Injectable Protein-Engineered Hydrogels to Improve Cell Transplantation

 Low post-transplantation cell viability presents a major roadblock in the success of cell therapy, as functional recovery is correlated with donor cell survival at the therapy site. While deemed minimally invasive, cell delivery by direct injection still result in significant cell death (<5% survival rate), owing to harsh mechanical forces during injection and hostile biochemical environment at the delivery site. Thixotropic hydrogels, characterized by the ability to undergo shearthinning and rapid post-shear recovery, have been used as cell carriers to rescue viability during injection.

We describe the design and characterization of mixing-induced two-component hydrogels (MITCH), which comprise two recombinant protein block copolymers with complementary peptide binding domains. Unlike common hydrogels that require extreme shifts in temperature, pH, or ionic strength for gelation, these physical hydrogels form by a simple mixing procedure, thereby enabling cell encapsulation at constant physiological conditions. Taking advantage of the specific and stoichiometric interactions between the peptide domains, hydrogels with predictable mechanical stiffness can be formed through systematic variation of component concentration and stoichiometric ratio. Crosslinked by reversible physical interactions, MITCH also exhibit thixotropic properties, thus promoting suitability as hand-injectable cell carriers.

Live cell tracking by bioluminescence imaging (BLI) revealed an immediate 1.5-fold improvement in in vitro post-injection viability when luciferase-positive rat mesenchymal stem cells were injected in MITCH, relative to saline. The protective effect of cell encapsulation and injection in MI TCH was also evaluated in vivo against saline, as well as alginate and collagen biomatrices by tracking the viability of luciferase-positive cells injected subcutaneously to the dorsum of athymic mice. Normalized to day 1 BLI signal, the retention of viable donor cells was significantly higher in MITCH throughout the two-week course of the experiment (3.4-fold improvement over alginate at day 3; 2-fold improvement over collagen at day 14). Work is currently underway to assess the ability of MITCH-delivered human iPS-derived endothelial cells to restore blood perfusion in a murine hindlimb ischemia injury model.

The constant physiological conditions under which cells are encapsulated in MITCH also allow peptide and protein drugs to be encapsulated and released in their native, bioactive state. The possibility of simultaneous cell and drug delivery to a therapy site further highlights MITCH as a versatile material for injectable therapy.