Regional Gene Therapy with Transduced Human Cells: The Influence of "Cell Dose" on Bone Repair

In vivo effects of cell seeding technique in an ex vivo regional gene therapy model for bone regeneration

When delivering cells on a scaffold to treat a bone defect, the cell seeding technique determines the number and distribution of cells within a scaffold, however the optimal technique has not been established. This study investigated if human adipose-derived stem cells (ASCs) transduced with a lentiviral vector to overexpress bone morphogenetic protein 2 (BMP-2) and loaded on a scaffold using dynamic orbital shaker could reduce the total cell dose required to heal a critical sized bone defect when compared with static seeding. Human ASCs were loaded onto a collagen/biphasic ceramic scaffold using static loading and dynamic orbital shaker techniques, compared with our labs standard loading technique, and implanted into femoral defects of nude rats. Both a low dose and standard dose of transduced cells were evaluated. Outcomes investigated included BMP-2 production, radiographic healing, micro-computerized tomography, histologic assessment, and biomechanical torsional testing. BMP-2 production was higher in the orbital shaker cohort compared with the static seeding cohort. No statistically significant differences were noted in radiographic, histomorphometric, and biomechanical outcomes between the low-dose static and dynamic seeding groups, however the standard-dose static seeding cohort had superior biomechanical properties. The standard-dose 5 million cell dose standard loading cohort had superior maximum torque and torsional stiffness on biomechanical testing. The use of orbital shaker technique was labor intensive and did not provide equivalent biomechanical results with the use of fewer cells.

Biodistribution of lentiviral transduced adipose-derived stem cells for "ex-vivo" regional gene therapy for bone repair

Ex-vivo gene therapy has been shown to be an effective method for treating bone defects in pre-clinical models. As gene therapy is explored as a potential treatment option in humans, an assessment of the safety profile becomes an important next step. The purpose of this study was to evaluate the biodistribution of viral particles at the defect site and various internal organs in a rat femoral defect model after implantation of human ASCs transduced with lentivirus (LV) with two-step transcriptional activation (TSTA) of bone morphogenetic protein-2 (LV-TSTA-BMP-2). Animals were sacrificed at 4-, 14-, 56-, and 84-days post implantation. The defects were treated with either a standard dose (SD) of 5 million cells or a high dose (HD) of 15 million cells to simulate a supratherapeutic dose. Treatment groups included (1) SD LV-TSTA-BMP-2 (2) HD LV-TSTA-BMP-2, (3) SD LV-TSTA-GFP (4) HD LV-TSTA-GFP and (5) SD nontransduced cells. The viral load at the defect site and ten organs was assessed at each timepoint. Histology of all organs, ipsilateral tibia, and femur were evaluated at each timepoint. There were nearly undetectable levels of LV-TSTA-BMP-2 transduced cells at the defect site at 84-days and no pathologic changes in any organ at all timepoints. In conclusion, human ASCs transduced with a lentiviral vector were both safe and effective in treating critical size bone defects in a pre-clinical model. These results suggest that regional gene therapy using lentiviral vector to treat bone defects has the potential to be a safe and effective treatment in humans.

Hydrogel armed with Bmp2 mRNA-enriched exosomes enhances bone regeneration

BACKGROUND

Sustained release of bioactive BMP2 (bone morphogenetic protein-2) is important for bone regeneration, while the intrinsic short half-life of BMP2 at protein level cannot meet the clinical need. In this study, we aimed to design Bmp2 mRNA-enriched engineered exosomes, which were then loaded into specific hydrogel to achieve sustained release for more efficient and safe bone regeneration.

RESULTS

Bmp2 mRNA was enriched into exosomes by selective inhibition of translation in donor cells, in which NoBody (non-annotated P-body dissociating polypeptide, a protein that inhibits mRNA translation) and modified engineered BMP2 plasmids were co-transfected. The derived exosomes were named Exo^BMP2+NoBody. In vitro experiments confirmed that Exo^BMP2+NoBody had higher abundance of Bmp2 mRNA and thus stronger osteogenic induction capacity. When loaded into GelMA hydrogel via ally-L-glycine modified CP05 linker, the exosomes could be slowly released and thus ensure prolonged effect of BMP2 when endocytosed by the recipient cells. In the in vivo calvarial defect model, Exo^BMP2+NoBody-loaded GelMA displayed great capacity in promoting bone regeneration.

CONCLUSIONS

Together, the proposed Exo^BMP2+NoBody-loaded GelMA can provide an efficient and innovative strategy for bone regeneration.

