Musculoskeletal tissue engineering with skeletal muscle-derived stem cells - Artificial Cells, Cell Engineering and Therapy - page 178

and the fat of adult rats and then genetically modified with an adenovirus to
express BMP2. 69 The BMP2-transduced cells and their unmodified controls
were implanted into an articular cartilage lesion made in the patellar groove of
the rat femur. The results from this study indicate that the periosteum and bone
marrow-derived cells that were genetically engineered to express BMP2
generated repair tissue that had properties similar to those of articular cartilage,
whereas the BMP2-transduced fat stromal cells formed mostly fibrous tissue.
This study is a clear indicator that not all cells will perform identically in vitro or
in vivo. For this reason, it is important to investigate and compare different cell
types and growth factors and identify the optimal combination.

7.5 Current progress with MDSCs

MDSCs were originally identified by researchers trying to find an optimal cell
source to be used for cell replacement therapy in Duchenne muscular dystrophy.
In the early stages of study, our group focused on characterizing the ability of
these cells to regenerate dystrophin-expressing muscle fibers in mdx mice. We
observed that transplanted MDSCs displayed a high degree of skeletal muscle
regeneration that was superior to that of transplanted satellite cells, a skeletal
muscle-committed progenitor cell. 17 In these experiments, MDSCs displayed
long-term proliferation in vivo, a strong capacity for self-renewal, multipotent
differentiation, and immune-privileged behavior. 17,18
Ex vivo gene therapy with MDSCs has been used to deliver dystrophin to mdx
mice. 70 MDSCs were isolated from mdx mice, genetically engineered with a
retrovirus carrying a functional human mini-dystrophin gene and then injected
into the skeletal muscle of mdx mice. Dystrophin-positive muscle fibers were
present up to 24 weeks postinjection, suggesting that ex vivo gene transfer may
aid in the delivery of dystrophin to skeletal muscle. To improve the trans-
plantation efficiency of MDSCs isolated from normal mice and injected into
dystrophic skeletal muscles, researchers recently used NGF to test the ex vivo
protein stimulation and ex vivo gene therapy of MDSCs. 71 MDSCs were either
stimulated with NGF in vitro before injection or were retrovirally transduced to
express NGF. In both cases, cells stimulated with NGF exhibited higher
engraftment in skeletal muscle than did unstimulated controls. Interestingly, the
ex vivo stimulation of MDSCs with NGF led to a higher regeneration of muscle
fibers than elicited by NGF-transduced MDSCs, suggesting that, in some cases,
simply preconditioning the cells with growth factors may be sufficient to induce
a beneficial effect.
In numerous other studies, MDSCs genetically engineered to express BMP2
or BMP4 have led to bone formation and healed critical-sized calvarial and long
bone defects. 21±24,72,73 In vivo tracking of the cells has demonstrated that more
than 95% of the implanted MDSCs expressing BMP2 were found in the newly
formed bone. 21 Some also appeared to co-localize with osteocalcin, suggesting