Characterizing the Impact of 2D and 3D Culture Conditions on the Therapeutic Effects of Mesenchymal Stem Cell Secretome on Corneal Wound Healing <i>in vitro</i> and <i>ex vivo</i>

Abstract

The therapeutic effects of secreted factors (secretome) produced by bone marrow-derived human mesenchymal stem cells (MSCs) were evaluated as a function of their growth in 2D culture conditions and on 3D electrospun fiber scaffolds.Electrospun fiber scaffolds composed of polycaprolactone and gelatin were fabricated to provide a 3D microenvironment for MSCs, and their mechanical properties were optimized to be similar to corneal tissue. The secretome produced by the MSCs cultured on 3D fiber matrices versus 2D culture dishes were analyzed by Luminex immunoassay, and the secretome of MSCs cultured on the 3D versus 2D substrates showed substantial compositional differences. Concentrations of factors such as HGF and ICAM-1 were increased over 5 times in 3D cultures compared to 2D cultures. In vitro proliferation and scratch-based wound healing assays were performed to compare the effects of the secretome on corneal fibroblast cells (CFCs) when delivered synchronously from co-cultured MSCs through a trans-well co-culture system versus asynchronously after harvesting the factors separately and adding them to the media. Cell viability of CFCs was sustained for 6 days when co-cultured with MSCs seeded on the fibers but decreased with time under other conditions. Scratch assays showed 95% closure at 48 hr when CFCs were co-cultured with MSCs seeded on fibers, while the control group only exhibited 50% closure at 48 hr. Electrospun fibers seeded with MSCs were then applied to a wounded rabbit corneal organ culture system, and MSCs seeded on fibers promoted faster epithelialization and less scarring. Corneas were fixed and stained for alpha smooth muscle actin (α-SMA) and keratan sulfate (KS), and then analyzed by confocal microscopy. Immunostaining showed that expression of alpha-SMA was lower in corneas treated with MSCs seeded on fibers, suggesting suppression of myofibroblastic transformation. KS expression was higher in corneas treated with MSCs seeded on fibers, suggesting that keratocytic phenotype was preserved.MSCs cultured on electrospun fibers facilitate wound healing in CFCs and on explanted corneas through differential secretome profiles compared to MSCs cultured on 2D substrates. Future work is merited to further understand the nature and basis of these differences and their effects in animal models.