Immune Tolerance Induction to Factor IX Through B Cell Gene Transfer – Delineating Between Tolerogenic and Immunogenic B Cells.

Abstract 3156 At present, the most serious complication in hemophilia therapy is the development of neutralizing antibodies (inhibitors) to intravenous administrated recombinant protein, which compromises therapy, creates immune-toxicity, and increases costs. Although inhibitor formation is less frequent in hemophilia B, it is more prevalent in severe hemophilia B patients, often with additional consequences - up to 50% patients with inhibitors to factor IX (F.IX) develop anaphylactic reactions. These further increase risks of morbidity and mortality. Available bypass therapy is expensive and at risk for thrombosis. Clinical immune tolerance induction (ITI) protocols are lengthy, expensive, and are often terminated in hemophilia B due to anaphylactic reactions or nephrotic syndrome. Therefore, effective protocols to induce immune tolerance to F.IX are urgently needed. B cells have been identified as antigen presenting cells with potentially immune suppressive/regulatory roles. Upon gene transfer, primary B cells were found to induce tolerance to the expressed transgene product. Hence, we use autologous gene-modified primary B cells expressing F.IX antigen fused with immunoglobulin-G heavy chain in a murine model of hemophilia B. Our murine hemophilia B model is unique in both developing high-titer inhibitors and fatal anaphylactic reactions to protein replacement therapy. Retroviral transduced B cells, expressing either full-length or shorter version of F.IX, markedly reduced inhibitor titers up to 30-fold and completely prevented fatal anaphylactic reactions. After 7 weeks of treatment with recombinant human F.IX (IV, 1 IU/mouse, once per week), mice receiving control B cells (n=6) had developed inhibitor titers of 23±8 BU, and 50% died after the last injection. Mice tolerized to F.IX by B cell transplant (n=7) had formed Disclosures: Herzog: Genzyme Corp.: Royalties, AAV-FIX technology, Royalties, AAV-FIX technology Patents & Royalties.