Induction of monocyte apoptosis by CD137. 10⁵primary monocytes were cultured on immobilized Fc or CD137-Fc protein or on untreated plates (control). The extent of apoptosis (top) and the number of living cells, determined by trypan blue exclusion (bottom) were evaluated at days 1, 3, 5, and 7. Error bars indicate standard deviation. The differences in rates of apoptosis and the number of living cells between Fc and CD137-Fc treated cultures were significant starting at days 3 and 5, respectively, with P < .05. Comparable results were obtained in 3 separate experiments.