Summary
The main impediments to clinical application of haematopoietic stem cell (HSC) gene
therapy for treatment of haemophilia A are the bone marrow transplant-related risks
and the potential for insertional muta-genesis caused by retroviral vectors. To circumvent
these limitations, we have adapted a non-myeloablative conditioning regimen and directed
factor VIII (FVIII) protein synthesis to B lineage cells using an insulated lentiviral
vector containing an immunoglobulin heavy chain enhancer-promoter. Transplantation
of lentiviral vector-modified HSCs resulted in therapeutic levels of FVIII in the
circulation of all transplanted mice for the duration of the study (six months). Immunostaining
of spleen cells showed that the majority of FVIII was synthesised by B220+ B cells and CD138+ plasma cells. Subsequent challenge with recombinant FVIII elicited at most a minor
anti-FVIII antibody response, demonstrating in-duction of immune hyporesponsiveness.
All transplant recipients exhibited clot formation and survived tail clipping, indicating
correction of their haemophilic phenotype. Therapeutic levels of FVIII could be transferred
to secondary recipients by bone marrow transplantation, confirming gene transfer into
long-term repopulating HSCs. Moreover, short-term therapeutic FVIII levels could also
be achieved in secondary recipients by adoptive transfer of HSC-derived splenic B
cells. Our findings support pursuit of B cell-directed protein delivery as a potential
clinical approach to treat haemophilia A and other disorders correctable by systemically
distributed proteins.
Keywords
Haemophilia A - animal models - haematopoietic stem cell gene therapy - B cell-targeted
transgene expression - bioengineered factor VIII