Inverse correlation of FOXP1 and BCL6 expression in GC. (A) Representative images of human tonsil reactive sections examined by IHC (left panels) using anti-FOXP1 and anti-BCL6 antibodies (brown). Also, FOXP1 (green) and BCL6 (red) expression were examined by immunofluorescence (right panels) in human tonsils. * denotes areas of positive signal. Arrows indicate cells that coexpress FOXP1 and BCL6 at the onset of the GC. (B) Double IHC in human tonsil sections for FOXP1 (blue) and other surface markers, including CD79α⁺ or IgD⁺ B cells, CD3⁺ T cells, CD21⁺ follicular dendritic cells, and CD68⁺ macrophages (brown). * denotes areas of positive FOXP1 signal (blue). (C) qRT-PCR (top) and immunoblot (bottom) analysis of FOXP1 and BCL6 expression from naïve (NBC), GC, and memory (MC) B cells isolated from human tonsils. qRT-PCR gene expression is shown relative to GAPDH. Bars show mean ± standard error of the mean (SEM) from at least 6 human tonsils. Significance was determined by 2-tailed unpaired Student t tests. Representative western blots from 2 independent experiments are shown. NBC, naïve B cells; MC, memory B cells; mRNA, messenger RNA.