Advances in combining gene therapy with cell and tissue engineering-based approaches to enhance healing of the meniscus

SUMMARY

Meniscal lesions are common problems in orthopaedic surgery and sports medicine, and injury or loss of the meniscus accelerates the onset of knee osteoarthritis. Despite a variety of therapeutic options in the clinics, there is a critical need for improved treatments to enhance meniscal repair. In this regard, combining gene-, cell-, and tissue engineering-based approaches is an attractive strategy to generate novel, effective therapies to treat meniscal lesions. In the present work, we provide an overview of the tools currently available to improve meniscal repair and discuss the progress and remaining challenges for potential future translation in patients.

Principles of meniscal repair

Structure and function of the meniscus and of the meniscal roots

The menisci are semilunar fibrocartilage structures that transmit joint forces and increase stability, facilitate nutrition, provide lubrication for the articular cartilage, and promote knee proprioception (Fig. 1). Type-I collagen is the predominant collagen of the meniscal extracellular matrix (ECM)¹. Collagen organization is paramount for meniscus mechanical function, with circumferential oriented bundles circumnavigating the c-shaped structure enabling efficient transfer of load in this direction. Additionally, interspersed radially oriented “tie” fibers interdigitate through this circumferential network and act to bind this primary collagen organization together². Other components of the meniscus ECM chiefly include proteoglycans and glycoproteins. The major role of the proteoglycans is to retain water (about 65–75% of its total weight) within the meniscal tissue. The interactions between the macromolecular ECM and the water enable the important viscoelastic properties of the menisci³.

Fig. 1.

(A) Normal human left medial meniscus attached to the medial tibial plateau. The meniscal body (MB) and the anterior (AMH) and posterior horn (PMH) of the meniscus are anchored into the tibial intercondylar region by the meniscal roots. The anterior root (AR) of the medial meniscus inserts into the anterior intercondylar area while its posterior root (PR) inserts on the posterior medial intercondylar eminence of the tibia. The central part of the medial tibial plateau (MTP-C) is not covered by the meniscus. Radially directed forces are presented by the arrows. Scale bar: 1 cm. (B) Visualization of the anterior root (AR) of the medial meniscus and its insertion into the anterior intercondylar area by pulling the medial meniscus towards the right side of the picture. The tibial aspect of the anterior horn of the medial meniscus (AMH) is shown. The peripheral part of the anterior medial tibial plateau (MTP-P) is now exposed, together with the central part of the medial tibial plateau (MTP-C). Scale bar: 0.5 cm.

The inner (central) parts of the menisci are mainly comprised of fibrochondrocytes and contain a higher proportion of type-II collagen and proteoglycans, whereas the outer (peripheral) region contains more fibroblastic cells and contains type-I collagen and a lower proteoglycan content⁴. Meniscal vascular supply originating from branches of the geniculate arteries is restricted to the peripheral 10–25% of the meniscal tissue⁵. If a small tear is present in this “red zone”, natural repair may occur. The nerve fibers mostly follow these blood vessels, with the two horns of the meniscus being the most richly innervated. The central (inner) third of the meniscus, called the “white zone”, is avascular and has no nerve supply. Therefore, no spontaneous healing of tears in this region occurs, although meniscal repair may be attempted (e.g. simultaneously with the reconstruction of a torn anterior cruciate ligament) also in the intermediate red-white or white-white zone applying novel suture techniques and/or anabolic factors⁶.

The meniscal roots are four ligamentous structures that firmly anchor the anterior and posterior horns of the menisci into the tibial intercondylar region⁷. The strong attachment of the medial meniscus to the medial collateral ligament is the reason for its decreased mobility during joint motion compared with the lateral meniscus, which is not attached to the lateral collateral ligament nor to the joint capsule at the site of the popliteus tendon hiatus⁸.

The menisci play a critical biomechanical role, contributing to joint congruence, load distribution, and stability in the knee⁹. Importantly, damage to the meniscus following trauma or with degeneration is associated with altered joint function that often leads to progressive joint degeneration¹⁰. The critical biomechanical functions of the menisci are provided by their geometry, attachments, and the unique mechanical properties of the tissue. The geometry and physical attachment of the menisci within the joint provide important restraints that govern the overall mechanical response and movement of the menisci to applied forces and provide for the complex kinematics of the tibiofemoral joint during the activities of daily living.

