IFN-γ inhibits macrophage proliferation in a Stat-1–dependent, p21^Waf1-independent manner. (A) Macrophages were obtained from wild-type (WT) and Stat-1–deficient mice. Quiescent macrophages (10⁵) were incubated for 24 hours with M-CSF either alone or together with the indicated concentrations of IFN-γ. Proliferation was determined by ³H-thymidine incorporation. Each point was represented as the percentage of ³H-thymidine incorporated in the absence of IFN-γ (M-CSF control). Mean values in wild-type and Stat-1–deficient macrophages were 83 000 and 71 000 cpm, respectively. (B) Quiescent cells (10⁵) were incubated with IFN-γ (10 ng/mL) for a range of time periods before and after stimulation with M-CSF. Proliferation was determined by ³H-thymidine incorporation. (C) The expression of p21^Waf1 was analyzed by Northern blot in WT and Stat-1–deficient macrophages treated with IFN-γ. Expression of 18S rRNA was used as a control for RNA loading and transfer. (D) Macrophages were obtained from WT and p21^Waf1 knockout mice. Macrophages (10⁵) were incubated for 24 hours with the indicated amounts of M-CSF either alone or in combination with IFN-γ. Proliferation was determined by ³H-thymidine incorporation. The experiments in panels A through C were performed 3 times with equivalent results. The data in panel D were reproduced twice. Error bars represent SDs from triplicates in 1 representative experiment.