Impact of the Chronic Ischemic Stroke Microenvironment on Silk Fibroin Hydrogel Biodegradation and De Novo Tissue Formation

The brain has limited spontaneous tissue regeneration capacity after stroke, partly due to the absence of an extracellular matrix in the stroke microenvironment. Self-assembling silk fibroin hydrogels can serve as a tissue-mimetic extracellular matrix; however, more information is needed on their behavior in the chronic stroke setting. We hypothesized that in the chronic stroke setting, self-assembling silk fibroin hydrogels serve as a reliable support matrix for regeneration in the stroke cavity. In this study, male Sprague–Dawley rats (240–290 g, 8–9 weeks old (n = 8) underwent transient middle cerebral artery occlusion 2 weeks before stereotactic injection of 4% w/v self-assembling silk fibroin hydrogels into the stroke cavity. Animals were randomly assigned to be terminated at 6– and 12–months postimplantation (n = 4/group) for blinded immunohistological analysis of the in situ distribution of the silk hydrogels and cellular infiltration and characterization. Results showed that robust in situ gelation with a good hydrogel–host tissue interface was observed with hydrogel remnants still evident at 1-year postgrafting. At 6 months postgrafting, most cells─primarily astrocytes and microglia/macrophages─were localized at the tissue–hydrogel interface and were CD206+ expressing, whereas the cells that substantially infiltrated the center of the hydrogels at 12 months showed a hybrid of CD86+ and CD206+ phenotypes. The hydrogel areas surrounded by macrophages showed evidence of degradation, potentially providing a niche for endogenous neuronal progenitor cell proliferation and migration (DCX+/Ki67+) that was evident in the hydrogels. These findings showed that self-assembling silk fibroin hydrogels effectively induce phenotypic changes in microglia and macrophages chronically after stroke that might favor tissue neurogenesis. These are important features for the development of next-generation stroke therapies.