Identification of barriers to retinal engraftment of transplanted stem cells.

PURPOSE Intraocular stem cell transplantation may be therapeutic for retinal neurodegenerative diseases such as glaucoma via neuronal replacement and/or neuroprotection. However, efficacy is hindered by extremely poor retinal graft integration. The purpose was to identify the major barrier to retinal integration of intravitreally transplanted stem cells, which was hypothesized to include the cellular and/or extracellular matrix (ECM) components of the inner limiting membrane (ILM). METHODS Mesenchymal stem cells (MSCs) were cocultured on the vitreal surface of retinal explants. Retinal MSC migration was compared between control explants and explants in which portions of the ILM were removed by mechanical peeling; the inner basal lamina was digested with collagenase; and glial cell reactivity was selectively modulated with alpha-aminoadipic acid (AAA). In vivo, the MSCs were transplanted after intravitreal AAA or saline injection into glaucomatous rat eyes. RESULTS Retinal MSC migration correlated positively with the amount of peeled ILM, whereas enzymatic digestion of the basal lamina was robust but did not enhance MSC entry. In contrast, AAA treatment suppressed glial cell reactivity and facilitated a >50-fold increase in MSC migration into retinal explants. In vivo analysis showed that AAA treatment led to a more than fourfold increase in retinal engraftment. CONCLUSIONS The results demonstrated that the ECM of the inner basal lamina is neither necessary nor sufficient to prevent migration of transplanted cells into the neural retina. In contrast, glial reactivity was associated with poor graft migration. Targeted disruption of glial reactivity dramatically improved the structural integration of intravitreally transplanted cells.