Due to the unique geometry of the menisci, loading of the knee joint gives rise to a vertical and radially-directed force component, which tends to outwardly displace the menisci. However, the meniscal attachments restrain this displacement, and give rise to a large circumferentially-directed force, and associated tensile “hoop stress” within the tissue. Under normal physiological loading of the knee, the menisci (and cells within the menisci) experience a complex mechanical environment consisting of large tensile, compressive, and shear stresses. Significant structural differences exist between the medial and lateral femorotibial compartments in the knee, including both the tibial plateaus and the menisci.

Meniscal lesions

The meniscus can tear either from trauma or as a consequence of age-related degenerative changes¹¹. Meniscal tears include radial, longitudinal, horizontal, complex and degenerative tears (Fig. 2–5). They are the most prevalent intraarticular knee injuries and the most frequent cause of orthopaedic surgical procedures, representing a significant risk factor for the development of knee osteoarthritis (OA)¹². While traumatic tears can occur at any age (even in children), patients over the age of 50–60 are more susceptible to degenerative meniscal tears. Traumatic tears have a better prognosis for reconstructive surgical options. Longitudinal and horizontal tears are amenable to repair, while symptomatic traumatic radial tears are often treated with partial meniscectomy. Of note, the extent of meniscal resection correlates with alterations in the stress distribution in the joint¹³, as well as the degree of OA development¹⁴. Medial meniscal injuries are much more common compared to those in the lateral meniscus¹⁵, potentially because of its lesser mobility¹⁶. However, lateral meniscus tears are associated more frequently with less favorable clinical outcomes than tears of the medial meniscus, including OA development and rapid chondrolysis¹⁴. As meniscal root tears disrupt the circumferential fibers that provide hoop strength, meniscal extrusion with altered biomechanical properties of the meniscus is a critical clinical consequence^17–19. Importantly, recent studies have shown that meniscal extrusion may be closely associated with progressive cartilage loss and accelerated OA²⁰. Furthermore, meniscal cysts may occur in both the lateral and medial sides. These are commonly associated with horizontal meniscal tears, and result in a void space within the meniscus²¹. The need for tailored tissue engineering-based approaches to enhance the healing of the meniscus is similar for all types of meniscal injuries.

Fig. 2.

Classification of meniscal tears. Meniscal tears include radial, longitudinal, horizontal, complex and degenerative tears.

Fig. 5.

Histological evaluation of a complex traumatic canine meniscal tear model. A full-thickness circumferential tear was performed as described in Figure 4. Formalin-fixed, paraffin-embedded sections (5 μm) were taken in the axial plane (12 weeks) and stained with toluidine blue (A) and trichrome (B), highlighting the circumferentially aligned collagen fibers. Evidence of incomplete healing in the tear is demonstrated by the asterisks in the mid-substance region. Scale bar: 500 μm.

An important factor that may influence meniscal repair is the inflammatory environment within the joint²². In particular, inflammatory cytokines, such as interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF-α), are elevated following injury or with OA in the knee joint^23,24. Importantly, these cytokines can increase tissue degradation, with increased matrix metalloproteinase (MMP) activity, sulfated glycosaminoglycan (S-GAG) release, and the presence of pro-inflammmatory mediators, such as nitric oxide and prostaglandin E2. These cytokines have also been shown to strongly inhibit integrative meniscal repair in vitro, by decreasing cell accumulation and tissue formation at the meniscal repair interface, ultimately compromising the shear strength of repair^25,26. Initial acute exposure to IL-1 for 1–3 days potently suppresses meniscal repair for at least 28 days²⁶, suggesting that the initial inflammatory environment in a joint post-injury may have long-term degenerative effects. In addition, IL-1 and TNF-α activate other degradative and pro-inflammatory pathways in the meniscus and other joint tissues that contribute to joint degeneration²⁷.

