Biomedical Engineering Reference
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Figure 2. Human osteoblast like cells retrovirally transduced with LacZ. Transduction was enhanced by the
cationic polymer Polybrene®, by temporary lowering of the temperature, and by mild centrifugation with
500 rpm. These measures were taken to improve contact of retroviral vectors with the cells. X-gal staining.
Compared to fig 1 there is an increase in the number of blue cells to approximately 60%. Micrograph
courtesy of Dr. Axel Baltzer. Reprinted with permission from Baltzer A et al: In vitro Transduktion humaner
osteoblastärer Zellpopulationen mit retroviralen Vektoren [In vitro transduction of human osteoblastic cells
using retroviral vectors]. Z Rheumatol 1999; 58: 88-94.
pluripotent and also contains osteoprogenitor cells. 32 These cells can act in paracrine and
autocrine mechanisms to enhance fracture healing. Balk and coworkers demonstrated that
mouse stromal cells can be retrovirally transduced. 33 Cells were transduced with a reporter
gene and cells maintained expression of osteoblast-specific marker genes in vitro following
supplementation with known osteogenic factors in the cell culture media. Similarly, Allay and
coworkers demonstrated that human bone marrow-derived mesenchymal progenitor cells are
susceptible to retroviral transduction. Cells retained their osteogenic potential and formed bone
in vivo after being seeded into porous calcium phosphate ceramic cubes and implanted subcu-
taneously into SCID mice. 34 Another interesting example of retroviral gene transfer is from
Oyama and coworkers. The authors retrovirally transduced murine stromal cells for a marker
gene ex vivo, and returned the genetically modified cells to syngeneic mice homozygous for the
osteogenesis imperfecta murine (oim) mutation by intramedullary injection. Interestingly, these
cells could be detected in both the marrow of the injected bones and also, remarkably, in
noninjected bones of the same animal. The authors concluded that the cells were able to traffic
between different bone marrow compartments. Furthermore, it was reported that the cells
adhered to the trabecular surfaces of the bone and appeared to be in the process of differentiat-
ing into osteoblasts in vivo. 35
In conclusion, retroviruses integrate their genomes into the chromosomal DNA of the host
cell. Although long term expression must be considered variable for retroviruses, 3 they may still
be advantageous for diseases that require prolonged expression of therapeutic proteins other
than adenoviral vectors, which integrate episomally. However, treatment of fractures and the
achievement of solid fusions in the spine or peripheral joints should not take longer than a few
weeks or a couple of months. Within this crucial time span, an increased expression of thera-
peutic agents may be of utmost benefit. Hence in these cases, gene expression following trans-
fer via adenoviral vector, which is typically high and transient, may be more appropriate. 36 In
contrast, prolonged delivery of beneficial drugs may even cause harm to the patient.
 
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