. Importantly, SAR augments MM cell lysis by CD8+ T and natural killer cells, as seen by enhanced cell surface CD107a for degranulation and IFNγ production. Pom/Len further enhances these effector functions induced by SAR. Ex vivo cocultures of MM cells with peripheral blood mononuclear cells (PBMCs) or Tcons significantly induce Tregs (iTregs) which express even higher CD38 than natural occurring Tregs (nTregs) in a time-dependent manner. CD38 is increased at even higher extent on iTregs induced from Tcons than PBMCs when cocultured with MM cells, indicating the conversion of Tcons into iTregs. This is associated with elevated circulating CD38+ Tregs in MM patients vs. normal donors. Besides upregulated CD38, iTregs, when compared with Tcons alone, express higher levels of CD25, Foxp3, CD44, ICOS, and PD1, while low CD127. PDL1 is concurrently increased on MM cell membrane in these cocultures. Since anti-TGFb, -PD1, or -PDL1 mAb, when added alone, partially blocks iTreg induction from Tcon, cell-cell contact via PD1/PDL1 interaction and TGFb are attributed to induction of iTregs. SAR decreases MM cell- and bone marrow stromal cell-induced iTregs and production of inhibitory cytokines TGFb and IL10, further indicating that SAR targets immunosuppressive function in CD38^high iTregs. Finally, CD38 levels correlate with differential inhibition by SAR on Tregs from MM vs normal donors. Taken together, these results show that targeting CD38 can preferentially block potent immunosuppressive Tregs while restore effector function to further against MM.