Repair of Osteochondral Lesions - Engineered Bone: Tissue Engineering Intelligence Unit - page 171

The delivery of the transgenes, reviewed by Evans, 62 can be either systemic or local. Sys-
temic delivery is performed by direct intraperitoneal or intravenous injection. However, sys-
temic gene transfer with viral vectors often results in high but transient serum levels of the
expressed proteins, an approach that is not likely to produce significant clinical benefit. More-
over, host response to viral components and diffuse expression of immuno-regulatory mol-
ecules can lead to tissue inflammation, widespread immunosuppression and toxicity when
high vector concentrations are used. 63,64 By contrast, the alternative local delivery within the
synovial lining of the joint would lead to the accumulation of the factor(s) in and around
articular tissues where they are needed the most. Genes can be locally transferred by two means:
either they are delivered directly to the target site (in vivo approach) or selected cells are har-
vested from the joint, expanded, genetically manipulated and reimplanted at the injury site (ex
vivo approach). Although both protocols have given encouraging results, 65-68 some concerns
cannot be denied: when using viral vectors, acute inflammation can be revealed at the site of
injection, in particular with adenoviruses that are highly immunogenic; moreover, inadvertent
spread of vector into nonarticular tissues like lung, liver and spleen has been reported. 69 Ex
vivo gene therapy has the advantage that a defined population of cells is genetically modulated
and that the effects and dosage for optimal proliferation and/or differentiation can be con-
trolled. However, the approach, while rather effective, is laborious and expensive, and thus less
appealing for a widespread scale application. Repair strategies aiming at inducing osteochon-
dral lesion repair by delivery of molecules have a great commercial potential, since they could
lead to the development of off-the-shelf products (e.g., “intelligent” matrices) readily available
for clinical use. However, in order to reach this goal in a safe way, further knowledge in the
long-term properties, mode of action and interactions of growth/differentiation factors, as well
as the development of safe DNA vectors, is required. Therefore, in the short-medium term we
foresee that the cellular approach will meet broader consent for clinical use. In the following
paragraphs, we review some of the issues that are likely to enhance osteochondral repair by
cell-based approaches.
Identification of Appropriate Cell Types
For all cartilage repair methods based on autologous cells, a critical issue is related to the
procurement of chondrogenic cells. In fact, a cartilage biopsy in the joint, necessary for autolo-
gous chondrocyte preparation, represents an additional injury to the cartilage surface, and has
been reported to be detrimental to the surrounding healthy articular cartilage. 70 In addition,
this approach is limited by the availability of cells, particularly in elderly individuals, and by
their capacity to proliferate and to differentiate after expansion in monolayers. Several alterna-
tives have been therefore proposed, either to use more efficiently a biopsy of articular cartilage
or to use different cell sources, including chondrocytes from nonarticular cartilages or mesen-
chymal progenitor cells.
Mature Chondrocytes
It is long known that human articular chondrocytes can undergo only a limited number of
cell divisions in vitro and that their proliferative potential decreases with age. 71 In addition, the
redifferentiation of expanded human chondrocytes was reported to be negligible, 72 to decrease
with serial passaging, 73 to fully develop only in long-term cultures, 74,75 and to be highly depen-
dent on the lot of serum used. 76 The use of specific growth factors (e.g., FGF-2, TGF β 1,
PDGFbb) during articular chondrocyte expansion was recently shown not only to increase the
cell proliferation rate, but also to enhance the ability of the cells to redifferentiate upon transfer
into a 3D environment (Fig. 2) and to respond to differentiating agents. 77,78 Human articular
chondrocytes were also seen to maintain a higher chondrogenic commitment, as indicated by
the expression of the transcription factor SOX-9, by using a defined serum-free medium dur-
ing cell expansion. 79 At present, however, there is no comparative animal or clinical study
reported in the literature concerning the use of articular chondrocytes expanded under condi-
tions favouring cell proliferation and maintenance of chondrogenic ability.