Decreasing the Humoral Response to Factor VIII By Targeted Deletion of Factor VIII - Specific B Cells

The development of neutralizing anti-factor VIII (fVIII) antibodies (inhibitors) remains the most significant complication in the treatment of hemophilia A patients. Treatment of inhibitor patients consists of management of bleeding episodes using bypassing agents or porcine fVIII. Inhibitors can be eradicated by immune tolerance induction (ITI) using thrice-weekly administration of large doses of fVIII. However, ITI fails in approximately 30% of patients. Additionally, the median time to tolerance in successful cases is ~18 months, making ITI expensive and inconvenient. In the current study, we used a murine E16 hemophilia A model to test a novel approach to both prevent and eradicate fVIII inhibitors. We hypothesized that conjugation of fVIII to the toxin saporin, a Type I ribosome-inactivating protein, would target fVIII-specific cell surface immunoglobulin and selectively delete fVIII-specific naïve and memory B cells. Recombinant full-length fVIII was covalently linked to saporin using the heterobifunctional crosslinker N-succinimidyl-3-(2-pyridyldithio)propionate (SPDP). To test for eradication of existing fVIII inhibitors by fVIII-saporin, an adoptive transfer protocol was developed to measure fVIII-specific memory B cells. Hemophilia A donor mice were immunized with 2 μg of full-length fVIII by intravenous injection every other week for 8 weeks, followed by a final dose of 4 μg at ten weeks. Four weeks later, the mice were randomized into three treatment groups to receive equimolar doses of saporin, fVIII, or fVIII-saporin. Seven days after treatment, the mice were sacrificed and 4 x 10⁶ plasma cell CD138⁺-depleted splenocytes were adoptively transferred as a source of fVIII-specific memory B cells into naïve recipient hemophilia A mice. At 24 hours, recipient mice were given a single injection of 0.5, 1.0 or 2.0 μg of recombinant full-length fVIII by tail vein injection. Anti-fVIII IgG antibodies in recipient mice were measured by ELISA 2 and 5 weeks following the fVIII injection. In the absence of fVIII-specific memory B cells from donor mice, naïve hemophilia A mice did not produce detectable anti-fVIII antibodies. Recipient hemophilia A mice receiving splenocytes from fVIII donor and saporin donor mice displayed a dose-dependent increase in anti-fVIII antibodies. In contrast, the slope of the anti-fVIII titer versus dose of fVIII was significantly decreased in recipient mice receiving splenocytes from fVIII-saporin donor mice. To test for prevention of fVIII inhibitor formation by fVIII-saporin, naïve hemophilia A mice were divided into three treatment groups to receive a single dose of saporin, fVIII, or fVIII-saporin by tail vein injection. Seven days after treatment, the mice were immunized by tail vein injection with 2 μg of full-length fVIII every other week for 10 weeks. Anti-fVIII IgG antibodies were measured 1 week after the fourth and sixth injections of fVIII. Anti-fVIII antibody titers were significantly lower in the fVIII-saporin group compared to the fVIII group (1,900 vs. 21,400 (p=0.027, n=4, Mann-Whitney test, see figure) after the fourth injection. After 6 injections, the average anti-fVIII titer of the fVIII group was 23,000 compared to 4,000 in the fVIII-saporin group (p=0.057, n=4, Mann-Whitney test, see figure). In conclusion, our results suggest that infusion of fVIII-saporin results in the depletion of both fVIII-specific naïve B cells and memory B cells. FVIII-saporin potentially could be used in the treatment of congenital hemophilia A patients with inhibitors and patients with acquired hemophilia A. In addition, fVIII-saporin potentially could be used in previously untreated patients with hemophilia A to prevent inhibitor development. Similar therapeutic strategies could be extended to other antigen-specific immune disorders.

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