Abstract
BACKGROUND: Hemophilia A (HA) inhibitor patients that fail traditional immune tolerance induction (ITI) have increased morbidity and mortality. Preclinical studies support factor VIII (FVIII) tolerance induction with a combined approach of anti-CD20 mediated transient B cell depletion and rapamycin mediated regulatory T cell (Treg) induction. METHODS: Two refractory HA inhibitor patients were treated with rituximab, rapamycin, and FVIII ITI. Their clinical course, anti-FVIII immunoglobulins, cytokines, and select lymphocytes were followed. RESULTS: One patient achieved complete and the other partial FVIII tolerance; both had marked annualized bleeding rate improvement. FVIII-specific immunoglobulins, but not total Treg counts, correlated with tolerance induction. IL-6 and IL-21 correlation with complete tolerance induction may support that down-regulation of T effectors and IgG4 production, respectively, contribute to the pathogenesis of tolerance induction. CONCLUSIONS: This regimen may be considered to induce FVIII tolerance in HA patients with refractory inhibitors. Further characterization of the FVIII-specific immune response is necessary to clarify the mechanism of immune tolerance.