Combined Administration of ASCs and BMP-12 Promotes an M2 Macrophage Phenotype and Enhances Tendon Healing

Outcomes after intrasynovial tendon repair are highly variable. An intense inflammatory cascade followed by a delayed healing response can cause adhesion formation and repair-site failure that severely impair the function of repaired digits. No effective remedies exist to fully address these issues. Cell- and growth factor-based therapies have been shown to modulate inflammation and improve cell proliferation and matrix synthesis and therefore are promising treatment approaches for intrasynovial tendon repair. (1) Can autologous adipose-derived mesenchymal stromal cells (ASCs) and recombinant bone morphogenetic protein-12 (rBMP-12) be effectively delivered to an intrasynovial flexor tendon repair without adverse effects? (2) Do autologous ASCs modulate the inflammatory response after intrasynovial tendon injury and repair? (3) Does the combined application of autologous ASCs and rBMP-12 modulate the proliferative and remodeling responses after intrasynovial tendon injury and repair? Sixteen 1- to 2-year-old female canines were used in this study. Autologous ASC sheets, with and without rBMP-12, were applied to the surface of sutured flexor tendons. Fourteen days after repair, the effects of treatment were determined using quantitative PCR (six per group) for the expression of genes related to macrophage phenotype or inflammation (IL-4, CD163, VEGF, NOS2, IL-1B, and IFNG), cell proliferation (CCND1), and tendon formation (SCX, TNMD, COL1A1 and COL3A1). Proteomics analysis (four per group) was performed to examine changes in tendon protein abundances. CD146 immunostaining and hematoxylin and eosin staining (four per group) were used to detect tendon stem or progenitor cells and to semiquantitatively evaluate cellularity at the tendon repair; analyses were done blinded to group. Gross inspection and cell tracing showed that autologous ASCs and rBMP-12 were delivered to the flexor tendon repair site without the deleterious effects of adhesion and repair-site gap formation. Quantitative assessment of gene and protein expression showed effects of treatment: ASC-sheet treatment modulated the postrepair inflammatory response and facilitated healing by increasing regenerative M2 macrophages (M2 marker CD204, twofold of normal, p = 0.030), inflammatory inhibitor (prostaglandin reductase 1 [PTRG1], 1.6-fold of normal, p = 0.026), and proteins involved in tendon formation (periostin [POSTN], 1.9-fold of normal, p = 0.035). Consistently, semiquantitative and qualitative evaluations of repaired tissue showed that ASC-sheet treatment reduced mononuclear cell infiltration (12% less than nontreated tendons, p = 0.021) and introduced CD146+ stem or progenitor cells to the repair site. The combined administration of ASCs and rBMP-12 further stimulated M2 macrophages by increasing IL-4 (116-fold of normal, p = 0.002) and led to the increase of M2 effector matrix metalloproteinase-12 involved in matrix remodeling (twofold of normal, p = 0.016) and reduction of a negative regulator of angiogenesis and cell migration (StAR-related lipid transfer domain protein13 [STARD13]; 84% of normal, p = 0.000), thus facilitating the proliferative stage of tendon repair. ASCs and BMP-12 accelerated the progression of healing in the proliferative stage of tendon repair. The effects of ASCs and BMP-12 on tendon functional recovery should be evaluated in future studies. The cell sheet approach is an effective, biocompatible, and surgeon-friendly approach for cell and growth factor delivery during tendon repair. Combined application of ASCs and BMP-12 may accelerate intrasynovial tendon healing while suppressing the adverse inflammatory response.