FOXP1 represses the PC gene signature and is expressed in human mature B-cell subsets but not in PCs. (A) Expression of genes that belong to the PC-2 PC gene signature^35,36  and were significantly repressed by FOXP1 in microarray analysis of 2 independent experiments of primary human MBCs, transduced with FOXP1 or control vector. Data are represented as z scores. The lower panel shows the mean relative expression values of the gene set. (B-C) Human CD19⁺ tonsil B-cell subsets, that is, naive (NBC) (IgD⁺CD38⁻), transitional (TBC) (IgD⁺CD38⁺), GC B (IgD⁻CD38⁺), class-switched MBCs (IgD⁻CD38⁻), and PCs (IgD⁻CD38⁺⁺), and peripheral blood B-cell subsets (MBC [CD27⁺] and naive enriched [CD27⁻]) were sorted. (B) Gene and protein expression levels of FOXP1 were analyzed in tonsillar and peripheral blood B-cell subsets. Gene expression levels in tonsillar B-cell subsets were quantified by qRT-PCR and normalized to expression levels in naive B cells. Means ± standard error of the mean (SEM) of 4 independent experiments are shown. Significant differences compared with naive B cells are indicated (1 sample t test, **P < .01). FOXP1 protein expression levels were analyzed by immunoblotting; β-actin was used as loading control. Representative blots of 2 independent experiments are shown. (C) Gene expression levels of BCL6, SPIB, PAX5, PRDM1, IRF4, and XBP1, were analyzed in tonsillar samples by qRT-PCR. Expression levels were normalized to expression levels in naive B cells. Means ± SEM of 4 independent experiments are shown. Significant differences compared with naive B cells are indicated (1 sample t test, *P < .05; **P < .01; ***P < .001).