TOWARD GENE-THERAPY FOR HEMOPHILIA-A - LONG-TERM PERSISTENCE OF FACTOR VIII-SECRETING FIBROBLASTS AFTER TRANSPLANTATION INTO IMMUNODEFICIENT MICE

Hemophilia A is caused by the lack of functional blood-clotting factor VIII. We have used retrovirus-mediated gene transfer to generate vari ous cell lines, rodent as well as human, that secrete the human factor VIII protein. To study whether transplantation of genetically modifie d fibroblasts is a feasible approach for gene therapy of hemophilia A, we implanted the factor VIII-secreting cells into immune-deficient mi ce. Implantation of factor VIII-secreting primary human skin fibroblas ts resulted in long-term persistence of the transplanted cells; cells recovered from the implants up to 2 months post-implantation still had the capacity to secrete factor VIII when regrown in tissue culture. H owever, we were unable to detect any human factor VIII in plasma sampl es of the recipient mice. The absence of human factor VIII in the reci pients' plasma is shown to be due neither to (epigenetic) inactivation of the retroviral vector in vivo, nor to inability of the stationary cells to secrete factor VIII protein. However, we did note a rapid cle aring of the human factor VIII : CAg from plasma upon intravenous inje ction of plasma-derived human factor VIII in mice (t1/2 < 60 min vs. 1 0 hr in humans). This phenomenon can fully explain the apparent absenc e of human factor VIII in the recipients' plasma.