CD137 induces proliferation of peripheral monocytes. (A) 10⁵ monocytes were cultured on 96-well plates coated with Fc or CD137-Fc protein. Proliferation was determined daily by a 24-hour pulse with 0.5 μCi ³H-thymidine and differed significantly from day 3 onward, with P < .02. (B) Colonies/aggregates formed on CD137-treated monocytes. Photographs were taken at day 10 of culture at a magnification of 200×.