Immune responses during healing of massive segmental femoral bone defects mediated by hybrid baculovirus-engineered ASCs

Baculovirus holds promise for genetic modification of adipose-derived stem cells (ASCs) and bone engineering. To explore the immune responses during bone healing and the cell fate, ASCs were mock-transduced (Mock group), transduced with the baculovirus transiently expressing growth factors promoting osteogenesis (BMP2) or angiogenesis (VEGF) (S group), or transduced with hybrid baculoviruses persistently expressing BMP2/VEGF (L group). After allotransplantation into massive femoral defects in rabbits, these 3 groups triggered similar degrees of transient inflammatory response (e.g. neutrophil proliferation and immune cell infiltration into the graft site), revealing that baculovirus and transgene products did not exacerbate the inflammation. The cells in all 3 groups underwent apoptosis initially, persisted for at least 4 weeks and were eradicated thereafter. The L group prolonged the in vivo BMP2/VEGF expression (up to 4 weeks), extended the antibody responses, and slightly enhanced the cell-mediated cytotoxicity. Nonetheless, the L group led to remarkably better bone healing and remodeling than the Mock and S groups. These data confirmed that the ASCs engineered with the hybrid BV imparted prolonged expression of BMP2/VEGF which, although stimulated low levels of humoral and cell-mediated immune responses, essentially augmented the healing of massive segmental bone defects.