Our lab is broadly focused on tissue regeneration and vascularization using biomaterials and biofabrication strategies. The lab employs an array of biomaterial, mammalian cell culture techniques, and animals models to develop translatable strategies. Specific areas of interests include:
1) Elucidating the role of immune cells in neotissue formation
2) Developing injectable biomaterial systems and other biofabrication strategies
3) Developing immunomodulatory strategies for guiding tissue regeneration
4) Developing bioresponsive drug-delivery vehicles
Nonunion of long bones is a debilitating injury and treatment for such recalcitrant wounds is an urgent clinical need. We have developed an injectable therapy for regenerating large and ischemic bone defects using mesenchymal stem cells. It has worked very well in small animals, and we are now in the process of testing them in larger animal models and human patients. Although they have shown a lot of promise, they often require some form of osteoinductive signals to differentiate the progenitor cells for robust bone formation. This prerequisite is not always practical in the clinic. So we are exploring ways to provide these osteoinductive signals through immune cells that naturally infiltrate the implanted tissue. To this end, we are investigating the role of inflammatory cells, mainly macrophages in physiological and pathological conditions. Macrophages are central to the vascularized bone formation during fracture healing. In healing fractures, inflammation, and osteogenesis are intimately linked, and the proper sequence and dose of inflammatory signals are critical for proper bone healing. If we understand how these cells recruit the progenitor cells to the target site and differentiate them to osteoblasts, we can potentially reproduce the same conditions in an engineered scaffold.
Harnessing inflammatory signals to promote endogenous cell recruitment and differentiation has a multitude of advantages over traditional regeneration approaches such as reducing the dependence on exogenous cells and growth factors. Further, our approach can avoid common limitations associated with cell-based therapies, including poor vascularization and immune rejections. Our preliminary work shows that macrophages can be modulated to secrete osteogenic and vasculogenic cytokines and consequently promote endochondral ossification.
A rapid and functional vascularization is indispensable for reinforcing the functionality of an engineered tissue construct and also restoring the functionality of ischemic tissue. We have developed different vascularization strategies using chitosan-glycosaminoglycan microcapsules and vasculogenic fibrin microtissues for creating a network of microvessels. These modules are injectable and once delivered within a scaffold or a tissue they generate microvessels with the lumen. These microvessels inosculate and form a vessel network resembling a terminal vascular bed. Although these strategies are very promising, often they require pre-culturing or preconditioning with growth factors such as PDGF or VEGF, which isn’t clinically feasible.
Currently, we are studying the contributions of immune cells, especially macrophages in vessel formation both in vitro and in vivo. Macrophages play a critical role in tip cell migration and anastomosis. Our preliminary studies show enhanced endothelial sprouting in the presence of macrophages. Further, under certain stimulatory conditions, macrophages are found to secrete vasculogenic cytokines including VEGF-A and PDGF. Using gene and protein arrays, we have investigated and identified phenotypic states of macrophages that can be harnessed for vascularization of tissue constructs and as potential targets for tumor suppression. Our research will shed light on such immune contributions to physiological and pathological vascularization to engineer material-based therapies for vascular diseases.
Macrophages and neutrophils infiltrate tissues swiftly after an injury or infection. Their invasiveness is mainly steered by their secretion of MMPs. Our recently published work showed that the material properties could be tuned to deliver growth factors in a spatiotemporally controlled manner by synchronizing their response with the local inflammatory milieu. Such ‘smart’ delivery vehicles are useful for targeted delivery of drugs and growth factors for healing recalcitrant wounds and inflammation-driven ailments. These ‘smart’ delivery vehicles can titrate the drug release to synchronize with the inflammatory response resulting in optimal therapeutic efficacy. This can reduce the washout of drugs during periods of low disease activity and hence prolong their therapeutic effect. Part of our research is to develop a flare-responsive ‘smart’ delivery system for osteoarthritis (OA). Our approach will tailor the chemistry of the microparticles to take advantage of the proteases-rich inflammatory milieu for the spatiotemporally controlled release of the loaded drugs.