Natural repair and current surgical options

Preserving as much meniscal tissue as possible is the current approach when performing arthroscopic partial meniscectomy, as total meniscectomy alone inevitably leads to osteoarthritic degeneration in the long-term^14,28,29. Partial meniscal resection is generally associated with a lesser degree of OA development, although it sometimes may occur in a rapid manner³⁰. Increased knowledge on the anatomy and function of the meniscus has led to the development of sophisticated techniques for meniscal repair and reconstruction to preserve or restore meniscus function^31,32. For the purpose of this review, we refer to meniscal repair when torn parts of the meniscus are put back and held in apposition, and refer to meniscal reconstruction when missing parts of the meniscus or the entire meniscus is replaced by an allograft or a meniscal substitute.

As a result of the differences in vascularization and blood supply, tears in the peripheral, vascularized portion of the meniscus may be successfully repaired using a variety of operative procedures, such as sutures³³. Lesions in the central, avascular area that has a poor healing capacity^34,35 may be treated by arthroscopic partial meniscectomy. Meniscal reconstruction is based on the use of allografts and substitutes. The use of meniscal allografts^36,37 is a therapeutic option especially for young patients with tibiofemoral joint pain and a history of meniscectomy in a normally aligned, stable joint without severe degenerative changes of the articular cartilage.

Meniscal allograft transplantation improves pain and function in the short and intermediate term³³. Meniscal substitutes have been shown to overcome the problems of allografting and to promote repair of segmental meniscal defects, for example, resulting from a partial meniscectomy³⁸. These replacement devices include porous bovine type-I collagen/GAG matrices³⁹ or polyurethane⁴⁰. More recently, artificial biomaterials have shown initial success in restoring meniscal function and preventing degenerative changes following meniscectomy⁴¹.

Strategies to improve meniscal repair

Preservation of meniscal tissue is paramount for long-term joint function and therefore the decisive guiding principle in the treatment of meniscal defects^33,42. While there are no established criteria for a successful meniscal replacement strategy, tests of stiffness in tension and compression are commonly used to assess the similarity of mechanical properties of repair tissue to the native tissue. Other outcomes may include pullout strength of meniscal attachment, cycles of loading to failure (fatigue), subsidence, and friction property. When mechanical integrity of meniscal repair strategies cannot be achieved, experimental approaches for replacement or regeneration of meniscal tissue are required. These may include the administration of cells or tissues, candidate factors or gene transfer vectors, and/or biocompatible materials (Fig. 6).

Fig. 6.

Principles of gene-, cell-, and tissue engineering-based approaches for meniscal repair. Schematic view of a bucket-handle meniscal tear. Gene-, cell-, and tissue engineering-based approaches for meniscal repair include the direct in vivo administration of a (viral) gene transfer vector or of ex vivo genetically modified cells within a meniscal lesion, and the implantation of biocompatible and bioactive materials in the form of scaffolds that deliver either a recombinant factor, gene transfer vector, or ex vivo genetically modified cells that may serve the dual role of providing the therapeutic factor and repopulating the lesion.

Target cells and tissues

Different cells or tissues may be targeted in approaches that aim at improving meniscal repair:

1. isolated meniscal fibrochondrocytes that may repopulate the injured meniscus or enhance integrative repair of a tear^43–52,

2. whole meniscal tissue^48,49,52, or

3. progenitor cells that may be induced towards a fibrochondrocyte-like phenotype, the most studied being mesenchymal stem cells (MSCs) from the bone marrow, adipose tissue, synovium, periosteum, trabecular bone, umbilical cord blood, amniotic fluid, Wharton’s jelly, or skeletal muscle^{48,50,53–59}; also notably, embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) have recently become a focus of stem cell-based regenerative medicine⁶⁰.

