Poster 136: Augmented Meniscus Radial Tear Healing via Local Transforming Growth Factor-beta 1 (TGF- β1) Gene Therapy in a Rat Model

Abstract

Objectives: Meniscus tears cause significant musculoskeletal impairment and are among the most common knee injuries. Tears in the inner third of the meniscus have intrinsically poor healing capacity due to insufficient blood supply. In an effort to improve healing, use of biologic augments is a promising area of investigation. Tissue engineering strategies, such as growth factor gene therapy, have emerged as encouraging approaches to improving meniscal healing. Transforming Growth Factor-Beta 1 (TGF-β1) gene therapy has shown promise in vitro and ex vivo as a therapeutic strategy, but its effects on meniscus healing in vivo remain unknown. This study aimed to determine whether local delivery of viral vectors carrying the TGF-β1 gene could improve the healing of a meniscus tear involving the avascular zone in vivo in a rat model. Methods: Recombinant adenoviruses were generated using the AdEasy system. Briefly, the coding regions of mouse TGFβ1 were PCR amplified and cloned into an adenoviral shuttle vector and subsequently used to generate recombinant adenoviruses in HEK293 cells. The resulting adenoviruses were designated as Adv-TGF-β1. An analogous adenovirus expressing only GFP (Adv-GFP) was used as mock virus controls. To achieve local delivery of viral vectors to the injury site, we utilized a thermoresponsive biodegradable polydiolcitrate-gelatin delivery scaffold (PPCNg). A full-thickness radial tear was made in the medial meniscus of the left hind leg of twelve Wistar rats. Rats were divided into two groups: one receiving a mixture of PPCNg+TGF-β1 (n = 3) and the other receiving PPCNg+GFP (n = 3). A 4ºC pre-chilled solution of PPCNg seeded with Adv-TGF-β1 or Adv-GFP at 1011 infectious viral particles/injection was administered at the site of the induced meniscus tear. After 3 weeks, rats were euthanized, and the menisci were histologically evaluated by hematoxylin and eosin (H&E) staining. The histologic assessment and scoring of meniscus tissue wound healing were done by one blinded and one unblinded observer, each using selected wound healing parameters as described in Table 1, and the results were averaged. The intra-class correlation coefficient (ICC) was computed based on a mean rating (k=2), absolute agreement, and two-way random effects model to assess the agreement between the wound healing score rating of the two observers. Statistical significance between groups was determined using the Mann-Whitney Wilcoxon rank sum test. A value of p < 0.05 was considered statistically significant. All animal procedures were performed according to an Institutional Animal Care and Use Committee protocol at the home institution. Results: Immediately after and 3 weeks post-surgery, there was no evidence of infection, dehiscence, or functional deficit. Histologic evaluation of the sections (n = 8/group) of harvested rat menisci (n = 2/group) revealed improved healing of tears in the PPCNg+TGF-β1 group compared to the control group (5.7 ± 0.48 vs. 5.0 ± 0.89, p = 0.011) (Figure 1B). The lesion area demonstrated significantly greater neovascularization (2.75 ± 0.45 vs. 2.06 ± 0.68, p = 0.004), better collagen organization (1.94 ± 0.25 vs. 1.62 ± 0.50, p = 0.038), and increased hypercellularity (1.0 ± 0.00 vs. 1.3 ± 0.48, p = 0.018) in the PPCNg+TGF-β1 group compared to the controls (Figure 1C). Moderate absolute agreement between the two raters was observed in their wound healing score ratings (ICC = 0.615). Conclusions: Our results indicate that PPCNg-mediated delivery of Adv-TGF-β1 enhanced the early (3 weeks) healing of the meniscus lesion in rats compared with control (Adv-GFP) gene therapy. Further, localized administration of Adv-TGF-β1 induced greater neovascularization, better organization of collagen fibers, and increased hypercellularity at the injury site in the Adv-TGFβ-1 rat group compared with controls at 3 weeks post-injury. These results together suggest that TGF-β1 gene therapy may enhance meniscus healing by inducing more cellular proliferation and infiltration and greater neovascularization at the injury site to serve as nourishment for the establishment of new wound matrix, leading to more significant deposition and remodeling of mature collagen matrix. Further study that includes larger sample sizes, confirmation of successful gene transfer to meniscal cells around the tear site, and investigations of both later time points of wound healing and the mechanisms by which local TGF-β1 gene therapy may modulate augmented meniscus healing in this rat model of meniscal defect is necessary to better determine the potential future clinical utility of local TGF-β1 gene therapy to treat of meniscus tears with poor healing capacity. [Table: see text]