CD137 induces proliferation of monocytes via autocrine induction of soluble factor(s). 10⁵ peripheral monocytes were cultured on immobilized Fc or CD137-Fc protein for 24 hours. 0, 10, 20, or 30 μL of conditioned supernatant of these cultures were transferred to 100 μL of new cultures with untreated monocytes. (A) After 8 days, the cells were photographed at a magnification of 300×. The bottom panel depicts monocytes grown on immobilized CD137-Fc protein for the 8 days. (B) Proliferation of the cultures from (A) was determined at day 8 by a 24-hour pulse with 0.5 μCi³H-thymidine. () Proliferation of monocytes on immobilized CD137-Fc protein. Cultures supplemented with conditioned supernatant from CD137-Fc–activated monocytes proliferated significantly stronger; P < .007. This experiment was repeated 3 times with similar results.