Candidate factors

Various factors have been identified that have a therapeutic potential to enhance meniscal repair by:

1. activating cell proliferation and survival: platelet-derived growth factor (PDGF)⁶¹, basic fibroblast growth factor (FGF-2)⁶², insulin-like growth factor I (IGF-I)⁶³, transforming growth factor beta (TGF-β)^22,64,65,

2. modulating cell migration: TGF-β^22,65, stromal cell derived factor-1 (SDF-1)⁶⁶,

3. stimulating anabolic pathways: PDGF^67,68, TGF-β^64,67, bone morphogenetic protein 7 (BMP-7)⁶⁷, hepatocyte growth factor (HGF)⁶⁸, FGF-2⁶⁹, IGF-I⁶³,

4. inhibiting inflammatory and catabolic pathways: IL-1 receptor antagonist (IL-1Ra)²⁵, TNF antibody²⁵, inhibitors of MMPs⁷⁰, TGF-β²²,

5. activating biomechanical signalling pathways that are pro-anabolic or anti-catabolic^71–73, and

6. combinations of such approaches: PDGF/HGF⁶⁸.

Direct application of recombinant factors to meniscal lesions is generally hindered by their short pharmacologic half-lives, highlighting the necessity for improving treatment by using gene delivery procedures and/or biomaterials coated with such factors to allow for sustained therapeutic activities.

Gene transfer vectors

Different classes of gene vehicles (nonviral and viral vectors) are currently available to genetically modify relevant target cells and tissues, with specific advantages and limitations listed in Table 1.

Table 1.

Overview of currently used gene transfer vectors for meniscal repair

Classes	Integration	Advantages	Shortcomings
NV	no	. nontoxic . large capacity	. relatively low efficiency . short-term expression
AdV	no	. high efficiency . large capacity	. possible replication . immunogenicity/toxicity . short-term expression
RV/LV	yes	. high efficiency . relatively large capacity . long-term expression	. possible replication . insertional mutagenesis
HSV	no	. high efficiency . large capacity	. possible replication . toxicity . short-term expression
rAAV	mostly episomal	. high efficiency . long-term expression . low immunogenicity/toxicity	. difficult to produce . size limitation . serotype specificity

AdV: adenoviral vectors; HSV: herpes simplex virus vectors; LV: lentiviral vectors; NV: nonviral vectors; rAAV: recombinant adeno-associated virus vectors; RV: retroviral vectors.

Nonviral vectors

Nonviral vectors are safe gene vehicles as they avoid the risk of acquiring replication competence inherent to viral vectors. These vectors can be repeatedly administered, but they mediate relatively low and short-term transgene expression (~10–38% transfection efficiency in meniscal cells and 14–25% in progenitor cells)^43,50,54,60.

Adenoviral vectors

Adenoviral vectors allow for high transduction efficiencies and levels of transgene expression in vitro (~80–100% efficiency in meniscal and progenitor cells) but they are immunogenic in vivo and promote very short-term efficacy (1 to 2 weeks maximum)^43,46,74.

Retro-/lentiviral vectors

Retroviral vectors allow for long-term maintenance of their transgenes by integration into the host cell genome. Nevertheless, such integration may lead to insertional mutagenesis and tumor gene activation. These vectors can only transduce dividing cells and at a relatively low efficacy (~20–30% efficiency before cell selection, reaching up to 90% upon selection)^43,46,74.

Lentiviral vectors have certain advantages over other viral approaches as they can integrate into the genome of nondividing cells and show high levels of transduction efficacy (up to 90% in progenitor cells)^55,56, but concerns remain for their potential for insertional mutagenesis.

Herpes simplex virus (HSV) vectors

HSV are large vectors that can deliver long transgenes in almost all known cell types, including nondividing cells (~70% transduction efficiency in meniscal cells). However, these vectors are relatively toxic and provide only transient transgene expression⁴³.

Recombinant adeno-associated virus (rAAV) vectors

rAAV vectors are derived from a replication-defective human parvovirus. These vectors are less immunogenic than adenoviral vectors and more effective than nonviral and retro-/lentiviral vectors for the transduction of both dividing and nondividing cells (up to 80% transduction efficiency in meniscal cells and up to 90% in progenitor cells), allowing for prolonged transgene expression. rAAV are maintained as stable, episomal constructs in their host, while permitting gene transfer in situ through the dense ECM due to their small size (20 nm)^{43,47,49,52,53}. Of note, the use of trans-splicing rAAV systems has allowed for the enhancement of the size capacity of the vectors. For all these reasons, rAAV has become a vector of choice for clinical applications.

