Killing 2 birds with 1 stone

Bone marrow transplantation with stem cells transgenically modified to express factor VIII in platelets corrects the bleeding tendency in hemophilic mice with neutralizing anti–factor VIII antibodies. Or, “Why the blind mice of Milwaukee didn't bleed to death after their tails were cut off.”

It has been sufficiently clear for some years that only 2 problems remain in the field of inherited bleeding disorders. These problems are easy to state but hard to solve. The first problem is delivery of the clotting factor in the right amount at the right time; that is to say, a normal level all the time. All that is required to correct the bleeding tendency in single factor deficiency is to deliver a normal level of factor, be it VIII, IX, Von Willebrand, or one of the 7 other Roman numeral–designated enzymes or cofactors that take part in fibrin formation. This might seem like a simple proposition given that safe effective preparations of nearly all those proteins are available. Sadly, most of the world's hemophiliacs have no access to factor replacement and solving this depends on new approaches that have a mainly political dimension. In economically advanced countries, there is now no shortage of most of the factors (factor V is the exception) but getting patients to adhere to a challenging routine of intravenous injections poses a separate set of problems with largely psychological/social aspects.

Many, myself included, consider that all these issues could be solved by gene therapy. Indeed, one of the earliest targets identified as being a sensible target for cure by DNA transfer was hemophilia. Although mice and dogs have now been cured of hemophilia by gene transfer, safe effective procedures for curing human hemophilia have yet to be demonstrated.

The second outstanding problem in clinical hemophilia care is that of inhibitory antibodies arising after replacement therapy. Patients who have developed such antibodies become resistant to standard treatment. They either undergo immune tolerance induction therapy, which is extremely expensive and sometimes fails, or are treated with bypassing agents that are also ruinously costly and not always effective. Patients with persistent inhibitors have shortened life expectancy.

In this issue of Blood, Shi et al, in a paper whose title says it all, now present proof of a principle that can solve the inhibitor problem and the delivery problem in one fell swoop for the majority of problem cases, that is to say, those with hemophilia A and antibodies to factor VIII (FVIII).

An ingenious transgenic mouse model is used in which a line of mice has been established where FVIII is ectopically expressed in megakaryocytes under the control of a platelet membrane receptor promoter sequence. Platelets formed by the megakaryocytes derived from hematopoietic stem cells of such mice contain FVIII stored in alpha granules along with Von Willebrand factor, the natural carrier for FVIII. This FVIII is both protected from plasma antibodies and released upon platelet activation at sites of vascular injury where haemostatic plugs are forming. With this line of mice on hand, a second line of mice that have had their factor VIII gene knocked out are stimulated to form high titer antibody to FVIII by infusing them with recombinant human B domain deleted FVIII. Subsequently, these mice that mimic human hemophilia A patients with inhibitors are rescued by transplanting them, after lethal irradiation with hematopoietic stem cells, from the mice that make platelets with releasable FVIII stored in platelet alpha granules. After successful engraftment of this modified bone marrow, the mice are no longer apt to bleed to death after minor injury. One can already see the outlines of a feasible clinical approach: Autologous hematopoietic stem cells from a patient with antibodies to FVIII are modified ex vivo with a lentivirus-based FVIII expression cassette under control of the GPIIb alpha promoter and then reinfused after irradiation to a level sufficient to produce partial engraftment. If the level of expression is high enough, it might be worth considering this as a general approach to hemophilia gene therapy.