Improving Lentiviral Transduction of Human Adipose-Derived Mesenchymal Stem Cells

Lentiviral transduction of human mesenchymal stem cells (MSCs) induces long-term transgene expression and holds great promise for multiple gene therapy applications. Polybrene is the most commonly used reagent to improve viral gene transfer efficiency in laboratory research; however, it is not approved for human use and has also been shown to impair MSC proliferation and differentiation. Therefore, there is a need for optimized transduction protocols that can also be adapted to clinical settings. LentiBOOST (LB) and protamine sulfate are alternative transduction enhancers (TEs) that can be manufactured to current Good Manufacturing Practice standards, are easily applied to existing protocols, and have been previously studied for the transduction of human CD34+ hematopoietic stem cells. In this study, we investigated these reagents for the enhancement of lentiviral transduction of adipose-derived MSCs. We found that the combination of LB and protamine sulfate could yield comparable or even superior transduction efficiency to polybrene, with no dose-dependent adverse effects on cell viability or stem cell characteristics. This combination of TEs represents a valuable clinically compatible alternative to polybrene with the potential to significantly improve the efficiency of lentiviral transduction of MSCs for gene therapy applications.

Effects of cell seeding technique and cell density on BMP-2 production in transduced human mesenchymal stem cells

Small animal models have demonstrated the efficacy of ex vivo regional gene therapy using scaffolds loaded with BMP-2-expressing mesenchymal stem cells (MSCs). Prior to clinical translation, optimization of seeding techniques of the transduced cells will be important to minimize time and resource expenditure, while maximizing cell delivery and BMP-2 production. No prior studies have investigated cell-seeding techniques in the setting of transduced cells for gene therapy applications. Using BMP-2-expressing transduced adipose-derived MSCs and a porous ceramic scaffold, this study compared previously described static and dynamic seeding techniques with respect to cell seeding efficiency, uniformity of cell distribution, and in vitro BMP-2 production. Static and negative pressure seeding techniques demonstrated the highest seeding efficiency, while orbital shaking was associated with the greatest increases in BMP-2 production per cell. Low density cell suspensions were associated with the highest seeding efficiency and uniformity of cell distribution, and the greatest increases in BMP-2 production from 2 to 7 days after seeding. Our results highlight the potential for development of an optimized cell density and seeding technique that could greatly reduce the number of MSCs needed to produce therapeutic BMP-2 levels in clinical situations. Further studies are needed to investigate in vivo effects of cell seeding techniques on bone healing.

Influence of donor age and comorbidities on transduced human adipose-derived stem cell in vitro osteogenic potential

Human adipose-derived mesenchymal stem cells (ASCs) transduced with a lentiviral vector system to express bone morphogenetic protein 2 (LV-BMP-2) have been shown to reliably heal bone defects in animal models. However, the influence of donor characteristics such as age, sex, race, and medical co-morbidities on ASC yield, growth and bone regenerative capacity, while critical to the successful clinical translation of stem cell-based therapies, are not well understood. Human ASCs isolated from the infrapatellar fat pads in 122 ASC donors were evaluated for cell growth characteristics; 44 underwent additional analyses to evaluate in vitro osteogenic potential, with and without LV-BMP-2 transduction. We found that while female donors demonstrated significantly higher cell yield and ASC growth rates, age, race, and the presence of co-morbid conditions were not associated with differences in proliferation. Donor demographics or the presence of comorbidities were not associated with differences in in vitro osteogenic potential or stem cell differentiation, except that transduced ASCs from healthy donors produced more BMP-2 at day 2. Overall, donor age, sex, race, and the presence of co-morbid conditions had a limited influence on cell yield, proliferation, self-renewal capacity, and osteogenic potential for non-transduced and transduced (LV-BMP-2) ASCs. These results suggest that ASCs are a promising resource for both autologous and allogeneic cell-based gene therapy applications.

Gene Therapy in Orthopaedics: Progress and Challenges in Pre-Clinical Development and Translation

In orthopaedics, gene-based treatment approaches are being investigated for an array of common -yet medically challenging- pathologic conditions of the skeletal connective tissues and structures (bone, cartilage, ligament, tendon, joints, intervertebral discs etc.). As the skeletal system protects the vital organs and provides weight-bearing structural support, the various tissues are principally composed of dense extracellular matrix (ECM), often with minimal cellularity and vasculature. Due to their functional roles, composition, and distribution throughout the body the skeletal tissues are prone to traumatic injury, and/or structural failure from chronic inflammation and matrix degradation. Due to a mixture of environment and endogenous factors repair processes are often slow and fail to restore the native quality of the ECM and its function. In other cases, large-scale lesions from severe trauma or tumor surgery, exceed the body’s healing and regenerative capacity. Although a wide range of exogenous gene products (proteins and RNAs) have the potential to enhance tissue repair/regeneration and inhibit degenerative disease their clinical use is hindered by the absence of practical methods for safe, effective delivery. Cumulatively, a large body of evidence demonstrates the capacity to transfer coding sequences for biologic agents to cells in the skeletal tissues to achieve prolonged delivery at functional levels to augment local repair or inhibit pathologic processes. With an eye toward clinical translation, we discuss the research progress in the primary injury and disease targets in orthopaedic gene therapy. Technical considerations important to the exploration and pre-clinical development are presented, with an emphasis on vector technologies and delivery strategies whose capacity to generate and sustain functional transgene expression in vivo is well-established.