Biocompatible materials

Numerous tissue engineering strategies have emerged for the replacement of meniscal tissue⁷⁵, based on the use of acellular⁷⁶ or cell-seeded matrices⁷⁷.

Several approaches for treating meniscal defects concentrate on meniscal replacement with acellular matrices^78,79, avoiding possible risks associated with transplantation of human allografts, such as high failure rate or immunoreaction and disease transmission⁸⁰. Different types of meniscal substitutes, such as autologous tissue^81–83, decellularized allogenic and xenogenic grafts^76,84,85, collagen grafts⁸⁶, permanent synthetic scaffolds³⁸, silk fibroin scaffolds^87,88, and biodegradable scaffolds based on small intestine submucosa⁸⁹, poly-lactic acid (PLA) or poly-glycolic acid (PGA)^90,91, have been used in experimental and clinical studies.

However, after transplantation of acellular meniscal constructs into defects, the transplants are populated by synovial fibroblasts, resulting in a scar tissue with poor biomechanical properties⁹². Therefore, some tissue engineering approaches focus on additional cell-seeding techniques prior to transplantation^38,93. Meniscal cells⁹⁴, articular chondrocytes⁹⁵, synovial fibroblasts⁹⁶, and MSCs⁹⁷ have been proposed as potential cell sources and have been cultivated in vivo and in vitro on various matrices. In addition, different environmental factors, such as growth factors, have been used to optimize cell proliferation in vitro⁹⁸.

Of note, different biomaterials have been tested in experimental settings in vitro, in situ, and in vivo for meniscal repair applications concomitantly with the use of cell- and gene-based approaches, including alginate^49,99, type-I collagen solutions⁷⁴ or type-I collagen/GAG matrices⁴⁸, and PGA scaffolds⁴⁵.

Strategies

Gene therapy may be applied directly or in combination with various cell- or tissue engineering-based approaches for meniscal repair (Fig. 6):

1. directly injecting a gene transfer vector,

2. administrating genetically modified cells,

3. implanting a biocompatible material that delivers a recombinant factor,

4. applying autologous platelet-rich plasma or fibrin clots,

5. providing a biomaterial that delivers a gene transfer vector, or

6. transplanting a material seeded with cells that have been genetically modified.

Cell-free strategies are less invasive, but the presence of cells in the therapeutic composition might be necessary as an effective means to repopulate meniscal lesions, particularly given the scarcity of fibrochondrocytes in mature tissue.

Evidence for gene transfer in vitro

All of the gene transfer vectors mentioned above have been successfully employed to target most of the cells relevant for meniscal repair in vitro including meniscal fibrochondrocytes^43–52,74 and various types of progenitor cells^53,54,60, and provide varying gene transfer efficiencies in vitro:

1. nonviral vectors: ~ 9–38% efficiency in meniscal cells (over less than a week)^43,99 and 14–25% in progenitor cells (bone marrow, perichondrium, umbilical cord blood) (between days 2 and 14)⁵⁴,

2. adenoviral vectors: up to 100% in meniscal cells (less than a week)^43,46,48,74,

3. retroviral vectors: up to 90% upon selection of meniscal cells^43,44,46,74,

4. lentiviral vectors: 10–90% in progenitor cells (bone marrow, iPSCs) depending on the cell source and time points evaluated (2–15 days)⁶⁰,

5. HSV vectors: ~ 70% in meniscal cells (less than a week)⁴³,

6. rAAV vectors: ~ 80% in meniscal cells (only 2 days tested)⁴⁷ and up to 90% in progenitor cells (bone marrow) (21 days)⁵³.

