Early PCs generated at the end of step 3 (day-10 cells) were FACS-sorted based on CD138 expression and lack of CD20 expression (> 95% purity). A. Purified early PCs were cultured for 4, 10 or 20 days with IL-6, APRIL and stromal cell-conditioned medium (SC-CM) in the presence of increasing concentrations of lenalidomide or the largest DMSO concentration used to dilute lenalidomide (DMSO). Fresh lenalidomide, DMSO, culture medium, cytokines and SC-CM were refreshed every week by replacing half of the culture medium. Counts of metabolically active LLPCs were assayed at day 14, 20 and 30 using a Cell Titer Glo Assay. Data are expressed as the percentage (mean ± SD; n= three separate experiments) of the values obtained in controls (DMSO). B. 30-day LLPCs were incubated with increasing lenalidomide concentrations or the largest DMSO concentration used to dilute lenalidomide (DMSO) for 4 or 11 days. Fresh lenalidomide, control DMSO, culture medium, cytokines and SC-CM were refreshed every week, by replacing half of the culture medium. Counts of metabolically active LLPCs were assayed at day 34 and 41. Data are expressed as the percentage (mean ± SD; n= three separate experiments) of the values obtained in controls (DMSO). * P ≤ .05, compared with the control group (DMSO) using a paired t-test.