FOXP1 inhibits PC differentiation of primary human MBCs. (A) CD19⁺CD27⁺ MBCs were sorted from human peripheral blood and cultured under conditions that promote PC differentiation. Gene expression levels of FOXP1, BCL6, PAX5, SPIB, PRDM1, IRF4, and XBP1 were analyzed by qRT-PCR in samples of cells that were cultured for 2, 5, or 8 days after sorting. Expression levels were normalized to levels in cells that were cultured for 2 days. Means ± SEM of 3 independent experiments are shown. (B) CD19⁺CD27⁺ MBCs were sorted from human peripheral blood and transduced with FOXP1-IRES-YFP, BCL6-IRES-GFP, or control empty vector and cultured under conditions that promote PC differentiation. Six days after transduction, yellow fluorescence protein (YFP)- or green fluorescence protein (GFP)-positive cells of cultures transduced with FOXP1, BCL6, or control vector were sorted (left). Gene expression levels of PRDM1, IRF4, XBP1, FOXP1, BCL6, and PAX5 were analyzed by qRT-PCR. Expression levels in FOXP1- and BCL6-transduced cells were normalized to β-actin and then to expression levels in control-transduced cells. Means ± SEM of 5 independent experiments are shown (right). Representative example of FOXP1 and BCL6 overexpression in primary YFP⁺ or GFP⁺ B cells as determined by immunoblotting. β-actin was used as loading control. (C) Six days after transduction, YFP- or GFP-positive cells were analyzed for CD20 and CD38 surface expression by flow cytometry. Representative density plots of 1 out of 13 independent experiments are shown (left). Percentages of CD38⁺CD20⁻ cells in FOXP1- and BCL6-transduced cultures were normalized to control cultures. Means ± SD values of 13 independent experiments are shown (right).