Various pathways as described below have been targeted to enhance the reparative capacities of both cell types via therapeutic gene delivery approaches (Table 2). Stimulation of proliferative activities in meniscal and progenitor cells (bone marrow) has been demonstrated following gene transfer of IGF-I⁵⁰, FGF-2^49,51, and TGF-β without⁵² or in a type-I collagen/GAG matrix⁴⁸ for up to 21 days^48,49,53 using nonviral⁵⁰, adenoviral⁴⁸, and rAAV vectors^49,52,53. Successful activation of anabolic processes has been reported in meniscal and progenitor cells (bone marrow) upon gene transfer of IGF-I or TGF-β without biomaterial^44,52 or with a type-I collagen/GAG matrix for up to 21 days⁴⁸ using adenoviral⁴⁸, retroviral⁴⁴, and rAAV vectors⁵². One potential therapeutic approach for enhancing meniscal repair may be through the controlled inhibition of pro-inflammatory mediators, either through direct protein delivery²⁵ or through gene therapy approaches. Successful co-delivery of different candidate genes has not been reported in meniscal or progenitor cells to date, but this might be achieved as evidenced in articular chondrocytes¹⁰⁰.

Table 2.

Overview of current in vitro gene-, cell-, and tissue engineering approaches for meniscal repair

Vector	Gene	Biomaterial	Cells	Effects	References
NV	IGF-I	-	meniscal cells	cell proliferation	50
	FGF-2	alginate	meniscal cells	cell proliferation	51
AdV	TGF-β	type-I collagen/GAG matrix	meniscal and progenitor cells	cell proliferation matrix synthesis	48
RV	TGF-β	-	meniscal cells	matrix synthesis	44
rAAV	FGF-2	-	meniscal and progenitor cells	cell proliferation	49,53
	TGF-β	-	meniscal cells	cell proliferation matrix synthesis	52

AdV: adenoviral vectors; FGF-2: basic fibroblast growth factor; GAG: glycosaminoglycans; IGF-I: insulin-like growth factor I; NV: nonviral vectors; PGA: poly-glycolic acid; rAAV: recombinant adeno-associated virus vectors; RV: retroviral vectors; TGF-β: transforming growth factor beta.

Evidence for gene transfer in situ and in vivo

Gene transfer in situ

Different gene transfer vectors have been tested via indirect (cell-based)^48,51,74 and direct (cell-free)^49,52,74 experimental procedures to target meniscal tissue, including:

1. nonviral vectors: ~ 10% transfection efficiency in meniscal cells transplanted in injured meniscal explants (8 days)⁵¹,

2. adenoviral vectors: ~ 80% in meniscal and progenitor cells (bone marrow) transplanted in injured meniscal explants using a type-I collagen/GAG matrix (21 days)⁴⁸ and ~ 40% by direct injection in intact meniscal explants (several weeks)⁷⁴,

3. retroviral vectors: ~ 30% in meniscal cells transplanted in intact meniscal explants using a type-I collagen solution (several weeks)⁷⁴,

4. rAAV vectors: ~ 70–75% by direct injection in intact or injured meniscal explants (15 days)^49,52.

Therapeutic gene transfer in situ has been attempted by transplanting meniscal and progenitor cells (MSCs) modified by a TGF-β rAAV vector⁵² or with an adenoviral vector using type-I collagen/GAG matrix in injured bovine meniscal explants⁴⁸ or by direct injection of rAAV FGF-2 and TGF-β vectors in human meniscal lesions^49,52 leading to an enhanced repair of the treated lesions over ~ 15–21 days (Table 3).

Table 3.

Overview of current gene-, cell-, and tissue engineering approaches for meniscal repair in situ and in vivo

System	Vector	Gene	Biomaterial	Cells	Effects	References
In situ	AdV	TGF-β	type-I collagen/GAG matrix	meniscal and progenitor cells	repair of meniscal lesions	48
	rAAV	FGF-2	-	-	cell proliferation, contraction, repair of meniscal lesions	49
		TGF-β	-	-	cell proliferation, contraction, repair of meniscal lesions	52
In vivo	NV	IGF-I	alginate	progenitor cells	repair of meniscal lesions	50
	AdV	HGF	PGA	meniscal cells	repair of meniscal lesions, vascularization	45

AdV: adenoviral vectors; FGF-2: basic fibroblast growth factor; GAG: gylcosaminoglycan; HGF: hepatocyte growth factor; IGF-I: insulin-like growth factor I; NV: nonviral vectors; PGA: poly-glycolic acid; rAAV: recombinant adeno-associated virus vectors; TGF-β: transforming growth factor beta.

