"Same day" ex-vivo regional gene therapy: a novel strategy to enhance bone repair

Abstract

Ex-vivo regional gene therapy with bone marrow cells (BMCs) overexpressing bone morphogenetic protein-2 (BMP-2) has demonstrated efficacy in healing critical sized bone defects in preclinical studies. The purpose of this preclinical study was to compare the osteoinductive potential of a novel "same day" ex-vivo regional gene therapy versus a traditional two-step approach, which involves culture expansion of the donor cells before implantation. In the "same day" strategy buffy coat cells were harvested from the rat bone marrow, transduced with a lentiviral vector-expressing BMP-2 for 1 hour and implanted into a rat femoral defect in the same sitting. There was no significant difference (P = 0.22) with respect to the radiographic healing rates between the femoral defects treated with the "same day" strategy (13/13; 100%) versus the traditional two-step approach (11/14; 78%). However, the femoral defects treated with the "same day" strategy induced earlier radiographic bone healing (P = 0.004) and higher bone volume (BV) [micro-computed tomography (micro-CT); P < 0.001]. The "same day" strategy represents a significant advance in the field of ex-vivo regional gene therapy because it offers a solution to limitations associated with the culture expansion process required in the traditional ex vivo approach. This strategy should be cost-effective when adapted for human use.

Figure 1

Steps involved in the “same day” ex vivo gene therapy. (a) Harvest of bone marrow from the rat femur; (b) Ficoll separation and preparation of rat “same day” bone marrow cells (SD-RBMCs) for viral transduction (time required 0.5 hour); (c) Short-duration viral transduction of SD-RBMCs (time required 1 hour); (d) Post-transduction preparation of SD-RBMCs (time required 1 hour); (e) Placement of transduced SD-RBMCs on a collagen-ceramic matrix and implantation into the femoral defect; (f) Healed femoral defect at 8 weeks after cell implantation. SD-RBMCs, “same day” rat bone marrow cells.

Figure 2

Structure of lentiviral two-step transcription amplification (TSTA) vector system. (a) The Top panel depicts the original lentiviral vector used in our laboratory which encodes bone morphogenetic protein-2 (BMP-2) complementary DNA (cDNA) downstream of RhMLV promoter. (b) The bottom panel shows the construct of the two-step transcriptional activation system (TSTA) that consists of two different lentiviral vectors namely the transactivator vector and the transgene expression vector. The cells are transduced with both vectors simultaneously. The GAL4-VP16 activates the G5 promoter in the transgene expression vector to amplify the expression of BMP-2. Both constructs contain the Rev-responsive element (RRE) and the central polyprine tract (cPPT), which enhance the efficiency of gene expression. LTR, long-terminal repeat; SIN, self-inactivating.

Figure 3

Plain radiographs. (a) Representative radiographic images of healed femoral defect in animals treated with transduced “same day” cells (SD-RBMCs + LV-TSTA-BMP-2). Double white arrows depict bridging bone across femoral defect and restoration of cortex. (b) Femoral defects treated with transduced cultured bone marrow cells (C-RBMCs + LV-TSTA-BMP-2) demonstrate bridging new bone formation (double white arrows) across the femoral defect 8 weeks after the cell implantation. The defects treated with (c) nontransduced “same day” cells (SD-RBMCs), (d) nontransduced cultured bone marrow cells (C-RBMCs) or (e) carrier alone demonstrated some periosteal new bone formation (red arrows) but none of these defects demonstrated complete radiographic healing. The composite carrier (compression resistant matrix) was used to deliver the cells in the femoral defect (yellow asterisk in d). BMP-2, bone morphogenetic protein-2; LV, lentiviral; SD-RBMCs, “same day” rat bone marrow cells; TSTA, two-step transcription amplification.

Figure 4

Micro-computed tomography (micro-CT) images. Representative micro-CT images obtained in the (a) longitudinal plane (blue color) and (b) axial planes (red color) of femur specimens from each study group at 8 weeks. (c–e) The delineation of quantified callus bone volume (enclosed within green color contour in e) from the native host bone and the scaffold material (gray scale material in d). The healed femora in the group I (f–h; RBMCs + LV-TSTA-BMP-2) and the group II (i–k; C-RBMCs + LV-TSTA-BMP-2) demonstrate abundant new bone bridging the femoral defect. There is formation of new cortices (red arrows in g, h, j, and k) and reconstitution of the medullary canal across the femoral defect. The carrier scaffold (yellow arrows in k and q) used to deliver the cells has higher radiodensity and is easily distinguished from the original cortex and the new bone formed in the defect. There is some periosteal new bone formed at the host defect interface and under the polyethylene plate (yellow asterisk) in the defects treated with the nontransduced “same day” cells (SD-RBMCs; l–n), nontransduced cultured bone marrow cells (C-RBMCs; o–q), or carrier alone (r–t) but none of these defects demonstrated any bridging new bone formation (longitudinal images l–p, r, and s). BMP-2, bone morphogenetic protein-2; LV, lentiviral; SD-RBMCs, “same day” rat bone marrow cells; TSTA, two-step transcription amplification.

Figure 5

Histologic analysis of study groups at 8 weeks. Representative longitudinal and transverse histologic sections stained with Mason trichrome were obtained in the longitudinal (blue color, p) and axial plane (red color, p) of the femur. The original host cortex is labeled with yellow asterisk in the longitudinal sections. The position of the polyethylene plate (construct shown in p) is labeled (§) in the transverse sections for the orientation purpose. Within each panel there are three images: a longitudinal section (×1), followed by a transverse section through the middle of the defect (×1) and lastly a higher magnification image (×10) of the inset. The healed femora in group I (a–c; SD-RBMCs + LV-TSTA-BMP-2) and group II (d–f; C-RBMCs + LV-TSTA-BMP-2) animals demonstrate abundant new bone formation at the host defect interface (red arrow in a and d) and across the defect (red arrows in b and e). There is reconstitution of the new cortices and the medullary canal in the region of the defect in groups I and II signifying complete healing. In contrast to the healed defects in groups I and II, the defects in group III (SD-RBMCs; g–i), group IV (C-RBMCs; j–l), and group V (carrier alone; m–o) demonstrated obliteration of the medullary canal at the host defect interface (black arrow heads in g, j, and m). There was fibrous tissue mixed with the composite carrier (yellow arrows in h, k, and n) and some new bone in the center of the defects in groups III, IV, and V (control groups). BMP-2, bone morphogenetic protein-2; LV, lentiviral; SD-RBMCs, “same day” rat bone marrow cells; TSTA, two-step transcription amplification.

Figure 6

Histomorphometric analysis (bone area/tissue area). The bone area/tissue area (BA/TA) in defects treated with transduced “same day” cells that overexpress BMP-2 (SD-RBMCs + LV-TSTA-BMP-2) was significantly higher (P < 0.01) than the defects treated with the nontransduced “same day” cells (SD-RBMCs) or defects treated with carrier alone. The defects treated with the transduced cultured bone marrow cells that overexpress BMP-2 (C-RBMCs + LV-TSTA-BMP-2) were significantly higher (P < 0.01) than the defects treated with nontransduced bone marrow cells (C-RBMCs) or treated with carrier alone. Furthermore, there was no significant difference (P > 0.05) between the defects treated with transduced “same day” cells and defects treated with transduced cultured bone marrow cells with respect to BA/TA. BMP-2, bone morphogenetic protein-2; LV, lentiviral; SD-RBMCs, “same day” rat bone marrow cells; TSTA, two-step transcription amplification.