Rituximab induces CD137 up-regulation on human NK cells after incubation with CD20⁺ tumor B cells. Peripheral blood from 3 healthy donors was analyzed for CD137 expression on CD3⁻CD56⁺ NK cells after 24-hour culture with lymphoma cell lines and trastuzumab or rituximab. (A) Percentage of CD137⁺ cells among CD3⁻CD56⁺ NK cells from 3 healthy donors cultured with a CD20⁻ lymphoma cell line (OCI-Ly19) or CD20⁺ lymphoma cell lines (Ramos, DHL-4, Raji). (B) CD20 surface expression on lymphoma cell lines (OCI-Ly19, Ramos, DHL-4, Raji). Histograms were colored according to the log10-fold increase in MFI of lymphoma cell lines relative to isotype. (C) CD137 expression on the NK-cell subsets CD3⁻CD56^bright and CD3⁻CD56^dim from a representative healthy donor after 24-hour culture with the CD20⁺ lymphoma cell line Ramos and rituximab.