Daratumumab masks the detection of CD38 with commercially available anti-CD38 flow cytometry antibodies. (A) Representative dot plots showing expression of CD38 on BM-localized MM cells (blue) by using a commercially available anti-CD38 flow cytometry antibody before start of daratumumab therapy (left dot plot) and after 10 infusions of daratumumab (right dot plot). The right dot plot demonstrates that, in daratumumab-treated patients, the CD38 antigen cannot be detected by flow cytometry with commercially available anti-CD38 antibodies due to epitope occupancy of the therapeutic antibody. (B) Dot plots from the same patient showing expression of CD38 on BM-localized MM cells (blue) by using the newly developed anti-CD38 monoclonal antibody, HuMax-003, which binds to a different epitope compared with daratumumab. This excludes the possibility that binding of daratumumab masks the detection of CD38. The left dot plot is obtained before start of daratumumab therapy and the right dot plot after 10 daratumumab infusions. These dot plots also illustrate that during daratumumab treatment, there is significant reduction of CD38 expression on residual MM cells.