The role of adipose-derived stem cells engineered with the persistently expressing hybrid baculovirus in the healing of massive bone defects.

Massive segmental defects arising from trauma or tumor resection remain a challenging clinical problem. To repair large, segmental bone defects using adipose-derived stem cells (ASCs) which alone cannot heal massive defects, we hypothesized that sustained expression of factors promoting osteogenesis (BMP2) and angiogenesis (VEGF) provides continuous stimuli to augment the healing. Baculovirus is a vector for gene delivery into stem cells, but it only mediates transient expression. Therefore we developed a dual system whereby one baculovirus expressed FLP recombinase (BacFLP) while the other hybrid baculovirus harbored an Frt-flanking transgene cassette. Within the ASCs transduced with BacFLP and the hybrid baculovirus, the transduction efficiency reached 98% and the FLP/Frt-mediated recombination efficiency approached 46%, leading to cassette excision off the baculovirus genome, enabling transgene persistence in episomal form and prolonging the expression to >28 days. ASCs engineered by the conventional baculovirus transiently expressing BMP2/VEGF (S group) only healed the critical-size (10mm) segmental femoral bone defects in 40% of New Zealand White rabbits at 12 weeks post-implantation, whereas ASCs engineered by the hybrid vectors persistently expressing BMP2/VEGF (L group) healed the critical-size defects in 12 out of 12 animals in 8 weeks. Compared with the S group, the L group not only accelerated the healing, but also ameliorated the bone quality (metabolism, volume, density, mechanical properties) and angiogenesis, thereby attesting our hypothesis that persistent BMP2/VEGF expression is essential. Use of ASCs engineered by the hybrid BV vector thus holds promise to treat massive segmental defects necessitating sustained stimuli.