Long-Term Effects of Intravitreal Injection of GMP-Grade Bone-Marrow–Derived CD34+ Cells in NOD-SCID Mice with Acute Ischemia-Reperfusion Injury

Retinal vascular diseases, such as retinal vascular occlusion and diabetic retinopathy, remain a common cause of blindness despite therapies available to treat associated complications such as macular edema and retinal neovascularization. Bone marrow stem cell (BMSC) therapy is a new area of research that is being investigated as possible therapy for various ischemic and degenerative diseases.1–5 A subpopulation of BMSCs, referred to as lineage negative in animals or CD34+ stem cells in humans, appears to be recruited to sites of ischemia and injury and play an important role in tissue healing through release of trophic factors.2,4–9 These adult stem cells have been investigated in clinical and preclinical trials as therapy for various ischemic or degenerative conditions because they are easily obtained. Infusion of autologous BMSCs into the coronary artery is being used in clinical trials in patients with recent myocardial infarct to minimize cardiac failure.2,5 Numerous clinical trials that use adult BMSCs are currently ongoing.10–15 The use of intravitreal BMSCs to treat retinal disease has been explored.4,16–20 This route of administration is appealing since it is easy and limited numbers of cells are needed. To date, BMSCs injected intravitreally after mechanical or laser retinal injury were found incorporated in the injured outer retina after a year.16,17 Intravitreal autologous lineage negative BMSCs in mice with retinal degeneration preserved some photoreceptors.4,18 The use of BMSCs in treating retinal vascular disease was explored by Caballero and colleagues20 using murine models of both acute and chronic retinal vascular pathology (ischemia-reperfusion injury and streptozotocin-induced diabetes, respectively). Intravitreal injection of human CD34+ cells resulted in rapid incorporation of these cells into the retinal vasculature within hours of injection. The retinal vasculature of treated eyes appeared to be less damaged than untreated eyes short-term. No long-term studies were done. The use of autologous intravitreal BMSCs to treat patients with vision loss from retinal diseases has been explored in two small pilot clinical trials outside the United States. Jonas et al.21 injected mononuclear cells intravitreally in three patients with end-stage macular degeneration, glaucoma, or diabetic retinopathy. No adverse event was noted during the follow-up period ranging from 2 to 12 months, but no visual benefit was reported. Siqueira et al.22 also used intravitreal autologous mononuclear cells in five eyes with advanced retinal degeneration. No adverse event was noted after 10 months. Two eyes had improvement in electroretinography (ERG). Based on these encouraging preclinical and clinical data, it is important to further explore the use of intravitreal BMSCs as treatment for retinal diseases, including retinal vasculopathy. Isolation of CD34+ cells for delivery into the eye might make this therapy more effective in a clinical trial.20 Before conducting a clinical trial, the current preclinical studies were conducted at the request of the U.S. Food and Drug Administration (FDA) to determine whether there are any long-term ocular and systemic side effects of intravitreal good manufacturing practice (GMP)–grade CD34+ BMSCs in a relevant in vivo model. NOD-SCID (nonobese diabetic–severe combined immunodeficiency) mice with acute ischemia-reperfusion injury were used as a model of acute retinal vasculopathy since diabetic retinopathy could not be induced in SCID mice (Grant MB, unpublished data, 2009).