Effect of daratumumab on CD38⁺ Tregs in PB. (A) In a representative experiment, the gating strategy of Tregs is shown (upper), and 18.2% of Tregs expressing CD38 were depleted after the first daratumumab infusion (lower). (B) CD38⁺ Tregs were reduced and remained low during weeks 1, 4, and 8 of daratumumab therapy. (C) In experiments performed on samples from multiple healthy donors, cell proliferation was assessed through the dilution of CFSE dye due to cell division; strong inhibition of cell division was observed in the presence of CD38⁺ Tregs, in contrast to the partial inhibition of proliferation observed in the presence of CD38⁻ Tregs or negative controls. Error bars represent standard error. Asterisks denote significant changes.