Future approaches to factor VIII inhibitor therapy

Abstract

Future progress in our ability to treat acquired factor VIII (FVIII) inhibitors must be based on advances in knowledge of both the FVIII molecule and the nature of the human immune response. New therapeutic approaches to patients with acquired FVIII inhibition likely will emphasize modifications of the immune response. This concept holds considerable promise, because studies have characterized the critical steps leading to tolerance of self-antigens. Development of FVIII inhibitors represents a loss of self-tolerance, which any successful therapy must restore. Conceivably, restoration of self-tolerance can be accomplished in many ways: prevention of antigen binding to helper T lymphocytes, deletion of self-antigen-reactive T cells, inhibition of major histocompatibility complex (MHC) recognition, or enhancement of the antigen-specific suppressor T lymphocyte population. Recent data have demonstrated that highly specific methods can suppress ongoing immune responses against defined autoantigens. Antibodies that inhibit T-cell activation, peptides that block self-antigen binding, and antibodies that inhibit MHC recognition all have been successful in modifying experimentally induced autoimmune diseases. Whether any of these immunotherapeutic approaches will be effective in the treatment of acquired FVIII inhibition remains to be determined. Until data from animal model systems establish the feasibility of immune intervention, scrutiny of other new therapeutic approaches to patients with spontaneous inhibitors will continue to be important. Administration of FVIII-bypassing procoagulant proteins shows promise, as does removal of inhibitors by affinity reagents, such as FVIII peptides containing relevant epitopes (antigenic sites). Farther on the horizon is development of recombinant FVIII molecules so modified as to remove antigenic determinants while preserving procoagulant function. Articles in this supplement summarize several avenues for treatment of patients with acquired FVIII inhibitors. Alternatives include treatment with sufficient human or porcine FVIII to offset inhibitors, use of materials that reestablish hemostasis even though FVIII levels are not increased (the so-called FVIII-bypassing agents), manipulation of immune responses through physical depletion of inhibitor by plasmapheresis or affinity chromatography, and administration of intravenous immunoglobulin or immunosuppressive cytotoxic drugs. Thus, the heterogeneous clinical presentation is paralleled by the wide range of available therapeutic approaches.