M-CSF and GM-CSF are essential, but not sufficient, for CD137-induced monocyte proliferation. (A) 10⁵ peripheral monocytes were cultured on immobilized Fc or CD137-Fc protein. Neutralizing anti–M-CSF antibody (2 μg/mL), anti–GM-CSF antibody (2 μg/mL), and anti–IL-3 antibody (2 μg/mL) or isotype control (IgG2a; 2 μg/mL) were added where indicated. Proliferation was determined at day 10 by ³H-thymidine incorporation in triplicate conditions. (B) M-CSF and GM-CSF do not induce proliferation of monocytes. Peripheral monocytes were cultured on 96-well plates coated with Fc or CD137-Fc protein (1 μg/mL) or M-CSF and GM-CSF at indicated concentrations (ng/mL). Proliferation was determined at day 10 by ³H-thymidine incorporation. Similar results were obtained in 3 separate experiments for (A) and (B).