INTRODUCTION: Post traumatic and chronic osteoarthritis (OA) share the underlying feature of damaged hyaline cartilage and exposure of type-II collagen (CII) in the damaged tissue due to erosive proteolytic degradation triggered by low grade ongoing inflammation. Adipose tissue derived mesenchymal stem cells (ADSC) have shown efficacy in the treatment of osteoarthritis through their ability to mitigate the host inflammatory response through paracrine signaling and possess capability for chondrogenic differentiation. However in vivo, very few ADSC bind to the damaged cartilage or differentiate in the OA lesion. Our study offers a solution to these obstacles through the creation of injectable type I collagen (Col I) microspheres loaded with ADSC and linked to an E4D4 IgG monoclonal antibody that specifically targets the type II collagen (MabCII) unmasked in osteoarthritic hyaline cartilage. Upon binding to damaged cartilage, the ADSC carried by the microsphere rapidly migrate onto the damaged cartilage surface. We propose that MabCII linked to Col I microspheres can serve as an intraarticular carrier for transport and targeting large numbers of ADSC to osteoarthritic tissue while promoting chondrogenic differentiation.

METHODS: Isolation and culture of ADSC: The ADSC were isolated from subcutaneous fat tissue of 3-4 months old pigs as described previously (1). All animal protocols and experimental procedures were approved by the Institutional Animal Care and Use Committee (IACUC) at the University of Tennessee Health Science Center. The adipose tissue was digested with 200 μg/mL collagenase II solution (Sigma) to yield ADSC that were cultured in Dulbecco's Modified Eagle Medium-High Glucose (DMEM-HG) (Gibco) supplemented with fetal bovine calf serum (FBS) and antibiotics at 37°C and 5% CO2. The ADSC were characterized by flow cytometry and for multigenic differentiation before experimental use.

Preparation of MabCII targeted Col I scaffold: Lyophilized purified native Col I from fetal calf skin was the gift of Dr. David Brand, Memphis VA Medical Center. The E4D4 IgG monoclonal antibody was isolated and characterized as described previously (2). The experimental group of MabCII targeted scaffolds were constructed using a two-step EDC/NHS crosslinking reaction (ThermoFischer) that bound hydroxyl groups on Col I to primary amine groups on MabCII in a 2:3 molar ratio. The efficiency of conjugation of MabCII conjugation on the Col I scaffold was measured by protein assay of the supernatant after pelleting the collagen by centrifugation at 1000g’s for 10 minutes. Cellular scaffolds were assembled in a solution of DMEM-HG + FBS using 4mg/ml homogenized MabCII-Col I and 16 million cells/ml ADSC.

Experimental group and analysis: 5ul droplets of the cells (80,000 cells) in a 4 mg/ml collagen suspension were plated on a non-tissue culture petri dish to form microsphere pellets and then cultured in DMEM-HG supplemented with FBS and antibiotics at 37°C and 5% CO2 for three days. At this time point, microsphere pellets were either collected for experimentation, or left on tissue culture plate and imaged under light microscopy after 4, 6, and 12 days (Figure 1). Control groups were established using pelleted ADSC (80,000 cells/ 5 microliters) without any Col I and ADSC integrated in a Col I scaffold without linked MabCII similarly cultured. To test the migration of ADSC from the scaffold onto damaged cartilage tissue, ADSC were stained with a fluorescent dye (Vivotrack680, PerkinElmer) before pellet formation. A 5ul MabCII linked scaffolded pellet was subsequently placed on to a 5 mm trypsinized cartilage explant and imaged after 6 days using a Keyence BZ-X800 microscope with a Cy5.5 filter (NIKON). To demonstrate targeting efficiency of our MabCII linked scaffolds, microspheres of fluorescent ADSC, pelleted alone or with Col I or MabCII/Col I were suspended in a 2 ml test tube on a rocker table with 5 mm damaged (trypsinized) cartilage explants. IVIS images were obtained at 4 and 24 hours and total fluorescence of ROI was calculated by Living Image 4.2 software.

RESULTS: ADSC integrated in Col I microspheres and cultured demonstrated release from microsphere and growth on tissue culture plate (Figure 1). Additionally, over 6 days in culture, ADSC migrated from their microsphere attached to the cartilage explant and spread on damaged cartilage tissue as seen by the spread of fluorescence outside of the stationary ADSC/MabCII microsphere (Figure 1). This migration was again demonstrated by scanning electron photography of stationary 5ul ADSC + Col I pellets after 4 days of incubation on trypsinized cartilage explants. The images exhibited ADSC viability on day 4 by capturing active cell division on the damaged cartilage (Figure 2). Finally, IVIS imaging of quantified fluorescence showed targeting of only ADSC/Col I/ MabCII microspheres to damaged cartilage at 4 hours, while the explants incubated alone or with ADSC or ADSC/Col I exhibited no florescence at this time point (Figure 3). ADSC/Col I linked MabCII pellets remained fluorescent on damaged cartilage at 24 hours.

DISCUSSION: Adipose tissue derived mesenchymal stem cell regenerative therapy for regrowth of damaged cartilage for osteoarthritis relies on ADSC’s ability to differentiate into chondrocytes on damaged hyaline cartilage tissue. We demonstrate in vitro that the use of a CII targeting MabCII linked to a Col I scaffold induced chondrogenesis of ADSC and increased the specificity and rapidity whereby ADSC can be delivered to osteoarthritic tissue. Our targeted pellets not only bound more rapidly to damaged cartilage, but they also remained attached after 24hrs and migrated onto the damaged cartilage. These results show promise for an in vivo study using our MabCII targeted Col I microspheres as an intra-articular injectable therapeutic treatment for osteoarthritis.

SIGNIFICANCE: The creation of an injectable cellular scaffold with ability to target ADSC to type-II collagen on damaged hyaline cartilage increases the localization of mesenchymal stem cells to an osteoarthritic lesion at early stages. This increases ADSC therapeutic potential as a treatment for OA.

REFERENCES: 1. Bhatti F, Kim SJ, Yi AK, Hasty K.A., Cho. H. “Cytoprotective Role of Vitamin E in Porcine Adipose-Tissue Derived Mesenchymal Stem Cells against Hydrogen-peroxide Induced Oxidative Stress” Cell and Tissue Research. 2018, 374(1):111-120. (PMID: 29951700). 2. Bedingfield SK, Colazo JM, Yu F, Liu DD, Jackson MA, Himmel LE, Cho H, Crofford LJ, Hasty KA, Duvall CL. Amelioration of post-traumatic osteoarthritis via nanoparticle depots delivering small interfering RNA to damaged cartilage. Nat Biomed Eng. 2021 Sep;5(9):1069-1083. (PMID: 34413494).