Elimination of FVIII-Specific B Cells By Immunotoxins Comprised of a Single FVIII Domain Fused to Pseudomonas Exotoxin a

The development of inhibitory antibodies (inhibitors) against coagulation factor VIII (FVIII) is the most serious complication for patients with hemophilia A that undergo FVIII replacement therapy. In addition, healthy individuals can spontaneously develop inhibitory anti-FVIII auto-antibodies, which results in acquired hemophilia A. The current standard therapy for patients with hemophilia A and inhibitors, named immune tolerance induction (ITI), is based on frequent and mostly high dose administrations of FVIII. Unfortunately, the eradication of inhibitors can only be achieved in about 70% of patients. Alternative treatment of inhibitor patients with the monoclonal anti-CD20 antibody rituximab results in complete eradication of inhibitors; however, depletion of the entire CD20-positive B cell population is potentially accompanied by severe side effects. Recent studies in hemophilic FVIII knockout mice showed that the application of a FVIII-toxin conjugate resulted in (i) prevention of inhibitor development in naïve mice and (ii) long-term eradication of inhibitors in FVIII-immunized mice.

As the use of FVIII for cell targeting of immunotoxins is presumably limited by its high molecular weight (250 kDa) and adhesiveness (off-target reactivity) we explored the potential use of alternative immunotoxins in the current study. The introduced immunotoxins are comprised of a single FVIII domain fused to the Exotoxin A (ETA) from Pseudomonas aeruginosa.The rationale for the use of a single domain instead of full length FVIII as cell-binding component is that immunodominant domains like A2 and C2 might still allow targeting of sufficient amounts of FVIII-specific B-cells by immunotoxins. For proof of concept studies, we generated a histidine-tagged C2 domain-ETA fusion protein (C2-ETA) that was bacterially expressed and purified by affinity chromatography. Purified C2-ETA was recognized by a panel of commercially available monoclonal anti-C2 antibodies in ELISA suggesting proper folding of the C2 domain in the bacterially expressed protein.

To test the capacity of C2-ETA to eliminate FVIII-specific B-cells, splenocytes of FVIII-immunized FVIII knockout mice were re-stimulated with FVIII ex vivo in presence and absence of different concentrations of C2-ETA and ETA alone (as control). Re-stimulation of FVIII-specific memory B cells to FVIII- and C2-specific antibody secreting cells (ASC) was analyzed in anEnzyme linked immunospot (ELISPOT) assay using FVIII and C2 as antigens. While differentiation to FVIII-specific ASC was only partially inhibited by C2-ETA, differentiation to C2-specific ASC was completely blocked in a dose-dependent manner. In contrast, the use of ETA alone had no effect. Further analysis of the FVIII domain specificity of antibodies in plasma of FVIII-immunized FVIII knockout mice used for depletion studies revealed a strong contribution of C2-specific antibodies to the overall FVIII-specific immune response.

In summary, our results show that the developed C2-ETA immunotoxin is able to specifically eliminate FVIII C2 domain-specific B cells ex vivo. Currently, C2-ETA is tested for its capacity to eliminate FVIII-specific B cells in FVIII knockout mice and additional FVIII domain-ETA immunotoxins are developed.