CD137 mAb inhibited progressive growth of P815R tumors that were induced by tolerogenic P1A peptide. (A) DBA/2 mice were immunized subcutaneously with 50 μg P1A peptide emulsified in IFA at 2 sites. Control mice received only IFA. On the day of peptide immunization, and again 3 days later, mice were given 100 μg rat IgG or anti-CD137 (2A). Ten days following peptide immunization, mice were challenged with 1.5 × 10⁴ P815R cells in the right flank. Tumor incidence of each group is plotted after the point when the maximal number of mice developed tumor. The difference between the rat IgG- and CD137-treated groups is statistically significant according to the log-rank test (P = .0038). (B) DBA/2 mice were immunized with P1A peptide emulsified in IFA as in panel A. Ten days later, the mice were challenged with 1.5 × 10⁴ P815R cells. Three days after tumor challenge, and again 3 days later, the mice were given rat IgG or anti-CD137 (2A). Mice were monitored closely for both tumor development and regression following challenge. The difference between the rat IgG- and CD137-treated groups is statistically significant according to the log-rank test (P < .0001).