BCL-6^-/- macrophages and determined macrophage progenitor cells enter senescence prematurely compared to wild-type controls. (A) Acidic β-galactosidase staining of in vitro differentiated macrophages on day 10 in CSF-1 medium. Representative images (original magnification ×10) from 6 independent experiments are shown. Wild-type macrophages in left panel; BCL-6^-/- macrophages, right panel. (B) Western blot analysis of p16 expression in in vitro differentiated wild-type and BCL-6^-/- macrophages. The result shown was obtained from day 10 macrophages, yet day 6 macrophages gave essentially the same result. (C) Colony-to-cluster ratios and colony numbers for cells of individual primary colonies picked from plates stimulated with either CSF-1 alone (left) or CSF-1 plus SCF (right). The primary colony cells were plated with CSF-1 on secondary plates for determination of their colony-forming (> 50 cells) and cluster-forming (< 50 cells) ability. Bars indicate the mean value for each mouse. Mean colony-to-cluster ratios and mean colony numbers ± SD for each genotype are presented under the graph. Because not all data sets conform to a gaussian distribution, the P values are from 2-sided Wilcoxon rank-sum tests performed to compare the results between BCL-6^+/+ and BCL-6^-/- cells.