Impact of the Chronic Ischemic Stroke Microenvironment on Silk Fibroin Hydrogel Biodegradation and <i>De Novo</i> 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.