Gene transfer in vivo

In vivo most of the gene transfer vectors have also been applied by indirect^46,50,74 and direct^47,74 approaches to target the meniscus or to repair meniscal lesions, such as:

1. nonviral vectors: ~ 22% transfection efficiency in meniscal cells transplanted in meniscal lesions using alginate (2 days)⁹⁹,

2. adenoviral vectors: ~ 40% by direct injection in meniscal lesions (several weeks)⁷⁴,

3. retroviral vectors: ~ 20% in meniscal cells transplanted in meniscal lesions using a type-I collagen solution (several weeks)⁷⁴ or ~ 50% by meniscal allograft transplantation (several weeks)⁴⁶,

4. rAAV vectors: ~ 50% by direct injection in meniscal lesions (at least 20 days)⁴⁷.

Therapeutic gene transfer in vivo has been performed by transplantation of meniscal cells modified by an HGF adenoviral vector using a PGA scaffold in an athymic nude mouse model⁴⁵ or by progenitor cells (MSCs) modified with an IGF-I nonviral vector using alginate in goat meniscal lesions⁵⁰, leading to an enhanced repair of the treated lesions for up to 16 weeks (Table 3). In general, more information will be needed on the possible deleterious effects of the different approaches especially when viral vectors are being manipulated in vivo for a safe future application in the patient.

Conclusions

Gene therapy - alone or in combination with cell or tissue engineering-based strategies - provides attractive approaches to enhance the repair of meniscal lesions in light of significant advances in experimental research in cell biology, molecular biology (therapeutic candidate factors and genes), biomaterials, and translational science. While there is an accumulating body of preclinical evidence showing the benefits of gene therapy to treat meniscal injury and various completed or ongoing clinical protocols using diverse scaffolds, implants, or biological compounds (www.clinicaltrials.gov), no trial of meniscal gene therapy has been initiated to the best of our knowledge. Among the open questions, the best suited candidate gene(s) and vector and the most adapted cell source and scaffold have to be clearly identified, as still relatively little information is available in experimental models in vivo. Furthermore, translational animal models need to specifically reflect the different lesion types, etiologies, and locations as seen in the clinical situation. It will also be essential to take into account that the composition will need to meet the challenging requirements of regulatory organizations prior to enrollment in a patient protocol. In this regard, combined efforts between scientists, clinicians, industry, and regulatory organizations will be necessary to tackle the question of treating such lesions in patients.

Fig. 3.

Traumatic meniscal tear. Arthroscopic view of a complex traumatic tear of the medial meniscus in a 29-year old man. The round medial femoral condyle (MFC) can be seen in the top left side of the picture, and the corresponding central part of the medial tibial plateau (MTP-C) on the bottom left side of the picture. Note the macroscopic good aspect of the articular cartilage in the medial femorotibial compartment. In the middle right, the complex rupture pattern of the meniscal tear (CMT) located in the pars intermedia can be appreciated (arrows), in part obscuring the view of the medial femoral condyle. The posterior horn of the medial meniscus (PMH) is shown on the left side of the picture and the anterior horn of the medial meniscus (AMH) extends on the right side of the picture.

Fig. 4.

Magnetic resonance imaging (MRI) of a complex traumatic canine meniscal tear model. A full-thickness circumferential tear (1/3 of the circumferential length) was initiated at at the posterior aspect of the meniscus and at 2–3 mm from the menisco-synovial junction (12 weeks). (A) A coronal MRI demonstrates the meniscal tear in the central (white-white) zone. Excised menisci were placed within a birdcage RF transmitter-receiver coil for imaging at 9.4 T (Oxford Instrument). The scan volume was 125 cm³ with a resolution of 0.02 cm/pixel. Serial MRI sections were processed in MATLAB to generate a three-dimensional reconstruction of the meniscus as shown. (B) Three-dimensional (3-D) MRI reconstruction of the meniscal tear. High resolution, 3-D MRI scans were performed on intact canine joints following sacrifice. Joints were imaged within a 7.1 T (300 MHz, 85 gauss/cm²) at up to 512³ isotropic resolution. (C) Normal canine meniscus.