p16^INK4a is expressed, but does not influence maturation or the cell cycle in macrophages. (A) p16^INK4a mRNA expression was measured in immune cells isolated from mice: dendritic cells (DC), BM-derived macrophages (BMDMs), neutrophils, B lymphocytes, and T lymphocytes. (B) p16^INK4a mRNA expression was measured at different stages during differentiation of BMDM isolated from p16^+/+ mice. Because p16^INK4a fold induction differs between experiments, data are from 1 of 3 distinct experiments. (C) p16^INK4a protein expression in p16^+/+ and p16^−/− BMDM was analyzed by Western blot with an anti-p16^INK4a Ab. Anti-actin Ab was used as loading control. (D-E) Increase of the macrophage marker F4/80 mRNA expression (D) and protein expression (E) measured by, respectively, QPCR and flow cytometry in p16^+/+ and p16^−/− BMDM. Data are shown from 1 of 3 independent experiments. (F) Cell-cycle analysis by propidium iodide staining of p16^+/+ and p16^−/− BMDM during differentiation expressed in % of events ± SD.