Tolerance induced by anti-ICOS immunomodulation is specific to FVIII. Six months after anti-ICOS mAb treatment, tolerized hemophilia A mice (n = 4) were challenged twice 4 weeks apart with the neoantigen bacteriophage Φx 174 (2 × 10⁸ PFU/each challenge). Untreated hemophilia A mice (n = 4) were used as control. Phage-neutralizing antibody activity was expressed as the rate of phage inactivation (Kv) using a standard formula. Mice receiving no bacteriophage did not produce neutralizing antibody (data not shown). All mice showed adequate antibody titers, amplification, and isotype switching (100% IgG) after secondary immunization.